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.

Impact/Purpose

Human land-uses, especially urbanization and agriculture, typically reduce richness and abundance of many taxa in terrestrial, marine, and freshwater ecosystems worldwide. Densely populated urban zones and intensively cultivated areas are also often associated with dramatic shifts in assemblage composition, for instance, species turnover toward more resistant and generalist species, triggering biotic homogenization. As such, mitigating human land-use impacts on biodiversity is a major challenge in resource management and for environmental protection and restoration. Physical habitat complexity is expected to provide a buffer against species loss under intensive human pressures. However, there are very few studies that have explicitly tested the capacity of habitat complexity to buffer streams against intensive basin and riparian anthropogenic pressure in “real-world ecosystems”, and none we are aware of demonstrate this over broad spatial extents. Since the first National Rivers and Streams Assessment (NRSA) in 2008-2009, EPA has included field measures of instream habitat complexity in its surveys.  At each survey field crews estimate the cover of six types of instream habitat complexity features: (i) undercut banks, (ii) rocks and boulders, (iii) large wood, (iv) brush, (v) instream live trees, and (vi) overhanging vegetation. In this paper the authors present evidence from a probability sample of 4093 NRSA river and stream sites across the conterminous USA, showing that habitat complexity mediates the impacts of urbanization and croplands on species richness, total density, and ecoregional and national beta-diversity of fish and insect assemblages. They show that high-habitat complexity sites support greater aquatic insect richness and density than low-habitat complexity sites. High-habitat complexity sites harbor greater ecological uniqueness of fish and insects compared to low-habitat complexity sites. However, as human land-uses intensify, losses in fish and insect richness and density, and reductions in insect uniqueness occur even in sites with high-habitat complexity. This illustrates that habitat complexity alone cannot effectively buffer the deleterious effects of human pressures on insect and fish biodiversity. Although high-habitat complexity seems to have buffered fish uniqueness against human land-use intensification, this is influenced by non-native generalist fish species that are masking a large-scale homogenization of fish assemblages. To our knowledge, this is the first continental-scale empirical evidence that high-habitat complexity itself cannot completely buffer aquatic biodiversity against the deleterious impacts of human land-uses. Fish and insects support multiple essential functions and services that freshwaters provide to human well-being, including detritus processing, food production, and water quality. Ensuring that these services remain available in urban and agricultural landscapes will benefit both people and nature. The authors findings highlight the urgent need for management actions aimed at (i) maintaining habitat complexity in streams in combination with (ii) reductions in the negative impacts of urban and cropland activities. In combination, these two measures would be a step forward in preserving the biodiversity of freshwaters worldwide.  

Citation

Moi, D., P. Kaufmann, L. Riato, G. Romero, P. Kratina, F. Teixeira de Mello, AND R. Hughes. Habitat Diversity Mitigates the Impacts of Human Pressure on Stream Biodiversity. Blackwell Publishing, Malden, MA, 30(10):e17534, (2024). [DOI: 10.1111/gcb.17534]

DOI: Habitat Diversity Mitigates the Impacts of Human Pressure on Stream Biodiversity