Spatial variation can boost temporal storage through spatial refuges

Background/Question/Methods

Environmental variation comes in many different forms. These types of variation may interact to affect species’ responses to competitors and local environmental conditions. For example, both spatial and temporal environmental variation are common in nature. The interaction between these two forms of variation and its effects on population growth are poorly understood. A species living in a variable environment must have an ability to take advantage of favorable environments whilst persisting through poor environments. A critical requirement of this is that the growth rate of a species be buffered from high competition in times of poor environmental conditions (i.e. be subadditive). To understand how spatial variation interacts with temporal variation to affect coexistence, we modeled population growth of competing species with spatial and temporal environmental variation. We investigated the resultant effects on subadditivity in the growth rate, a critical component of the temporal storage effect coexistence mechanism. Specifically, we asked how variation in space in both the environmental characteristics of a location and local population density can modify the strength of the temporal storage effect.

Results/Conclusions

We found that a species’ ability to take advantage of favorable years and persist through poor years can be decomposed into three components: aspects of a species life history that lead to subadditivity, a species' sensitivity to the temporal environmental fluctuations in space, and a species' sensitivity to temporal fluctuations in competition in space. Stronger subadditivity in the growth rate occurs when individuals are concentrated in locations where they are strongly sensitive to the environment and weakly sensitive to competition. This can be quantified through the covariance between relative density within a site and site-specific sensitivity to competition and the environment. This result presents the first explicit, quantitative description of the importance of spatial refuges to a species both in terms of regional persistence and stable coexistence. This suggests ways to experimentally determine the importance of spatial refuges for a given species. Because the distribution of individuals in space affects the strength of subadditivity, traits associated with spatial distribution can affect the temporal storage effect. This provides a link between spatial environmental variation, traits that respond to that variation, and consequent effects for competitive interactions in time.