2017 Nov. Pub. Lecture—Sea Otters: Confessions of a Keystone Carnivore
- Sea otters are perhaps the best-known example of a "keystone predator".
- Sea otter behavior -- in particular diet specialization and limited mobility -- can mediate their effects on ecosystem dynamics.
- Other predators, especially large sea stars, can complement and reinforce the keystone role of sea otters: this became apparent with the loss of all sea stars from wasting disease.
Location Taken: Menlo Park, CA, US
[ Silence ]
Welcome – good evening – to the
United States Geological Survey.
And I have to first applaud all of you
because you saw the lights dim,
and you all hushed. [chuckles]
And I hadn’t even gotten up here.
- [inaudible] sound is not working.
- It’s on.
Maybe I’m not close enough?
Thank you. And thank you for
feedback, so that’s great. [laughs]
So I’m Diane Garcia,
and I work for our
Science Information Services office,
and I’m so glad to see you here.
Before we introduce this evening’s
speaker, I want to go ahead and
let you know there will not be a
December public lecture, but stay tuned.
We will be back in 2018
with a January lecture.
It’s going to be by Doug Given
with our Earthquake Science Center,
and he’s going to talk about ShakeAlert,
the path to West Cost earthquake
early warning – how a few seconds
can save lives and property.
So I hope to see you all here on
January 2-5, 2018, for that. Yeah.
I want to make sure I give the right date.
But what we’re really here for tonight
is a lecture, Sea Otters –
Confessions of a Keystone Carnivore.
And it’s going to be
presented by Dr. Tim Tinker
Tim is a research wildlife biologist with
the Western Ecological Research Center
for USGS and an adjunct professor
in the Department of Ecology and
Evolutionary Biology at the
University of California-Santa Cruz.
Since 1993, he’s been studying
sea otter populations and their roles
as top predators in California,
Alaska, British Columbia,
and the Russian Commander Islands.
Dr. Tinker is a project leader for
federal research on sea otters
in California and heads a multi-agency
study investigating the
factors limiting the recovery
of this threatened sub-species.
Tim’s research also focuses on the
ecology of coastal marine communities,
particularly the suite of direct
and indirect interactions
between sea otters and other species
in the near-shore environment.
And he uses this as a model system
to elucidate the influence
of high trophic level consumers
on organization of the communities
within which they live.
Tim utilizes quantitative modeling
approaches to integrate diverse data sets,
exploring how individual-level processes,
like physiology behavior,
translate into population-level dynamics
like recovery, decline, and redistribution,
and they can affect food,
web structure, and ecosystem health.
So the USGS’ monthly public lecture
series is pleased to bring you a program
this evening about sea otters and
the ecosystem with which they live.
Let’s give Tim a
big round of applause.
[ Applause ]
- Thank you. Well done.
[ Applause ]
Well, thank you very much for
that wonderful introduction.
And the round of applause even before
I’ve said anything. That’s remarkable.
[laughter] Thank you all for
coming out here to hear tonight
the confessions of our
friends, the sea otters.
Before I begin, how many of you
here have seen sea otters before?
Okay, so I’m talking to a knowledgeable
audience about sea otters.
How many people here like sea otters?
Wow, okay, that’s good to
know even before we begin.
All right, how many here – people here
have been bitten by a sea otter before?
Fewer. That explains why you
all like sea otters. [laughter]
I have been bitten by sea otters,
and I still like sea otters.
Although I just have
a healthy respect for them,
and we try and avoid
the sharp end, as we say.
So you may wonder, why would I spend,
though, 25 years studying sea otters?
That seems like an awful long time.
You’d think I – one would know
everything that there is possibly to
know about sea otters after that time.
And of course, we don’t.
And they are fascinating animals.
All aspects of their behavior,
their population, biology, is fascinating.
And that alone would keep any
studying them for 25 years.
But in fact, that’s not all we’re studying.
And the reason we have spent
so much time and effort studying
this particular organism is because
we really use them as a window
into how the near-shore ecosystem
that they live in functions.
You can do that, of course,
with many different species,
but as I think you’ll – hopefully you
will see tonight, sea otters have sort of
a unique place in that near-shore marine
ecosystem that most of you are familiar
with anytime you go to the ocean here.
If you just look out that sort of first,
you know, 200, 300 yards out to sea,
that’s the environment
that sea otters live in.
They don’t swimming – they’re not
like other marine mammals swimming
hundreds of miles offshore.
They spend their lives in that
really narrow fringe of habitat that
they basically share with us and
with a lot of other
creatures in that environment.
And, as I’m going to – the purpose
tonight is to talk about the important
ecological functions that they
play in those ecosystems and
how we have learned about that
through a variety of different methods.
And I guess I also want to say that
what I’m going to be presenting
to you tonight is not my work at all.
It’s the work of an incredibly
dedicated collaborative team of people.
So I just want you to imagine about
with me because it’s really on
their shoulders that I’m standing
and presenting this work.
It’s taken a long time,
and I’m very proud of the
things that we’ve found, but it’s
definitely been a team effort.
Okay, so let’s jump
into the confessions now.
And I’m going to
give you a little roadmap
of what I want to try
and cover tonight.
It’s quite a bit of ground, and so I’m
speaking slowly now, but by the end
of the evening, I’m probably going to
be talking very quickly, so be prepared.
The first topic I’d like to cover
is a bit of background context.
And that is, why the fuss about sea
otters? What are – do we even mean
when we refer to them as a “keystone”
species or a “keystone” predator?
And what exactly is a trophic cascade?
So we’re going to cover that and give
you a little – sort of a primary
ecology lesson on near-shore ecology
and sea otters’ role in that.
And then, with that background,
we’re then going to plunge forward
into some of our current research
So the next topic will be San Nicolas
Island – the Return of the Keystone.
Some unexpected and
San Nicolas Island is the most
remote of the channel islands
in the Southern California Bight.
Most of you are – will be familiar with
some of the other channel islands, like
San Clemente, Santa Rosa, San Miguel.
Many of you won’t know San Nicolas
because you can’t go there.
It’s a Navy base, and so getting out there
basically involves that you either
work for the Navy or you have
permission for the Navy to get out there,
as we do, to do research on some of
the animals out there. So it’s way, way
out there, way offshore, straight off
of Long Beach, basically.
And I’ll give you more background
about that island and about the unique
population of sea otters out there, and
they’re an experimental population that
were translocated out there and what
we have learned from that translocation.
And some of the things we’ve learned
were things that we expected based on
the theory that we covered in number 1.
Some of the things we found
were very unexpected.
And so this third topic is going to
sort of be delving into some of the
complications and complexities of
behavior that we think explain
some of those unexpected patterns.
And the particular complexities
I’m going to cover today will be
individual diet specialization and the sort
of unique spatial ecology of sea otters.
Both those things are topics that
will make sense when we get there.
They’re topics that we’ve learned a
great deal about over the last 20 years.
We didn’t know anything
about 20 years ago.
And as a result, a lot of our predictions,
well, were frankly wrong because
we really didn’t account for
those complexities of behavior.
So that would be topic 3.
And then finally, building on
those complexities, I’m going to
talk to you about some work –
a project that’s going on right now.
We just started about a year ago.
Some exciting and quite remarkable
changes happening in the near-shore
environment, really up and down the
entire west coast of North America.
We’re studying it in-depth right
around the Monterey Peninsula.
It involves kelp forests, urchins,
sea otters, sea stars, and a lot of
changes in that ecosystem.
And, again, some unexpected
findings that relate to
those behavioral complexities.
So there you go. There’s a roadmap of
what we’re going to try to cover tonight.
But we’re going to start
by way of an introduction.
So first of all,
what is a keystone species?
And some of you here probably
know that already, but just in case,
I’m going to sort of
give you a brief definition.
A keystone is generally understood to
be a species, often a high trophic level
consumer, that has strong effects on
community structure and dynamics –
very strong relative
to its abundance, okay?
So what do I mean by
high trophic level consumer?
“Trophic” is just a fancy
way of saying food or eating.
So a trophic level is
a level in a food web.
A high trophic level consumer is
something that is a predator up on –
sitting up on top of the food web.
Nothing eats it. It eats – it eats things
that are below it. So just sort of think
of it as the capstone of the food web.
Decades ago, it was generally
understood in ecology that that capstone
of a food web was kind of like
an ornament on a Christmas tree.
It looked pretty, but it
didn’t really do very much.
And so, in fact, you could
remove predators from ecosystems
and not worry about
having much of an effect.
We now understood that to be
completely and utterly wrong,
and there are many cases now
where we realize that those
predators were not just ornaments.
They were doing really important things.
And when we remove them, whether
those were grizzly bears or wolves,
sea otters, coyotes – lots of other
big species – mountain lions,
that were removed from systems,
those systems changed in ways
that were often unexpected
and not always good for us.
And so, over time, there’s been a lot
of movement to try and recover these
top predators to the – to their
functional roles within ecosystems.
And some of those predators of the top
systems have really, really big effects
on their systems. That’s like sea otters,
and that’s why we call them keystones.
In many cases, what makes them
a keystone is that they can trigger
a shift in the ecosystem between
alternative stable states.
Okay, what do I mean
by alternative stable states?
I’m going to illustrate that with an
example that’s relevant to tonight.
So if you go out to the rocky reefs
that you can find offshore – if you
go over to Santa Cruz or Monterey
or anywhere along the coast here of
California, some areas are sandy, but
then you see other areas where there’s
sort of rocky reefs, both onshore and
offshore, if you dive under the water.
And if you do that, if you go out
and you snorkel or dive down,
you’re going to find those rocky
reefs in one of two basic states.
Either they’re going to be
urchin-dominated – that means
there’s going to be a lot of those things –
purple or red urchins here.
If you go farther north,
they’re going to be green urchins.
There’s different species of urchins,
but in general, they’re the dominant
herbivore that graze down various types
of algae that grow on the bottom.
And so, if you find an urchin-dominated
state, you’re going to find little bits
of kelp here and there, but for the
most part, you’re not going to find
a ton of algae, and the algae
you do find will mostly be small,
low algae like
You’ll find lots of urchins and then
different species that go along with this.
So there are a whole bunch of other
invertebrates and some small fish,
but it’s going to be mostly an
Alternatively, you may go to that same
reef at a different point in time, and
you won’t find it dominated by urchins.
You’ll find it dominated by kelps.
So this is the kelp-dominated state.
