New research shows that highways have reduced the genetic diversity of desert bighorn (Ovis canadensis nelsoni) sheep in southern California. The effects of barriers on these at-risk bighorns are particularly dire because the populations live in scattered mountain ranges and so are already naturally isolated.
“Interstate highways, canals, and developed areas, where present, have apparently eliminated gene flow,” say Clinton Epps of the University of California, Berkeley, and five coauthors in Ecology Letters.
“Anecdotal evidence suggests that bighorn sheep rarely cross [roads],” say the researchers. Yet desert bighorn populations are naturally small and local extinctions are common, so they depend on migration for gene flow and recolonization. This leaves them especially vulnerable to isolation by highways with their multiple lanes of whizzing traffic, concrete median dividers, and fences. California has an estimated 3,400 desert bighorns living in about 50 populations.
To see how highways affect the bighorns, the researchers determined the genetic diversity of 27 populations in the Mojave and Sonoran desert regions of southern California. The researchers analyzed DNA from 461 scat samples and 47 blood or tissue samples. The study area was divided by four major highways, and the researchers assessed the effects of these barriers by comparing the expected genetic diversity to the actual genetic diversity for each population. Expected diversity reflected natural geographic isolation, whereas actual diversity also reflected the additional isolation due to man-made barriers. The difference between expected and actual diversity demonstrated the genetic impact of these barriers alone.
The researchers found that genetic diversity was lower than expected based on natural geographic isolation alone. Desert bighorn populations that were completely isolated by highways fared the worst: they were estimated to have lost as much as 15 percent of their genetic diversity in only about 40 years.
The presence of barriers was the equivalent of increasing the distance between desert bighorn populations by 40 km, which would effectively stop gene flow between populations. Bighorn dispersal is most common between populations that are within 15 km of each other; the populations in the study area were from 2 to 80 km apart.
“This study shows that the scale at which wildlife conservation must be applied is much greater than that of the day-to-day habitat used by desert bighorns,” says Marco Festa-Bianchet of the Université de Sherbrooke in Québec, Canada.
A relatively simple fix would be to reroute the fences along highways so bighorns can reach the other side via underground drainage tunnels. Other approaches include underpasses and overpasses, which have helped large animals from elk to bears cross highways in Canada’s Banff National Park.
By Robin Meadows
Epps, C.W. et al. 2005. Highways block gene flow and cause a rapid decline in genetic diversity of desert bighorn sheep. Ecology Letters 8:1029-1038.
Photo by Julia Perkins