Species may coexist in a fluctuating environment via the storage effect, a coexistence mechanism describing temporal niche partitioning. To do so, species need to exhibit 1) differences in their patterns of activity and 2) a life history which allows a species to persist through periods of poor environmental conditions and high competition. Natural selection has the potential to shape both species activity patterns and their life histories, but evaluation of the effects of natural selection on activity patterns and the consequences for species coexistence have come to conflicting conclusions about the maintenance of the storage effect under natural selection. Given that the storage effect has been implicated as an explanation for diversity maintenance in highly diverse communities, we asked under what conditions the fluctuating patterns of activity would be favored by natural selection and when they would not.
We modeled population growth and fitness of two species of annual plants which are phenotypically variable in their germination variation. We derived an approximate expression for fitness, and using an adaptive dynamics approach, evaluated the general conditions under which germination variation is maintained by natural selection and the consequences for species coexistence.
Results/Conclusions
We find that when germination variance is the only phenotypically variable trait, species evolve constant germination rates, erasing the storage effect and leading to exclusion of one competitor. Alternatively, germination variance can be favored by natural selection when it occurs with an associated decrease in the correlation between species' germination rates or an increase in average seed production. In the former case, competing species coexist. In the latter, one species excludes the other as selection favors increased competitive ability.
Our results are best understood as the temporal analogue to character displacement on different resource types. Here germination variation represents the relative specialization and intensity of resource consumption. Variable germination is resource use that is temporally specialized whereas constant germination is a generalist strategy. Without an associated change in the timing of resource use, interspecific competition is strengthened by germination variance and one species is driven to extinction. By reducing germination correlations, germination variance specializes resource use in time just as character displacement specializes resource use by type. Our results tie together disparate conclusions from previous analyses of the evolution of the storage effect and shows that coexistence mechanisms may be strengthened when within-species trade-offs increase intraspecific and decrease interspecific density-dependence.