Vertebrate disturbances in forest ecosystems: An experimental case study in Pennsylvania and a worldwide review

Background/Question/Methods

Much of our understanding of the indirect effects of vertebrates on forest ecosystems comes primarily from studies of how the direct consumption of plants impacts community composition. Indeed, much of the literature considers trophic effects of vertebrate herbivores to be the sole process shaping forest communities. However, animal disturbances have largely been ignored as a driving agent of community change. Vertebrates alter the physical environment through sexual displays, nest building, burrowing, bedding, trampling, urination, defecation, and rooting. All of these are relatively discrete disturbance events that damage and kill plants and change resource availability at various spatial and temporal scales. We tested the hypothesis that the exclusion of vertebrates (primarily deer and turkeys but also humans) decreases the frequency and magnitude of litter and soil disturbances. We argue that the impact of these disturbances is rarely considered when evaluating the impact of vertebrate browsers, particularly in fenced exclusion experiments. To address our hypothesis, we examined litter disturbances inside and outside of six long-term exclosures over a period of two years. In addition, we reviewed the degree to which vertebrates cause physical damage via disturbances in forests worldwide. 

Results/Conclusions

We found that the mean size of physical disturbances to the litter layer was between 5 and 20 times higher and the number of disturbances was between 2 and 8 times higher within control plots versus paired vertebrate exclosures. In a review of more than 60 studies, we found that vertebrates (particularly suids, large galliforms, and elephants) cause important disturbances in forests worldwide and these disturbances should be considered simultaneously with their trophic impacts. In eastern deciduous forests in the U.S., the increased frequency of litter disturbances may limit the germination rates of large seeded tree species (oaks) and favor small seeded species such as maple and beech. Finally, we reinterpret the results of several studies that failed to consider that exclosures, particularly ones designed to exclude deer, exclude other vertebrates (e.g., humans) that have both trophic and non-trophic effects.