The importance of species diversity and dominance in limiting and facilitating invasions

Despite years of research, there is no consensus on the hypothesis that high species diversity increases community uptake of resources and decreases the probability of species invasion. Here we outline and test two contentions with this hypothesis. First, in natural communities, high native richness is typically associated with large numbers of rare species, but it is the abundant species -not the rare species- that are expected to be better competitors for resources. Second, recent research emphasizes the importance of facilitation in communities, which challenges the competition framework on which the diversity hypothesis is based. We tested the effects of diversity and dominance on invader establishment in a forest understory in northern Canada, and tracked concurrent changes in resource availability. We separated the effects of species’ abundances and diversity in natural communities by removing a constant amount of biomass from each treatment as either a portion of one of two dominant species or a number of less abundant species. Two control plots were also created: the extant community untreated, and all plants removed. Invasion (both native and exotic) was simulated through seed addition or transplant addition. Invasion levels among treatments were used to determine if net interactions were facilitative or competitive. We found evidence of both competition and facilitation. Removal of part of the dominant species resulted in the highest level of invasion, indicating that dominant species are more important to invader suppression than species diversity.  Although removing all plants from a plot generated the highest level of resource availability, invasion in these plots was less than 10% of the partial removals. The change from an increase in invasion with partial removals to a decrease with complete removals indicates that the relative importance of facilitation and competition changes in a non-linear manner as the density of the extant community is reduced.