Invasive Plants Have Scale-Dependent Effects on Diversity by Altering Species-Area Relationships

The Scale of Plant Invasions Many studies have shown large effects of invasive species on native species diversity, but at the same time, invaders (especially plants) have rarely been implicated in the extinctions of native species. Powell et al. (p. 316) noted that studies showing large effects have tended to be focused on smaller spatial scales, while those showing smaller effects have usually been on broader spatial scales. For such a scale-dependent effect to occur, invasive species must alter the shape of the species-area relationship (SAR). Comparing the differences in SARs with invaded and uninvaded communities of plants in three different ecosystem types in the United States revealed smaller effects of invasive species on biodiversity at increasingly broader spatial scales. Empirical data and simulations suggested that the observed patterns result from disproportionately greater influences of invasive species on common relative to rare species that result from a combination of sampling effects and differential responses to invasion. In three different biomes, invasive plant species cause larger declines in common species than in rare species. Although invasive plant species often reduce diversity, they rarely cause plant extinctions. We surveyed paired invaded and uninvaded plant communities from three biomes. We reconcile the discrepancy in diversity loss from invaders by showing that invaded communities have lower local richness but steeper species accumulation with area than that of uninvaded communities, leading to proportionately fewer species loss at broader spatial scales. We show that invaders drive scale-dependent biodiversity loss through strong neutral sampling effects on the number of individuals in a community. We also show that nonneutral species extirpations are due to a proportionately larger effect of invaders on common species, suggesting that rare species are buffered against extinction. Our study provides a synthetic perspective on the threat of invasions to biodiversity loss across spatial scales.

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