1. 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.

2. 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.

3. Using 204 seedling transects (1 x 10 m) at 51 sites across a 900-hectare 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 ~15,000 individual seedlings were monitored over three growing seasons.

4. Demographic neighborhood models were constructed to examine the influence of neighborhood composition and density on seedling survival and growth. An interaction term between conspecific neighbor density and fencing treatment was included to test the hypothesis that large herbivores cause CNDD.

5. We found that seedling survival was influenced by both conspecific neighbor 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.

6. Synthesis: Debate regarding the role of large herbivores in fostering the diversity and stability of forest communities continues. Our results indicate that large mammalian herbivores can cause CNDD in some temperate tree species, but whether they help to promote community diversity over time remains unclear. Further research is needed to assess the role of large mammalian herbivores in driving conspecific negative density dependence in plant communities.

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