Environmental Engineering Reference
In-Depth Information
competitively exclude corals. Calcium carbonate deposition would remain high
because of the proportion of calcifying algae and because bioerosion would
decrease due to low sea urchin and fish abundance.
McClanahan's model has the general applicability to assess coral reef food
web interactions through a somewhat holistic approach rather than focusing on a
limited number of organisms and interactions. However, a major shortcoming of
this type of modelling is presented by the fact that it is highly aggregated. Treating
several components under the umbrella of one variable, i.e. the grouping of various
components according to functional types or trophic groups severely reduces
natural variability and may therefore easily mask processes which are crucial
attributes to the system's dynamics. Considering that different species or functional
groups of algae and corals, do exhibit very different types of tolerance to a given
environmental condition, the lack of distinction can lead to the oversimplification
of a given scenario.
17.3 Spatial Competition Between Coral Species: Application
of a Cellular Automaton Modelling Approach
Langmead and Sheppard (2004) designed a classical cellular automaton model
(Chap. 8) to assess the effect of natural background disturbance (e.g. sedimentation,
predation) on a coral community on a Caribbean fore-reef slope. Each disturbance
event can be set to occur over different spatial scales and at varying levels of
intensity. The model comprises ten different coral species that compete for space.
Each of the species has a specific pattern of recruitment, growth, mortality rate and
aggression (i.e. competitive potential).
The simulation area of the model reef comprises a torus with a total size of 9 m
2
that is subdivided into 1 cm
2
cells corresponding to a median sized coral polyp.
Each of these cells can contain either bare substratum or one of ten coral species at
a time.
The model was exclusively parameterized with data obtained from the literature.
Coral growth was based on annual skeletal extension rates of each species and was
expressed as radial expansion. Growth was determined by the rules for competitive
interactions between corals: Colonies could only grow into adjacent cells if they
were either unoccupied (bare substratum) or occupied by a competitively subordi-
nate species (species were ranked according to their aggressiveness). Depending on
the differential susceptibilities to disturbance and varying mortality rates, each
species was assigned a probability of mortality if impacted by disturbance. Based
on data for larval settlement in the study area, each coral species was set to be
present at a specific density and the number of potential recruits was then deter-
mined annually using a Poisson probability distribution. Recruitment success was
determined by larvae abundance and the amount of free space in the plot as recruits
are only allowed to settle on bare substratum.
