Experimental isolation of factors increasing invader success: The role of the abiotic environment independent of propagule pressure

Individual ecosystems vary greatly in the degree to which they have been invaded. Although understanding the factors that contribute to this pattern has major ecological and conservation implications, it has remained difficult to isolate mechanisms influencing invader success. One common observation related to invasions is that degraded areas will accumulate more invaders than less-impacted sites. However, the role of the physical environment alone in shaping invasions has rarely been tested directly, principally because the supply of potential invaders is frequently confounded with habitat conditions thought to increase invasibility. Here, we experimentally controlled for invader supply to test how the diversity of exotic versus native marine invertebrates changed during community assembly on initially bare fouling plates under different exposure levels to a common aquatic pollutant, copper. The results demonstrate that natives and exotics respond differently to increasing pollutant loads. Native diversity experienced a significant decline with increasing copper concentration, while exotic diversity did not. Despite these underlying differences, the diversities of exotics and natives were actually positively correlated on a per-plate basis. While such correlations are commonly used to suggest that species are responding similarly to the environment, our results suggests that plots of native vs. invader diversity might mask disparate responses to important abiotic factors. Overall, these results confirm that anthropogenic stress can affect the relative success of invaders, indicating that management strategies to improve environmental conditions and reduce invader supply may each reduce invasion risk in vulnerable systems.