Thesis

Effects of ecosystem engineering by the giant kangaroo rat on the common side-blotched lizard

Ecosystem engineers are organisms that control the availability of resources to other species by creating and modifying habitat (Jones et al. 1994). Ecosystem engineers alter habitat in such a way as to reduce physical and biological stresses for other organisms (Crain & Bertness 2006). Burrowing mammals are keystone ecosystem engineers in many communities because burrowing is an engineering activity that can directly and indirectly alter the availability of resources, have effects at multiple spatial and temporal scales, and have a significant role in community organization (Prugh & Brashares 2012). Giant kangaroo rats (Dipodomys ingens), are a federally listed endangered species and keystone ecosystem engineers that modify habitat by building extensive burrowing systems. They are associated with greater density and diversity of vertebrate, invertebrate, and plant populations. However, few studies have addressed the functional mechanisms behind these associations and how ecosystem engineering affects the behavior of associated species. The purpose of this study was to investigate the functional attributes of ecosystem engineering by giant kangaroo rats and quantify their effects on the common side-blotched lizard. Lizard abundance is positively correlated with giant kangaroo rat density in this system, and although an increased presence of arthropods has been suggested as the mechanism, the causative factor has not been determined (Prugh & Brashares 2012). I investigated both landscape and local scale effects of habitat modification by giant kangaroo rats on the density, microhabitat use, and behavior of the common side-blotched lizard in 2012 and 2013. I determined that lizards are associated with greater burrow tunnel density at the landscape scale and are found more often on precincts on a local scale. A translocation experiment revealed that lizards prefer precincts with more burrow tunnels and remain at those precincts longer. I investigated whether giant kangaroo rats facilitate lizard density by three possible mechanisms: by providing refuge from predators, by providing thermal refuge, or by providing increased arthropod prey resources. Additionally, I investigated whether lizards altered their behavioral response to predation risk in the presence of burrow tunnels. I found that lizards use burrow tunnels as refuge from predators and display differences in escape behavior on and off precincts. Precincts also provide thermal refuge where temperature and humidity is more stable than the outside environment. However, at the local scale, there were no differences in arthropod resources. Giant kangaroo rats likely facilitate common side-blotched lizards by a combination of supplying refuge from predators and from extreme temperature and humidity.

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