Climate change affects the dynamics and structure of communities across different environments and scales. Warming conditions can impose physiological stress, affecting population growth rates. In addition, habitat loss affects connectivity and can limit dispersal between the communities, reducing potential for demographic rescue effects. Therefore, when occurring simultaneously, warming and changes in dispersal are expected to alter the spatial scale of diversity-maintaining processes, thereby affecting community resilience to extreme events such as heat waves. Metacommunity theory allows us to understand how communities connected by dispersal respond to environmental change. We tested the stated above hypothesis using experimental pond metacommunities, each comprised of four-1000L mesocosms spanning a 4.5°C spatial temperature gradient and connected by one of three dispersal rates. This spatial temperature gradient was maintained, while allowing the mesocosms to fluctuate temporally with seasonal temperature changes. The mesocosms were sampled throughout the summer for zooplankton, phytoplankton and bacteria to evaluate whether dispersal could buffer these communities against seasonal fluctuations in temperature, which included a mid-summer heat wave.
Results/Conclusions
We found different responses to dispersal and temperature among the three examined organismal groups. Dispersal had a positive effect on phytoplankton abundance with increasing temperature, but we found no clear effect of dispersal on the composition or abundance of zooplankton. Instead, there were strong impacts of both spatial and temporal variation in temperature on the zooplankton community. Initially, there were clear differences in composition of the communities across the spatial temperature gradient, with species sorting based on clear preferences for warm and cold conditions. A mid-summer heat wave then caused the zooplankton communities to converge on a single compositional state that was dominated by a single heat tolerant taxon, Diaphanosoma. Following the heat wave, the communities diverged from this compositional state with other taxa becoming more abundant, but the communities did not return to their pre-heat wave composition and their composition was no longer clearly determined by the spatial temperature gradient. As for bacteria, we observed a high turnover in the dominant taxa from month to month. Our data suggest that both temperature and dispersal treatments had a less apparent effect on community structure for bacteria than for zooplankton. These results illustrate how temperature can act as a strong driver of community composition across trophic levels, but also suggest that the effect of dispersal on communities may depend on a trophic level as well as on the environmental conditions.