Coral-associated richness on live versus dead corals
As with standing dead wood forests on land that have died due to disease or herbivorous insect attacks (Stokland et al. 2012, Seibold et al. 2015, Thorn et al. 2020), bleached and dead finger corals retained their biodiversity-enhancing function for multiple years after cold-water bleaching and death. This is both related to the habitat provided by the complex branching structure of the finger corals, as well as the food provided by sessile invertebrates and algae that rapidly recruited to and grew on the dead and undefended corals (Hadfield & Paul 2001, McCook et al. 2001). In this study, both overhead surface area and maximum branch length of Pocillopora corals significantly predicted the number of associated species, aligning with theory that habitat complexity is a leading predictor of species diversity (MacArthur & MacArthur 1961), and that coral colony surface area and branch length are indicators of the habitable area or volume for associated fishes and invertebrates (Abele & Patton 1976, Britayev et al. 2017). Our study further demonstrated that live tissue area was particularly important in determining the richness of associated species; surface area (and coral branch length, to a lesser extent) accounted for a much greater proportion of the total variation in species richness on live corals versus dead corals (26-62%versus 15%). As with other recently disturbed and dead foundational habitats, live coral habitat has greater structural integrity and complexity relative to dead coral structure, such that live coral area is more representative of habitat area. Additionally, dead corals provide supplementary, ephemeral functions related to provisioning of food, which differ from the habitat and food functions provided by live corals, which attract a disproportionately rich assemblage of opportunistic coral associates.
Accordingly, the richness and composition of the associated communities differed between live and dead finger coral habitats, and these differences magnified over time. Though the composition and abundance of fishes (dominated by coral hawkfishes and juvenile damselfishes) was similar on live and dead corals, abundance was highest at the first community census and then fluctuated over time on live corals, while both fish species decreased in abundance over time on dead corals. Moreover, live and dead corals exhibited distinct assemblages of invertebrates, with symbionts and specialists occupying live corals, versus opportunistic generalists occupying dead corals, and each of these assemblages became more and more distinct as live corals recovered and grew, while dead corals simultaneously eroded into rubble and disappeared.
Species composition on recovered, live corals exhibited a shift over time toward total dominance by xanthid crabs (genus Trapezia ), highlighting the importance of live coral habitat for this species. Trapeziid crabs are obligate residents that feed on live coral tissue while also defending corals from attack (Stewart et al. 2006, Stella et al. 2011). Trapezia crabs exhibited a striking increase in abundance on live corals between July 2008 and February 2010, possibly due to redistribution of individuals from recently dead to remaining live (recovered) corals, which has been found to occur after widespread bleaching (Glynn et al. 2017), according to colony size (Canizales-Flores et al. 2021). It is possible that the large increase in the number of Trapezia occupying the live coral habitats from July 2008 to February 2010 at the Galápagos sites reflects habitat limitation due to the reduction of live coral habitats after the bleaching-induced mortality of Pocillopora spp ., in addition to growth of live tissue on those colonies over time. Overall high mobility of the fish, crustacean, and echinoderm fauna associated with live and dead corals suggests that active dispersal could explain the surge of species richness in the dead coral habitats between July 2008-January 2009 and the increase in abundance of Trapezia crabs in live corals. Our results also suggest that a metapopulation perspective (Hanski & Ovaskainen 2002) may be a useful conceptual framework for investigating post-disturbance patterns of live and dead coral habitat occupancy, at least in this system of discrete finger corals in the Galápagos, and for tropical reefs of other regions where disturbances result in increasingly patchily and sparsely-distributed branching coral colonies.
By contrast, bleached and dead corals exhibited rapid colonization by opportunistic and transient urchins, crabs, and gastropod snails, which ultimately declined due to loss of structure. These mobile macroinvertebrates contributed to a marked, non-linear change in associated community richness; first an increase in total species richness on recently dead corals between the first two community surveys (July 2008-January 2009), and thereafter a striking decrease over the next 13 months to February 2010. These species included large numbers of the well-known pencil urchin bioeroder, Eucidaris galapagensis , as well as hermit crabs and a number of species of gastropod snails. These species were observed to be feeding on dead coral tissue and sessile invertebrates, which may have arrived via migration of juveniles and adults and/or larval recruitment onto undefended coral skeleton habitats. These species declined in abundance and richness over time as corals disappeared into rubble, serving to temporarily enhance overall coral-associated community abundance and richness until obligate specialists recovered on live corals.