In this case, you’re going to
see large macroalgae.
We call these giant kelp.
Some species are – don’t go to the
surface, so they’re sub-canopy kelps.
Other kelps grow up and form a dense
layer along the surface of the water.
We call those canopy-forming kelps.
But there’s a host of other plants and
animals that are living within that kelp
forest – invertebrates and fish that
rely on the structure of that kelp forest.
So in this kelp-dominated state,
it’s not just the kelp that’s different –
not just the urchins that are different.
It’s really the whole suite of
plants and animals that differ
between these two states.
And they’re stable in the sense that,
once they’re in one of those states,
they tend to remain in those –
that state for a long period of time,
and it take some sort of big perturbation
to kick them from one
state into the other state.
So either state is stable, but it can –
sometimes there’s a big perturbation
that can shift them from
one state to the other,
and one of those perturbations is the
introduction of a keystone predator.
So the sea otter, being a keystone
predator, as you might guess,
is one of those things that can
shift a rocky reef system
from an urchin-dominated
state to a kelp-dominated state.
It does this primarily by
eating a lot of urchins –
more urchins than anything else.
There are other species that do
eat urchins, but they’re just –
they’re a tiny fraction of a percent
as much urchins
as sea otters eat.
And that’s because they have such
an incredibly high metabolic rate.
So when otters come to a system,
rather quickly, it shifts from
the urchin-dominated state
to a kelp-dominated state.
This paradigm is based on a suite
of what I’m going to refer to
as opportunistic experiments
in Alaska and British Columbia.
What do I mean by an
I mean an experiment that we really
had no control of, although people
did have control of this
particular experiment because
it was the
north Pacific fur trade.
So prior to 1740, this is the Aleutian
Island Archipelago in Alaska.
There’s the state of Alaska, and that tail
of Alaska stretching out towards Russia
is the Aleutian Archipelago. And there’s
hundreds and hundreds of islands.
They’re volcanic islands.
They’re very beautiful.
Imagine Hawaii, but much windier
and much colder. [laughter]
That’s the Aleutians.
Not a big tourist trade there.
I’ve spent a lot of time up there.
Prior to 1740, we know sea otters
existed throughout that island because
there are human midden sites on those
islands from past civilizations, basically.
And they – in those midden sites,
there’s sea otter bones throughout the
entire chain. So we know otters were
present throughout the entire chain.
It was the Bering expedition from
Russia set out – Vitus Bering
and crew, including George Steller,
set out from Russia from
Petropavlovsk and came to Alaska.
They only spent about 18 hours on the
shores of Alaska – a pretty brief visit.
Although George Steller, the ship’s
naturalist, named lots of things
after himself during that time.
And then they turned around
and headed back.
They didn’t make it back that year.
They were shipwrecked in Bering Island.
And actually, Bering himself died
on Bering Island over that winter,
as did a lot of others.
But they did lots of things.
They discovered the Steller’s sea cow,
which they ate a lot.
And the Steller’s sea cow
was due to go extinct
in about 15 years
after that, sadly.
And they also hunted a lot of sea otters,
and they brought back those pelts to
Russia, and that set off the fur trade –
the European fur trade for the next
valuable commodity on the planet Earth.
And so every major nation set out to
get as many sea otter furs as they could.
And so, by the time 1900 rolled around,
sea otters were virtually extinct
throughout their entire range in the
north Pacific, with the exception of
a few little colonies where –
these arrows are pointing at places
where we’re pretty certain
there were remnant colonies
of maybe a few dozen animals.
And these were generally places
that you just couldn’t get a ship
because they’re just too reefy.
They couldn’t anchor the ships there.
So they couldn’t get there to
kill those last sea otters,
which was very lucky for sea otters.
They were protected by
international treaty in 1911.
And then they began to recover,
as otters do often – or, animals do that.
When you stop killing them,
they’re often pretty good at recovering.
And over the next 70 years,
they recovered over much of
the range in Alaska.
I’ll talk a little bit more about
their recovery in other
places in North America.
But in the Aleutian Islands,
the yellow islands here are
showing islands where, by 1970,
there were either growing or,
in some cases, even stable
populations of sea otters.
They had already reached
equilibrium at some islands.
At other islands, they’re at low density,
and at many other islands –
all these ones that are white –
they hadn’t reached there yet.
And so what you have is
a natural experiment,
essentially controls and treatments
of predator removals
and then predator
returns to some of these islands.
If you were going to do a
huge Pacific-wide experiment,
it might be
something like that.
Again, I don’t recommend that,
but that’s what the fur trade had done.
So we took advantage of that to
do comparisons of the near-shore
ecosystem at islands with sea otters –
with abundant sea otters,
with low-density populations,
and with no sea otters
to see if they were different.
And they were incredibly different.
And in fact, this was
one of the things that led to
the concept of these
alternative stable states.
Islands that didn’t have sea otters
universally looked like this.
You could dive down anywhere around
the island – anywhere you want –
throw down a random site, and what
you’d find was solid carpet of urchins.
Occasionally there would be a patch
of rock that urchins couldn’t get to,
usually because there’s lots of sand
around it that sort of protected it.
And then you’d find little patches
of kelp, but for the most part,
When you went to islands such as
Amchitka, where otters had recovered
first and were already at high population
density, it was completely different.
When you dove down anywhere around
the island, you found these thick,
luxuriant kelp forests with a whole suite
of different animals around them.
And then, over the – otters were
just arriving at Attu in 1972.
And by the late 1980s,
the Attu environment had shifted
from this to this –
to the kelp-dominated.
So it was actually an experiment to
actually see what happened when you
return sea otters to that environment.
So really dramatic changes.
And then, in the late 1990s –
well, actually, in the early 1990s,
we were up there studying the
populations at a couple of these islands,
Amchitka Island and Adak Island.
Mainly we were studying those
populations because we wanted
to compare a stable population
with the population in California
to get a sense of what was
different about, you know,
a healthy, stable population.
And surprisingly, it turned out –
well, it started out – when the study
started, they were healthy, stable
populations, but something changed.
And what changed was a behavioral
innovation by another predator –
probably ultimately just one pod of
killer whales, although this probably
spread to other pods of killer
whales by cultural transmission –
discovered that they could go
into kelp beds and eat sea otters.
Probably because a lot of their
other prey – Steller’s sea lions,
particularly – had been declining
for the previous 20 years before that.
This behavioral innovation spread,
as I mentioned, and killer whales
ate a lot of sea otters.
So many, in fact, that they caused
about a 90% population
decline within just five years.
So the population plummeted at
these islands that we were working.
And we just happened – I mean,
we could have missed it if we hadn’t
have been out there doing these studies.
But luckily we were, and so we saw
this happening in real time, and so
we were going around and surveying
these islands as fast as we could to try
and document what was happening.
And what was happening was
what you might expect would happen
when you add another trophic
level above the sea otters.
The killer whales controlled
the abundance of sea otters.
Sea otters no longer controlled urchins.
Urchins increased very rapidly
because there’s extremely high
urchin recruitment in the Aleutians.
And the kelp was decimated.
And so, by 1999, all the sites that,
over the previous 50 years,
had transitioned from urchins to kelp
transitioned right back again to urchins.
And if you go to any of the Aleutians
today, anywhere in the range,
you will see very, very little kelp.
You’ll just basically see these
vast urchin barrens underwater.
So it’s a very different situation,
and that, again, gave us
further understanding of
how these ecological interactions
between sea otters and their prey,
and now their predators, worked.
So that same general paradigm seems
to apply very well throughout Alaska,
southeast Alaska, British Columbia –
there’s been a lot of work over the
last decade or so at the – looking at
the recovery of sea otters there.
And the same phenomenon happens.
As otters recover, urchins decline,
the kelp forests come back,
there’s increases in rockfish
and all the other species that
depend on the kelp forests.
So it seems to be a pretty solid
paradigm for the northern areas.
Where it becomes more contentious
and complex is in California.
In the 1970s and 1980s, there was
a whole suite of papers going back
and forth arguing about how applicable
this paradigm of keystone predators
and alternative states was in California.
One of the arguments was just,
California was a lot more complicated,
and otters were just another
brick in the wall. They weren’t really –
that was an allusion to a popular album
at the time, of course.
That sea otters were really not more
important than any other species.
Others felt that they were,
and that it might just be sort of
a different aspect of that.
And then that debate just kind of ended.
It never really was resolved,
although there’s generally an assumption
by most ecologists that otters do play
an outsized role even here in California.
But we realized we had a chance to sort
of resolve this question through another
opportunistic experiment. And this
one a little bit nicer than the fur trade.
This was the translocation of sea
otters out to San Nicolas Island –
this most remote of the channel islands.
And San Nicolas – the purpose of
this translocation was not
for the experiment, actually.
The purpose of the translocation was to –
as part of the recovery plan for the
sea otters, it was realized that their
distribution in central California
matched that almost perfectly
of the area affected by oil
in the Exxon Valdez oil spill.
So if there was an oil spill of the
same magnitude of the Exxon Valdez,
it could literally kill 100% of sea otters
And that was, of course, a concern.
Because oil spills
actually do happen,
and they – we wish they didn’t,
but they happen periodically.
So it was – it was decided by the
Fish and Wildlife Service to
establish an independent population
geographically separated from
this mainland population,
essentially as an insurance.
So before that translocation happened
in the late 1980s, early 1990s,
it was realized this would be
an opportunity to experimentally
see the effect of sea otters
on the sub-tidal community.
And so a monitoring program was
set up to study that sub-tidal community
around the island to see what
sorts of changes would happen
when sea otters were brought out there.
And the methodology was similar to
the methodology we used in Alaska,
which means lots of sub-tidal sampling
like this, setting down these swaths
and random quadrats to measure the
relative abundance of different species
of plants and animals on the bottom.
And then – and then return
to those sites year after year –
actually two times a year –
every spring, every fall,
for the last 37 years,
those sites have been sampled
by these techniques to look at
community changes over time.
And so that brings us to number 2 –
San Nicolas Island –
the Return of the Keystone. Some
expected and some unexpected patterns.
So here’s our crew.
And actually, a lot of these guys are –
that’s Mike Kenner
in the center.
Mike has been sort of heading up
that sampling program since 1987.
He’s got a few more gray hairs than
he had back then, but don’t we all?
And he – this is some of his
younger folks here beside him.
They’re out there actually right now,
as we speak, this week, sampling.
