Paved Roadways Can Disrupt Movement of Small Wildlife

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When does the pocket mouse cross the road? If the road is paved, it may not. As evidenced by many a roadkill carcass, high traffic roads pose an obvious barrier to large wildlife species. Yet few studies to date have examined how different types of roadways impact smaller wildlife species like rodents and lizards. 

A new study by U.S. Geological Survey and San Diego State University scientists now demonstrates that paved roads large and small can deter small wildlife species, creating a barrier that may deter them from reaching neighboring habitats. 

The findings offer insights on the road types that present the most significant movement barriers to various species, and reinforce past research which found genetic isolation in small species with ranges divided by paved highways.

predicted movement pattern of San Diego pocket mouse

The predicted movement pattern of the San Diego pocket mouse (Chaetodipus fallax) at the top left, compared with actual movement patterns observed across road types. Image Credit: USGS/Conservation Biology.

“Species that avoid roadways can become isolated, resulting in fragmented populations — whereas species that are not deterred by roads can be at risk of higher mortality rates and even local extirpation due to wildlife-vehicle collisions,” says ecologist Cheryl Brehme of the USGS Western Ecological Research Center, the study’s lead author. “In addition, we found that the way these different animals tend to move and forage within their scrub habitat may be predictive of whether or not they tend to avoid roads.”

WERC scientists Jeff Tracey and Robert Fisher also contributed to the study.

In this study, scientists observed how rodent and lizard species in San Diego County avoided or crossed three different types of roadways:

  • unimproved dirt roads with almost no traffic
  • a narrow low-use two-lane paved secondary road with low traffic
  • State Highway 94, a two-lane rural paved highway with moderate to heavy traffic 

All three roadways pass through scrubland habitat in San Diego National Wildlife Refuge or Rancho Jamul.

road types

Road types examined as part of Brehme et al. 2013. (Public domain.)

The experiments took place at night, when many species are most active. Scientists captured animals at each roadway study site and dusted them with special fluorescent tracking powder, allowing scientists to follow and track their paths using ultraviolet flashlights the following night after each animal was released. 

Scientists studied 17 species but obtained the most data from five: San Diego pocket mouse (Chaetodipus fallax), cactus mouse (Peromyscus eremicus),western fence lizard (Sceloporus occidentalis), orange-throated whiptail (Aspidoscelis hyperythrus) and Dulzura kangaroo rat (Dipodomys simulans). All five are common to the sage-scrub habitat of San Diego County.

whiptail lizard with fluorescent powder

A whiptail lizard dusted with fluorescent powder as part of the study. (Credit: Cheryl Brehme, USGS Western Ecological Research Center. Public domain.)

Searching for fluorescent trails from study animals

USGS ecologist Cheryl Brehme and colleague search for fluorescent trails created by the marked study animals. (Public domain.)

All species readily crossed the dirt roads to habitat on the other side. The lizards also regularly ventured out onto the low-use paved secondary road of similar width, possibly using this road for both movement and thermoregulation.

The mice, on the other hand, avoided moving onto this road, suggesting that they avoid paved road substrate. All of the lizards and mice avoided the paved highway — clearly turning away when they reached the edge of the road. 

Brehme and colleagues also observed some exceptions, such as in the Dulzura kangaroo rat (Dipodomys simulans).

This kangaroo rat did not cross the road...

This kangaroo rat did not cross the road... (Image credit: USGS)

“We knew this species likes open space, and in our dirt road study site we tracked one to its burrow that was literally in the middle of the road,” says Brehme. “Two out of three ran right out onto the low-use paved road, and at our highway study site, we couldn't catch any to powder and track — yet we counted quite a few dead ones on the road itself. So clearly this species isn't deterred by paved roads, but the car traffic nevertheless forms somewhat of a barrier to their movements and increases the mortality rate of this local population.”

The study is part of a growing discipline called road ecology, which brings together biologists, civil engineers and planning agencies to find new ways to improve roadway designs that both conserve wildlife populations and reduce harm to humans.

Cheryl S. Brehme, Jeff A. Tracey, Leroy R. McClenaghan, Robert N. Fisher. 2013. Permeability of roads to movement of scrubland lizards and small mammals. Conservation Biology. 

-- Ben Young Landis

woodrat with fluorescent powder

"Forget crossing a road -- with all this glow powder, get me to a rave party!" A woodrat (Neotoma) dusted with fluorescent powder as party of a USGS road ecology study. (Credit: Cheryl Brehme, USGS Western Ecological Research Center. Public domain.)