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 calculated the SAR as a sample-based rarefaction curve using 60 streams within landscape windows (scales) in the US, ranging from 160,000 to 6,760,000 km². First, we found that all models provided good fits (R² ≥ 0.93), but the frequency of the best-fitting model was strongly dependent on organism, scale, and metacommunity properties. Michaelis-Menten model was most common in fish, at the largest scales, and at the highest levels of intraspecific spatial aggregation. The power model was most frequent in diatoms and insects, at smaller scales, and in metacommunities with the lowest evenness. The logarithmic model was best fitting exclusively at the smallest scales and in species-poor metacommunities, primarily fish. Second, we tested our framework with the parameters of the most broadly used SAR model, the log-log form of the power model using a structural equation model. This model supported our framework and revealed that the SAR slope was best predicted by scale- and organism-dependent metacommunity properties, particularly spatial aggregation, while the intercept responded most strongly to species group and γ-diversity. Future research should investigate from the perspective of our framework how shifts in metacommunity properties due to climate change would alter the SAR.

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.

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