How the spatial scales of interactions with natural enemies influence both their contributions to diversity maintenance and our ability to measure these contributions – and implications for the interpretation of Janzen-Connell studies

Background/Question/Methods

Over 35 years ago, Janzen and Connell suggested that specialized natural enemies concentrate around adult trees, that these enemies elevate seed and seedling mortality near conspecific adults, and that this contributes to the maintenance of tree species diversity in tropical forests by preventing any one species from becoming very common. While we now have extensive empirical evidence for elevated mortality near conspecific adults and theoretical demonstration that such mortality contributes to diversity maintenance, we lack tools for extrapolating from quantities measured in the field to community-level effects on diversity. Fundamentally, the contribution of spatial Janzen-Connell effects to diversity maintenance depends on the strength of the resulting stabilizing effect of population-level negative frequency-dependence on the change in reproductive rate with relative abundance. Empirical studies have quantified not whole-population effects but local effects, specifically, changes in establishment probability, growth, and survival as a function of the distance to conspecific adults or the local density of conspecific seeds or adults. 

Results/Conclusions

Here, we use models to illuminate the links or rather, lack thereof between Janzen-Connell patterns that are or could be measured in the field and larger-scale influences on population regulation and community diversity maintenance. We first investigate how characteristics of specialized natural enemies and their host plant species influence plant populations and communities in models designed to capture key characteristics of tropical forests, including local resource competition. We then do field sampling within the simulated landscape to see what traditional empirical studies of Janzen-Connell effects might find, and evaluate the relationship between these observed patterns and actual effects on population regulation and species diversity. We find that the correlations are quite poor, and insofar as they exist, are often in the opposite direction of what might be expected. We close by discussing the implications for interpreting empirical patterns of density-dependence.