Two critical drivers of forest biodiversity are disturbance and conspecific negative density dependence (CNDD; a reduction in performance with high conspecific densities). Numerous studies have investigated these processes independently, but little is known about how they interact. This is a surprising research gap since several lines of evidence suggest disturbance may alter CNDD strength (e.g. if a species-specific pathogen thrives only in shade). In addition, complex interactions between disturbance and CNDD likely drive diversity, and thus any attempt to explain diversity with one process could be confounded by the other. For instance, an understanding of CNDD-disturbance interactions may be essential for resolving decades-old debates about how diversity is affected by disturbance frequency, scale, and/or severity. Here we describe a novel quantitative framework for simultaneously investigating CNDD and disturbance, and present results from a custom-built, individual-based, forest simulation model parameterized for the eastern United States with native species that vary in traits such as seed dispersal distance and shade tolerance. Using this model, we explore biodiversity dynamics across scenarios that systematically vary: a) baseline conspecific effects (negative, neutral, or positive), b) disturbance characteristics, and c) the effects of disturbance on conspecific effects (negative shift, no change, or positive shift).
Results/Conclusions
Our results illustrate complex emergent outcomes of modeled scenarios combined with intrinsic species-specific traits. First, even in the absence of disturbance-driven variation in conspecific effects (“no change” scenario), or species-specific differences in conspecific effects, we found clear complementary effects of disturbance and CNDD; different species responded in different ways, and diversity was highest when landscapes experienced both disturbance and CNDD. When we allowed disturbance to alter conspecific effects, we found that the effects of disturbance frequency on diversity could change direction; more frequent disturbance increased alpha diversity (and decreased beta diversity) only when the net effect of disturbance was an increase in CNDD strength, and vice versa. Another notable finding was that species characterized by short-distance dispersal and strong baseline CNDD were able to persist through repeated contiguous disturbance events only if CNDD strength was weakened by disturbance. Previous forest simulation models have not allowed conspecific effects to vary with disturbance, despite the fact that this almost certainly occurs in real ecosystems, and thus our work reveals numerous unexplored possibilities. Moving forward, we will incorporate data from an associated field experiment (currently underway) to better parameterize disturbance-driven conspecific effects and empirically constrain simulated biodiversity dynamics.