Habitat Diversity Mitigates the Impacts of Human Pressure on Stream Biodiversity.

Recent decades have witnessed substantial changes in freshwater biodiversity worldwide. Although research has shown that freshwater biodiversity can be shaped by changes in habitat diversity and human-induced pressure, the potentials for interaction between these drivers and freshwater biodiversity at large spatial extents remain unclear. To address these issues, we employed a spatially extensive multitrophic fish and insect database from 3323 stream sites across the United States, to investigate the ability of habitat diversity to modulate the effect of human pressure on the richness and abundance of fish and insects. We found evidence that high levels of habitat diversity were associated with increased richness and abundance of fish and insects (including whole-assemblage and individual trophic guilds). We also show that the effects of human pressure on the richness and abundance of fish and insects tend to become positive at high levels of habitat diversity. Where habitat diversity is low, human pressure strongly reduces insect richness and abundance, whereas these reductions are attenuated at high levels of habitat diversity. Structural equation modeling revealed that human pressure reduced habitat diversity, indirectly negatively affecting the richness and abundance of fish and insects. These findings illustrate that, in addition to promoting greater fish and insect biodiversity, habitat diversity may mitigate the deleterious effects of human pressures on these two stream assemblages. Overall, our study suggests that maintaining high levels of habitat diversity is a useful way to protect freshwater biodiversity from ongoing increases in human pressure. However, if human pressures continue to increase, this will reduce habitat diversity, further threatening stream assemblages.