Disturbance has many effects on natural communities, such as removal of biomass, reduction of fecundity, or stimulation of growth. A physical characterization of a disturbance event must describe its intensity, duration, and extent. Moreover, species' responses to disturbance depend on within-season timing, and time since last disturbance (i.e. frequency). The ecological effects of a disturbance regime can depend upon these five aspects, both singly and in concert. To investigate how the different aspects of a disturbance can affect competitive outcomes, we analyze a model of annual plant species which are subjected to periodic disturbance and compete via a reciprocal yield law. By quantifying the distinct mechanisms of coexistence that contribute to long-term low density growth rates, we are able to understand why different disturbance regimes produce different coexistence outcomes.
Results/Conclusions
Soil disturbance provides an example of how timing can alter plant response: early disturbance affects germination rates, while late disturbance affects seed yield. Though timing has rarely been considered in theoretical studies, we find that it can have significant effects on competitive outcomes. This is because timing of disturbance can control the action of coexistence mechanisms. Specifically, we show that early soil disturbance generates coexistence via the storage effect, which is a mechanism of coexistence in variable environments which quantifies how covariance between environmental and competitive responses can increase species' growth rates. We also show how that late disturbance facilitates coexistence through a different mechanism, relative nonlinearity. While each single aspect of disturbance can influence community dynamics, the interactions between these aspects may also be important. Though there are many potential interactions, we document how the effects of timing change depending on disturbance intensity, and also explore frequency-intensity interactions. By explicitly linking a disturbance aspect to a mechanism of coexistence which it affects, we extend our basic understanding of how disturbance can shape ecological systems. Additionally, a theoretical basis for describing how aspects of disturbance affects competitive outcomes may inform the use of disturbance as a management tool for conservation of natural communities.