They’re doing the inner-tidal
sampling, actually, right now.
But they’ve been out there
twice a year doing the work.
And this is San Nicolas Island.
As mentioned, it’s a Navy base.
You can see some of
the landing strips there.
So that means that it’s
very hard to get out there.
But also it means that it’s a great place to
do research because there’s not a lot of
people there. It’s highly protected. So
essentially, a very pristine environment.
If any of you have ever read the Island
of the Blue Dolphins, that’s the Island
of the Blue Dolphins where the
lone woman lived – on that island.
And there’s some really
cool archeological relics
to do with that as well,
but that’s a story for another day.
Okay, so what – our experimental
treatment, again, is the addition
of a predator to this island to see
how it would change over time.
And we had variation
in the abundance of that predator –
the sea otter –
over both time and space.
The over time, of course,
is with the re-introduction
and then their increase over time.
I should mention that about
brought out to San Nicolas Island.
We discovered after that,
really an amazing behavior
that sea otters had was the ability
to home back to the original –
the original territories.
Even though they had been flown out
in crates on a cargo plane, you know,
hundreds of kilometers, they had an
amazing ability to just, like, open it up,
and pffsh, off they’d swim –
show back up in their territories,
which is pretty amazing. [laughter]
And some of them –
some of the younger ones
didn’t find their way home
and were returned a couple times.
Eventually, about a
dozen of them stayed.
So by 1992, there were
just 12 sea otters on the island.
But they – as you
can see from this chart,
they increased over time, and now
we’re up to almost 100 sea otters.
So it was a slower increase
than we expected, but nonetheless,
they’re now reaching the abundance
that we might expect them
to start having effects
on the sub-tidal community.
And so that’s the variation over time.
The variation over space
we were also not expecting.
We kind of expected – you know,
the island is only, you know,
about 20 kilometers long.
We figured there’d be an equal effect of
sea otters all the way around the island.
It turns out, no, that was not the case.
Sea otters – what you see here is sort of
a heat map showing the relative density
of sea otters over time, with all these
dots being sea otter groups over the
last three of four years of surveys.
And you can see they
spend almost all their time
out at the west end
of the island.
There’s almost no use at all on the
north side of the island and very little
use by sea otters of the southern
and southwest side of the island.
A little bit of use down here, especially
in the winter when there’s big storms.
But for the most part,
this is where sea otters are
spending all their time,
doing all their feeding,
and therefore, that’s where we’d
expect them to be having effects.
So part of the paradigm, of course,
requires that sea otters would have
their effects on the kelp
forests by eating a lot of urchins.
They – luckily, here’s one thing that
they did that we expected them to do.
At last, they did something right.
They ate a lot of urchins.
And we went there in the early 2000s
and conducted a study looking at their
movements, their behavior, their survival,
their reproduction, and
most importantly, their diet.
And I’ll tell you in a few minutes
how we do those dietary studies.
But basically, it turns out that
about 80% of their diet were
these big urchins – red urchins.
There’s also purple urchins out there.
They didn’t eat as many purple
urchins because they like these
big red ones because they’re full
of calories. They’re pretty big.
They’re just chock-full of calories.
They’re like little energy bombs.
But most of their diet, of the
urchin diet, were these red urchins,
but some purple urchins as well.
Especially now that the red urchins are
mostly depleted by sea otters, they’re
starting to eat more purple urchins.
Okay, so what I’m now going to do
is I’m going to show you – I’m going to
walk you through the data from three
of these sites around the island.
Nav Fac is a site along the north side
of the island, or the northeast side of
the island, and that’s, you remember,
is where the otters have used
that site hardly at all.
So any variation we see here does not
have to do with sea otters because
they’re just – they’re just not there.
The south side of the island,
Dutch Harbor, there is some use
by sea otters, though not
nearly as much as the west end.
So we might expect them to
maybe have some small effects there,
but we don’t know that they
spend enough time feeding there.
And then the West End.
This is where sea otters
spend most of their time,
so if there are going to be effects
of sea otters in the sub-tidal community,
we’d expect it to be out here.
Okay, so how do we measure
variation in community state?
I’ve been talking – you know,
mentioned the urchin-dominated
and kelp-dominated, and I’m referring
to sort of these alternative stable states.
But when I’m talking about a
community, and I’m of course talking
about a huge suite of plants and animals
that make up a community, okay?
So to try and describe that,
we have a – we have a real challenge.
We can actually show,
individually, variation –
you know, 100 different species,
but that would be extremely
confusing and hard to
make any sort of sense of.
So we use statistical techniques to
sort of boil those down to a sort of
a smaller conceptual and mathematical
space that we can make sense of.
And now let me explain the –
so the technique is called non-metric
multi-dimensional scaling, or NMDS.
That’s a mouthful, I know.
Basically – but it’s a mathematical
approach using some other
very illustrative terms,
like Bray-Curtis non-similarity indices
and things like that to take a whole
bunch of different dimensions and
collapse those dimensions down into just
two dimensions. It’s a really neat trick.
And in two dimensions,
it’s a lot easier to measure
differences between communities.
So each of these dots here could be
a particular community – a particular
reef at a given time and space, right?
So Community A and Community B
and Community C, we’ll call them.
And what we can see from
this simple graph now – again,
this is all of those different species
boiled down into just two dimensions.
Community A and Community B
are very similar because they’re
close together. Community C is
different because it’s farther apart.
So that’s all you really
need to know is that these two –
these different dimensions are
ways of looking at similarity
or differences between communities
over space and over time.
So there’s all the data in that
multi-dimensional scaling space.
And it’s still a big mess.
And it’s a big mess because I –
on purpose. I’ve thrown everything
on there so it looks just sort of like
the dog’s breakfast, as they say.
But what you can see – if you
start to sort of squint your eyes,
you can see that
there are certain clusters.
So this is one of the sites,
and you see that it’s in one single cluster.
That means, over time – over 37 years,
the community state stayed –
for that particular site, stayed
really similar in one place in time.
And then another ones of the sites,
you see that it sort of clusters
into three different groups.
So that tells you that that community
seemed to vary in different ways
over time, and there were sort of
three different clusters of
community states over time.
So that’s just to sort of
give you a preview.
Now I’m going to actually start to delve
into each one of these sites and kind of
walk you through what happened
and why we think it happened.
So the north side –
this is the Nav Fac site.
First of all, I’m just going to show
you the sort of spaghetti plot.
It’s just showing
the relative abundance –
each line is showing the
relative abundance over time
with one of the main species – the most
dominant species in the community.
So we have Macrocsytis, which is the
giant kelp. That’s the light green.
Purple urchins is the purple line.
They’re the most dominant grazer.
Red urchins are the red line.
As you can see, they’re not very
abundant relative to the purple urchins.
And finally, the understory kelps,
which is a whole suite of kelps
that don’t make it up to the surface,
but they’re really important
down low to the ground.
So those are – you can see there’s
a lot of variation over time.
Particularly, you can see there’s
these great big booms and crashes
of the purple urchins.
And at a couple points in time,
those purple urchins crashed
down to almost zero.
And you might think, wow,
is that driven by otters?
But of course, then you remember,
no, because the otters aren’t there,
at least on the
north side of the island.
So it’s probably not sea otters
causing those crashes.
In fact, what we think is driving
those crashes is sea urchin disease –
a disease that, when urchins are
really abundant, sweeps through the
population really fast, kills about
in that brief time after all the urchins are
gone, you have a little boom of kelp.
But then the urchins begin to return,
and so you see that subsequent increase.
So that’s – so that’s what we see when
you sort of look at one species at a time.
Now let’s sort of go back
out to this two-dimensional
nMDS space –
the community space.
And I want to show you that
this is actually a dynamic thing.
I’ve arranged this incredibly complicated
cartoon that you’re going to see.
I’m very proud of this animation.
You can see up here – that little
rocky cartoon, there’s our
community right there –
rocks and kelps and algaes
and some other little things.
And that community is going to show –
it’s going to follow the trajectory
of the actual community over 35 years,
but it’s going to do it really fast –
much faster than 35 years. [laughter]
Okay, here we go.
Spending some time over there, then
over here, then down here, then back to
there, and then down, and that’s where
it finally ended up right there. [laughter]
So you see that?
Oops. We can even do it again.
Again, this cluster, that cluster,
back to this cluster, and down.
So it seems that this – at this site,
the community is transitioned
between two different sort of
clusters of community states.
And each of those clusters
might represent a particular
sort of stable
In fact, it looks a lot like what we
were describing as our alternative states.
And when we look at the actual relative
abundance of, say, some of the main
species on there – so the size and color
here is referring to the different species.
So there’s the purple urchins right there.
There’s the giant kelp right there.
What you see is that the
community cluster on the left
is the urchin-dominated state.
The community cluster on the
right is the kelp-dominated state.
So basically, this community
was bouncing back and forth between
urchin-dominated and kelp-dominated,
but it was doing so without the help
of sea otters. So that was interesting.
Then we go down to Dutch
Harbor on the south side.
we’re going to do this –
I don’t – this will go faster because you
now know what everything means.
Here, again, is sort of those
four dominant species.
And you see a very different pattern.
There’s no huge crashes and booms
of urchins. There’s no huge
crashes and booms of everything.
Instead, you just sort of
see fluctuation over time
of all these species
around sort of a stable point.
And when we look at that in the nMDS
space, we see a really different pattern.
We don’t see two different clusters
and a community bouncing
back and forth between them.
In fact, we see there’s really just
sort of one cluster, and the – we call this,
in ecology, a basin of attraction.
Because you can kind of – you see
these sort of contour lines I’ve drawn
around here. Actually, those represent
contours of a bivariate normal plot.
Those contour – and sort of –
you can think of it as a basin, and the
community is caught in that basin.
We called these basins of attraction.
And when you have different stable
states, you have two basins of attraction.
And if you had a little
ball in there, it might jump
from one basin to another basin.
But it tends not to want to leave a basin.
This community is stuck
within one basin of attraction.
So that seems to – that suggests
it’s a very stable place.
And when we look at
urchins and kelp,
we see that they’re both
present within that basin.
So what defines Dutch Harbor is that it’s
the most topologically complex area.
The bottom of that –
there’s huge pinnacles.
There’s all kinds of
really cool rock formations.
And in that complexity,
there’s sort of micro-habitats
It’s also the place where we have
the most abundant fish populations.
