Indirect competitive advantages of invasive over native plants under multiple stressors.

The success of invasive alien plants is often ascribed to competitive advantages they have over native plants. Based on differences in intra- and interspecific competition, we propose three mechanisms: invasive species may alleviate intraspecific competition (self-regulation hypothesis); invasive species exert stronger suppressive effects on native species than other invasive species (competitive suppression hypothesis); invasive species tend to exhibit greater tolerance to native neighbors than native species (competitive tolerance hypothesis). In addition, we consider that competitive advantages can also be dependent on environmental factors, although few studies have tested this. Here, we tested the different mechanisms and their context dependence in a large competition experiment using two aliens and five co-occurring native plants in the presence and absence of an herbivorous snail, under drought and well-watered conditions, and with low and high phosphorus availability. Although intraspecific competition did not significantly differ between the alien and native species, we found that natives had a more negative neighbor effect on other natives than on aliens under well-watered conditions, particularly when there was also phosphorus limitation or herbivory. Similarly, aliens were worse neighbors for natives than for other aliens under well-watered conditions. Overall, these results suggest that conditions of non-limiting water availability, particularly when combined with phosphorus limitation or the presence of herbivores, might result in successful invasion by alien species.