Ecological monitoring and object-oriented simulation studies on stability and alternative stable states of coral reef communities.

摘要

Coral reef communities have high ecological and economic values but are under increasing human-induced stress locally and worldwide. Long-term monitoring of the communities is thus essential to understand the natural variation of the communities in order to provide objective assessment on the impacts of human on them. Furthermore, anthropogenic disturbances are known to cause phase shifts of coral reef communities that suggests the potential existence of alternative stable states in these communities. Examining this diagnosis by modelling studies is important for their conservation.;A two year (1997--1999) monitoring programme of the coral communities at two sites, A Ma Wan (AMW) and A Ye Wan (AYW), in Tung Ping Chau, Hong Kong was carried out. The study identified a total of 45 scleractinian coral species in the study areas, in which 40 species were recorded in AMW and 32 species were recorded in AYW. Multidimensional Scaling (MDS) ordinations showed that distinct spatial variations occurred in the coral communities within and between sites and were probably due to the differential effects of the repeated severe cyclone impacts on the communities. The seasonal influence of corallivorous gastropods (Drupella rugosa and Cronia margariticola ) and macroalgae also caused substantial decrease in percent cover of various dominant species in the communities.;Another two year (1998--1999) monitoring programme on the reef fish communities in the study areas was also carried out. The study identified a total of 106 species in the study areas, in which 87 species were recorded in AMW and 78 species were recorded in AYW. A seasonal pattern was observed in the abundances and species richness of all fishes and of most of the frequently encountered families/trophic groups in both study areas. This pattern may be related to the seasonal fluctuation of macroalgae, the influence of recruitment and post-settlement of fish larvae, and the seasonal variation in the behaviour of fishes. MDS ordinations demonstrated that spatial variation in fish community structure existed within and between sites and were possibly related to the spatial variation of the coral community structure therein. The ordinations also showed that the fish community structures were not seasonally stable throughout the study period.;A 3-dimensional individual-based model, the ReefModel, was developed to simulate the interaction among six functional groups of coral reef organisms (tabular coral, foliaceous coral, massive coral, macroalga, corallivorous gastropod and herbivorous fish) using an object-oriented technique. The simulation results suggest that (i) fast-growing habit with overtopping competitive mechanism is effective for corals to gain dominance in a stable coral community; (ii) the integration of physical disturbance and the differential responses of corals play an important role in structuring coral communities; (iii) macroalgal influence can cause significant shift in coral community structure, but the influence can be mediated by herbivory; (iv) gastropod predation provides differential effects on different coral groups; (v) alternative stable states can occur in coral reef communities and result from the influence of small random spatial events that occur early on during the interactions among the reef organisms.;In conclusion, the coral communities in Tung Ping Chau, Hong Kong may have low resilience towards severe disturbances. Preserving the coral resilience should be one of the main goals of any strategy aimed at conserving the coral communities around the island. Precautionary principle should also be adopted in their conservation as the alternative stable states may appear in the communities after exposure to human or naturally induced disturbances.