DOI: 10.5061/dryad.4tmpg4fdq
title: Data from: On the shape and origins of the freshwater species-area relationship

description: 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.
description: 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.
description: Microsoft Excel, R