No consistent small-scale native–exotic relationships

Abstract

The perception that native and exotic species are negatively related to each other at small scales has proliferated in the ecological literature. Although mounting evidence shows that such a perception is not always reality, the degree to which the mismatch occurs and the causes are not clear. Here, I compile and synthesize data based on the smallest scales used in 75 case studies in plant communities around the world, and analyze detailed data from a study in early successional California chaparral. I show that (1) different metrics (community variables) yielded different results, (2) native biomass and cover had much stronger negative effects on exotic richness than native richness and density, (3) there is no consistent correlation between native and exotic richness, especially in natural and immature communities, and (4) proportionally more experimental studies revealed negative relations than field observations. Collectively, these results reveal and confirm a high degree of mismatch between perceptions and reality in small-scale patterns of biotic invasions which have management implications.