So there’s lots of kelp. There’s some
pinnacles that the urchins sort of
have taken over, and there – so there’s
urchin areas, there’s kelp areas.
There’s kind of everything
within this one area.
So it seems that habitat
complexity is really important
in conferring stability
to different places.
So that’s the story of Dutch Harbor.
Again, not much of a signal of sea otters
at all. Probably not surprising because
otters still hardly use that area.
So now we come to the West End site.
And again, just this little cartoon here
is showing you the relative abundance
of sea otters, going from almost nothing
up to about 70 otters that are
feeding within the area
of those West End
So we might expect, if there’s
going to be an effect of sea otters,
that over the latter half of this period
is where we might expect to see it.
So let’s start again.
Let’s look at those individual species.
If you look just to the left-hand
side of the plot, you see something
that looks a lot like Nav Fac.
You see these bust – boom-and-bust
cycle of the purple urchins, with
crashes suggestive of sea urchin disease.
And of course, after the crashes,
you get a temporary boom in kelp.
But then, after the second crash of
urchins around 2000, that crash
was persistent. The urchins never
bounced back from that crash.
You do remember, over on the north
side of the island, they did bounce back.
But here, once they got low,
they stayed low. Why might that be?
Well, remember, the sea otters
are increasing over this time,
and it may be that, once the urchins got
low enough in abundance, otters were –
at this abundance, were able to
control them and keep them down.
Once they were down,
otters kind of held them down.
So now let’s look at
the community state.
So I’ve got a little –
our bouncing cartoon is back now.
And you’ll notice, this is a lot –
a bit more complicated.
There’s – rather than just two sites,
it looks like there’s really three.
Two sites here, and then this
sort of additional sort of cluster of
community states over to the right.
So let’s watch what happens.
Okay, and it looks like it’s
bouncing back between two.
And then it ends up
over at that third site.
And the growing otter is
just showing the otter population.
If you watch it again, it’s bouncing
back and forth between those two,
and then, just right at the end,
it goes over to this new state
that it hadn’t been
at ever before.
And if we look at the rate of abundance
of some of the main constituents here,
we see that left side is really
the super urchin-dominant site.
Urchins are less abundant at the – at the
site at the right. Kelp is more abundant.
But kelp is even more abundant
still at this far-right cluster.
And here’s what’s really interesting.
These sub-tidal kelps become
super abundant at this far-right
cluster that the community ended at.
This is a greater abundance of these
sub-tidal kelps than we’d seen at
any time anywhere around
San Nicolas over the last 37 years.
And it seems to be something that’s due
to the recovery of sea otters, that site.
Once they kept urchins low for
long enough, then you have
competition between the kelps.
And then those sub-tidal kelps
eventually begin to
out-compete the Macrocystis,
and you have a whole new kelp
community beginning to occur.
So in summary – San Nicolas Island –
what did we learn?
Well, we [chuckles] – we learned
that we were wrong about
a lot of our expectations.
That’s always a good thing to learn.
We learned the sub-tidal communities
did indeed shift between alternative
states at some sites. However,
the number of basins of attraction
varied between the different sites.
Some sites, there’s only one basin
of attraction, and the community
stayed there the whole time.
Other locations, there was indeed
a fluctuation between kelp- and
urchin-dominated, but otters did not
seem to be necessary to be driving that.
Looked like – looked like
disease could play a role.
However, sea otters probably
did play a role in driving that
West End site to a whole new state
that we hadn’t seen before
characterized by really abundant
understory kelps and fleshy red algae.
So, again, other factors also mediate
state shifts, like habitat complexity
and disease. And the high site
fidelity of sea otters also
made their impacts a lot more
localized than we had expected
when we set out at the
beginning of this experiment.
So why are –
why are otters so local?
Why aren’t they just feeding all
around the – around the island?
Well, that sort of gets at topic number 3,
the complications added from behavior.
And I’m going to talk a little bit – both
about the spatial ecology of sea otters,
but also about this phenomenon,
So, you know, before I get to what I
mean by diet specialization, I’m going to
give you a really quick crash course
in sea otter foraging ecology.
This has been something that
we’ve been looking at a lot
over the last 20 years with sea otters.
And in fact, I would say, out of
all large carnivores, we probably
know more about sea otter diets
and foraging behavior and
foraging ecology than any other
terrestrial or marine.
And the reason I say that is
because otters are pretty unique,
for any carnivore –
again, terrestrial or marine.
They dive down to the bottom to
get their food, and we can’t really
see that part, but then they
bring it all to the surface,
and they eat their prey
lying on their backs
holding it up on
their chests, eating it.
And they often even do this thing where
they actually hold it up so we can see it.
So if you’re on shore with a telescope,
you can see every single prey that they
bring up. You can even measure it
relative to their paw width.
And we know – since we’ve measured
a lot of sea otter paws, we know
exactly how wide each paw is.
So they have a little ruler.
They measure the prey for us.
Then they eat it. [laughter]
So basically, [laughs]
it’s almost kind of crazy.
They’re just this perfect thing to
measure how much different food of
different types that they’re consuming.
And so we’ve obliged them by
studying that over the last 20 years,
and we’ve learned a lot about that.
And so we’ve – one of the first
things we wanted to do
as we’ve studied this was sort of
compare them to foraging theory.
And foraging theory suggests
that predator populations,
as they increase in abundance,
are expected to expand their
dietary niche as their preferred
prey become depleted.
Basically, as you have an increase
in interest-specific competition,
you expect more and more
less-preferred prey to be –
to enter into the diet, and therefore,
an increase in their dietary niche.
So when food abundance is
really low, as the populations
reach carrying capacity, we expect
to see the most diverse diets.
So we did – we were working –
I did my Ph.D., actually, in the
early 2000s doing exactly this –
comparing different sites that
were at different abundances,
including Alaska, but also in California,
to see if sea otters
fit this pattern.
So how do we actually go
about collecting these data?
Well, we can – you can go and measure –
one way you can do it is just go and
collect data from untagged otters.
And we do a lot of that, actually,
especially in places in Alaska.
But here in California, where it’s
possible, actually, to go out and
capture these otters and tag them,
you can actually learn a lot more
when you follow individually
marked animals over time.
And this is true of a lot of
different wildlife studies.
Studying tagged animals over time
is how we learn about survival,
reproduction, foraging ecology,
movement ecology –
all kinds of different things.
So to capture sea otters, there’s a –
we have a technique that’s been sort of
perfected over many decades using
scuba divers with specialized equipment.
They’re rebreather equipment,
so there’s no bubbles,
so sea otters can’t smell
them from underneath.
And these electronic scooters with
traps attached come up underneath,
capture them in the trap.
We then bring them ashore,
either to the Monterey Bay
Aquarium if we’re in Monterey,
or to a ship if
we’re somewhere else.
We tag them.
We take a bunch of different samples.
And then we return them to the wild,
and we study them for about the next
three to five years, or as long as we can.
And we collect a lot of different data,
like their movements and their survival,
their behavior, their interactions.
But particularly their foraging ecology.
And this is all you need.
There’s Michelle Staedler from
the aquarium with a high-powered
telescope and some radio
tracking equipment collecting data
on a sea otter and
recording what it’s consuming.
As well as the observational data,
we also – for a smaller subset of animals,
we deploy these time-depth recorders.
That gives us information on how deep
they’re diving, how long they’re diving,
and a variety of other information.
And when we have the otters in hand,
as well as taking all our measurements
and samples, one of the samples we
take is a single whisker. Go, bleek!
And the whiskers grow back pretty fast,
so – but I wouldn’t want someone
pulling my whisker, but they are
unconscious when we do it.
And that whisker is very valuable
because it gives us about a year and
a half record of the life of the animal
of what the animal was eating.
In fact, if I did go out and pluck one of
your whiskers, I could analyze it in a
similar way with stable isotope analysis,
and I could see what you’ve been eating
over the – over some period of time.
Depends how long your whiskers are.
But with sea otters, we know that
those whiskers take about a year and
a half to grow. So it gives us this
record of – in the life of their diet, and
we can compare that to the foraging.
The observational behavior is sort of
a test to make sure what we’re seeing is
reflective of what the animal is doing.
So diet diversity in
different types of populations.
We compared a number of different
populations of different density.
And here I’m showing you a low-density
rapidly growing population –
San Nicolas, where we believe
food was really abundant,
compared to San Simeon,
a place in the middle of the range –
high density, where we believe
the preferred prey was depleted.
And the patterns are exactly
predicted by foraging theory.
We have a low-density – at the
low-density population, we have
a very low-diversity diet dominated by
red urchins – the most profitable prey.
And, at the high-density site,
where preferred prey are depleted,
you see some diverse diet.
But when you look at individual animals,
we saw something that
we were not expecting.
And these, I should mention,
are three adult females and just
pretty much selected at random.
You can choose any three females.
These ones I chose specifically
because their home ranges were
They basically had the exact
same home range. They rested
every day in the same kelp beds.
And yet, when you looked at their diet,
collected over a five-year period,
They’re eating completely
different things, even though –
so it’s not because they’re using –
they’re living in different areas.
It’s not because
they’re different ages.
They just have
completely different diets.
And so this greater diversity we see at
the population level is not being driven
by every otter becoming a generalist.
It’s actually being driven by every otter
becoming a specialist, but different
otters are specializing in different things.
And we were – we were quite
gobsmacked when we found this,
I can say – in the –
this was in the early 2000s.
And we published this, and right
around the time we published this,
there was a couple other publications
came out at the same time,
and it kicked – another species
showing similar patterns, and that
kicked off a whole flurry of studies
of people going out to their own
study organisms and marking their
animals and following them over time.
And it turns out this is
a really general phenomenon.
It’s not just sea otters.
It’s not just one or two species.
This sort of individual diet specialization
is pretty ubiquitous in nature.
And that – I mean, it seemed surprising
at first, although we specialize,
if you think about it, and so I think
we sort of thought, well, we’re special.
You know, we do things – but animals,
you know, they’re all the same.
They’re just kind of like machines.
But, no, they’re a lot like us.
develop different preferences,
and they do
So we’ve done a number of different
studies of this phenomenon.
And, as we’ve looked
at different populations,
what we’ve found consistently here
in California – and we have less data
from northern populations, although we
see the same types of phenomenon –
places we’ve looked in Alaska where
populations are reaching high density.
