Challenges with assessing long-term trends
To track Caribbean coral community change prior to and since the arrival
of humans, we used data from multiple sources, including uplifted fossil
reefs, reef matrix cores, qualitative historical data, and underwater
survey data. However, the prevalence of most of coral taxa are
comparable across the paleo, historical, and early modern time bins,
suggesting that overall trends were not artifacts of comparing differing
data types (Figures 2, 3). Exceptions include (1) the conspicuously
lower within-country community dissimilarity observed within the fossil
versus non-fossil time bins, which is likely due to the greater time
averaging within the former and (2) significant differences in the
prevalence of multiple coral taxa between the Pleistocene and Holocene.
Significant Pleistocene-Holocene differences included declining
prevalence of A. cervicornis at the midslope zone and increased
prevalence of a number of stress-tolerant and weedy taxa at both zones
(Table 2, Figures 2,3). These differences could reflect declining rates
of sea level rise during the late Holocene, which would favor increased
dominance of more slowly growing species with massive colony forms
[68]—a trend we observe in our data from the midslope zone (Figure
3b). Lower A. cervicornis prevalence in the Holocene compared to
the Pleistocene could also be a result of inaccurate paleodepth
estimates and/or underestimation of A. cervicornis abundance from
the narrow-diameter core tubes which comprise most of the Holocene data.
However, A. palmata dominance increased slightly at the reef
crest between the Pleistocene and Holocene, demonstrating that there was
no bias against sampling this coral in the Holocene reef cores [13].
Importantly, when data from the Pleistocene and Holocene periods are
combined, overall trends in prevalence of coral functional and species
groups are largely identical to those with these time periods
separated–declines in Acropora prevalence first occurred in the
1960s, followed by increases in stress-tolerant and weedy species in the
1970s and 1980s (Table S6, Figures S3,S4).
Although abundance data allows for a more robust assessment of community
composition than presence/absence data, the exceptionally broad
temporal, taxonomic, and geographic scales covered in this study
necessitated the utilization of the latter. Prevalence does not equal
abundance; although we found that the prevalence of stress-tolerant and
weedy corals has increased over the most recent decades, the total
abundance of living coral has declined by 50-80% across the Caribbean
since the initiation of quantitative surveys in the 1970s [1-2,8].
However, ecological studies show that, on large spatial scales,trends in prevalence (proportion of sites occupied) are
correlated with trends in abundance [69-70]. This suggests
that the long-term trends shown in this study are reliable proxies of
qualitative trends in relative (but not absolute) abundance.