Coral life history strategies
To infer potential environmental changes driving coral community shifts,
we also tracked trends in coral life history groups. We adapted a
trait-based classification approach that grouped scleractinians into
four life history strategies influenced primarily by colony morphology,
growth rate, and reproductive mode [23]. This configuration roughly
follows Grime’s arrangement of plant species into three basic life
history strategies: competitive species that maximize growth, stress
tolerant species that maximize survival, and ruderal or weedy species
that maximize fecundity [24-25]. A fourth category, generalist
species, represents a mixture of these strategies. However, because the
generalist group was composed primarily of corals that are sub-dominant
in the Caribbean, this group was not included in our analyses. To add
ecological context to these groupings, we collated from the literature
taxon-specific, qualitative measures of additional life history
characteristics including sexual reproductive output (larval
recruitment), asexual propagation via colony fragmentation,
interspecific aggression, and susceptibility to disturbances such as
sedimentation and bleaching (Tables 1,S2). Although larval recruitment
represents the end-point of fecundity, fertilization, dispersal, and
early post-settlement mortality and is not a life history characteristic
per se, this metric is closely linked to life history strategy [26]
and provided valuable ecological context for observed community change.
Our life history groupings closely followed those determined by refs 23
and 27, with two exceptions described below.
The competitive life history group included Acropora cervicornis,A. palmata, and the hydrozoan Millepora spp . This group is
distinguished by fast growth rates, large branching morphologies that
can outcompete other corals for light and/or space, medium to high
levels of aggression, a spawning mode of reproduction but low rates of
sexual recruitment, high propensity for asexual reproduction via
fragmentation, and low tolerance to disturbances such as sedimentation
and thermal stress (Tables S2,S3). This combination of traits
historically allowed Acropora corals to dominate shallow, high
energy reef environments prior to local and global anthropogenic
stressors [28-30]. Although not included in the previous analyses of
coral life history guilds, we included Millepora in the
competitive category because of its Acropora -like ability to
preempt space on reefs due primarily to fragmentation and fast growth
and its high susceptibility to bleaching (Tables 1,S2, refs 31-32).
The stress-tolerant life history group includes Colpophyllia
natans , Diploria spp., Meandrina spp., Montastrea
cavernosa , Orbicella spp., Siderastrea spp., andStephanocoenia spp. This group is distinguished by slow to
moderate growth rates, large and domed morphologies with higher ability
to clear sediment and other particles and resistant to storm damage, a
spawning mode of reproduction with low to moderate sexual recruitment,
low to high interspecific aggression, and relatively higher tolerance
for sedimentation and thermal stress (Tables 1,S2). Although some
stress-tolerant corals have a higher susceptibility to bleaching, colony
survival rates are typically high within this group [33]. This
combination of traits historically allowed these species to persist and
dominate in environments subject to frequent, low-magnitude disturbances
such as sediment resuspension and temperature stress [30,34-35].
Because the three extant Orbicella species were until recently
classified as Montastrea annularis [21], we assignedOrbicella spp. to the stress-tolerant category in accordance with
the classification for M. annularis [23]. However, this
genera’s historical dominance on midslope zones on many Caribbean reefs
and its high levels of interspecific aggression via mesenterial
filaments [36] suggest it may also be considered as a highly
competitive taxon. Therefore, we ran two sets of analyses of trends in
coral life history and species prevalence: one that includedOrbicella in the stress-tolerant group and one that includedOrbicella in the competitive group.
The weedy life history group includes Agaricia spp.,Madracis spp., branching Porites spp., and P.
astreoides . This group is distinguished by lower-relief plating,
foliose, branching, and domed morphologies with slow to fast growth
rates, a brooding mode of reproduction that allows for rapid
colonization at low population densities, generally high rates of sexual
recruitment, high to low occurrence of asexual reproduction via
fragmentation, low interspecific aggression, generally high
susceptibility to bleaching, and high tolerance of sedimentation (Tables
1,S2). This combination of traits historically allowed these
early-successional species to opportunistically and rapidly colonize
open spaces cleared by high-magnitude acute disturbances [28,37-38).