Large mammalian herbivores contribute to conspecific negative density dependence in a temperate forest

The Janzen-Connell Hypothesis (JCH) predicts that density-responsive and host-specific natural enemies limit the population sizes of abundant species. Importantly, these interactions help to maintain local community diversity through time. While ample evidence exists for the demographic predictions of the JCH, it remains unclear which natural enemies drive these dynamics across different plant communities. While large mammalian herbivores are often assumed to lack the specialized diet needed to drive Janzen-Connell effects, they do show a degree of host-preference that could drive density-dependent plant demography. However, the potential role of large mammalian herbivores in Janzen-Connell interactions has only rarely been investigated. Using 204 seedling transects (1 m x 10 m) at 51 sites across a 900-ha forested reserve in southwestern Pennsylvania (USA), we examined the role that large mammals play in driving conspecific negative density dependence (CNDD) in temperate tree seedlings. Individual fences were erected around half of the transects (n = 102) to exclude large mammals, and were paired with adjacent unfenced transects. Within transects, a total of similar to 15,000 individual seedlings were monitored over three growing seasons. Demographic neighbourhood models were constructed to examine the influence of neighbourhood composition and density on seedling survival and growth. An interaction term between conspecific neighbour density and fencing treatment was included to test the hypothesis that large herbivores cause CNDD. We found that seedling survival was influenced by both conspecific neighbour density and fencing. CNDD was strongest when large mammals were allowed access to seedlings, and these results were driven by two abundant taxa (Prunus serotina and Fraxinus spp). Despite evidence that large mammals mediate CNDD, we found no effect of fencing on rarified species richness or evenness in seedling transects during the study. Synthesis. Understanding the specific natural enemies driving conspecific negative density dependence remains vital for understanding the maintenance of forest diversity across the globe. Our results indicate that large mammalian herbivores are capable of driving CNDD in temperate tree species. These results suggest that large mammals may be an important and generally overlooked agent contributing to Janzen-Connell interactions in forest communities. We expect that further research examining large mammals in other systems will be important in the future.

Automated summary

The Janzen-Connell Hypothesis predicts that natural enemies limit the population sizes of abundant species, maintaining local community diversity. This study found that large mammalian herbivores can drive conspecific negative density dependence in temperate tree seedlings. Further research on large mammals in other systems is important for understanding forest diversity maintenance globally.