Data from: Correlation of native and exotic species richness: a global meta-analysis finds no invasion paradox across scales

Support for the “biotic resistance hypothesis,” that species-rich
communities are more successful at resisting invasion by exotic species
than are species-poor communities, has long been debated. It has been
argued that native-exotic richness relationships (NERR) are negative at
small spatial scales and positive at large scales, but evidence for the
role of spatial scale on NERR has been contradictory. However, no formal
quantitative synthesis has previously examined whether NERR is
scale-dependent across multiple studies, and previous studies on NERR have
not distinguished spatial grain and extent, which may drive very different
ecological processes. We used a global systematic review and hierarchical
mixed-effects meta-analysis to provide a comprehensive quantitative
assessment of the patterns of NERR over a range of spatial grain sizes and
spatial extents, based on 204 individual cases of observational
(non-experimental) NERRs from 101 publications. We show that NERR was
indeed highly scale-dependent across studies and increased with the log of
grain size. However, mean NERR was not negative at any grain size,
although there was high heterogeneity at small grain sizes. We found no
clear patterns of NERR across different spatial extents, suggesting that
extent plays a less important role in determining NERR than does grain,
although there was a complex interaction between extent and grain size.
Almost all studies on NERR were conducted in North America, western
Europe, and a few other regions, with little information on tropical or
Arctic regions. We did find that NERR increased northward in temperate
regions and also varied with longitude. We discuss possible explanations
for the patterns we found, and caution that our results do not show that
invasive species are benign or have no negative consequences for
biodiversity preservation. This study represents the first global
quantitative analysis of scale-based NERR and casts doubt on the existence
of an "invasion paradox" of negative NERR at small scales and
positive correlations at large scales in non-experimental studies.