Environmental Engineering Reference
In-Depth Information
of 20 years. It is important to bear in mind that the model was only simulated on the
reef dynamics of initially 'healthy' reefs. The most important findings of the
simulations may be summarized as follows:
First, in the absence of any acute disturbance event and the urchin
D. antillarum
,
coral cover always increased when grazing was carried out by an unexploited
community of parrotfish. This in turn had a positive influence on recruit survival
where highest densities correlated positively with highest coral cover. Second, in
the absence of
D. antillarum
, the dynamics of coral cover were highly sensitive to
changes in hurricane frequencies. Reefs that experienced hurricanes on a decadal
basis showed a net decline in coral cover whereas a hurricane frequency of 20 years
allowed for full recovery akin to the initial 30% coral cover. Reefs that were
subjected to hurricanes at even lesser frequencies (e.g. 40-60 years) exhibited
rapid reef growth. On the other hand, the inclusion of
D. antillarum
enabled reefs
to withstand hurricanes on a decadal basis and the results showed that overall
diversion in reef recovery (i.e. reef trajectory) between different hurricane frequen-
cies decreased. Third, a reduction in parrotfish biomass lead to substantial changes
in reef community. High parrotfish biomass (i.e. high grazing) resulted in a 25%
increase in coral cover. Conversely, coral cover decreased from 30 to 7% when
parrotfish were heavily depleted. Furthermore, the results demonstrate that grazers
(or the depletion of them) is a fundamental and overarching factor in shaping the
trajectory of reef development in Caribbean forereefs. All other parameters, such as
whole-colony mortality rates, connectivity, larval retention and dispersal had a
negligible effect on coral cover. Last, reefs that maintained a healthy parrotfish
population showed clear phases of reef growth in between hurricane disturbances
which recurred at 40-year intervals. In contrast, reefs with a partially depleted
parrotfish population being subjected to the same hurricane frequency exhibited a
steady decline in coral cover.
Interestingly, examination of temporal shifts in the relative size-frequency
distribution of corals under different disturbance scenarios also indicates that
hurricanes and the exploitation of grazers had very contrasting effects on coral
populations: Under intense exploitation of grazers, the coral size distribution
become bimodal and the population experienced a bottleneck among the juvenile
size classes. On reefs with high levels of grazers but frequent hurricane distur-
bances, populations were characterized by high numbers of juvenile and pubescent
colonies while the adult part of the population experienced a bottleneck.
Improvement and Adjustment to Different Questions
This study illustrates that the overall outcome of the model could not have been
predicted by simply examining the parametrisation due to the intricate nature of
biotic and abiotic interactions across spatial and temporal scales. The results of this
model received strong support by proceeding studies using modified versions of the
same simulation model. The importance of sea urchins for the ecological balance
