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Competition for limited space is an important driver of benthic community structure on coral reefs. Studies of coral-algae and coral-sponge interactions often show competitive dominance of algae and sponges over corals, but little is known about the outcomes when these groups compete in a multispecies context. Multispecies competition is increasingly common on Caribbean coral reefs as environmental degradation drives loss of reef-building corals and proliferation of alternative organisms such as algae and sponges. New methods are needed to understand multispecies competition, whose outcomes can differ widely from pairwise competition and range from coexistence to exclusion. In this study, we used 3D photogrammetry and image analyses to compare pairwise and multispecies competition on reefs in the US Virgin Islands. Sponges ( Desmapsamma anchorata, Aplysina cauliformis ) and macroalgae ( Lobophora variegata ) were attached to coral ( Porites astreoides ) and arranged to simulate multispecies (coral-sponge-algae) and pairwise (coral-sponge, coral-algae) competition. Photogrammetric 3D models were produced to measure surface area change of coral and sponges, and photographs were analyzed to measure sponge-coral, algae-coral, and algae-sponge overgrowth. Coral lost more surface area and was overgrown more rapidly by the sponge D. anchorata in multispecies treatments, when the sponge was also in contact with algae. Algae contact may confer a competitive advantage to the sponge D. anchorata, but not to A. cauliformis , underscoring the species-specificity of these interactions. This first application of photogrammetry to study competition showed meaningful losses of living coral that, combined with significant overgrowths by competitors detected from image analyses, exposed a novel outcome of multispecies competition. 

Many studies have evaluated the impacts of hurricanes on coral communities, but far less is known about impacts, recovery, and resilience of sponge communities to these extreme events. In September 2017, St. Thomas, U.S. Virgin Islands, was impacted by two Category 5 hurricanes within 2 weeks: Hurricanes Irma and Maria. Such extreme events occurring in such rapid succession are virtually unprecedented. Pre-hurricane (2015, 2016) surveys of permanent transects at six sites around St. Thomas were compared with those at 10 weeks post-hurricanes (December 2017) to evaluate storm impacts on sponges and on benthic coral reef constituents. These surveys also established a baseline for evaluating future recovery. Percent cover of sponges declined by 24.9% post-hurricanes. In contrast, sponge density increased by 43.9% from 2015 to 2016 and declined slightly after the hurricanes. Overall sponge volume did not vary over time, and whereas sponge diversity was similar in 2015 and 2016, it increased post-hurricanes. Sponge morphologies were differentially affected by the hurricanes; the proportion of upright sponges declined by 36.9%, while there was a 24.4% increase in encrusting sponges. Coral and macroalgal cover did not change significantly over the sampling period, while percent cover of epilithic algae increased and non-living substrata decreased from 2015 to 2016 but did not change further post-hurricanes. At all sites, recruitment and/or regrowth of sponges was observed within 10 weeks post-hurricanes, indicating potential resilience in Caribbean sponge communities. Whether these sponge communities return to pre-hurricane conditions and how long that will take remains to be seen.