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An experimental test of how microbial communities respond to warming in systems connected by dispersal Yangel, Evgeniya


Community dynamics and structure are greatly affected by climate change through warming. Temperature directly affects the rates of biochemical reactions that in return affect growth, resource use, organismal abundance, species interactions, and, therefore, communities and biodiversity. In addition, low connectivity can limit dispersal between communities, reducing the potential for demographic rescue effects. Therefore, the effects of temperature on diversity and community structure in patchy landscapes can depend on the degree of connectivity among landscapes. We tested whether the effects of temperature on communities contingent on the degree of connectivity 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 also allowing the mesocosm temperatures to fluctuate temporally with seasonal weather variation. Bacterial communities in the mesocosms were sampled in the summer to evaluate whether dispersal rate at the metacommunity level affects local and regional community response to seasonal fluctuations in temperature. We predicted that higher levels of dispersal would raise local (alpha) diversity and decrease species turnover among ecosystems (beta diversity) and metacommunity-level (gamma) diversity. However, we found no effect of dispersal on local and regional diversity metrics. We also predicted that dispersal rates would differently affect species compositional differences along the thermal gradient. At low dispersal rates among communities, we observed differences in species composition associated with temperature. At higher dispersal rates, communities were not structured by temperature and composition was similar within a metacommunity, which was not observed in any other dispersal treatment. This emphasizes the homogenizing effect high dispersal has on bacterial community structure. Our findings demonstrate that bacterial diversity metrics do not follow metacommunity predictions about dispersal effects on diversity. However, we found support for the hypothesis of high dispersal homogenizing communities. This suggests there are other processes that influence bacterial community diversity patterns, but dispersal can erode the effect of the environment on bacterial community structure.

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