Interference competition between coyotes and raccoons: a test of the mesopredator release hypothesis

Some predator species appear to conform to the mesopredator release hypothesis (MRH), in which larger predators help limit populations of smaller predators. This hypothesis has been used to explain the possible relationship between coyotes, mesopredators, and resultant cascades involving nonpredators. However, relationships between coyotes and noncanid mesopredators are poorly understood, and predictions from the MRH have rarely been rigorously tested. We monitored sympatric raccoon and coyote populations to assess 2 predictions derived from the MRH: coyote predation is an important cause of mortality in raccoon populations or raccoons avoid areas used by coyotes. Between March 2000 and September 2001, we recorded 3553 locations for 27 radio-collared raccoons and 1393 locations for 13 coyotes captured on the Max McGraw Wildlife Foundation in Illinois, USA. No raccoon mortality from coyote predation was observed during the study, and raccoon survival was > 0.7 each season. All raccoon 95% home ranges exhibited overlap with 95% coyote home ranges in each season. The mean proportion of raccoon locations within 95% coyote home ranges did not vary by sex but did vary by season. Raccoon overlap of coyote core areas varied considerably among individuals within seasons, ranging from 0% to 83%. However, 45% of raccoons had < 10% overlap with coyote core areas, whereas only 14% of raccoons exhibited > 50% overlap. Mean overlap with core areas did not vary by season or sex. For those raccoons with home ranges overlapping coyote core areas, mean proportion of observed raccoon locations within coyote core areas was generally greater than the mean proportion of random locations. Scent-station experiments failed to document raccoon avoidance of specific sites that had been marked with coyote urine. We did not find support for a mortality prediction or avoidance prediction to support MRH with regard to raccoons and coyotes. These results suggest that relationships among mammalian predators may not be simply dictated by body size, particularly for species outside the Canidae.