Invasion paradox, a change of direction in native-exotic richness relationships (NERRs) from negative at small scales to positive at large scales, has been shown to exist in many environments. A major explanation for these patterns is environmental heterogeneity, and to a lesser extent, species pool size. It is likely that a combination of these factors is producing these patterns and determines if and how they appear in certain environments. Beta diversity, which describes the heterogeneity of species composition among assemblages and as the scalar between local richness and regional richness (species pool size), could serve as a proxy for these factors, and therefore assess their combined influence on native-exotic richness relationships. Here, we hypothesize that the changes observed in the directional changes in NERRs can be explained by the degree of beta diversity among the communities being compared. We used data from the Forest Inventory and Analysis (FIA) dataset, a spatially nested, national inventory of forests in the United States to test this hypothesis. We evaluated the relationship between regional beta diversity (Raup-Crick metric) and the slope of the NERR for different units at multiple scales (seven distinct spatial grain/extent configurations) using generalized linear mixed models.
Results/Conclusions
Results show that inconsistent patterns in NERR observed across scales and across ecosystems can be partially explained by variations in the level of beta diversity and the relationship between beta diversity and scale. As scale increases, so will heterogeneity of the area studied, and consequently beta diversity as well. Once the contribution of the beta diversity component to the observed native gamma diversity becomes large enough, beta diversity is able to drive the relationship between native and exotic diversity at the gamma level. This hypothesis may serve as a unifying theory to explain the positive relationship between native and exotic diversity at large scales.