Consequences Of Habitat Variation And Habitat Association For Coral Reef Assemblages

We defined habitat as the place where an organism lives, yet discussed habitat patches and zones (e.g. reef slope, rubble grooves) relatively independently of any particular organism. This is because defining a habitat based on where an organism lives introduces a curious circularity in its definition. For example, the damselfish Pomacentrus coelestis is primarily found living on rubble. This introduces a problem because it implies there could be 'empty' habitats in the environment: if a patch of rubble doesn't contain P. coelestis then is the patch suitable habitat for the species? So the challenge lies in defining habitats even though they may be unoccupied. Of course in the example above, this is not too difficult if we view suitable habitat as a probability of occupation, rather than strict occupancy of any single particular piece of space. While there is spatial and temporal variability in habitats, there is also greater spatial and temporal variability in species abundance. In general, rubble patches on reefs contain P. coelestis, and conversely P. coelestis is usually found on rubble patches. The abundances of course may vary on leeward v. seaward sides of the reef, across the continental shelf, and latitudinally because of increasingly larger scales of environmental variation. This introduces another problem though: if habitat is the place where an organism generally lives, then we need to identify the scales at which an organism perceives and responds to the habitat characteristics, and also identify what the important characteristics are.

Determining what exactly is important about the habitat to an organism can be surprisingly difficult for some species. In an obligate species such as a coral goby or coral crab, it is easy—a particular species of coral needs to be present. For species with a wider range of movement such as wrasses, then simple correlations between species abundance and different types of coral cover can be misleading. For example, many fish species appear to be positively correlated with soft coral cover, but removal of soft corals at small scales may result in no change to the fish assemblage. The apparent correlation may simply be due to wrasses preferring habitats within a diverse range of substratum types, coupled with soft corals also being positively correlated with a diverse range of substratum types. Prior knowledge of the habitat characteristics that are actually important to organisms is essential for identifying and predicting responses of organisms to habitat changes.

Identifying coral reef habitats, environments and cross shelf and latitudinal gradients is important both for pure ecological studies, and management of the GBR. Many ecological questions and management solutions require the habitats of a range of organisms to be identified or protected, so measuring habitat diversity is an important task because it is closely linked to species diversity. This is often known as a community-level approach, although a community in a strict sense is a complete suite of interacting species. On coral reefs 'resident' species also interact with nonresident species. For example planktivores eat pelagic species, and pelagic fishes such as jacks (Carangidae) may eat resident fishes despite not being considered 'coral reef' fishes themselves. Additionally, some fish species such as some wrasses feed on hard reef and soft bottom substratum, thus linking the hard and soft bottom 'communities' energetically. In reality, the community-level approach is really an assemblage-level approach, in which we consider a co-occurring subset of the entire range of interacting species, so we will use the word assemblage in preference to community. Measuring or managing the habitats of a wide range of species raises some interesting problems. First, if habitat can only be defined as the place where an organism lives, do we need to identify the typical range at which each of the organisms in the community perceive their habitat? Obviously we could not do this easily with such a diverse flora and fauna, with each species interacting at different scales with their habitat and each other. While this may be an important nuisance to coral reef ecologists, for GBR managers the solution is rather easier. Simply recognising habitat as a spatial concept, and recognising that reefs vary along the GBR, a spatial management protocol can use an 'umbrella' approach. If large spatial units (e.g. individual reefs) can be managed or protected from some type of anthropogenic pressure, then all habitats contained within that spatial unit will be automatically protected as well. In addition, if these units can be placed along all axes of the main environmental gradients such as latitude and shelf position, then in all probability this protection will extend over the entire range of habitats without really needing to know exactly how to define habitats for each species, or even know which habitats occur on each reef. Such an approach was used in the zoning of the GBR in which large regional areas with clusters of similar habitats ('bioregions') were identified and different zoning levels applied to protect representative habitats across the reef (see Chapter 12).

To summarise: on coral reefs living organisms contribute to the habitat structure of a vast array of organisms, and so ecological processes such as competition, predation, and biological disturbance interact with physical processes such as water movement and physical disturbance to alter the physical structure of the habitat. In this way, local biological processes, perhaps correlated over large spatial and temporal scales, can exert very strong effects on those organisms that respond to the habitat-forming organisms. The diversity of habitat-responding organisms is not restricted to places in which the habitat-former is present; many 'coral reef' organisms are not associated with corals themselves, but rather the habitat heterogeneity they provide. Temporal and spatial variability in habitat structure generates a range of opportunities for habitat responding organisms to coexist at a range of spatial scales. Although defining habitats for individuals and assemblages is problematic, recognising that habitat is a spatial concept means that management of habitats can be a relatively simple process, as long as big enough areas of space are controlled. This diversity of places to live, in combination with the vast array of organisms that are available and able to live in these places undoubtedly combine to maintain biodiversity on the GBR.

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