Janzen-Connell effects and habitat-induced aggregation synergistically promote species coexistence

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.