Human settlements in headwater catchments are associated with generalist stream food webs

Abstract

While a variety of anthropogenic impacts on lotic biodiversity have been documented, food-web responses to catchment development are poorly understood. We selected 27 stream food webs of comparable quality and conducted an analysis to assess the effect of catchment development on food-web structure. We quantified population densities, built-up area, and proximity to urban centres, and calculated a Settlement Index for each catchment using Principal Components Analysis. We also calculated the percentage of agricultural land cover in each catchment. Next, we assessed the correlations between the structural properties of food-webs (species richness, connectance, mean food-chain length, linkage density, trophic generality, and trophic vulnerability) and the Settlement Index as well as agricultural land cover. We found that linkage density, trophic vulnerability (number of consumers per species), and trophic generality (number of resources per species) were higher in streams with greater Settlement Index, indicating a reduction in specialisation. However, no clear trends were observed for species richness, connectance, and mean food-chain length. Agricultural land cover was also not related to food-web structure. We propose that the reduction in specialisation may be driven by species turnover and feeding plasticity, as biotic invasion or species impoverishment was not evident.