4 | DISCUSSION
Behavioral adjustments in diet, spatial, and temporal use can reduce competition for resources to promote coexistence (Inouye 1978). We tested for spatial and interannual variation in the time use of raccoons across an urban-rural gradient and measured the use of temporal refuges by raccoons in the presence of coyotes across that same gradient. As expected, we found that raccoon time use varied both across the gradient and between years. More importantly, we highlight that there were consistent patterns across the urban-rural gradient in raccoon temporal response to coyotes. We found that at the most urban site (DMP), raccoons consistently did not shift their temporal activity in response to coyotes, despite significant interannual variation in raccoon activity. In contrast, all other sites showed some evidence of behavioral plasticity in raccoon time use with the intensity of coyote spatial use. These results complement other findings that: a) non-consumptive effects impact the spatial use within the carnivore guild (Newsome and Ripple 2015); and b) that non-consumptive effects (fear effects) are present within the hierarchy of the carnivore guild (Gordon et al. 2015).
Urban systems represent an extreme of human pressures, and the continuing increase in urban habitat makes understanding the unique behaviors and ecologies of wildlife in urban spaces such as Detroit, Michigan particularly important. Breck et al. (2019) found that coyotes at urban sites are bolder in comparison to their rural counterparts, which would support their role as a fear source in cities. In absence of shifts in raccoon activity at our DMP site, it seems that this fear effect does not extend to raccoons. Given that we did find some evidence of temporal avoidance at our other sites, a more plausible explanation is that fear of coyotes is not strong enough to elicit a shift in raccoon time use in the face of a stronger force; the most obvious in an urban system being humans and domestic dogs, as reflected by raccoons at DMP having the least diurnal activity (Figure 5) (Gaynor et al. 2018; Nix et al. 2018; Sévêque et al. 2021). Despite raccoon activity consistently being similar between zones of coyote intensity of use, raccoon activity did seem to show somewhat reduced overlap with coyote activity in the high coyote intensity of use areas. This implies that coyotes were potentially using time differently depending on how heavily used an area was by conspecifics. A plausible explanation would be intraspecific competition (Cunningham et al. 2019), or this result could more generally suggest coyotes are more plastic in their time use than raccoons in urban systems (McClennen et al. 2001). The latter would make sense; although both species are cosmopolitan, raccoons are more human tolerant than coyotes (Crooks 2002; Randa and Yunger 2006).
Surprisingly, it was not the human-dominated urban system that was the most unique in raccoon temporal use amongst the sites, but instead the more pristine HMC in northern Michigan. The overall raccoon activity pattern showed considerable use of the diurnal period during which humans tend to be most active (Figure 6), resulting in low overlap with other sites. HMC also showed the greatest interannual variation in raccoon response to coyotes out of the four sites, once again perhaps reflecting a lack of human impact in the form of food subsidies (Manlick and Pauli 2020). The availability of resources can modulate the strength of competition, and so annual variation in food resources could drive the avoidance response of raccoons to coyotes (Newsome et al. 2015). At the other three sites, human food waste and other human-derived subsidies likely offset years that may otherwise be relatively resource-poor for raccoons (Oro et al. 2013). Unlike UMBS and SNWR, which have nearby towns, HMC is isolated, surrounded by forest and with the few cabins on the property only seasonally occupied.
Our results highlight broad patterns in raccoon temporal use between zones of high and low coyote activity. The mechanisms that underlie these patterns require further study and a temporal shift could very likely have more nuance than simple avoidance by a subordinate carnivore. A shift in temporal use by a subordinate (as shown in our SNWR and DMP sites) might instead reflect indirect avoidance of competition with a larger competitor rather than direct avoidance of antagonistic interactions (Newsome et al. 2015). While our results indicate the response of the raccoon to be driven by a larger predator, it does not preclude an interaction between top-down and bottom-up forces, which may be important to understanding what raccoons are directly responding to across sites and survey seasons (Elmhagen and Rushton 2007). For example, resource availability, such as the abundance of small mammal prey, fluctuates with season and could be a driver of varying levels of competition between coyotes and raccoons (Batzli 1992; Fedriani et al. 2000; Neale and Sacks 2001). At an urban site (e.g., DMP), food subsidies in the form of trash could reduce seasonal variation in resource competition (Oro et al. 2013; Newsome et al. 2015). Thus, we would expect patterns of temporal use, particularly in the presence of a competitor, to vary seasonally (Sovie et al. 2019). Seasonal variation in temporal response may explain the divergent result for the 2017 SNWR survey, which occurred during the summer months. The other two surveys at the site occurred during the fall and the spring, periods which are associated with heightened resource gathering for the imminent winter, and heightened coyote aggression because of the coyote breeding season (Way 2001). Pairing dietary studies that explore the seasonal variation in coyote and raccoon diets across all sites with spatiotemporal analyses would elucidate if seasonal variation in resource availability drives resource partitioning between these species.
Though the two sites at the opposite ends of the gradient (i.e., HMC and DMP) best highlight the variation in raccoon temporal activity and temporal response to coyotes, there were site specific patterns for the entire gradient. We intended for our sampling sites to represent opposing gradients of humans and native apex predator presence, which were reflected in the amount of built structures and which carnivores were captured on camera at each site. However, given that we did not test for the effect of the relative activity of apex predators and humans , we cannot discount the possibility that factors other than top-down forces drove the urban-rural gradient we observed in our results. Sites varied in vegetative cover, topography, latitude, and distribution of resources. Though, differences in the sources of top-down forces are the most obvious and likely ecological factor that differs between the sites for generalist species such as raccoons and coyotes. Similar outcomes have been reported for other coyote-subordinate predator systems when compared across sites that vary in the presence of an apex predator (Shores et al. 2019).