Is biotic resistance to invaders dependent upon local environmental conditions or primary productivity? A meta-analysis

Biotic resistance is an important component to limiting the spread of invasive plants into native communities. However, under certain conditions native plants may also facilitate invasion, a process identified as biotic assistance. Identifying the conditions that associate with resistance and assistance is needed to better understand the factors driving species invasion. Substantial theory and empirical work suggests net effects of neighbors may be dependent upon habitat productivity and environmental conditions. How that applies to the interaction among the resident community and the invaders is largely untested. Here we compiled data from 23 articles, which had experimentally determined the strength of biotic resistance and/or assistance. We then combined these data with remote-sensing estimates of productivity, precipitation and temperature at each study site, Using standard meta-analytical techniques we determined the overall effect resident communities had on the emergence, growth, reproduction and survival of non-native invaders. Further, we tested whether the interaction between resident communities and invasive species was influenced by primary productivity, temperature and precipitation. Across all sites, we found broad support for biotic resistance, while evidence for biotic assistance was rare. However, we found the relative magnitude of biotic resistance on invaders increased with temperature or precipitation; a pattern consistent with the stress gradient hypothesis. In contrast we found no evidence that the strength of biotic resistance varied as a function of primary productivity. Further evaluation of the relationship between productivity and environmental conditions on the direction and strength of the effect of resident species on invaders may help predict invasion establishment and success. Understanding or predicting the susceptibility of communities to invasion may help prioritize management efforts.