Environmental Engineering Reference
In-Depth Information
Chronic alterations of a reef environment including increased levels of nutrients,
overfishing and the release of toxic compounds can severely undermine reef
resilience and thus the ability of reef communities to cope with new disturbances
superimposed onto those already existing (Nystroem et al. 2000). Reduced resil-
ience inhibits or delays reef regeneration after a disturbance event, which can lead
to long-lasting or even irreversible changes in community structure; so-called phase
shifts to alternative stable states (Hughes and Connell 1999; Hughes et al. 2007).
The resultant alternative state is manifest in either a new dominant coral species
(Aronson et al. 2004) or an alternative life-form, like corallimorpharians (Kuguru
et al. 2004), ascidians, soft corals, sponges and urchin 'barrens' (Norstroem et al.
2009) and very often algae (McManus and Polsenberg 2004). Regardless of the
nature of these shifts, they generally all culminate in a conspicuous loss of benthic
invertebrate and fish diversity as well as a decrease in inorganic carbonate deposi-
tion, which in turn reduces reef complexity, overall species richness and increases
shoreline erosion.
Important factors that mediate resilience include (1) the degree of diversity
within functional groups, functional redundancy and the response diversity within
each group, (2) demographic structure of populations, (3) recruitment success, and
(4) ecosystem connectivity, i.e. exchange processes among reefs or between reefs
and adjacent habitats within a given seascape.
When diversity of coral reef species is high and species interact in a highly
structured environment, feedback loops occur over a wide range of scales. Thus,
descriptive approaches using mean average measures or starting from reduced
statistical assumptions might not be appropriate for analysing the complex structure
and underlying processes. Here modelling may help to integrate the multitude of
components, relevant variables and parameters to describe and visualize complex
ecological processes and the driving forces which shape the resilience of a system.
Models may also be used to simulate the behaviour of specific system components
in response to a changing environment (Fig.
17.1
).
In the following subsections we describe different approaches to modelling reef
resilience including examples of a trophic model which is based exclusively on
differential equations (Sect.
17.2
) and a Cellular Automaton (CA) model which
allows spatial explicit analysis (Sect.
17.3
). Section
17.4
introduces how Individual
Based Modelling (IBM) can facilitate the implementation of direct individual
interactions of organisms and Sect.
17.5
gives an example of a grid based commu-
nity model which combines differential equations and a CA approach. We have
chosen these examples to illustrate and discuss the possible advantages and draw-
backs of presently applied ecological modelling techniques.
17.2 Equation-Based Modelling of a Coral Reef Food Web
McClanahan (1995) developed a differential equation model to evaluate the impact
of fishing on Kenyan coral reefs. The model simulates the food web of a virtual reef
ecosystem of undefined spatial extent in which corals and algae comprise the primary
