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

Abstract

Janzen-Connell Effects and Habitat Partitioning have both been proposed as potential stabilizing mechanisms in species-rich forest communities. Janzen-Connell Effects describe the process in which specialized predators are attracted to adult trees, which reduce the survivorship of nearby conspecific juveniles. This is thought to generate stabilizing negative frequency dependence. Habitation Partitioning describes when species exhibit different fitness responses to spatially heterogeneous environmental factors, typically causing species to aggregate into favorable habitat types. Despite substantial empirical evidence that both processes occur simultaneously in empirical systems, the theoretical implications of how their interactions shape species diversity remain underdeveloped. In this study, I examine a spatially explicit model that incorporates both processes. I show that Janzen-Connell Effects and Habitat Partitioning can act synergistically to promote coexistence such that their contribution to species coexistence is greater than the sum of their parts when environmental spatial heterogeneity is positively autocorrelated. The results of this study highlight the need to explicitly model the interaction between co-occurring spatially dependent coexistence mechanisms to understand the role they play in the maintenance of species diversity.

Habitat heterogeneity, environmental feedbacks, and species coexistence across timescales

“…We also treat space implicitly in our model, but the spatial structure of ecological landscapes could shape how patch modification and dispersal interact. For example, a recent theoretical analysis incorporating both habitat partitioning and Janzen-Connell effects (endogenous heterogeneity) showed that these processes can combine to promote coexistence in a strongly synergistic manner when the spatial autocorrelation of patches is accounted for (Smith 2022). Even when only one source of heterogeneity is present, the spatial arrangement of patches could modulate our understanding of coexistence, for instance by reducing source-sink effects that limit the coexistence of imperfect habitat specialists (Snyder & Chesson 2003).…”

“…Additionally, many studies – and even sub-fields of ecology – focus on a single source of heterogeneity, making it hard to draw general conclusions that cut across system specifics. In particular, fixed or “exogenous” heterogeneity and biotically-generated or “endogenous” heterogeneity have often been approached from very different perspectives (Bolker 2003; Smith 2022). Our models and analysis help overcome these challenges by providing coexistence criteria that extend naturally to communities of arbitrary size, and by building endogenous environmental feedbacks directly into a core model for landscapes with exogenous heterogeneity.…”