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.