Smith, D. J. B. | Search bioRxiv

bioRxiv2022

On the interaction between Janzen-Connell effects and habitat partitioning in spatially structured environments

Janzen-Connell (JC) effects, habitat partitioning (HP), and dispersal limitation are three processes thought to shape tropical forest diversity and patterns of species aggregation, yet how they jointly influence species coexistence remains poorly understood. I analyze a spatially explicit model that incorporates all three processes, tracking propagules subject to habitat filtering and conspecific density dependence. The model shows that dispersal limitation and habitat filtering interact with JC-effects in qualitatively different ways. Aggregation driven by dispersal limitation weakens the ability of JC-effects to maintain coexistence, whereas when JC-effects and HP operate together in spatially autocorrelated habitats, richness increases with conspecific aggregation. JC-HP interactions strongly depend on habitat spatial structure. If habitats are not spatially autocorrelated, JC-effects and HP minimally interact (or act antagonistically when coupled with dispersal limitation) and do not maintain high species richness. However, when habitats are positively autocorrelated the two mechanisms act synergistically. To explain these patterns, I introduce a novel metric -- the spatial JC-HP covariance -- which quantifies how strongly JC-effects are concentrated in favorable versus unfavorable environments, providing a direct measure of the extent to which JC-effects and HP synergistically promote coexistence.

ecology↗

bioRxiv2021

The functional form of specialized predation dramatically affects whether Janzen-Connell effects can prevent competitive exclusion

Janzen Connell Effects (JCEs), specialized predation of seeds and seedlings near conspecific trees, are hypothesized to promote high species richness. While past modeling studies show JCEs can maintain higher diversity than a neutral community, recent theoretical work indicates JCEs may weakly inhibit competitive exclusion when species exhibit interspecific fitness variation. However, recent models make somewhat restrictive assumptions about the functional form of specialized predation - that JCEs occur at a fixed rate when seeds/seedlings are within a fixed distance of a conspecific tree. Using a theoretical model, I show that the functional form of JCEs largely impacts their ability to promote coexistence. If specialized predation pressure increases additively with adult tree density and decays exponentially with distance, JCEs maintain considerably higher diversity than predicted by recent models. Parameterizing the model with values from a Panamanian tree community indicates JCEs can maintain high diversity in communities exhibiting high interspecific fitness variation.

ecology↗