Home & Garden Columns
I’ve seen only a handful of bobcats in my life, most of them in or around Point Reyes and the Marin Headlands. My one East Bay encounter was about a decade ago, while heading out to Briones Regional Park on a spring morning. The cat was crossing Bear Creek Road near the reservoir, not being in a particular hurry about it. The first reaction in such sightings tends to be “funny-looking dog,” and then you notice the pointed ears and the abbreviated tail.
I thought about that bobcat recently when I read a commentary in Nature about a study that appeared in the journal Molecular Ecology. It was about the impact of the Ventura Freeway on the population genetics of mid-sized predatory mammals, specifically bobcats and coyotes.
The Ventura, with 10 to 12 lanes and a daily load of 150,000 vehicles, is a much bigger deal than Bear Creek Road. It slices between the Santa Monica Mountains to the south and additional undisturbed (for now) habitat to the north. To some creatures, it’s a barrier as absolute as an ocean or a mountain range. To others, it’s more of a filter. The study in question, led by S. P. D. Riley, tried to quantify just how such a manmade filter works.
Riley and colleagues spent seven years trapping bobcats and coyotes on both sides of the freeway, taking samples for genetic analysis, and rigging them with radio transmitters. Telemetry showed that there was some cross-road traffic: at some point during the study period, 11.5 percent of the bobcats and 4.5 percent of the coyotes crossed the freeway. That’s not a lot, but it might be enough to help maintain connectivity between the populations on either side and reduce inbreeding.
But the genetic picture didn’t exactly mirror the crossing statistics. Looking at seven microsatellite loci—highly variable DNA markers that provide good clues to population structure—the authors saw significant differences between populations north and south of the Ventura for both bobcats and coyotes. Estimates of migration rates from genetic data alone were three to 18 times lower than estimates based on the radiotelemetry results. It looked as if both species were crossing the freeway but not sticking around to establish territories, mate, and rear offspring. They were tourists, not colonizers.
Six of the 10 tagged bobcats that made the crossing returned to their point of origin, and neither of two females that settled in on the other side produced litters the next spring. Only one of five coyotes remained on the other side during the mating season. Mapping coyote and bobcat territories, the biologists discovered what they called a “home-range pile-up” effect. Territories didn’t straddle the freeway, and those that bordered it overlapped with territories farther away. So a young, ambitious bobcat venturing from south of the Ventura to the north side would find the suitable habitat filled up, and would be unable to stake out turf of its own, find a mate, and produce more bobcats.
It’s not like the Ventura Freeway is the only constraint to the movement of bobcats and coyotes, of course. These creatures are effectively island dwellers, hemmed in by roads, houses, and malls. And as E. O. Wilson and Robert MacArthur proposed back in 1963 and Robert Soulé and other biologists have confirmed, island populations have trouble maintaining themselves without an inflow of immigrants. The smaller the “island”—whether surrounded by water or concrete—the greater the risk of inbreeding, depression, losses to disease or other stochastic factors, and eventual extinction.
That’s the whole point of the Wildlands Project, still hanging in there although its estimable magazine Wild Earth folded last year, trying to bridge the isolated fragments of wildlife habitat so genes can still flow among the pieces of a metapopulation. Soulé and his partners in the project have grandiose visions about linking wolf populations from Maine to New Mexico. More pragmatically, they—and mainstream groups like The Nature Conservancy—have helped establish local landscape corridors all over North America, from the Boundary Waters to the Rio Grande Valley.
No one was thinking about habitat connectivity when the Interstate Highway System was built, and the freeways aren’t coming down any time soon, regardless of the price of gas. But it’s possible to tinker with the system, add overcrossings and undercrossings that will allow animals to disperse and establish new territories. The scale will vary, of course, and a toad or snake crossing won’t look much like a bobcat or mountain lion corridor.
It’s ironic that while environmental groups have been working to mitigate the consequences of our fragmentation of the landscape, the politicians are pushing for the biggest barrier yet, the Great Wall of Separation along the U.S.-Mexican border. What would keep out illegal immigrants would also affect endangered borderlands species like the jaguar, ocelot, and Sonoran pronghorn, dooming some populations to extinction. When you reckon up the cost of xenophobia, don’t forget the collateral damage to wildlife. Some things are even worse than freeways.
Photograph Courtesy http://philip.greenspun.com
A bobcat keeps a wary watch from its arboreal perch.?