Invasion hotspots and ecological saturation of streams across the Hawaiian archipelago

Species introductions are a widely recognized threat to global freshwater biodiversity. The proliferation of non-native species can result in the loss of native species through direct and indirect interactions with predators, competitors, pathogens and parasites. Thus identifying invasion hotspots and understanding the capacity of vulnerable ecosystems to absorb new invasions is fundamental to conserving native biodiversity and preventing further introductions. Here, we assess whether endemic biodiversity, land-use and human population density predict the location of invasion hotspots and ecological saturation in streams across the Hawaiian archipelago. We found that non-native fishes, mollusks, crustaceans, and insects are prevalent in Hawaiian streams across the archipelago, whereas the distributions of native species appear to be constrained by urbanization and habitat alteration. We detected a strong link between invasion hotspots and human population densities, and we found a positive relationship between the number of non-native species and native species present in watersheds, suggesting that Hawaiian streams are not ecologically saturated. Though native species richness explained more than half of the variance in non-native mollusks and crustaceans, it explained a low proportion of the variance in non-native fish and insect richness, indicating that a compilation of factors influence total non-native species richness in Hawaiian streams. Our findings reveal that Hawaiian streams remain vulnerable to further species introductions, and that conservation of endemic Hawaiian stream fauna can be improved by addressing interactions between introductions and degradation that can arise from human habitation.