Effects of disturbance on conspecific negative density dependence: a conceptual review and synthesis

Background/Question/Methods

A central goal in ecology is to understand patterns of species relative abundance and diversity. In recent years, density-dependent mechanisms have emerged as an important framework for understanding these patterns at local scales. However, how density-dependent mechanisms, such as conspecific negative density dependence (CNDD), will contribute to biodiversity maintenance in the face of more frequent and intense ecological disturbance remains unclear. CNDD maintains species diversity by stabilizing population abundances of species in a community, decreasing per-capita population growth rates of locally common species and increasing population growth rates of locally rare species. CNDD is driven natural enemies and mutualists whose effects differ among host species, as well as strong intraspecific competition. In this review, we synthesize a conceptual framework for predicting the influence of disturbance on CNDD. We present a theoretical understanding of how fire, drought, and logging may influence the strength of CNDD drivers, including plant-soil feedbacks and intraspecific competition. We also discuss the implications of this framework for forest recovery and biodiversity. A deeper understanding of how disturbance impacts the role of these drivers in generating CNDD is especially relevant given that disturbances, such as wildfire and drought, are expected to increase in frequency and intensity under climate change.

Results/Conclusions

We suggest that the impact of disturbance on CNDD is variable and will reflect the type, intensity, and frequency of disturbance. Ultimately, however, the effects of disturbance on CNDD will depend on how particular disturbances influence abiotic conditions and the species interactions that drive CNDD. For example, ectomycorrhizae and fungal pathogens have positive and negative effects, respectively, that are relatively host-specific across plant species. Thus, decreases in the number and types of ectomycorrhizae is expected to strengthen CNDD whereas decreases in the number and types of fungal pathogens is expected to weaken CNDD. Studies indicate that ectomycorrhizae and fungal pathogens initially decline after wildfire but that fungal pathogens recover more quickly than ectomycorrhizae, leaving the overall impact of fire on CNDD dependent on relative shifts in ectomycorrhizae and pathogen communities following wildfire. In some cases, logging may have less detrimental effects on mycorrhizae and fungal pathogens than wildfire, suggesting that legacy assemblages of fungi associated with particular tree species may contribute to strong CNDD post-logging. Other important effects of disturbances on CNDD are considered, and a conceptual framework for predicting effects of disturbances, including drought, logging, and wildfire, on CNDD will be presented.