The role of local and global human stressors
Although climate change is currently imperiling reef ecosystems globally
[49], the early timing of the initial declines in CaribbeanAcropora corals suggests that climate change was not responsible
for this first phase of Caribbean coral community transformation. In the
Caribbean, anthropogenic ocean warming did not become significant until
the 1970s [50-51] and warming-related coral bleaching was not
observed until the late 1980s [52]. Our recent analysis of long-term
trends in the dominance of A. cervicornis and A. palmatashowed that initial declines in the 1950s and 1960s were unrelated to
regional or global stressors (i.e., anthropogenic temperature stress or
hurricane exposure; ref 13).
Instead, the early timing of initial changes in Caribbean coral
communities implicates long-standing local stressors such as fishing and
land-based pollution. However, the paucity of long-term data on fishing
effort/reef fish abundance or reef water quality precludes a
quantitative assessment of the role of these activities in recent
Caribbean-wide reef ecosystem change. Consequently, despite the
well-established relationships between hermatypic coral persistence and
abundant herbivorous reef fish populations and low-sediment,
low-nutrient waters [3,53-55], historical fishing and land clearing
have been largely ignored in most analyses of Caribbean coral declines
[56]. Fortunately, a few longer-term datasets on water quality at
various Caribbean reefs provide valuable insights into the role of
land-based runoff in coral community change. An analysis of seawater and
macroalgae nitrogen content since the 1990s from the Florida Keys
implicates land-based nutrients from agriculture and development in the
decades-long coral declines within that reef tract [57]. Studies
based on historical and paleontological data also suggest that early
reef ecosystem declines in Barbados and Panama may be attributed to
increases in coastal runoff from historical land clearing for
agriculture [17,58-59].
In contrast to the early transformation of Caribbean coral communities
following the initial loss of Acropora in the 1950s/1960s, more
recent changes since the 1980s/1990s demonstrate the heightened effects
of local stressors and climate change acting on reefs simultaneously.
Although our study suggests that White Band Disease was not the cause of
initial Acropora declines, it confirms that it has unequivocally
contributed to the loss of this genus: the second significantAcropora decline observed in our time series in the early 1980s
immediately followed the first instances of this disease reported in the
late 1970s [9]. Land-based runoff has been shown to exacerbate coral
bleaching and disease [57,60-61], suggesting that reef
eutrophication played a role in the emergence of these morbidities.
Similarly, the region-wide declines in stress-tolerant and weedy corals
we observed since the 1980s/1990s reveal that local and global stressors
are making Caribbean reef environments unsuitable even for those corals
with the hardiest of life history strategies. Indeed, recent monitoring
efforts have documented declines in several stress-tolerant taxa from
bleaching and disease that were initiated two decades ago [62-63]
and show that several stress-tolerant species are currently rapidly
succumbing to the highly lethal Stony Coral Tissue Loss Disease that
does not affect Acropora [64-66]. Monitoring efforts are also
documenting declines in weedy corals such as Agaricia due to
recent Caribbean-wide bleaching events [67]. Thus, the shifts
documented in our 131,000-year record indicate a long history of
increasingly stressful environmental conditions on Caribbean reefs that
began with local human disturbances and have culminated in the combined
effects of local and global change.