Location of demographic sources affects the distributions of a vulnerable native fish in invaded river networks

Abstract

Summary 1. Invasive predators negatively affect native prey to varying degrees across landscapes, and spatial configuration of invader-free refugia may affect prey distributions across invaded river networks. In New Zealand, introduced trout (Salmo trutta and Oncorhynchus mykiss) create source-sink dynamics in native Galaxias vulgaris populations, and their co-occurrence with trout may be enhanced by immigration from trout-free reaches. 2. We investigated how network configuration of trout-free demographic sources affected the distribution of G. vulgaris across trout-invaded riverscapes. Using quantitative biomass surveys and spatially extensive presence–absence surveys, the interaction between habitat variability and location relative to sources in limiting distributions of G. vulgaris in trout-invaded reaches was assessed. 3. Galaxias vulgaris biomass at invaded sink sites decreased with increasing network distance to the nearest trout barrier. The maximum distance to barriers at which G. vulgaris occurred in the riverscape was limited, so that galaxiids were excluded from small and stable streams far from sources. 4. Large predatory trout (i.e. >150 mm fork length) occurred in high densities at stable sites all year round and were seasonally excluded from sites disturbed by flooding. Large streams probably provide increased refugia for galaxiids to avoid predation from trout, but narrowness and habitat stability may act synergistically to extirpate G. vulgaris from sites that are too far from sources to receive regular immigrants. 5. The interaction between immigration and habitat configuration in mediating effects of trout on G. vulgaris distributions indicates the spatial dependency of predator–prey interactions in river networks. 6. These results indicate creating new invader-free source habitat should enhance co-occurrence in nearby invader-occupied reaches. Moreover, adding source habitat in sink streams far from existing sources, and ensuring barriers prevent future invasion, will also allow native fish dispersal between sources and sinks and will maximise the conservation gains from management across invaded riverscapes.