Could spatial heterogeneity in flow disturbance drive temporal stability of native-invasive species co-occurrence in riverscapes?

Spatial heterogeneity of abiotic influences like disturbance in riverscapes could play an important role in dispersal-aided community stability. We tested if higher spatial variability in conditions around river confluences caused by different flood disturbance regimes in branches could possibly influence the stability of a fish assemblage dominated by strong negative interactions between non-native trout and native galaxiid fishes. We used field surveys to evaluate the effects of disturbance-driven differences between river branches on fish densities and temporal variation in the relative abundance of galaxiids. Three repeated electrofishing surveys were conducted in two reaches of all branches of eight confluences in Canterbury, New Zealand. We found the differences in flood disturbance regimes between the two stream branches joining at a confluence were associated with higher mean fish abundance and more temporal stability in the relative abundance of native fish. Conversely, when flood disturbance conditions in the two streams joining were similar, resulting in a homogeneous confluence, fish abundance was lower and temporal variability was higher. For homogeneous confluences, strong interactions between native galaxiids and invasive trout probably produced lower fish densities where rivers with low flow disturbance met, whereas when two highly flow-disturbed rivers met, environmental harshness probably lowered fish densities. By comparison, enhanced galaxiid temporal stability at confluences containing both disturbed and stable river branches probably reflected the combined effects of refugia from trout predation in disturbed branches and refugia from flooding in stable branches. Overall, these results are likely to reflect the combined effects of population-specific responses to physical heterogeneity and spatially variable interspecific interactions. They may mean that river network configuration, and confluences especially, could influence community stability through a habitat portfolio effect. Furthermore, it will be worth investigating further whether preservation or restoration of differences in flow disturbance conditions in riverscapes encourages higher ecosystem resilience in the face of global change.

Automated summary

The spatial heterogeneity of abiotic influences like disturbance in riverscapes may influence the stability of fish assemblages, with higher mean fish abundance and more temporal stability observed in confluences with different flood disturbance regimes in branches. Conversely, confluences with similar flood disturbance conditions in the two stream branches tend to have lower fish abundance and higher temporal variability in fish density.