Long-term change in coral community composition
Since the mid-20th century, shallow water reefs across the Caribbean have transformed from systems dominated by competitive corals to systems dominated by stress-tolerant and weedy corals (Figures 2, 3). At both the reef crest and midslope zones, coral community change occurred via three stages: (1) significant declines in competitive corals relative to the pre-human baseline occurring by the 1960s; (2) significant increases in stress-tolerant and weedy corals occurring by the 1970s and 1980s; and (3) significant declines or leveling-off of stress-tolerant and weedy corals since the 1980s or 1990s (Table 2, Figures 2, 3). These trends were consistent whether Millepora was included or excluded from the competitive life history group and whetherOrbicella was included in the competitive or stress-tolerant group, with the exception that the inclusion of both Orbicellaand Millepora in the competitive group rendered overall declines in competitive corals non-significant at the reef crest zone (Figures S1,S2, Table S5). For competitive corals in both zones and stress-tolerant and weedy corals in the midslope zone, the first significant change relative to the pre-human baseline occurred in the Holocene, reflecting contrasting environmental conditions and/or data types between the Pleistocene and Holocene (Figures 2,3).
The assessment of trends for individual species groups revealed that at both reef zones, the prevalence of competitive Acropora palmataand A. cervicornis declined significantly between the Pleistocene and Holocene periods. After the Holocene, the next significant decline in competitive Acropora corals occurred in the 1960s; the prevalence of these corals remained significantly lower than pre-human levels from this point forward (Table 2, Figures 2a, 3a). In contrast, prevalence of the competitive hydrozoan Millepora increased significantly across the full time series, peaking in the late 1990s (crest) or early 2000s (midslope). At the reef crest zone,Siderastrea was the only stress-tolerant taxon that increased significantly from the pre-human period to present, while at the midslope zone all stress-tolerant taxa except for Diploria andOrbicella increased significantly across the full time series (Table 2, Figures 2b,3b). The post-1980s declines in the prevalence of stress-tolerant corals that occurred at the reef crest zone were driven by declines in Colpophyllia, Diploria, Montastrea cavernosa , and Siderastrea while post-1990s leveling off in the prevalence of this group at the midslope zone was driven by declining or consistent prevalence of Colpophyllia, Diploria, andMontastrea cavernosa . At both reef zones, all weedy taxa increased significantly across the full time series with the exception of Madracis in the reef crest (Table 2, Figures 2c,3c). Initial significant increases in individual weedy coral species primarily occurred in the 1980s (crest) and in the 1970s (midslope). Significant post-1980s declines in the weedy group at the reef crest reflect contemporaneous declines in Agaricia , branching Porites , and P. astreoides , while significant post-1990s declines in the weedy group at the midslope reflect declines in Agaricia and branching Porites .