Urbanization reduces and homogenizes trait diversity in stream macroinvertebrate communities

Abstract

More than one-half of the world's population lives in urban areas, so quantifying the effects of urbanization on ecological communities is important for understanding whether anthropogenic stressors homogenize communities across environmental and climatic gradients. We examined the relationship of impervious surface coverage (a marker of urbanization) and the structure of stream macroinvertebrate communities across the state of Maryland and within each of Maryland's three ecoregions: Coastal Plain, Piedmont, and Appalachian, which differ in stream geomorphology and community composition. We considered three levels of trait organization: individual traits, unique combinations of traits, and community metrics (functional richness, functional evenness, and functional divergence) and three levels of impervious surface coverage (low [<2.5%], medium [2.5% to 10%], and high [>10%]). The prevalence of an individual trait differed very little between low impervious surface and high impervious surface sites. The arrangement of trait combinations in community trait space for each ecoregion differed when impervious surface coverage was low, but the arrangement became more similar among ecoregions as impervious surface coverage increased. Furthermore, trait combinations that occurred only at low or medium impervious surface coverage were clustered in a subset of the community trait space, indicating that impervious surface affected the presence of only a subset of trait combinations. Functional richness declined with increasing impervious surface, providing evidence for environmental filtering. Community metrics that include abundance were also sensitive to increasing impervious surface coverage: functional divergence decreased while functional evenness increased. These changes demonstrate that increasing impervious surface coverage homogenizes the trait diversity of macroinvertebrate communities in streams, despite differences in initial community composition and stream geomorphology among ecoregions. Community metrics were also more sensitive to changes in the abundance rather than the gain or loss of trait combinations, showing the potential for trait-based approaches to serve as early warning indicators of environmental stress for monitoring and biological assessment programs.

Habitat‐specific invertebrate responses to hydrological variability, anthropogenic flow alterations, and hydraulic conditions

“…Gibbins, Vericat, & Batalla, 2007), higher FEve values within fine sediments indirectly contradicts previous findings highlighting that FEve decreases with higher disturbance frequencies (e.g. Barnum et al, 2017;Schriever et al, 2015).…”

“…As such, the latter must be true for invertebrate communities sampled from fine sediments given that greater FEve values occurred (relative to other HGs) despite exhibiting high Berger‐Parker values (indicating a lower taxonomic evenness). The more even distribution of taxa across functional trait space (indicated by higher FEve values) within fine sediments suggests that the loss of taxa (TaxRic) occurred randomly, rather than clusters of taxa exhibiting comparable functional niches being extirpated (Barnum, Weller, & Williams, ). Larsen and Ormerod () highlighted that fine sediment deposition led to random co‐occurrences of species as biotic interactions weakened.…”

“…Greater FEve values occur when there is a high degree of taxonomic evenness or when functional distances among species are more regularly distributed (Villéger et al, 2008). As such, the latter must be true for invertebrate communities sampled from fine sediments given that (Barnum, Weller, & Williams, 2017). Larsen and Ormerod (2014) highlighted that fine sediment deposition led to random co-occurrences of species as biotic interactions weakened.…”

Leaf litter decomposition and macroinvertebrate assemblages along an urban stream gradient in Puerto Rico

“…Environmental factors, such as warmer water temperatures and higher nutrient levels in urban streams, facilitate microbial colonization and activity on leaves (Fernandes, Seena, Pascoal & Cássio, ; Gulis, Ferreira & Graça, ). Stream‐flow and scouring influence leaf litter breakdown and urban streams tend to have variable and flashy hydrological regimes and leaf breakdown can occur during high flow events (Barnum, Weller & Williams, ; Paul, Meyer & Couch, ). However, macroinvertebrates are the main consumers of leaf detritus and aid in breakdown rates.…”

“…Those factors interact with each other and determine rates at which this process occurs and because functional diversity is regarded as key to understanding ecosystem processes, their response to environmental stress proves to be essential for biomonitoring ecosystem services (Gutiérrez‐Fonseca & Ramírez, ; Ramírez & Gutiérrez‐Fonseca, ). Therefore, by assessing leaf decomposition we can evaluate whether stream macroinvertebrate functional diversity and ecological functions are negatively affected by urbanization with an increase in impervious surface cover (Barnum et al., ; Cook & Hoellein, ; Torres & Ramírez, ).…”