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).