Environmental Engineering Reference
In-Depth Information
between corals and macroalgae on Caribbean reefs was confirmed by Mumby et al.
(2007) who investigated the susceptibility to and persistence of macroalgal domi-
nance on Caribbean reefs. Mumby and Hastings (2008) extended the model by
including vicinity to mangroves as an additional factor and two different depths
as additional parameters in order to assess the relative importance of mangroves
(they function as nursery grounds for Scarid fish) on the abundance of parrotfish on
adjacent reefs. In another recent study Mumby (2009) uses the same model to assess
the stability of alternative stable states of Caribbean reefs. Here, the parameters
pertaining to external disturbances were omitted and instead the model concentrates
on the inherent parameters of coral community dynamics. In contrast to the model
discussed here, growth rates, sizes at maturity, overgrowth and mortality were now
not set as fixed values but determined probabilistically, which allows for a higher
degree of natural behaviour of the model due to a broader range of natural
variability. The reef community dynamics were investigated with regard to differ-
ent levels of grazing intensity ranging from 5 to 40%. Simulations were run in
6 month intervals for a total of 36 single reefs. With this approach, Mumby was able
to identify clear threshold levels of grazing intensity (i.e. the level of grazing
necessary to prevent the shift to an alternative state) for different sets of initial
coral cover and different levels of grazing.
Spatially explicit modelling approaches to understand the impact of grazers on
coral reefs have only emerged over the past 5 years. The models by Mumby offer a
novel approach to overcome the problem of reef complexity, which to date has
complicated experimental studies of the interactions of multiple disturbances.
However, a word of warning should be issued regarding the organisation in a
lattice. The complexity of the model may lead to complications in the definition
of clear rules for interactions between individual organisms which are not located in
the same cell. For example, if a coral outgrows its cell, the part that protrudes into
the adjacent cell becomes an integral component of that cell and thus 'fragmented'
from its original colony. It starts to function as a new and smaller entity with rules
and trajectories of a juvenile coral colony, since age is determined via size. The fact
that most processes are determined via a von-Neumann-Neighbourhood might
compensate for the loss in detail to some extent, however, depending on the
questions asked, there may still be a risk to ignore certain important processes.
17.6 Summary and Conclusions
The techniques currently available in ecological modelling all bear certain limita-
tions and the choice of a given approach depends on the question of interest.
The exclusive utilization of differential equations (Sect.
17.2
) can provide
interesting results for different fishing scenarios in a relatively complex simulation
environment, that may help to improve fisheries practices. Often these techniques
lack resolution, however, as all components are aggregated into functional (larger)
groups, which substantially obscure the inherent natural variability among relevant
