Life history theory predictions for hydrologic filtering of fish assemblages are rarely tested with historical time series data. We retrospectively analyzed flow regime and fish assemblage data from the Sabine River, USA, to test relationships between life history strategies and hydrologic variability altered by impoundment construction. Downstream flow variability, but not magnitude, was altered by completion of Toledo Bend Reservoir (TBR) in 1966. Consistent with life history theory, occurrence of opportunistic strategists declined while equilibrium strategists increased as the fish assemblage was transformed between periods immediately after (1967–1973) and approximately one decade after (1979–1982) completion of TBR. Assemblage transformation was related to decline of opportunistic strategists throughout 250 km of river downstream of TBR. Temporal trajectories for opportunistic and intermediate strategist richness modelled as a function of flow variability converged 12 years postimpoundment. The spatiotemporal scaling of our study is novel among tests of life history theory, and results suggest impoundment-induced alteration to natural hydrologic filtering of fish assemblages can operate on the scale of hundreds of stream kilometres and manifest within approximately one decade.

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Summary Disturbances caused by drying can eliminate fully aquatic organisms and alter wetland communities. However, the net effects of pulsed drying followed by re‐wetting on populations of benthic macroinvertebrates with greater tolerance to drying have been poorly studied. We quantified the population responses of two large macroinvertebrates, dragonflies (a number of species; Family: Libellulidae) and a crayfish (Procambarus fallax), to variable drying history and predatory sunfish (Family: Centrarchidae) in experimental subtropical wetlands. To simulate naturally occurring combinations of drying and sunfish presence, the experimental treatments included a drying and re‐wetting sequence that either eliminated sunfish or did not, with a third treatment of continuously flooded conditions with sunfish present. The activity of adult dragonflies was similar over all experimental wetlands, but larval density after 6 months was highest in continuously flooded wetlands (with sunfish) and some species (Erythemis simplicicollis, Celithemis eponina) were absent from wetlands that experienced drying. There was no evidence that sunfish limited larval density. After 6 months, overall crayfish biomass was greatest in wetlands that had dried and lacked sunfish. Crayfish density was similar in all wetlands, but individual crayfish body mass (g) was greatest in wetlands that lacked sunfish. Because of sunfish predation, few crayfish survived to full juvenile size. In the presence of sunfish, drying did not affect crayfish biomass or individual body mass. The abundance of libellulid dragonfly larvae was indirectly reduced by the drying. In wetlands where submerged vegetation was reduced by drying, we suggest that libellulid populations were limited by a small‐bodied fish (Gambusia holbrooki). In contrast, survival of juvenile crayfish was directly limited by sunfish, and therefore, crayfish (but not libellulid) populations could be temporarily enhanced by drying events when and where sunfish populations are reduced.

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