But here in California, there’s six
different sort of diet modules,
or diet specialization types, that we
see consistently up and down the coast
wherever you go, and they occur
at about the same frequency.
So you have urchin specialists,
mussel specialists, crab specialists,
turban snail specialists,
and then soft-sediment specialists that
feed on a variety of clams, fat innkeeper
worms, and other species that you
find in soft-sediment-type habitats.
And these different – as mentioned,
these specialist types seem to
occur pretty predictably at every –
all the different areas that they occur.
So that is really intriguing. One question
is, might we just be being biased by
our observational data? It’s good that
we have the stable isotope data.
When we look at that,
do they tell the same story?
Yes, they do.
They tell exactly the same story.
When we look at – so what [chuckles] –
this is showing you the relative
abundances in one whisker
over a year and a half period
of stable isotopes of nitrogen and
carbon, which may not mean a lot.
So if we translate this
into isotopic space,
each one of these
circles is an animal.
In the center of the range are areas
where there’s high-density populations.
You see there’s very little overlap
between otters in terms of their
isotopic space, which corresponds
to their foraging niche.
But when you go to a place like
San Nicolas, where food is abundant,
there’s a lot of overlap.
Basically, all animals have these
overlapping isotope niches
because they have similar diets.
And we’ve actually now seen
some of these switch from this to this
as the population
becomes more abundant.
So the isotopes confirm what we’ve
been seeing with the observational data.
So why are
we seeing this?
Why specialize when prey becomes
scarce and not when prey is abundant?
we believe – well,
there’s sort of an ultimate
and an approximate answer.
The ultimate answer is, it improves
their foraging efficiency.
Specialists are just really
good at what they do.
Approximately – a lot of females
are specializing in what their mothers
are specializing on. So they get a
head start in life learning from their
mothers because they spend six months
with their mothers learning to feed.
And so a lot – what we found is a lot of
females, more so than you’d expect by
chance, tend to specialize in the same
prey that their mothers specialized on.
So it’s essentially a cultural
transmission of foraging.
And it’s not 100%, so some females
end up specializing in different prey.
definitely a tendency.
What we – the thing that is consistent is,
when we compare the speed of
prey handling – so, like, for instance,
this animal, who is handling a
[inaudible] clam, or if it’s a
clam specialist, or whatever it’s
feeding on, if you compare the rate
at which they’re handling prey
to generalists or other non-specialists
who just feed on that prey occasionally,
That means they’re consuming 40 to
So the reason they’re specializing
basically is because it’s energetically
profitable for them to do so.
Even though they are forgoing
some prey, they actually do better
by just specializing in the prey
that they’re good at.
And it means that they’re
more likely to ignore prey that
they’re not specializing on.
This leads to a number of different
and unexpected predictions.
One is that it’s going to greatly increase
the complexity in food web interactions.
It’s going to make our job a lot harder
trying to make predictions from
food webs because we can
no longer assume that a predator
is just, like, a single node interacting
with different prey species.
Now we have to think of it as actually
a whole bunch of different nodes
interacting with different prey species,
and that can – that can differ over time
and space and, again, make our job
a lot harder. But more interesting.
It also had some unexpected advantages.
Around the same time we were
doing this work, we were actually
doing some disease studies
trying to understand how
certain types of infectious diseases
that occur on land were
getting into marine food webs
and affecting a variety of species,
including sea otters.
This was – a couple of these were
protozoa parasites – Toxoplasma gondii
and Sarcocystis neurona, both of
which can cause brain infections.
And they’re both terrestrial parasites.
For Toxoplasma, the dominant –
the ultimate host is members of the
cat family, so wild felids like
mountain lions, bobcats,
and domestic cats.
And in the case
of Sarcocystis neurona,
the ultimate host is the opossum,
so it’s a – which is an invasive species.
So, in both cases, these are not parasites
that you might expect to find out in the
kelp forest, and yet we are finding them
in a rather large proportion of sea otters
and other marine species,
as mentioned, that were having
We’re trying to figure out, essentially,
the mechanisms by which they’re
getting from land into the ocean,
or into these marine food webs.
And the – as mentioned, so we’re doing
these studies, and at the same time we
were finding this diet specialization,
we were working on epidemiological
models with colleagues at UC-Davis.
And we thought, why not just throw in
diet specialization into the mix and see
if maybe that will explain some of it?
And we did, and it
explained almost all of it.
Almost all variation in terms of
infection rates, it turns out,
was explained by what diet
specialization that particular otter had.
So for instance, in the case of
Toxoplasma gondii, snails specialists
were 12 to 24 times more likely to
be infected with Toxoplasma gondii
than were all the otters from
any other diet specialization.
And we found – and in
the case of Sarcocystis neurona,
it was actually clam – particularly
razor clams, but clam and
worm specialists that had that
much higher prevalence of infection.
So it turned out – we realized,
and we published in this paper –
Prey choice and habitat use
drive sea otter pathogen exposure.
We realized that this finding of
diet specialization had given us
a whole new tool to try to understand
how different types of diseases,
and in fact, pollutants, were getting
from the land into the sea and
particularly into marine food webs,
just by knowing the different prey
that they’re consuming and the different
micro-habitats that they were utilizing.
So we’ve been – we’ve had many other
studies following up on these findings.
So that’s a little bit –
a crash course in diet specialization.
So I mentioned diet – I also –
we talked about high site fidelity.
Why are sea otters so –
essentially such stay-at-homes?
Why – and particularly for the
females, why do they have
such small home ranges?
We now believe that actually
has to do with this high site –
this diet specialization.
And we believe that, actually,
because the animals that have the
most dietary specialization can have
the most constrained home ranges.
Males that have less diet specialization
move over much greater areas.
And that’s probably because they
can consume – they can switch
between different types of prey
when they’re in different areas.
Females, once they specialize in a prey,
are pretty much locked into not only
their particular prey, but also to the
habitat that they’re very familiar with.
And so what we find with adult
females is that, over their entire lifetime,
they live within about
So there’s the lifetime home range for
one female right around Carmel Bay,
just going from Cyrpess Point here –
the golf club over there,
through Stillwater Cove, Carmel Bay,
down to Point Lobos.
And that’s – she won’t –
she doesn’t go beyond that.
She’s never been over to Cannery Row.
And she’s never been down to Big Sur.
Poor girl. [laughter]
And that’s completely normal and
typical of all other adult females.
So when you go and look at the
population at Big Sur – or as – well,
Big Sur or Monterey,
or drive down to San Simeon,
you’re looking at completely
distinct sea otter populations.
It’s not just one population of animals
swimming and mixing all the time.
These are – essentially, you can
sort of think of these as villages
between which there’s a little bit of
movement on the – a few males.
But for the most part,
they’re completely different animals.
They probably speak different languages.
And that’s really fascinating.
And it also has really important
implications for both the regulation
of the population – it means they’re
going to be regulated locally,
not at large scales,
and also for their ecological effects.
And that sort of explains what we were
seeing at San Nicolas, why they were
so localized to one part of the island.
I mean, so that’s what we –
one of the terms we use
for that is cryptic
When you look out at the population,
you see what appears to be just a
solid continuous band of otters,
but in fact, what you’re not seeing
is that that’s actually
broken up by the sort of
cryptic hidden structure
throughout the population.
And we’re actually now doing
some genetic studies,
and those are supporting this
completely genetically as well.
So there’s this genetic diversity that
varies over the course of the population.
So armed with this behavior,
this complexities in behavior
and this new knowledge, I’m going to
now tell you about a study that –
a final project that
we’re doing right now.
And it was another
unexpected opportunistic experiment
that presented itself to us.
And we’re using our knowledge of
diet specialization and spatial ecology
to try and understand how sea otters
are responding to this perturbation –
how the ecosystem is responding.
And what we’re learning
is that, again, what seemed –
what we thought was a
pretty simple trophic cascade
of sea otters, urchins, and kelp is
not at all as simple as we thought.
So here’s the Monterey kelp forests,
and if you went and put on a mask
and snorkel and dove down –
went off of Cannery Row or
Point Pinos or anywhere along the
shoreline there, this – for about 50 years,
from about 1960, when sea otters
arrived for the first time,
up to the present, this is what you
would see pretty much everywhere –
this beautiful kelp forest.
Tons of fish and different invertebrates.
Now, it seemed – and we kind of
assumed that was the way it was
going to be forever until it wasn’t.
And about 1940 – or, 1940 – 2014,
we started to see this, particularly
around Carmel Bay, but it quickly
spread all around 17-Mile Drive
and then came around the corner.
Now, right off Hopkins,
we’re seeing this, and it’s spreading
down towards the aquarium in
Monterey, Monterey Bay Inn.
These urchin barrens that aren’t
supposed to occur where sea otters
are abundant, they haven’t
read the textbooks,
and they’re occurring all over
the place. [laughter]
So we kind of had to go back
to the old drawing board and
figure out what was going on.
And we did – we do –
did what scientists often do
is we quickly sat down and
wrote some grant proposals.
one of them was funded.
So this is an ongoing study we’re doing.
It’s a collaboration between myself
and Mark Carr, who is a professor
at the Department of Ecology and
Evolutionary Biology at Santa Cruz,
to basically understand the potential role
of predator complementarity
affecting ecosystem resilience
in a California kelp forest.
So I’ll explain what I mean
about predator complementarity
is just a second.
So, again, starting in 2014, ’15,
many areas transitioned to this –
from kelp-dominated state to
the urchin-dominated state,
even though sea otters were abundant,
and they weren’t supposed to do that.
Possible causes – well, one of the
possible causes was something that
you might have heard about – the blob.
This big warm-water mass that sat off a
large portion of North America for the –
for a few years, actually – 2014 and ’15.
And that blob changed a lot
of oceanographic patterns.
And one of the things we thought
it might have done was increased
recruitment of urchins, so maybe
there’s just a lot more baby urchins.
That certainly might have contributed
a bit, although the pattern we saw
in these urchin barrens was not
just a whole bunch of little ones.
It was all different-sized classes. And
they can’t get to a big size class that fast.
So it turns out that the increased
recruitment explanation is not capable
of explaining the speed with which
we saw the transition to urchin barrens.
The other possibility is another
phenomenon that happened.
Started with 2013, went through –
right through the present, actually,
but 2013 and ’14 were
the big years for this,
and that was the loss of predatory
sea stars from wasting disease.
So this beautiful animal you see here
is a sunflower star – Pycnopodia.
