Evaluating top down and bottom-up effects on a generalist mesocarnivore in winter

Background/Question/Methods

Investigating how species respond to changes in top-down and bottom-up factors is essential to understanding their ecology and informing management and conservation actions. Carnivore guilds are typically composed of at least one larger, dominant species and one or more smaller, subordinate species that compete with their larger counterparts for resources. This competition may result in agonistic interactions and intraguild predation, and both factors can lead to top-down regulation of subordinate carnivore populations. Subordinate carnivore populations can also be regulated by bottom-up factors, such as the availability of their preferred prey, habitat features, and climatic conditions. Nevertheless, substantial ambiguity remains regarding the relative roles these complex factors play in shaping subordinate carnivore populations, especially during the winter. To investigate this issue, we conducted a large-scale camera-trapping study (n=197 sites) following a natural experimental design wherein we contrasted forests that were more recently vs less recently harvested. We used a dynamic occupancy model to examine the influences of top-down (interference competition) and bottom-up (primary prey, habitat, and climate) factors on a widespread, generalist mesocarnivore, the red fox (Vulpes vulpes), across three successive winters in the Maine, USA.

Results/Conclusions

Between 2017 and 2020, we collected 145 red fox and 435 coyote (Canis latrans) daily encounters, and 6389 snowshoe hare (Lepus americanus) detections. We found evidence for top-down effects of coyotes on red fox site use intensity and site colonization probability. Contrary to our predictions, the association between the occurrence of this top-down competitor and red foxes was positive rather than negative. Snowshoe hares had a positive association with red fox site abandonment probability, which is also surprising given that snowshoe hares are the preferred winter prey of red foxes in this ecosystem. The intensity of forest disturbance and the proportion of conifer forest had negative associations with both the site use intensity and occurrence of red foxes. Together, these results suggest red foxes may be limited more by bottom-up factors related to habitat than by the top-down effect of risk of interference competition or the availability of primary prey in winter. This study supports recent findings that bottom-up factors may be more influential during less productive times of year. This work also highlights how caution is needed when extrapolating previous results from summer studies to winter, as the role of top-down and bottom-up factors may change seasonally.