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Summary Drought disturbances can have strong but variable effects on aquatic communities and little is understood about the impacts of drought, fragmentation and habitat reconnectivity on the dynamics of intermittent stream fish metacommunities.
We performed two experiments using outdoor stream mesocosms to test the effects of drought‐mediated connectivity and habitat heterogeneity on realistic stream fish assemblages at local (pool/patch) and regional (stream unit) scales under non‐drought versus drought conditions (Exp1) and under drought conditions with ‘pulse’ flow connectivity versus ‘non‐pulse’ (Exp2).
Survivorship,
‐diversity and γ‐diversity differed little at the unit level between treatments and experiments, but significant interactions between treatments and pool position were observed for species richness and abundances at the pool‐level. Specifically, drought (Exp1) and non‐pulse (Exp2) treatments had consistently higher species richness and abundances in deeper downstream pools due to downstream‐biased immigration during the onset of drought and higher residency among fishes in downstream pools. Species‐specific responses in these treatments resulted in downstream pools that were characterised by species pre‐adapted to lentic conditions (e.g. sunfishes), whereas upstream pools were characterised by smaller‐bodied top‐water and pelagic species. Non‐drought (Exp1) and pulse (Exp2) treatments showed no difference in richness or abundances among pool positions and assemblages were generally well‐mixed, indicating that connectivity (even a brief pulse) of pool refugia was important for determining local and regional assemblage structure and mediating the impacts of drought. These experiments demonstrate that stream fish assemblages responded strongly to reduced flows and fragmentation, and that metacommunity dynamics were structured by differential emigration and immigration rates and directionality among heterogeneous and variably connected pool habitats.
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Abstract The Amazon rainforest has experienced rapid land‐use changes over the last few decades, including extensive deforestation that can affect riparian habitats and streams. The aim of this study was to assess responses of stream fish assemblages to deforestation and land cover change in the eastern Amazon. We expected that percentage of forest in the catchment is correlated with local habitat complexity, which in turn determines fish assemblage composition and structure. We sampled 71 streams in areas with different land uses and tested for relationships between stream fish assemblages and local habitat and landscape variables while controlling for the effect of intersite distance. Fish assemblage composition and structure were correlated with forest coverage, but local habitat variables explained more of the variation in both assemblage composition and structure than landscape variables. Intersite distance contributed to variance explained by local habitat and landscape variables, and the percentage of variance explained by the unique contribution of local habitat was approximately equivalent to the shared variance explained by all three factors in the model. In these streams of the eastern Amazon, fish assemblages were most strongly influenced by features of instream and riparian habitats, yet indirect effects of deforestation on fish assemblage composition and structure were observed even though intact riparian zones were present at most sites. Long‐term monitoring of the hydrographic basin, instream habitat and aquatic fauna is needed to test for potential legacy effects and time lags, as well as assess species responses to continuing deforestation and land‐use changes in the Amazon.