You wouldn’t see – these are different
than the stars you’d see in tide pools.
These are sub-tidal stars,
so they occur deeper.
Although you might be lucky enough
to see one occasionally, and if you go
out diving or snorkeling, you can see –
well, you used to be able to see these.
They are gorgeous.
They’re quite big.
And they’re voracious terrifying
predators of sea urchins.
When these guys come over a reef,
all the urchins literally flee.
They run away on their little tube
feet as fast as they can. [laughter]
There’s pictures of these –
it’s just incredible.
You’ll see one of these in the reef.
There might be five of them.
And around each of them, there’s this
halo formed by a lack of urchins as the
urchins are fleeing as fast as they can.
So these guys are – yeah, these are
basically Godzilla to urchins. [laughter]
To little urchins.
They can’t eat the really big urchins.
But they eat the little ones and the –
and the middle-sized ones,
basically, up to about that size.
So gorgeous animals, great predators.
Except in 2013, this started
happening to them.
They melted and became piles of ooze.
And this happened to 100% of
Pycnopodia between Mexico and
southeast Alaska. It also happened
to almost all other sea stars as well.
There’s a few different species,
like bat stars, that survived a little bit.
But basically, 99.9% of sea stars of
all species, from Mexico to Alaska,
died over about a one- to two-year
period from sea star wasting disease.
So if you happen to be near tide pools
over the last few years and noticed
you weren’t seeing any sea stars,
it’s not just you. They’re gone.
And this one of the – one of the most
traumatic events that – you know,
that I’ve seen within my lifetime.
There’s been diseases like this before,
but never this broad and this,
you know, complete.
There are some species coming back,
particularly up in Alaska
in the northernmost part.
Not Pycnopodia yet.
We’ve been talking to our colleagues
and hoping we’re going to start to
see these again soon,
but no sign of them as yet.
Occasionally we see one or two ones
that look like they’re coming back,
but then they die of the disease still.
So the virus is still out there.
So that was – that was a remarkable
thing to witness, but the question is,
did that have something to do
with the increase in urchin barrens.
Well, it seems like it might, given that these
are another predator of urchins.
But, again, even though everyone
who studies these knows they
eat urchins, no one thought
that they were nearly as important
as sea otters in limiting
What we’re wondering is whether
these guys were sort of like the hidden –
the hidden sidekick of sea otters.
Sea otters, you know,
get all the credit for limiting urchins.
But up until the sea star wasting disease,
Pycnopodia were always there.
You know, right, they’re sort of
like Batman and Robin here.
They’re kind of the hidden Robin.
While otters get the credit for eating
all the big urchins, those Pycnopodia
were busily eating away all
the little and middle-size urchins.
And maybe, for really
effective control of urchins,
you need to have both sea otters
and stars in the population.
So that’s our primary hypothesis that
we want to investigate with this project.
But we’re also really interested in
why sea otters were not able to
respond faster to this
increase in urchin abundance.
And we’re wondering if it
has to do with some of these
that I’ve mentioned before.
In fact – so particularly,
does dietary specialization
in this population – in this
abundant population in Monterey
actually inhibit their ability to keep
up with the increase in sea urchins?
Okay, so now this brings us to the
concept of predator complementarity.
What exactly do I mean that?
There are three different
ways that two – that if you have
two different predators –
sea otters and sea stars –
that they can – that those two
predators together can affect urchins.
One way is that their effect on
urchin abundance can be additive.
So if otters have this much of
an effect on urchin abundance,
and sea stars have this much
of an effect on urchin abundance,
when you have otters and sea stars
together, you basically just add up
their mortality rates, and there
you have the total effect on sea stars.
Another idea, though,
is this – multiplicative.
And that is that the sum of
sea otters and sea stars together
is greater than their independent
contributions to limiting urchins.
So when they’re both together, they have
a much greater effect in limiting urchins
than you would expect as – based on
either one of them by themselves.
And, of course, the third
possibility is the opposite.
That would be interference –
So that, when otters or sea stars are
by themselves, they have one effect,
but when they’re together,
they inhibit each other,
and so therefore, their ability
to control urchins is even less
than you would expect
as compared to the additive.
We’re pretty sure this is
not happening, but what we’re
wondering is whether this is happening.
Whether otters and sea stars together
do a better job of limiting urchins and
therefore conferring resilience to the
kelp forest than they do when either
one of them is all by themselves.
And, again, we’ve never
seen this before simply because
stars have always been there.
You know, so they’re the old reliables.
Right up until the
time that they weren’t.
So to answer this, we’re going about it
in a couple of different ways.
Again, we’re doing – aside from
writing grant proposals, another thing
ecologists love to do is to
do these types of experiments.
So we’re setting up these
underwater cages just off of –
if you go to Cannery Row, and you
look off, you’ll see a bunch of different
buoys, but one of those little buoys
is marking our sub-tidal cages.
So right now, out there, underwater,
there’s 20 of these cages right now
set up. And they’re set up not
over existing reefs. We had –
we had to build our own reefs. So we
set these up outside the rocky reef.
We built our own rocky reefs
inside each one of these cages,
within which we can stock
a known number of sea urchins.
And then, some of those cages,
we can place – we were hoping
by this time, the Pycnopodia
would have recovered.
They haven’t, so we’ve having to
use some other surrogate star species
in place of Pycnopodia until
such a time as they do recover.
So we have some treatments where
we have stars but not otters,
and we keep the otters out because
we can put covers over those cages.
These are – again, these are –
these are all cyclone fencing.
So that the water can move through
there, but we can keep sea otters out.
We can have stars and otters
by having the cage open.
Or we can have otters only by having
the cage open and not putting stars in.
And we know no stars will go in by
themselves because there are no stars.
And then we can have
no stars and no otters.
And this sort of fully factorial
experiment, we can actually measure
the independent feeding rates of otters
and stars by themselves or together.
And we can sort of get at this idea of
complementarity versus additiveness.
And this is what they
look like underwater.
And a kind of sediment-y day,
so it’s a muddy – but this is when
we were actually in the process of
moving the great huge rocks into the –
into the fenced enclosures,
which was a monumental task.
[laughs] But now they’re out there, and
there’s – like, there’s little web cams
set up on these to record when otters
come in there, and we’re already
beginning to find some cool things.
But I’m going to – I’m not going
to tell you about that part yet.
You’re going to have to wait
until a future talk until we have
all the results in. But I can tell you,
that’s going to be – it is really cool.
So the other part of the study, though,
was to actually look at what’s
happening on top of the surface.
What’s happening with the sea otters?
So we did what we’ve done in the past.
We went out. We tagged 25 otters.
And now – and we’re following them
over time and recording their diet.
And because we have this historical diet
at Monterey, we can sort of see,
are there changes on both the
abundance of otters and the diet
of sea otters and the survival
of sea otters and all these other
parameters over time in response
to this increase in urchins?
So one of the first things we did see was
an increase in the abundance of otters
that seemed to track – and this is just
specifically at Monterey Peninsula – that
seemed to track the decline of sea stars
and the increase in urchin abundance.
And this increase appeared when we
went out and actually did surveys
of these groups. What we saw was
a lot of juvenile and sub-adult animals.
And these were animals
that basically had grown up
in this urchin-rich
And we started – we coined the
term “urchin millennials.” [laughter]
Because – not because they were
looking at their phones all the time,
but because they were eating –
they were young millennials,
and they were eating a lot of urchins.
But – so that was kind of interesting.
So we see – this is what we
would call a numerical response –
that is, the response of the population
abundance to the increase in prey.
The other thing we might expect is
what we call a functional response,
and that’s not changing the numbers,
but rather changing the behavior
of the otters that are there.
And what we might expect, of course,
is an increase in the
amount of urchins in the diet
if they’re going to respond,
again, to this glut of urchins.
And when we look at the population
level, in fact, we did see that.
When we compared the proportion of
urchins in the diet from 2004 to 2012,
urchins made up only just
about 5 or 6% of the diet.
And why so small?
Because there just weren’t many urchins.
But from 2016 to ’17, they’ve increased,
not to, like, 100% of the diet,
like you might expect given
their super abundance,
but at least to about
So that is about a doubling of the –
in terms of the proportion of
sea urchins in the population-level
diet consumed by sea otters.
We also saw more size selectivity.
So urchin – otters always tend to
choose the most energy-rich prey types.
And with more urchins available,
they’re able to
be more selective.
And so, instead of just eating all urchins,
they’re tending to eat the larger urchins.
So this is predictable, perhaps,
based on some of the optimal foraging
behavior of sea otters, but it means
that they’re not going to be doing
a very good job at controlling
the small urchins when they’re
just limiting themselves
to eating the big ones.
Okay, so if there’s more urchins in the
diet, does this mean that all otters are
just eating a little bit more urchins?
Or does it mean just a few otters are
eating a lot more urchins, and other
otters are not changing their diets at all?
So in other words, has there been
a change in the degree of diet
specialization at Monterey?
So one of the ways you can measure
this statistically is with a statistical
metric called the proportional similarity.
And that’s – and it’s basically
just a number that allows you,
with one number, to determine –
to distinguish between –
generalization would be
when you have high numbers
of the index means that
no individuals are specializing,
right down to extreme specialization
as you approach zero.
So, if there’s – what we
would expect if all otters
were becoming more generalized
and just starting to consume
more urchins is that
this line should go up.
And does it? No, it does not.
It remains exactly the same.
So that means that the
level of individual specialization
has not changed at all in the
face of this increase in urchins.
And particularly, some of
the otters that we had –
the older animals that we had
tagged from previous studies,
they just continued doing
what they had always did.
So the crab specialists
kept specializing in crabs.
Snail specialists just
kept specializing in snails.
They weren’t increasing their
consumption of urchins at all.
So that actually sort of confirmed
one of our hypotheses that, in fact,
specialization does inhibit
the ability of sea otters
to respond to a sudden perturbation
in terms of the increase in urchins.
However, what we did –
the change that we did see
was an increase in the proportion –
at the population level in the proportion
of animals specializing in urchins.
And you can see this is – each of these
clusters of bars is just showing you the
proportion of each specialist type –
generalists, abalones, cancer crab,
clams, kelp crabs, mussel –
I can’t – something I
can’t even read from …
- Thank you. Snail [laughs] and urchins.
And what we see is that an increase in
the frequency of both urchin specialists
and mussel specialists in the diet.
