Competition vs. spatial insurance as determinants of community stability and composition in experimentally warmed pond zooplankton metacommunities

Background/Question/Methods

Climate warming is resulting in the reorganization of ecological communities at local and regional scales. The spatial insurance hypothesis suggests that dispersal of organisms between communities should increase the stability of total community biomass through the colonization of tolerant genotypes of species not initially present locally. On the other hand, colonization of new species may increase the number and strength of competitive interactions, which are thought to be destabilizing for both populations and communities. We tested the theory of how dispersal affects the stability and composition of communities in warming environments using replicate two-patch experimental pond zooplankton metacommunities. Initial differences in local community composition and abiotic conditions were established by seeding each patch in the metacommunities with plankton and sediment from one of two naturally differing ponds. We subjected the metacommunities to a 2.5°C increase in ambient temperature, crossed with three levels of dispersal (none, low, high). Community stability (variability) and composition were determined from samples collected every ten days over the 90-day experiment. 

Results/Conclusions

The greatest determinant of community stability was the initial pond type from which mesocosms were seeded, which obscured any effect of warming or dispersal on stability. Contrary to the spatial insurance hypothesis, we found that the stability of total zooplankton biomass was negatively related to the diversity of the community. Rather, community stability was dependant upon the stability of the most abundant species, Ceriodaphnia spp. (64% of total biomass), which was most stable when other species were in low abundance. These differences in community stability were due to changes in variance rather than in abundance or in the covariance between species, suggesting that changes in stability were due to competition rather than differences in species-specific responses to warming. 

However, warming increased the abundance of rare species by 34%, indicating that there were differences in the species-specific responses to warming. These increases occurred in all but the high dispersal treatment, suggesting an increase in competitive ability with the dominant Ceriodaphnia except when high dispersal resulted in mass effects.

These results highlight how competition, especially in communities with uneven species abundance, can counteract the insurance effect of biodiversity for the stability of communities. Furthermore, they illustrate a difficult scenario that managers may face as they try to maintain both ecosystem stability and biodiversity as conditions change.