Coral Community Adaptability to Environmental Change at the Scales of Regions, Reefs and Reef Zones(1).

INTRODUCTION

I use the term "coral community" to describe the local assemblage of stony coral and other conspicuous benthic populations which can co-habit with them (e.g., algae, soft corals, zooanthids) on reefal or non-reefal substrates (Done, 1992, Devantier et al., 1998). By "local" coral community I refer to the places that people anchor their boats at and swim around in.

On coral reefs, coral communities dominated by stony corals and encrusting coralline algae are best developed on seaward slopes, and on the margins of shallow sub-tidal reef flats. Coral communities are the aesthetic face of coral reefs, and this, along with their major role as reef builders and complex three dimensional habitats for other reef organisms, is good reason to consider the environmental changes they will face over coming decades, and how they may respond.

At the local scale, the environment endured and exploited by the coral community is defined by a long list of variables, many of which may change under global change scenarios (Brown, 1997a): the prevailing day to day and season to season regime of waves, light, nutritional input and aragonite saturation state of the water; the regime of natural catastrophic disturbances, be they decadal or less frequent; the rates and interannual variability in supply of larvae and spores of benthic biota capable of settling and surviving at that place; the incidence of pathogens which are fatal for key groups such as corals; the local ecological determinants of space occupancy by competing benthic groups, such as physical interactions between them, and the daily rate of scraping of the substratum by fishes and invertebrates (e.g., Sammarco et al., 1974; Hatcher 1993; McCook, 1996).

Changes in coral communities attributable to global environmental change per se may be difficult to recognise because coral communities are highly dynamic on ecological time scales (Fig. 1), and their dynamic properties vary substantially among regions and habitats (e.g., Scoffin, 1993; Dollar and Tribble, 1993; Done and Potts, 1992). They exhibit changes in cover, vertical relief, bio-mass, relative abundance of species and diversity over time scales of years to decades, slowly increasing in undisturbed periods, and often crashing following catastrophic disturbances (e.g., Dollar and Tribble, 1993). In those areas where global change is manifest as a reduction in the return interval between highly destructive events such as cyclones, floods (Pittock, 1999) and outbreaks of coral predators (e.g., Done, 1988), one might predict that the stage, age and size frequency distributions of coral populations and communities across regional seascapes will be skewed more to earlier successional stages (Fig. 1b) than is presently observed. Such changes, though perhaps mere noise in the context of the global scene and of geological and evolutionary time (e.g., Grigg and Dollar, 1990; Done, 1997), are major determinants of how reefs are perceived and managed.

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There is a long list of direct human impacts on coral reefs that act locally but are globally widespread and ubiquitous (Brown, 1987; Ginsburg, 1994): sedimentation, overfishing and destructive fishing; pollution, to name just a few. In this paper, it is useful to view global changes as an overlay on both the natural spatial and temporal variability in coral communities, plus the localised human impacts. At the organizational level of communities and populations, net outcomes of all these three sources of change may most strongly be manifest in community succession and population dynamics viewed over local to regional space scales, and decadal time scales.

One measure of the reef-building potential of coral-dominated zones and other reef zones is their community calcification rate. Reef zones dominated by corals and/or coralline algae calcify at rates up to 10 kg[multiplied by]Ca[CO.sub.3][multiplied by][m.sup.-2][multiplied by][y.sup.-1] (Kinsey, 1991). The rate appears to be largely independent of the precise taxonomic composition and relative abundance of the calcifying species present (Kinsey, 1991). However community calcifying rate can presumably be reduced to lower (including negative) levels where disturbances or other ecological or environmental changes cause calcifying corals and coralline algae to be replaced by fleshy algae, bioeroders, filter feeders and other non-calcifying or slowly calcifying organisms (Kinsey, 1988). The possibility of such transitions are thus of more than academic interest, affecting not only reef aesthetics, habitat amenity, and biodiversity issues, but also the maintenance of reef mass and volume.

NICHES AND ENVIRONMENTAL CHANGE

The environmental niches for coral communities are defined by a wide range of physical, chemical and ecological variables (Brown, 1997a). Key parameters include prevailing waves forces, temperature, aragonite saturation state, turbidity and supply of organic and inorganic nutrients, and planktonic productivity. The extent to which a potential habitat satisfying the physical/nutritional niche is actually occupied by a coral dominated community is strongly predicated on demographic and ecological considerations, especially the rate and reliability of supply of viable propagules, and on rates of scraping of the substratum for epilithic algae by herbivorous fishes and invertebrates (Hatcher, 1983 and see "phase shift" below).

Many environmental variables are projected to substantially change over the next 50 to 100 years. Contrasting changes in light penetration and wave exposure on steep and flat topographies are predicted as a result of rising sea level per se (Fig. 2). Hopley and Kinsey (1988) have pointed out that on reef flats, it is likely corals will flourish in some previously high intertidal areas from which they were formerly excluded or limited by excessive exposure to the air, while on deeper slopes, phototrophic corals may need to become increasingly heterotrophic to survive in a regime of reduced light.

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However, of the climate change factors projected for the next century, …