And by the way, mussels are another
thing that have increased in abundance
with the loss of stars, and that’s
primarily from the loss of
Pisaster that are mussel consumers.
So there’s a lot more mussels
and urchins, and that means
there’s more urchin
and mussel specialists.
But all these new specialists
are younger animals, basically.
So the older ones just kept on
doing what they’re doing.
And it’s these younger millennials,
as they come up in the population, and
they’re born into a world where there’s
just tons of urchins and tons of mussels.
And not surprisingly, that’s what
they’re choosing to specialize on.
So those are some of the
complexities we’re seeing.
One final thing we’re looking at, again,
is this idea of selectivity by otters.
When you have urchin barrens,
and those urchin barrens get big enough,
the urchins in the middle of the barrens
don’t have any kelp to eat, basically.
So they’re just – they’re sort of
in the middle of an urchin desert.
They’re feeding on a little bit
of coral and algae, but basically,
they have almost
nothing to eat.
And with urchins, unlike with
homeotherms, that’s not a problem.
When you don’t have anything to eat,
you can just sort of remain in a state
of suspended animation
Because they can just turn their
metabolic rates down to zero.
So, but what it does mean is,
when you look inside those urchins,
you see almost exactly nothing.
You see a few little structures,
but it’s mostly just hollow.
When you go to the edge of the urchin
barrens, where they’re grazing
on kelp, then they’re full of –
they’re mainly full of gonads, basically.
They’re eating up tons of food,
producing gonads and eggs, and then –
and making more little baby urchins.
So otters, being clever like they are, are
not going to be eating the empty urchins.
They’re going to be – tend –
we would expect them to feed on
the urchins that are full of energy.
So we’re testing that by recording
all the places that otters are feeding –
or where we see them –
our tagged otters eating urchins.
And then we have a dive team go in
that same day, dive at that exact spot,
and compare – take a selection of
urchins, measure their gonad index –
their composition, and then go
do random sites all around,
and we compare that.
And what we’re finding,
basically, is none of our tagged otters
are feeding in the urchin barrens.
They’re just feeding along the edges
where – and, yeah, they’re doing what
you might expect a smart otter to do.
They’re feeding on the urchins that are
in the kelp that are full of energy.
And they’re ignoring
the ones in the barrens.
And that’s, again, going to be
problematic in terms of otters limiting
these urchin barrens. Because they’re
just not going to be feeding there.
So these are all some of the dynamics
that we’re measuring right now
at the site. So I think, at this point,
I have to wrap it up.
So I’m going to sort of step through
some – our overall conclusions from
what we’ve learned and sort of try to
tie together these different studies.
So in general, sea otters, in fact,
do exert very strong influences
on near-shore communities.
And I am – you know, I didn’t cover
all the different ways that they affect it,
but as you saw from those studies
in Alaska and British Columbia,
they can completely change the
nature of the ecosystem.
Today I didn’t talk about some of the –
another study we’re doing in
Elkhorn Slough, but we’re finding –
in that study, we found that sea otters
have similar really big effects in
systems as well.
We’ve seen an increase in the eel grass
as the otters move into the system from
a very different trophic cascade, and
that’s sea otters controlling cancer crabs.
But, in any event, something that
is unique about sea otters
is their ability to really
shake things up, basically.
And, again, that really all comes back
to their incredibly high metabolic rate.
When otters come into a system,
they tend to change it dramatically.
However, that said, that simple cartoon
that we had before of just, otters come in,
urchins disappear, kelp forests, that –
as with many simplified cartoons,
that turns out to be
a little bit too simple.
It’s actually a lot more
complicated than that.
And particularly in California,
where there’s a lot more species,
we’re finding that behavioral
complexities of otters – in particular
the diet specialization and site fidelity
and also the fact that there’s multiple
predators, not just otters,
that are controlling urchins,
are affecting the degree to which
sea otter – these sea otters are
having their food web impacts.
And it looks like sea otters interacting
with other predators,
particularly with Pycnopodia –
the giant sea star, might be
what leads to really effective
control of urchins and kelp forest
resilience in California.
We don’t just need one predator.
We need multiple predators.
And finally, I guess sort of to take
a big step back, one of the things
that I can sort of say after 25 years
of studying these guys is that
[chuckles] being wrong is okay.
In fact, it has to be okay.
You don’t have much choice with otters.
They love to prove you wrong. [laughter]
But ecological surprises –
and those – and that’s how
we characterize all the different
things that we found with these otters –
are really, really useful
And so one of the things –
when you’re – for any wannabe
ecologists in the crowd is,
be attuned to these ecological surprises.
Because when things –
unexpected things happen,
that’s an opportunity to learn
something new that you didn’t know
before about the – about the ecosystem.
And with that, I would like to say
thank you to all the different groups
providing funding for all this research.
And of course, for all my collaborators
and the field assistants and
graduate students and all the different
people that made all this work possible.
Thank you very much, and I’d
be happy to take any questions.
[ Applause ]
- Thanks, Tim.
[ Applause ]
If you’d like to ask a question,
I want to ask you to please
go to the
microphone in the stand.
So our online
listeners can hear you.
- I was wondering if the otters that
specialize in their diet are more at
risk if the prey they prefer goes
away or becomes not abundant.
- That’s – it’s a great question.
And in fact, that’s why –
if you looked at those bar graphs,
you’ll notice there was generally one,
or sometimes two, dominant prey.
We call those their core prey species.
But then you’ll
notice that there’s –
it’s not like they weren’t
eating anything else at all.
And that’s – we refer to those
as their peripheral prey species.
And if they were true specialists –
like a snail specialist would
eat nothing but turban snails,
They eat about – snail specialists
are probably the most specialized.
They eat about
But then that means they’re
eating 30 to 40% other stuff.
And that other stuff is
pretty much everything else.
So they are, in fact, always
sampling their environment.
And, under extreme circumstances,
of course, when their prey completely
disappear, then they will switch.
We also see females who specialize
in large prey that are in deeper water,
when they have pups, when the pups
are small, they actually switch
to smaller prey that occur in kelp beds.
So we actually see them relax their
specialization when they have a pup.
And they’ll start
eating a lot more mussels
and smaller things that
they get in kelp forests.
As their pups gets bigger, then they
start to slowly move into deeper water.
And then eventually,
once the pups are weaned,
the females go back to specializing in
what they were specializing on before.
So you – so you’re right.
I mean, if you – if you are a specialist,
it does behoove you to keep an eye
on what everything else is doing.
And it may be – it may be that
they need to practice just enough
so that they sort of have an insurance
policy if their preferred prey disappears.
So that was a great question.
- Another diet question.
You mentioned the
San Nicolas Island isn’t very big.
So when they start running out of
their preferred food in the areas they’re
used to hunting in, do they move?
Or do they stay where
they are and change diet?
- Yeah, that’s a great question.
We think – well, what I would predict,
and we haven’t seen it yet –
what I predict is both.
We actually have seen some increase –
nothing like the diverse diet
we see in the mainland yet.
And that’s because there’s
still a ton of urchins out there.
One of the things about San Nicolas
is there was really two species of urchins.
So they actually did deplete all the
red urchins – not all of them,
but the red urchin abundance,
we’re really seen a decrease in
abundance from otter predation,
particularly at the west end of the island.
And only now are they now beginning
to consume the purple urchins.
So that’s why, I think,
they didn’t control urchins
completely at first was
those two different species.
As the purple urchins get low,
we are starting to see them eat
more large Lithopoma snails and a few –
and also some more kelp crabs.
So they’re beginning to diversify their
diet, but we’re – we totally expect them
to start to spread around the island.
They just really haven’t done it yet.
We actually expected
that a few years ago.
And then, of course, they’re
going to spread to other islands.
And we already – in fact,
I’m sending one of my biologists
out next week to have
a look at Santa Rosa
where we have reports of two
probably male otters out there.
There’s a couple at San Miguel
that go periodically.
So we’re just right at the
edge of them beginning to
colonize some of the other channel
islands, which will be very exciting.
- Yeah. With the diet specialization,
do you see, like, different species
have different diseases?
And also life span and also physical …
- … differences among different species
because of the diet specialization?
- Yes, so …
- Tim, can you repeat the question?
- I’ll repeat the question, yes.
So the question – the question was,
with these different diet specializations,
do they translate into different
health differences, different
different morphological characteristics
or survival or reproduction?
Which is – yeah,
so that’s a great question.
I already mentioned the
differences in disease prevalence.
We’ve seen that with
a couple of different things –
the protozoan parasites.
We’re starting to see that and some of
the – some other disease types as well.
And also in terms of
domoic acid exposure,
which they get from, you know,
the diatom Pseudo-nitzschia
that produces this toxin
when it has big blooms.
It looks like different diet –
different types of diet specialists
have different risks of exposure to
that based on what they’re consuming.
So definitely, they have
different health risks.
In terms of reproductive success,
we actually – we have now
got reproductive histories for about
I think it’s about 1,000 –
over 1,000 different pups.
So we’re beginning to analyze that
to look at exactly that question.
And what we’re seeing in general is no.
There’s no significant differences
in reproductive success between
different diet specialist types.
Within each specialist type, you have
both winners and losers, basically.
So that, again, sort of suggests
that there is some sort of frequency
dependence in the relative abundance
of the different prey specialists.
If everybody was trying to be a
snail specialist, or everybody was
trying to be an abalone specialist, at that
point, they would deplete those prey,
and they wouldn’t be as profitable.
But when you have sort of this
balance between the relative
numbers of specialist types,
then they all tend to
do just about as well.
- Please go to the microphone
if you do have a question.
- No more questions?
I’m happy to stick around and talk
to people or answer some questions.
- Is there anybody who would like
me to bring the microphone to them?
Because they couldn’t get to …
- I wondered – you mentioned that
you only had 12 adults stay at that –
at the island. And that’s a
real small founder population.
I wonder if you saw any signs of –
it’s grown very well,
but I wonder if there were
any potential problems
that you began to see from
the small founder population.
- Yeah. Yeah, I know.
That is a very small population.
In fact, probably that’s comparable
to almost all the founder populations
post-fur trade. So the – in terms of
having – losing genetic diversity,
there’s two things
you need for that to happen.
One is to reduce to a very small size,
but the other is for it to remain
at that small size long enough for it to
lose alleles in genetic diversity by drift.
And, in that case, it seems that it
was probably not low long enough.
