Ecology and Conservation of Amphibians in Northern California

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This project improves our understanding of the ecology of amphibians in northern California and evaluates methods of managing landscapes and these imperiled species. In particular, Dr. Brian Halstead examines the distribution and demography of amphibians to understand factors that affect where amphibians are found and how populations change. He further explores the relationships of amphibians with their abiotic and biotic environments to understand important components of habitats for amphibians, how they interact with competitors and predators in the environment, and how disease and individual characteristics affect vital rates.

Image: California Red-legged Frog

California red-legged frog (Rana draytonii) surfaces in a pond in Point Reyes National Seashore, CA.(Credit: Gary M. Fellers, USGS. Public domain.)

Amphibians are declining worldwide, and although many factors have been proposed as causative agents of declines, none has been identified as the smoking gun. Indeed, recent research suggests that different agents are responsible for declines in different regions of North America. The variety of factors affecting amphibian populations is facilitated by the biphasic life cycle of many species, which exposes them to many different stressors as they progress from aquatic eggs and tadpoles to terrestrial juveniles and adults. Amphibians are therefore not only of conservation concern themselves, but they also serve as sentinel species that could be early indicators of ecosystem change and stress.

Studies under this project improve our understanding of ecosystem structure, function, and processes; advance our understanding of how drivers influence ecosystem change; and encourage the application of science to enhance management, conservation, and restoration strategies for ecosystems.

Occupancy: To estimate the probability of occurrence of amphibians, we typically use replicated surveys at multiple sites to account for the imperfect detectability of amphibians. Shorelines of ponds, streams, or meadow pools are carefully surveyed for all life stages of all amphibian species.  If necessary, observed amphibians are captured by hand or with a dip net to verify species. Observers keep a tally of each life stage of each species. Alternative survey methods, such as detecting amphibian DNA in water samples (eDNA) are also used in some circumstances. 

In addition to species detection data, we record many site- and survey-specific environmental variables at each survey to explain differences in detection probabilities among sites or surveys and differences in the probability of occurrence among sites.

frog egg mass

A frog egg mass spotted along the edge of a water body.(Credit: Pat Kleeman, USGS, WERC. Public domain.)

Abundance and Demography: Estimation of abundance and demographic rates of amphibians is typically accomplished with capture-mark-recapture (CMR) methods. Like occupancy surveys, CMR surveys consist of visual encounter surveys of pond, stream, and pool margins; unlike occupancy surveys, however, an attempt is made to capture by hand or dip net each observed amphibian of the target species.  Captured individuals are implanted with a unique passive integrated transponder (PIT) tag, measured, examined to determine sex, and released at the location of capture. The location of each capture is determined with handheld global positioning system (GPS) receivers.

In some cases, we use counts of egg masses as an index of abundance. The advantages of egg mass counts are that egg masses have higher detection probabilities than individual adult amphibians, surveys require less time and expense, and because they are stationary and persistent, they can be surveyed under ideal conditions for detection. The disadvantages of egg mass counts are that individual survival probabilities of adults cannot be estimated and they assume that each individual female deposits one egg mass per year, which is difficult to verify.

Egg mass counts are conducted throughout the breeding season of the target amphibian species, and consist of a single observer systematically searching an entire body of water (pond or pool) or section of stream for egg masses.  Each observed egg mass is given a unique identifying number with a flag, measured for volume (cm3), and the number and Gosner stage of developing embryos is estimated. Visual surveys are repeated throughout the breeding period to document the deposition of new egg masses and the disappearance or hatching of previously observed egg masses.

surveying frogs

A biological technician, Liz Williams, surveys a pond in Yosemite National Park(Credit: Mike McDonald, USGS, Western Ecological Research Center. Public domain.)

Population Modelling: Ultimately, we hope to use data from demographic studies to construct models of amphibian population dynamics and analyze them to determine the life stages that contribute most to population growth rates.  These life stages can then be targeted for research and management to increase population growth rates of rare species or reduce population growth rates of invasive species. In some cases, integral projection models, which use continuous predictors of survival and transition probabilities and fecundity, might also be useful.  Regardless of the form of the model, sensitivity and elasticity analyses will indicate the life stages most important for population growth.

Amphibian Habitat Relationships: Amphibian-habitat relationships will be evaluated using the occupancy and demographic studies outlined above when habitat characteristics are used as predictor variables for occurrence or demographic rates. In addition to these studies, we examine amphibian-habitat relationships in more detail using radio telemetry. To estimate habitat use and selection, we estimate the percent cover of structural microhabitats and specific vegetation types or species within an individual’s location, with exact microhabitats and vegetation types recorded depending on the species and site.  We then collect the same data at a nearby random site to estimate the microhabitats and vegetation types available to that individual in a paired case (amphibian location)-control (random location) design. In many cases, these microhabitat analyses are supplemented with GIS-based habitat selection studies at larger spatial scales.

In the lab we analyze radio telemetry data by several different methods, depending on the response variable. Movement distances and rates are examined to evaluate the influence of individual characteristics, date, and environmental characteristics on amphibian movements.  We also estimate activity centers and home ranges, selection of microhabitats, and survival rates with the data we collect.

An invasive American bullfrog with tracking device.

An invasive American bullfrog with tracking device.  (Public domain.)

Species Interactions: Amphibian interactions with other species are typically evaluated as part of other studies, including occupancy, demography, and radio telemetry studies.  In these cases, we often use the presence or abundance of an interacting species to evaluate their effects on amphibian occurrence or demography.  Examples of this approach would be to evaluate whether the presence of predators, such as fish or snakes, affects the probability of occurrence of amphibians or their demographic rates.  In other cases, we employ additional methods to examine species interactions.  For example, we often swab amphibians to test for the presence of pathogens such as Batrachochytrium dendrobatidis.  The presence or quantity of pathogens can then be used as predictor variables for survival probabilities.  We submit dead or dying amphibians to the U.S. Geological Survey National Wildlife Health Center for necropsy and culture to evaluate potential pathogens, and provide information to national databases on the occurrence of amphibian diseases.

List of specific projects:

  1. Anuran Occupancy in Yosemite National Park
  2. Sierra Nevada Yellow-legged Frog (Rana sierrae) Demography in Yosemite National Park
  3. California Red-legged Frog (Rana draytonii) Abundance in Point Reyes National Seashore
  4. Foothill Yellow-legged Frog (Rana boylii) Demography at the Angelo Coast Range Reserve
  5. Surveillance Monitoring for Batrachochytrium salamandrivorans in Northern California
  6. Amphibian Occurrence in California Coastal Dune Drainages
  7. California Red-legged Frog Ecology (Rana draytonii) in Coastal Dune Ecosystems
  8. California Red-legged Frog (Rana draytonii) and Northern Red-legged Frog (R. aurora) Occurrence in Forest Streams in Mendocino County
  9. Yosemite Toad (Anaxyrus canorus) Distribution and Demography in Yosemite National Park [Proposed]