干旱区生态环境知识资源中心

I measured harvest rates and maximum running-escape speeds of two species of ground-dwelling squirrels, the least chipmunk (Tamias minimus) and the golden-mantled ground squirrel (Spermophilus lateralis). Using seed trays in which foragers experienced diminishing returns, I found that S. lateralis harvested seeds faster than did T. minimus. This difference was due to a significantly higher rate of encounter of seeds by S. lateralis. There was no significant difference in handling times between species. Higher harvest rates may reduce predation risk by decreasing the amount of time spent exposed to predators while foraging. I also measured maximum running speeds by chasing animals over short distances. I found that S. lateralis ran significantly faster than did T. minimus (3.14 and 2.13 m/s, respectively). A faster maximum running speed could enable S. lateralis to escape pursuit, but also to exploit the meadow habitat farther from the safety of the forested edge. In previous studies, I found that T. minimus perceived a greater level of predation risk than did S. lateralis in areas 20 m from protective cover and that they were competitively inferior there. The results from this study provide a possible mechanism for this difference. One generalization from this study is that, if larger body size usually confers an anti-predator advantage, larger-bodied species should more often exploit riskier (often open) habitats.