OOS 26-6 - Pulse and press disturbances have different effects on transient community dynamics

Background/Question/Methods
Disturbances are important determinants of diversity, and the combination of their aspects (e.g., disturbance intensity, frequency) can result in complex diversity patterns. Here, we leverage an important approach to classifying disturbances in terms of temporal span, to understand the implications for species coexistence: pulse disturbances are acute and discrete events, while press disturbances occur continuously through time. We incorporate the resultant mortality rates into a common framework involving disturbance frequency and intensity. Press disturbances can be encoded into models in two distinct ways, and we show that the appropriateness of each depends on the type of data available. Using this framework, we compare the effects of pulse vs. press disturbance on both asymptotic and transient dynamics of a two-species Lotka-Volterra competition model to understand how they engage with equalizing mechanisms of coexistence.

Results/Conclusions
We show that press and pulse disturbances differ in transient behavior, though their asymptotic diversity patterns are similar. Specifically, we show that the co-occurrence times of the two species diverge markedly between press and pulse disturbances. Pulse disturbances can lengthen co-occurrence times by a factor of four when compared to an equivalent press disturbance regime; however the opposite may also occur. Our work shows that these differences depend on how the underlying disturbance aspects interact, and that the two ways of characterizing press disturbances can lead to contrasting interpretations of disturbance-diversity relationships. Our work demonstrates how theoretical modelling can strategically guide and help the interpretation of empirical work.