Spatial and environmental influences on macroinvertebrate community structure through a stream network

Abstract

Stream ecosystems provide a unique framework for examining spatial variation in community structure. Streams generally have semi-predictable changes in abiotic characteristics from headwaters to mainstems, which leads to specific expectations for how longitudinal position along the stream gradient may shape benthic macroinvertebrate community structure. Streams also have a dendritic network configuration and directional connectivity, which may generate spatially-structured communities that are influenced by rates of dispersal between sites. In this study, we evaluated how benthic macroinvertebrate communities change along a longitudinal stream gradient, and we assessed the relative influence of local environmental variables versus dispersal-related mechanisms in shaping community structure. We hypothesized that (1) the position along the longitudinal stream gradient would be primary driver in shaping both alpha diversity (i.e., local taxonomic richness) and beta diversity (i.e., community variation between sites). Specifically, we expected that alpha diversity would increase and beta diversity decrease as stream order increased. Additionally, we expected that (2) pairwise differences in community structure between sites would increase with both greater geographic distances and greater environmental dissimilarity, reflecting the influence of both dispersal-related and local environmental filtering mechanisms. To assess the hypotheses, we collected macroinvertebrates and environmental data from 63 sites spanning 1st, 2nd, and 3rd order streams across three watersheds in the Sangre de Cristo Mountains of southern Colorado. Community structure was characterized using measures of density, biomass, alpha diversity, beta diversity, and pairwise community dissimilarity between sites (based on Bray-Curtis dissimilarity). Environmental variables were summarized using Principal Components Analysis, and pairwise environmental distances were used to evaluate the role of local environmental conditions relative to geographic distances between sites in shaping community structure. Geographic distances were quantified as both instream and overland distances, which were expected to affect aquatic and terrestrially dispersing taxa. Our results showed that alpha and beta diversity remained relatively consistent across stream orders along the longitudinal gradient. Density and biomass peaked in 2nd order sites, potentially due to increasing anthropogenic influences in 3rd order sites and challenging natural environmental conditions in 1st order sites. Although measures of diversity did not shift, taxonomic composition differed significantly between stream orders, indicating species turnover was happening along the stream gradient likely due to a shift from more specialist taxa to more generalist taxa at higher stream orders. Environmental dissimilarity and instream geographic distances were both significantly correlated with community dissimilarity, suggesting that local environmental conditions and dispersal-related factors affected community structure. Importantly, however, environmental dissimilarity was more strongly correlated than measures of geographic distance, indicating that local conditions play a more important role than dispersal at the spatial scale of our surveys. These findings highlight the role of environmental heterogeneity in shaping macroinvertebrate metacommunities within headwater stream networks. Additionally, the turnover in composition along the stream gradient highlights the importance of conservation measures that consider biodiversity patterns across the whole stream network, rather than specific isolated reaches.