Keeping Rattlesnakes Connected



Conservation biologists have long been concerned about how the growing human footprint will affect vulnerable species. A lot of this focus has been on protecting core habitat and this is obviously very important. But we have learned that simply protecting isolated patches of habitat isn’t always good enough. Isolated populations may be wiped out by a catastrophic event or may decline slowly as genetic inbreeding takes place. Because of this, keeping connections is important even if the animals only use those corridors for a short time. Protecting habitat corridors is a tough business, though, because roads, houses, and farms are spread out all over the place. On a map, try drawing a line that represents several miles between two locations that doesn’t cross some sort of human development. Unless you are looking at a large national park or wilderness area, you probably can’t do it.

In the state of Washington, there is a group that is trying to help with conservation corridor planning for many wildlife species. This group is called the Washington Wildlife Habitat Connectivity Working Group. The group is made up of representatives from different agencies and conservation groups working in the northwest region. The group decided that the area of eastern Washington, northeast Oregon and north Idaho (known as the Columbia River Plateau) was a critical area to identify important movement corridors because it is one of the least protected areas in the Pacific Northwest and has many declining species associated with Sagebrush habitat, such as Sage Grouse and Ground Squirrels. The Northern Pacific Rattlesnake (Crotalus oreganus oreganus), a subspecies of the western rattlesnake, is a key species of the Columbia Plateau that has likely declined in the region due to habitat loss and human persecution. Therefore, it was chosen as one of the focal species for the project. Because of my work with Midget Faded Rattlesnakes (another western rattlesnake subspecies) for the Orianne Society, I was asked to participate in the Columbia Plateau project and serve as the species lead for the rattlesnake.

We used a two-stage method to figure out the important connections for rattlesnakes. First, we had to figure out what the important habitat patches were that we needed to connect. We were lucky enough to have a database of rattlesnake observations from across the state of Washington that we could use to model the most important habitat. There were GIS specialists that were part of the working group that compiled a group of environmental layers for the entire Columbia plateau. This allowed me to use a computer program that identified what the environment was like at the spots where we knew rattlesnakes were, and then projected on a map the areas that were most similar. These similar areas would have the best rattlesnake habitat. I had used this method for our previous Midget Faded Rattlesnake study and it worked really well, so I thought it would be good for Northern Pacific Rattlesnakes as well. And we did produce a habitat map that other rattlesnake experts thought looked quite adequate. Our habitat map showed us that rocky slopes along rivers were the best habitat for rattlesnakes in the area.

  Enlarge Photo
A longleaf pine oversees a vast south
Georgia sandhill known to support an
Indigo Snake population

Once we had our map of the habitat patches, we could then predict what the best linkages among those patches should be. But unlike the habitat model, we didn’t have a database that showed us where rattlesnakes actually moved. So we had to make a prediction of what the cost would be for a rattlesnake to move across the landscape, a tool known as a resistance surface. A resistance surface can then be used to show the easiest path to cross (a least cost path) and that is the movement corridor for the species. Of course, in reality it’s pretty difficult to come up with a single surface that represents all the movement costs for the species! I tackled this problem by getting help from other rattlesnake experts (including Orianne Society staff), and together we decided on how costly different parts of the environment are for snakes to cross. Not surprisingly, we gave roads, houses, high elevations, and farmland some of the higher costs, and intact sagebrush and rocky areas the lowest cost. Our corridor map (pictured here) showed us that there are still a number of corridor connections for rattlesnakes, but that there are also some big areas where there aren’t any connections. You can see on the map how corridors are usually far away from roads and cities, and I bet you can guess from the map where the agricultural areas are too.

This map and other species models can immediately help land managers in the Pacific Northwest prioritize conservation actions and resources to the most important areas. But at this point, it is still a model. That is why the next step for the working group is to do validation to see if the predicted corridors actually serve this purpose. We are now working to develop a field validation study for rattlesnakes in which we will use genetic markers to see if there is gene flow along the predicted corridors. If we see this, then we can feel confident that our corridor model is accurate and can be used reliably for rattlesnake conservation. And that way, we can help ensure that rattlesnake populations that are now connected will stay connected.

To learn more about this project, and to view the full sized version of this map, you can download the report Here.