Species coexistence and environmental variation: Not all variation is created equal

Background/Question/Methods

Environmental variation has a strong potential to promote coexistence, but different types of variation have differing properties. In any natural system, the total environmental variation has three components:  a fixed spatial pattern, such as pools and riffles along a stream; temporal fluctuations that affect all sites, such as the weather in a given year; and spatio-temporal fluctuations, which affect each site independently in each year. Studies of each type of variation in isolation demonstrate that although they each affect coexistence, they act in different ways. Temporal variation separates competition between species in time, leading to the temporal storage effect. Spatial and spatio-temporal variation can lead to a spatial storage effect by spatially separating competition between species. In addition, spatial variation can generate the fitness-density covariance coexistence mechanism, which arises when organisms build up in good habitats.

Each type of variation affects coexistence mechanisms differently. It is not well understood how these effects will interact. We use a simulation model to quantify the effect on coexistence of adding temporal and spatio-temporal variation to an underlying pattern of fixed spatial variation. We specifically examine how the storage effect and fitness-density covariance respond to different combinations of environmental variation.

Results/Conclusions

While all three types of variation can promote coexistence, their effects are not additive. The effect of mixtures of different sorts of variation depends strongly on the types of variation, local dynamics, and dispersal characteristics.

Given limited dispersal, spatial variation promotes the fitness-density covariance because organisms can build up in good environments. This tends to promote coexistence more than other mechanisms or sources of variance. Temporal variation does not alter where organisms concentrate, as the locations of good habitats are unchanged. Absent habitat selection, organisms cannot build up in response to spatio-temporal variation, preventing it from affecting the fitness-density covariance.

The storage effect reacts differently than the fitness-density covariance. Temporal variation does not alter the spatial storage effect because it does not generate increased spatial separation from competing species. However, spatio-temporal variation does increase spatial separation, promoting the spatial storage effect. This increases the relative importance of the spatial storage effect. We expect that community dynamics permitting a temporal storage effect will increase the ability of temporal and spatio-temporal variation to promote coexistence.

The composition of environmental variation has important coexistence implications due to the different ability of each type of variation to generate coexistence mechanisms.