Abstract
Natural and anthropogenic disturbances commonly alter patterns of biodiversity and ecosystem functioning. However, how food webs respond to these changes remains poorly understood. Here, we have described aquatic food webs using invertebrate and fish community composition, functional traits, and stable isotopes from twelve agricultural streams along a landscape disturbance gradient. We predicted that excessive inputs of fine inorganic sediment (sedimentation) associated with agricultural land uses would negatively influence stream trophic diversity (e.g., reduced vertical and horizontal trophic niche breadths). Food-web properties based on Bayesian analyses of stable isotope data (δ13C and δ15N) from consumers showed that increasing sediment disturbance was associated with reduced trophic diversity, indicated by the whole community (fish and invertebrates combined) occupying a smaller area in isotopic niche space. Reductions in trophic diversity were best explained by a narrowing of the consumer δ13C range, and to a lesser extent, consumer δ15N range along the sedimentation gradient. We hypothesized that multiple mechanisms associated with sedimentation may have caused trophic niche "compression". Decreased niche partitioning, driven by increasing habitat homogeneity, environmental filtering, and resource scarcity seemingly lead to a greater similarity in trophic roles. These pathways may have contributed to a reduction in trophic diversity, whereas increased resource homogeneity was seemingly less important. Our results also indicate downward shifts in the vertical trophic position of benthic meospredators and invertebrate prey relative to higher consumers. This "trophic decoupling" suggests that terrestrial resource subsidies may offset reductions of aquatic prey for larger stream fishes. Sedimentation was associated with reduced trophic diversity, which may affect the functioning and stability of stream ecosystems. Our study helps explain how multiple mechanisms can influence food-web properties in response to this type of disturbance.
