Between 2008 and 2011, we conducted several translocations of the endangered Stephens’ kangaroo rat, creating or supplementing populations in the Southwestern Riverside County Multi-Species Reserve. This fall, we continued to monitor their success by trapping at each of the sites to check on their progress in the wild. When we capture a “roo-rat,” we always record its sex, age, weight, location, and tag number if it has one or tag it if it’s new. This provides us with valuable information about each population.
This year, we successfully trapped hundreds of roo-rats, including many reproductive adults and independent young of the year. More than half of these animals were new roo-rats that we tagged during the trapping week. This suggests that despite this being a year with heavy predation pressure, Stephen’s kangaroo rats are persisting at each site in good numbers. Their natural predators include coyotes, owls, and rattlesnakes.
Our North Shore site was interesting because there were so many cottontail rabbits there. They often ate the trap bait, a combination of white millet and oats, and often bumped or turned over our traps. At all of our sites this season, captures of other species such as deer mice and San Diego pocket mice were low. This suggests that their numbers may be low due to competition with the roo-rats.
At Bachelor Mountain, near Lake Skinner, herbicide had been used to remove invasive plants and grasses, resulting in open ground with small native shrubs such as doveweed. This was an effort to increase possible habitat area and create corridors between the sites for the roo-rats. Their colonization appears to be restricted by dense grassland with complete ground cover. We found that a few had moved across a road into a corridor area. Furthermore, in 2011 a few San Diego kangaroo rats, a common species, had been trapped at site D. This season, we did not trap any San Diego roo-rats at site D. We only trapped one of them overall, suggesting this species is not moving into Stephen’s kangaroo rat habitat.
Crown Valley proved to be a difficult site to trap this year. We had coyotes hunting and trying to steal traps. We had to use a hand-held spotlight and air horns to chase them away. This meant little sleep and long, vigilant watches over our little critters. A few rattlesnakes were also spotted at night during trapping. Crown Valley has six pie-shaped research sites with slices that have been mowed, grazed, and burned to create open grassland environments. In 2010 and 2011, we planted nearly 20,000 native grasses at these sites (see Kangaroo Rats Get Home Improvement). It was initially obvious that there were many more burrows on the slices with native plants, thus we put 52% more traps on those areas. Burrow counts on each site support this. For instance, Site 5 slices with plantings had 109, 95, and 108 burrows, and those without had 20, 17, and 10 burrows. Furthermore, we had 454 captures on the slices with plantings compared to 97 on slices without plant restoration, a four-fold differences in animal captures. This suggests that the native plants are really doing their job at making a nice habitat for roo-rats.
Our last site of the season was the Schoolhouse Plateau at Lake Skinner. This site is so large that every year we randomly trap half the site and estimate the population. The plateau is the size of several football fields. This site had many jackrabbits, but they did not bother the traps much. However, we did have issues with ants stealing the bait and infesting our traps. This site is also known for black widows, as they commonly coexist with roo-rats by sharing burrows. The roo-rats often use borrows with black widow spider webs across the entrance. We always had to look before we sat down, especially in the dark!
We will continue to track the progress of Stephen’s kangaroo rats with annual trapping and burrow counts for population estimates. In addition, we will continue to conduct behavioral research (see No Night-lights for Kangaroo Rats) and participate in an inter-departmental study on the species’ landscape genetics. Continuing to increase knowledge about these little animals is vital in protecting an endangered species and in making successful management plans.
Christine Slocomb is a senior research technician in the Applied Animal Ecology Division at the San Diego Zoo Institute for Conservation Research. Read her previous post, Endangered Rats and Mice: Unexpected Results.