Effects of masting hardwoods on stand-use by tree squirrels and fishers in Northern California.

Abstract

According to the prey-habitat hypothesis, predatory species may associate with specific habitats that are of high value to their prey; a relationship often overlooked when considering carnivore foraging strategies that can explain how landscape characteristics influence individual fitness. In western North America, western gray (Sciurus griseus) and Douglas's squirrels (Tamiasciurus douglasii) are important prey for fishers (Pekania pennanti), particularly western gray squirrels due to their large body size. Masting trees including Black Oak (Quercus kelloggii) and Tanoak (Notholithocarpus densiflorus) produce an important food source for tree squirrels. Therefore, forest stands containing these trees may be useful to foraging fishers. We hypothesized that: (1) occupancy of a forested stand by western gray and Douglas's squirrels increases with increasing mast production potential of that stand; (2) Fisher stand-use increases with increasing tree squirrel occupancy in that stand; and (3) Fisher stand-use is conditional on only the Western Gray Squirrel occupancy status in that stand. We deployed remote cameras in the northern Sierra Nevada of California in 3 different forest stand types: conifer dominant, conifer co-dominant with tanoak, and conifer co-dominant with Black Oak. We tested the relationships of stand type and other covariates on tree squirrel and Fisher occupancy and detection using single-species occupancy models. We also tested the effect of Western Gray Squirrel presence on Fisher occupancy probability using 2-species co-occurrence models. Douglas's squirrels occupied most sites regardless of stand type. Gray squirrels and fishers had highest rates of occupancy in tanoak co-dominant stands. Fisher stand-use patterns suggested both conditional and unconditional occupancy with western gray squirrels, signifying fishers and gray squirrels may use tanoak stands at high rates irrespective of each other as these stands potentially provide increased access to diverse food resources. Forested stands containing masting trees may support greater numbers of western gray squirrels than other habitats, and retention of this type of tree across the landscape may improve foraging habitat for fishers and other carniovres. Determining which habitat metrics influence prey availability will contribute to understanding carnivore species foraging ecology and inform management and conservation strategies.