Understanding the processes of diversity maintenance and competitive coexistence remains a central goal in theoretical ecology. Generally, environmental variation is a key factor in forming mechanisms of diversity maintenance. In fact, many such mechanisms, such as the storage effect, are variation dependent, meaning that they cannot act in systems with constant environments. While many types of variation are found in natural systems, of particular interest are the types of variation brought about by disturbance, and how disturbance processes interact with life history traits to structure communities. Several explanations of species coexistence have been given in terms of disturbance processes. These explanations are largely phenomenological, such as the intermediate disturbance hypothesis, but some are mechanistic, such as the successional mosaic hypothesis. There is still much confusion in this area, arising in part from differences in definitions and modeling conventions. Mechanistic descriptions are important, because they facilitate comparisons between different models, or different systems.

Results/Conclusions

We present a model for n-species competition under the effects of disturbance, and characterize the action of two mechanisms: the spatial storage effect, and fitness-density covariance. The model is kept general, and could apply to various communities subject to strong disturbance. Likewise, the disturbance is also general, and can represent fires, floods, or grazing. We consider the most common effects of disturbance, which alter species' mortality, fecundity, and free space in which to establish. For the two species case, we have shown how life a history trade-off generates the storage effect in a model for Fynbos shrubs. In that model, a resistant species avoids dying in fires, while a resilient species responds to burning with higher fecundity. It is this resistance-resilience trade-off which forms the biological basis for the storage effect. In this manner, frequent disturbance provides niche opportunities for species, by allowing them to distinguish themselves by the life history-strategies they employ in response to disturbance. This remains true in the n-species model, but, in addition to the storage effect, there are also effects from fitness-density covariance. We utilize a general theory of competition in variable environments to derive exact expressions for the storage effect, and we use a quadratic approximation to detail the effects of fitness-density covariance. Lastly, we illustrate some simple applications of the model to different communities and disturbance types.