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Negative co-occurrence patterns are intriguing because they may reflect the outcome of interspecific interactions and therefore signal how competition shapes communities. However, other factors also contribute to these patterns. For example, theoretical studies as well as two survey-based studies have all suggested that dispersal may also impact these patterns. While natural communities commonly have nonrandom patterns of negative co-occurrence, understanding how different processes drive these patterns requires further research. We tested the influence of dispersal on co-occurrence patterns using a zooplankton mesocosm experiment with four different dispersal treatments varying in the number of dispersers delivered into mesocosms on regular intervals. Our dispersal treatments were intended to adjust the relative importance of dispersal and competition experienced within mesocosms (i.e., high dispersal results in a relatively low influence of competition on species composition and vice versa). Higher dispersal translated into increased zooplankton species richness and inter-mesocosm compositional similarity, and also changed species occupancy patterns such that species occurrences were more even across mesocosms in higherdispersal treatments. Dispersal treatments also differed markedly in species co-occurrence patterns. Negative co-occurrence patterns were significant for all but the lowest-dispersal treatment, peaked in the intermediate-dispersal treatments, and declined in the highestdispersal treatment. Stability analyses illustrate that co-occurrence differences are robust to the exclusion of any single mesocosm in null model analyses. Dispersal treatments did not significantly differ with respect to abiotic variation, which has been recognized as a potential driver of negative co-occurrence patterns. These results suggest that not only can dispersal influence patterns of negative co-occurrence via changes to species richness and distribution (occupancy patterns among mesocosms), but the degree to which they do so varies nonlinearly with the strength of dispersal. Critically, because negative co-occurrence patterns were nonsignificant when the contribution of dispersal was lowest, it is possible that dispersal contributes strongly to many observed patterns of negative co-occurrence. Consequently, great care should be taken prior to interpreting significant co-occurrence tests as a product of species interactions. [ABSTRACT FROM AUTHOR]

The article offers a study on the impact of dispersal limitation on the recovery of zooplankton communities in Killarney Provincial Park in Ontario that were affected by a regional stressor. It says that a variety of zooplanktons were dispersing to recovering lakes, while some were dispersing overland. It adds that six colonist species were identified from the traps such as Mesocyclops edax and Tropocyclops extensus, which did not disperse in numbers large enough to surpass the critical density.

Community responses to acidification and recovery of boreal lakes are poorly understood, particularly after several years of recovered lake-water pH (pH ≥ 6.0). We tested if zooplankton communities in two circumneutral lakes with different acidification histories were adapted to historical lake-water pH with a reciprocal transplant field enclosure experiment. A second objective was to assess the influence of local environments on zooplankton survival and abundance. Differences in acid tolerance could be detected in some zooplankton among the two lakes with different acidification histories — zooplankton from acid-recovering Carlyle Lake, recovered to pH ≥ 6.0 for 6 years were more speciose and maintained higher total abundance at pH 4.8 than the community from a buffered lake. The zooplankton community in this historically acidified lake was comprised of two dominant species with acid-adaptable tolerances, Holopedium gibberum and Leptodiaptomus minutus. High establishment of transplanted zooplankton in our experiment has important implications for the recovery of zooplankton communities because it suggests that local conditions are suitable for most species in acid-recovering lakes with pH ≥ 6.0 and that other factors such as dispersal limitation and biotic interactions may be impeding recovery. [ABSTRACT FROM AUTHOR]

Invasive species introductions into freshwater ecosystems have had a multitude of effects on aquatic communities. Few studies, however, have directly compared the impact of an invader on communities with contrasting structure. Historically high levels and subsequent reductions of acid deposition have produced landscapes of lakes of varying acidity and zooplankton community structure. We conducted a 30-day enclosure experiment in Killarney Provincial Park, Ontario, Canada, to test the effects of Bythotrephes longimanus, an invasive invertebrate predator, on two contrasting zooplankton communities at different stages of recovery from acidification: recovered and acid damaged. Bythotrephes significantly decreased zooplankton biomass and abundance in both communities but had a greater negative effect on the abundance of zooplankton in the recovered community. Bythotrephes reduced species diversity of the recovered zooplankton community but not of the acid-damaged community. Species richness of both community types was unaffected by Bythotrephes predation. The effect of Bythotrephes on small cladocerans, a preferred prey type, differed between the community types and appeared to be related to density-dependent predation by Bythotrephes. Both community- and species-level results suggest that recovered and acid-damaged zooplankton assemblages may be negatively affected by an invasion of Bythotrephes but that the specific response is dependent on the original community structure. [ABSTRACT FROM AUTHOR]