The maintenance of clonal diversity and stability in environmentally forced metapopulations

Background/Question/Methods

How populations maintain diversity and stability in temporally varying environments is an issue of long-standing and continuing interest in ecology. Metapopulation models that incorporate temporal variability show that dispersal can promote stability by increasing population minima and reducing extinction probability, an effect that becomes more probable when environmental perturbations are spatially asynchronous. Less apparent is how intraspecific variation and genetic diversity contribute to and are impacted by such processes. Prior models have shown that inter-individual variation in responses to changing conditions can interact with dispersal to promote diversity, compensatory dynamics and stability. However, such models only consider a limited range of phenotypic traits and trade-offs. A fundamental ecological trade-off is the ability of populations to increase rapidly under high resource availability versus the ability to persist under low availability (a r_max-Rstar or gleaner-opportunist trade-off). How population stability and genetic diversity respond to dispersal and environmental perturbations when individuals exhibit such trade-offs is unclear. Here we present the results of field experiments in which we examined the effects of environmental perturbations (nutrient pulses), dispersal and spatial covariation in perturbations on the stability and clonal diversity of aquatic metapopulations (using Daphnia pulex which exhibits clonal variation in response to resource availability).

Results/Conclusions

Our experiment provided support for a stabilizing effect of dispersal on temporal variability of D. pulex populations. However, effects were highly dependent on the form of spatial covariation in nutrient perturbations. Stabilizing effects only emerged in the absence of perturbations or when pulse events were spatially asynchronous. Dispersal in the presence of spatially synchronous perturbations destabilized populations (increasing temporal variability). In contrast to prior model predictions, stabilization by dispersal under asynchronous perturbations did not occur via promotion of clonal diversity and compensatory dynamic responses among clones. Instead, dispersal resulted in a decline in clonal diversity and selection for a single clone in these treatments. Laboratory-based trait assays of clones isolated from the experiment showed that dispersal and asynchronous perturbations selected for clones that performed best under low resource availability (a Rstar strategy). In contrast, dispersal under synchronous nutrient perturbations or in the absence of perturbations promoted clonal diversity and a mix of clonal traits (both Rstar and r_max strategies). These results indicate that the factors promoting population stability and diversity in varying environments may run counter to each other.