When we compare the diversity, at least
across all the micro-satellite sites that we
have, we see the same level of diversity
at San Nicolas as on the mainland range.
That said, the mainland range itself is
a pretty low level of diversity as well.
So it doesn’t look like they –
over that period of time, they lost
too much in the way of diversity.
But, yeah, it is certainly something
that we’re concerned about –
and also in the mainland range.
We recognize that there’s pretty low
genetic diversity throughout California.
And in other sea otter populations
to the north as well, actually.
- One more question.
I can just shout.
- No, we have online
viewers who won’t hear it.
- How does a little sea otter
pry up a great big abalone?
It takes divers a pry bar to get them up.
How do they do it?
- [laughs] That is a great question.
So I’ll start by saying sea otters
are really freakishly strong.
They have short, little, stubby forearms.
But their muscles are just huge.
And so even a small –
you don’t want to challenge
a sea otter to an arm wrestling contest.
They also have
semi-retractable claws like a cat.
So when we’re handling them,
their claws are generally retracted
at that point, and their paws
are incredibly soft.
But, when they want to,
they can exert those claws.
So basically, when they’re
prying up abalone, claws come out,
and they just have
this incredible strength.
But they have another
thing in addition to all of that.
And those – in many cases, that’s
enough for them to pry off an abalone
that you or I would not have a hope
of prying off. They can use tools.
And that’s – I didn’t mention that
tonight, but that’s another series of
studies we’ve done in their foraging
ecology is differences in tool use.
And it turns out that, for the otters that
we see using tools at the surface, about –
almost all of them are snail specialists.
So when you see an otter with a rock,
or if you’re out in Monterey early in the
morning, and it’s foggy, and you hear
this kind of toc-toc-toc-toc-toc-toc,
that’s a tool-using sea otter.
But you – there’s about
a 95% chance that it’s –
that’s a snail specialist
that’s using that tool.
Snail specialists use tools about
and other specialists use it very little.
It depends what they’re specializing on.
The next most frequents
are mussel and clam specialists.
Urchin specialists use tools
almost zero percent of the time.
So that tool use allows –
gives them – increases their efficiency
with certain prey. The one prey that it’s
really hard to tell are abalone specialists.
We don’t – they don’t use tools at
the surface, but we do sometimes –
when they start making dives in
one location, and we know they’re
going to come up with an abalone,
it takes them about 20 dives,
especially for, like, a really big red
abalone, like a dinner-plate-size abalone.
When we’re watching
one of their specialists,
they’re going along the coast –
dive, dive, dive – and then we
start to see them – start to see
them make dives in one location.
And when they come up,
they’re not getting anything.
They’re out of breath. They’re literally –
they come up to the surface, and they’re,
like – [breathing hard] –
and they’re rubbing their arms.
And sometimes they’re carrying a rock.
And then they go down.
So we can’t say for sure,
but we’re pretty darn certain that
sometimes they’re using – the abalone
specialists are hidden tool users.
They’re using tools at the
bottom to pry off the abalone.
But they – when they drop – when they
finally do get the abalone, they drop the
rock, they bring up the abalone, and then
they’re just eating it like a dinner plate.
you know, scooping it out.
Yeah, so that’s – yeah, that’s – the tools
are their hidden – their hidden weapon.
- You mentioned that the whiskers have
a history of what they ate, or their diet.
- Does it give you an average of the last
you know, stages of what they …
- It’s sequential.
Just like fingernails.
It’s only – it’s only
growing at the cuticle.
So once it grows out, it’s inert.
So the tips of a whisker is a –
tells us about the stable isotope
concentrations a year and a half ago.
And then, right at the follicle, that tells
us about what they ate very recently.
So we chop it into, just by convention,
And for every one of them –
and we tend to take the longest whiskers
as well because we want
to get the longest samples.
So we analyze that with
those 20 different samples.
And that’s how we measure
It’s a comparison of within
individual variance in the isotopic
niche width compared to
between individual variance. Yeah.
- I have a question.
- Back to eating.
- How does a sea otter
eat an urchin?
- So I mentioned they don’t use tools.
You might think they might.
So urchins have these great spines, right?
But their actual tests – the shells –
are incredibly thin and brittle.
Anybody – if it wasn’t for those spines,
anybody can crack an urchin’s test.
It’s like – unlike a turban snail –
they need to use tools there because
the turban snail has one of the most –
you cannot – try as you might,
if you had a turban snail, you could
not crush it between your fingers.
Or if you were,
you’re really, really strong.
Otters can’t either.
And if they tried to bite it,
they would probably crack their dentin,
but that’s why they use rocks.
Urchins, on the other hand – easy to
break, but they’re protected by
these spines. What they do is they
roll them between their paws.
And even with the big ones,
they bring them up, they just –
if it’s a red urchin with huge spines, they
sort of flatten it down with one paw.
But for most of the smaller ones, they
just bring it up, they have their little
armpit pouches. So they can carry
about 20 urchins or so in a dive.
They’ll come up, just reach into their pit,
ch-ch-ch-ch – crack in the side,
suck out all the insides, drop it, grab
the next one, ch-ch-ch-ch – like that.
So they can – they can process them
really fast. [laughter and conversation]
So if you – and that’s how you
process and urchin. [laughter]
- Why do they have
such a high metabolism?
It that because of a loss of heat?
- Yes. Yes. In short.
So they’re the – they’re the
marine mammal that shouldn’t be.
You know, as compared to other
marine mammals, they don’t –
they’re relatively recently evolved
back to marine existence,
although they’ve still been –
they’ve had – you know,
had quite a bit of time in the ocean.
But, unlike all the other mammals that
went back to the ocean that developed
thick blubber layers being their
primary insulation, sea otters did not.
They have no subcutaneous
fat almost whatsoever.
They’re reliant on the thickest fur
of any mammal – thus the fur trade.
It’s very luxuriant fur.
And then they have –
it’s a very expensive type of insulation
because you have to groom about,
you know, 10 times a day or so to keep
it in that perfectly waterproof state.
And then the other thing they do is they
have about 3-1/2 times the metabolic rate
of a terrestrial carnivore of the same size.
And they maintain that by eating a ton.
So between that thick fur and
that really high metabolic rate,
they are able to maintain about the
same level of thermal regulation
as a harbor seal. But, as I say,
it’s a very expensive way to do it.
- What are their –
what are their natural enemies that
keep the population under control?
- So as a – that’s a
very interesting question.
So apex predators, by virtue
of being an apex predator,
don’t have natural enemies.
And they keep themselves under control.
Or, more specifically,
they’re limited by something else.
They’re generally limited by
a limiting resource.
For some predators, that may be space,
or it may be something else.
But for a lot of top predators,
their limiting resource are
their prey populations.
So they reach an equilibrium with
their prey populations, and that –
so those dynamics I was mentioning –
as they deplete their most
preferred prey, they start to
diversify on less and less profitable prey.
What that means is they’re eating
less and less profitable prey –
any one animal.
Their consumption goes down,
their body condition goes down,
their survival rates go down.
And eventually, their survival
and their births equal,
and they’re at carrying capacity.
So they’re limited mostly – mostly –
from below, basically.
It’s a bottom-up limitation.
There are instances
where sea otters are – where they are
suddenly not the apex predator.
One of those happened in the Aleutian
Islands when another predator started to
consume them – the killer whales.
And at that point – and that
changes everything, right?
And now their equilibrium
is not being set by their prey.
It’s being set by the amount of refuge
habitat they have from killer whales.
So when you go to
the Aleutians now,
there are these little –
we call them otter hot spots.
They’re generally in either completely –
lagoons that you just can’t get into,
or they’re in these reefy areas where
the otters will rest over the top of reefs
that are only just about, you know,
a meter or so below them.
And then they’ll feed just a couple
meters away from where
they’re resting. So they’re just –
they’ve learned not to go very far away
from these sort of secure areas.
So that’s in the Aleutians.
Down here, there’s areas where
they’re not being limited by their prey.
They’re being limited by shark bite
mortality, particularly down around –
well, the south end of the range around
Point Conception, and then the
north end of the range around
Half Moon Bay, down to Año Nuevo.
And those areas – sharks aren’t
eating otters. They’re biting them.
They eat other things, but they go up
and they do these investigatory bites.
And they do – not just with a few otters,
but – well, hundreds – hundreds a year.
We’ve got now thousands
of shark-bit carcasses.
So in those two areas I just mentioned,
the north and south end of the range,
shark bite right now is
the primary source of mortality.
And it’s what’s keeping
the population from spreading
to the north or south now.
So that’s – it’s not really predation,
but it’s kind of a special
type of predator limitation.
- I read recently – but I can’t remember
where, that, on some instances,
the orcas are coming in closer to the
shore and actually capturing sea otters.
Is that true?
And if so, to what extent?
- Not that we know of
here in California.
There have been a – well, so first off,
again, backing up, in the Aleutians, yes,
that’s sort of what –
that’s what we saw in the early ’90s.
Before then, they hadn’t really
been coming in – well, they had been
along the edge of the kelp beds
eating sea lions and harbor seals.
But what we saw – the change in
behavior we saw then in the early ’90s
and, you know, right through the ’90s
and into the early 2000s was they began
to come right into the kelp beds and
eat sea otters right in front of our eyes.
I mean, one of the times,
I would – we were watching it
literally from a cliff top –
one of our tagged otters.
And we were watching the
killer whales come along the coast,
but the otters couldn’t see them because
they were sort of in this little cove.
And we – all three of us were just
sort of watching this little – this vignette
happening right below us, and the killer
whale came up, right in the kelp bed,
right in the middle of the group of
sea otters. So they figured out this
new hunting technique pretty quickly,
and it had a huge effect.
We haven’t seen that outside
of southwest Alaska,
except periodically in Glacier Bay.
There’s quite a few killer whales that
come in there and feed on other marine
mammals, primarily harbor seals.
But periodically, they have been seen
going into otter groups and taking
sea otters. It doesn’t seem to have –
be frequent enough there to limit
the abundance of sea otters, but it
definitely shows that they are capable of
doing that, so it’s something we’re
always kind of watching out for.
[ Silence ]
- Going one, going twice?
- People are getting tired.
[ Applause ]
- Thanks, Tim.
- Thank you very much.
[ Applause ]
- And I hope – safe and
happy holidays to all,
and I hope to see you January 25th
for Doug Given’s talk on ShakeAlert.
[ Silence ]