Community rescue in pond metacommunities facing severe acidification

V. Fugere, R. Barrett, B. Beisner, G. Bell, M. Cristescu, G. Fussmann, J. Shapiro, A. Gonzalez

Rapid adaptive evolution can sometimes prevent local extinction in degraded environments, a process termed ‘evolutionary rescue’ (ER). Theory and laboratory experiments with microorganisms have shown that prior exposure to moderate (sublethal) stress can facilitate ER from more severe (lethal) stress. Furthermore, once a local population undergoes ER, dispersal of resistant individuals across habitat patches can then confer stress resistance to an entire meta-population/community of connected patches. To extend these results to more natural environments, we conducted an experiment with 96 pond mesocosms sustaining complex communities of phyto and zooplankton. Mesocosms inoculated with a pristine lake community were arrayed in 4-patch metacommunities. We then tested whether exposure to sublethal stress and dispersal would influence the likelihood of rescue from severe acidification, a stressor to which our source community should be naïve. Prior exposure to sublethal stress influenced the response of phytoplankton to severe acidification (pH of 3). Although phytoplankton from all pH treatments eventually recovered, communities that had previously experienced a low pH (4) were initially more resistant when exposed to a pH of 3. In contrast, zooplankton biomass entirely collapsed at pH 4 and lower and never recovered. Dispersal had no influence on the response of either group to the pH treatments. Our results are only partly consistent with microcosms studies and suggest that the likelihood of rescue could vary across trophic levels in semi-natural communities with trophic structure.