High densities of conspecifics buffer native fish from negative interactions with an ecologically similar invasive

Invasive species are a leading cause for native species declines, but it remains unclear whether maintenance of high native densities influence native persistence in freshwater systems. We designed complementary laboratory and field experiments to test whether high native cutthroat trout (Oncorhynchus clarkii) densities affect competition with invasive brown trout (Salmo trutta). We manipulated density of native cutthroat trout while holding brown trout density constant. Interspecific aggressive interactions towards native cutthroat trout were 66% fewer in the highest cutthroat trout density treatment compared to the lowest density treatment. At high densities of conspecifics, cutthroat trout, sympatric with brown trout, lost 33-81% less weight and demonstrated 1.5 times greater survival in both experiments than at low conspecific densities. Cutthroat trout held at low densities in laboratory experiments experienced greater stress-related disease virulence and negative effects of harassment, apparently owing to more frequent aggressive interactions with brown trout. These results support the hypothesis that reduced per capita effects of invaders on native fish at high densities buffers native species from negative effects of invaders, reducing the potential for invasive fish to displace high density native fish populations.