Invasional meltdown in southern California marine communities: Could greater community diversity facilitate invader dominance?

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Ryan A. Beshai, Danny A. Truong, Amy K. Henry and Cascade J. B. Sorte, Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA

Background/Question/Methods
High community diversity may either prevent or promote the establishment of invasive species. The Biotic Resistance Hypothesis holds that species-rich communities are more resistant to invasion than species-poor communities due to greater interspecific competition. Conversely, the Invasional Meltdown Hypothesis (IMH) proposes that, because invaders contribute to community richness, greater invader abundance or diversity could increase invasibility via facilitative interactions between invaders. A recent literature review found relatively strong support for IMH in terrestrial habitats but that few studies have examined marine habitats. To evaluate whether invasional meltdown influences marine community structure, we studied the development of invertebrate fouling communities at two southern California sites, Huntington and Newport Harbors, over 4 months (August-December 2018). We photographed n=8 experimental plates/site biweekly and utilized CoralNet image analysis software for species identification and abundance (cover) estimates. We hypothesized that the invasibility of marine communities would increase with community richness. We therefore predicted that (1) the rate of increase in invasive species cover would relate to the total community richness as well as (2) to the invasive proportion of community richness, leading to (3) an increase in the invasive:native cover ratio over time.
Results/Conclusions
We found that the sign (positive or negative) and magnitude of the relationship between richness and invasive cover tended to differ over time and by richness metric (total community richness or invasive proportion of richness). Total richness was more frequently predictive of invasive cover changes in the early and intermediate stages of community development, with the strength of the relationship decreasing or becoming slightly negative in the late stages. Interestingly, although there were some differences between our two study sites, invasive dominance did not increase consistently through community development. Most (75%) of Huntington Harbor plates contained greater invasive than native cover early in community development compared to only 20% of Newport Harbor plates. The ratio of invasive:native cover in Huntington communities was generally invader-dominated across all development stages. In contrast, the invasive:native cover ratio fluctuated greatly in Newport communities with the greatest invader-dominance in the intermediate stages of community development. Together, our results suggest a complex relationship between community richness and invasiveness in which factors that vary across both time and/or community structure are contributing to invasibility, highlighting the need for further interrogation of IMH in marine communities.