Data from: Opportunistic data reveal widespread species turnover in Enallagma damselflies at biogeographical scales

An information tradeoff exists between systematic presence/absence surveys
and purely opportunistic (presence-only) records for investigating the
geography of community structure. Opportunistic species occurrence data
may be of relatively limited quality, but typically involves numerous
observations and species. Given the quality-quantity tradeoff, what can
opportunistic data reveal about spatial patterns in community structure?
Here we explore opportunistic data in describing geographic patterns of
species composition, using over 4,600 occurrence records of Enallagma
damselflies in the United States. We tested phylogenetic scale (genus
level, Enallagma major clades, Enallagma subclades) and spatial extent
(U.S. vs. watershed regions), hypothesizing that nonrandom structure is
more likely at larger spatial extents. We also used three sets of
systematic presence/absence surveys as a benchmark for validating
opportunistic presence-only records. Null model analysis of matrix
coherence and species replacements showed many cases of nonrandom
structure and widespread species turnover. This outcome was repeated
across spatial and environmental gradients and community composition
scenarios. Turnover dominated across the U.S. and two watersheds spanning
biogeographic boundaries, but random assemblages were prevalent in a third
watershed with limited longitudinal extent. Turnover also pervaded each
level of phylogeny. Opportunistic presence-only datasets showed identical
patterns as systematic presence/absence datasets. These results indicate
that extensive opportunistic data can be used to detect species turnover,
especially at geographic scales where range margins are crossed.