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Why scientists have mapped 29,777 instances of roadkill across California

Scientists in California have spent the past six years carefully tracking and mapping 29,777 instances of roadkill across the state.

The reason? The animals we kill on the roads, it turns out, can tell biologists a lot about the animals living in the wild as a whole.

roadkill map 2

(California Roadkill Observation System)

This map, from the California Roadkill Observation System a detailed database of volunteer-submitted information on roadkill — shows all observations from just the past 90 days. The medium-size mammals (dark blue) are mostly possums, raccoons, bobcats, and skunks, while the smaller ones (light blue) are mainly squirrels and rabbits. The large mammals (orange) are almost all deer.

If you go to the interactive map and zoom in, you can see all sorts of interesting patterns, such as a concentration of snakes and other reptiles near Santa Maria (you can also click on each recording, and in many cases see an actual photo of the animal):

roadkill reptiles

(California Roadkill Observation System)

"I think of roads as a continuous wildlife sampling device," says Fraser Shilling, the UC Davis professor who operates the database. "Our system is the largest, most taxonomically broad wildlife monitoring system in the state."

Most wildlife-monitoring systems focus on specific species, often using motion-detecting cameras or people manually counting. But because virtually all species are prone to getting run over from time to time, mapping roadkill instead can give you information about a huge range of animals: the UC Davis system, the largest of several around the country, includes roadkill data on 350 of California's 680 native vertebrate species.

What's more, in any given specific area if a particular species starts showing up far more (or less) often as roadkill, it's usually a reflection of that species' changing abundance in the wild. As a result, counting roadkill has given Shilling and other researchers a few interesting insights into the ever-changing state of California's wildlife.

Mapping roadkill can reveal the spread of invasive species

Shilling has used the database to map the spread of the Eastern gray squirrel and Eastern fox squirrel in California, among other invasive species:

eastern fox squirrel

Observations of the Eastern fox squirrel as roadkill in Northern California. (California Roadkill Observation System)

"They're both invading different parts of the Western gray squirrel's habitat, and we can see that in the roadkill data," he says.

Elsewhere, other ecologists have used roadkill data to map the spread of the red fox in Tasmania and Burmese pythons in the Everglades.

California's wildlife corridors don't seem to be working

Across California, state officials have mapped out wildlife corridors: strips of relatively undeveloped, sometimes protected land that connect parks and other wildlife reserves. In theory, these corridors could allow wildlife to travel between habitats, instead of having to thread their way through suburbs or industrial areas.

Except for one thing: according to the roadkill data, animals don't seem to be using them. When Shilling analyzed data along more than 4,000 miles of California highway, "there was really no difference between the areas inside and outside corridors, in terms of roadkill," he says.

Shilling is far from the first to critique these corridors' effectiveness, but despite some being in place for decades, there's extremely little hard data on how effective they really are. "It's tempting to draw lines between habitat patches or reserve areas," he says, "but for the vast majority of species in the world, there's no ecological meaning to the term 'wildlife corridor.'"

Is California's drought taking a toll on wildlife?

California is in the midst of a truly historic drought — perhaps the worst it's seen in the past 1,200 years. And it appears to be affecting the overall abundance of wildlife.

Shilling's observations on this are preliminary, and haven't yet been published and peer-reviewed. But early on in the drought, he and the other researchers saw a spike in the amount of roadkill, especially deer. Shilling speculates this might have been driven by the animals' increased migration as they looked for food sources in a wilting environment.

More recently, however, the data has shown the opposite trend. "We've seen a significant decline in roadkill rates across all species," he says. "It's probably because we have a decline in wildlife populations in general."

How you can help track roadkill

The California system — along with a partner system in Maine — relies heavily on citizen observers (that is, everyday, normal people) to provide data. If you live in one of these areas, these scientists want you to stop and take note the next time you see roadkill, and enter the data here.

It helps to have some training or familiarity with wildlife, so you can accurately identify the correct species. But even if all you know is that the animal is some type of rabbit, you can write that in, and it still helps. If you're the kind of person who enjoys taking pictures of roadkill (who isn't?), you can also upload a photo.