A simulated heatwave and dispersal enhance dominance and decrease community productivity in experimental metacommunities
Sarah Lena Eggers · Helmholtz Centre for Ocean Research Kiel (GEOMAR) · 2009
Global change and habitat destruction are probably the most important drivers of changing diversity. Diversity might get lost due to consecutive species extinctions and homogenisation of previously heterogeneous environments. Consequently, the question rises whether this alteration of diversity affects ecosystem functioning. It is hypothesized that ecosystems comprising higher diversity not only in terms of species richness but also evenness are more resilient to environmental change which is often characterized by both increasing mean temperature and increasing numbers of extreme weather events. Many of the minor species are analogues of the dominants in terms of the ecosystem functions they perform, but differ in terms of their capabilities to respond to environmental stresses and disturbance. Under changing conditions, ecosystem functions thus can be maintained when dominants decline or are lost because functionally equivalent minor species are able to substitute for them. That is, high diversity has the potential to secure essential ecosystem functions by enhancing the communities' recover ability in variable environments and uncertain futures. I set out to experimentally test whether diversity in terms of richness and evenness, and biomass in metacommunities shows a response to dispersal and I or temperature stress and whether communities with enhanced realised richness and evenness show faster recover ability after a heatwave. Therefore I used metacommunities with ten species of naturally co-occuring marine benthic microalgae from the western Baltic Sea. I found that local and regional evenness and biomass decreased with increasing dispersal rate at constant temperature. After being exposed to the simulated heatwave local and regional evenness similarly decreased with increasing dispersal rate but there was no effect on biomass. I found that local and regional evenness and biomass were overall lower within the heatwave treatment. Additionally, ten days after the heatwave regional richness increased with increasing dispersal rate. Furthermore, I found a positive correlation between evenness and community productivity at constant temperature that became significant with time. This positive correlation vanished when communities were exposed to the heatwave. Other than expected the benthic microalgae communities showed no sign of recovery after the simulated heatwave. Thus, the recovery potential in this system can be rendered as independent of the respective community's state of evenness and richness. The results showed that only one extreme heatwave was sufficient to cause a dominance shift in benthic microalgae communities and a consequent negative effect on ecosystem functioning in terms of reduced biomass production. The results also suggest that the relationship between evenness and biomass became decoupled when communities were exposed to the heatwave. The results for mean local Bray-Curtis dissimilarities that were calculated between temperature treatments at high and no dispersal and between no and daily dispersal within each temperature treatment suggest that temperature is stronger than dispersal as a community structuring factor. Actually, the impact was twice as high when both factors were combined. That is, the dispersal and the temperature effect added up. The results show that only one extreme weather event in combination with community homogenisation was sufficient to cause a shift in community composition and functioning. I conclude that the recover ability of communities after extreme weather events is not to be overestimated.