Abstract
Human activities are changing the environments that animal communities utilize, altering competition and co-existence dynamics as well as changing species distributions. A global consequence of human activity is increased woody cover, which changes environmental conditions on landscape, local, and fine spatial scales. These multi-scale processes are likely to change species interactions and distributions in unexpected ways. To elucidate these effects, we investigated the co-occurrence of two interacting species in a rapidly changing ecosystem. Eastern fox (Sciurus niger) and gray (S. carolinensis) squirrels occur sympatrically throughout the southeastern United States. We used single-season, two-species occupancy modeling to understand what factors influence competition between the squirrels at three spatial scales. We found evidence that fox and gray squirrels compete at fine scales but not at local and landscape scales. The best model to explain fox and gray squirrel co-occurrence at fine scales was an additive fine scale understory and tree canopy closure model. Increased fine-scale canopy closure correlated with increased fox and gray squirrel occupancy while increased fine-scale understory cover correlated with decreased gray and fox squirrel occupancy. Fox squirrel occupancy probability declined in the presence of gray squirrels. The intensity of interaction was modified by fine-scale canopy cover and understory density. These findings support the hypothesis that species interactions can be an important factor in structuring biotic communities, however the strength of the effect changes across spatial scales. Our results support the importance of considering species interactions at multiple scales when predicting community composition in human modified and managed systems.
Highlights
Human activities are changing the environments that animal communities utilize (Lewis and Maslin, 2015)
We found patterns of fox and gray squirrels co-occurrence that are indicative of potential competition at fine scales (0.01 ha) but not at local (5.3 ha) or landscape scales (765 ha)
Fox squirrels were less likely to occur in areas occupied by gray squirrels, while gray squirrel occupancy was not influenced by the presence of fox squirrels
Summary
Human activities are changing the environments that animal communities utilize (Lewis and Maslin, 2015). These activities can alter species interactions and co-existence dynamics and change species distributions and community composition (Lyons et al, 2016). Changes in community composition are likely to be pronounced for ecologically similar species that often co-occur through specializations on multiple dimensions which result in multi-scale “trade-offs” (Kneitel and Chase, 2004). It is hypothesized that these multiscale and dimensional competitive advantages allow similar species to co-occur (Kneitel and Chase, 2004). The mechanism by which common, human-driven environmental changes alter how species interact at multiple scales is poorly understood (Tews et al, 2004)
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