On the shape and origins of the freshwater species-area relationship

The species-area relationship (SAR) has over a 150-year-long history in ecology, but how its shape and origins vary across scales and organisms is still not fully understood. This is the first subcontinental freshwater study to examine both properties of the SAR in a spatially explicit way across major organismal groups (diatoms, insects, and fish), differing in body size and dispersal capacity. First, to describe the SAR shape, we evaluated the fit of three commonly used models, logarithmic, power, and Michaelis-Menten. Second, we proposed a hierarchical framework to explain the variability in the SAR shape, captured by the parameters of the SAR model. According to this framework, scale and species group were the top predictors of the SAR shape, climatic factors (heterogeneity and median conditions) represented the second predictor level, and metacommunity properties (intraspecific spatial aggregation, γ-diversity, and species abundance distribution), the third predictor level. We calc..., Diatom, insect, and fish data were collected during the warmer months (May-September) between 1993 and 2019 from streams in all major watersheds in the US by the National Water-Quality Assessment (NAWQA) Program of the US Geological Survey and the National Rivers and Streams Assessment (NRSA) of the US Environmental Protection Agency. Diatoms and insects were sampled from a predefined area of substrate in 2,278 and 2,270 distinct localities, respectively. Fish were collected by electrofishing and seines from 2296 distinct localities. Diatoms and fish were identified to species, and insects, to genus. Taxonomic data include counts of taxa in a total count of 400 individuals per site for diatoms and 100 individuals per site for insects and fish. Bioclimatic data for each locality were retrieved from the WorldClim database., Microsoft Excel, R