Abstract
AbstractTo better understand phase shifts on Caribbean reefs, we quantified community structure on shallow reefs over 27 yr in St. John, U.S. Virgin Islands, contrasted the community dynamics of scleractinians and octocorals, and evaluated the extent to which community structure was associated with rainfall, temperature, and hurricanes. To gain insight into the likely abundance of octocorals on future reefs with low scleractinian cover, we compared two sites dominated by the major Caribbean reef‐building coral Orbicella annularis. Between 1987 and 2013, scleractinian cover declined from 45% to 6% at Yawzi Point, but remained at ~30% at Tektite. We compared changes in community structure using four benthic assemblage constructs—“scleractinian‐focused” (cover of scleractinians, macroalgae, and CTB i.e. crustose coralline algae, algal turf, and bare space), “octocoral‐focused” (abundance of octocorals, cover of macroalgae, and CTB), “octocoral genera” (abundance by genus), and a “complete” approach (all taxa)—to reveal how a consideration of octocoral abundance influenced the interpretation of coral reef community dynamics. Overall, temporal variation in community structure differed among the four assemblage constructs at both sites and was associated with rainfall and mean seawater temperature. These results suggest that: (1) scleractinian‐ and octocoral‐focused communities in the same location responded differentially to the same environmental conditions, (2) their communities generally were influenced more by the chronic effects of rainfall and temperature than acute effects of storms, and (3) octocoral‐focused communities were more resilient to environmental conditions than scleractinian‐focused communities. With further declines in cover of scleractinians, octocoral communities are likely to become more common throughout the Caribbean.
Highlights
Many Caribbean coral reefs provide examples of complex communities that have undergone dramatic changes over decades (Jackson et al 2014) that have been defined by declines in cover of scleractinians (Hughes 1994, Gardner et al 2003), and reductions in population size of macroscopic vertebrates (Jackson et al 2014)
The changes affecting Caribbean reefs over the last 40+ years provide examples of phase changes in community structure (Done 1992), the best-known of which involves a transition in spatial dominance from scleractinians to macroalgae (Hughes 1994, Mumby et al 2007, Jackson et al 2014)
To expand the taxonomic scope of analyses of Caribbean reefs, we have started to augment our studies of scleractinian-focused communities (e.g., Edmunds 2015) with a consideration of octocorals (Lenz et al 2015, Edmunds and Lasker 2016, Edmunds et al 2016)
Summary
Many Caribbean coral reefs provide examples of complex communities that have undergone dramatic changes over decades (Jackson et al 2014) that have been defined by declines in cover of scleractinians (Hughes 1994, Gardner et al 2003), and reductions in population size of macroscopic vertebrates (Jackson et al 2014). Studies of taxa other than scleractinians on coral reefs are important, because they have the potential to support a broader interpretation of the notion of ecological winners (or losers) on contemporary reefs (sensu Loya et al 2001), and can provide insight into the community structure that is likely to prevail in the future. As it is unknown whether the selective environment on future reefs will favor the taxa that are “winning” on contemporary reefs (Van Woesik et al 2011, Brown and Phongsuwan 2012), it remains possible that novel taxa will emerge as ecologically important community members
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