In the long term, the persistence of a coral reef requires that its overall rate of mass and volumetric growth of framework equals or cxcecds losses to biological and physical erosion and transport of sediment away from the framework zones. Persistence is thus a function of benthic community structure as well as the vitality of associations between zooxanthellae and individual hosts. Mass bleaching can facilitate the switch between net accretion and net decay. This switch may also be mediated by a bio tic functional group called the facilitators.
Facilitators are biota whose actions directly affect benthic community structure, and hence the apportionment of carbon between and within pathways with bioconstructional outputs, and those without (e.g. the various configurations of Figure 15.3). Grazing herbivores, for example, facilitate coral growth by preventing algal overgrowth of newly settled and small corals (Hatcher 1983). However, excessively high densities of urchin grazers can lead to destruction of corals (Sammarco 1982. and above), and exces-
Figure 15.4 (opposite) Functional groups having bioconstructional roles on coral reefs. Reef construction is a result of the activities of stony coral framework builders which are influenced by "modifier groups" such as calcification enhancers, eroders and sediment operators (see text). Arrows (see also Figure 15.3) signify biological groups and processes as follows: 1, carbon fixed by symbiotic zooxanthellae and coralline algae, 2, carbon fixed into small skeletal elements of foraminifera, calcareous algae and other non-framework builders; 3, carbon fixed in fleshy plants sively low densities can lead to undergoing and overgrowth of corals by algae (Cuet et al. 1988: Hughes 1989). Certain species of abundant territorial damcselfish are also important facilitators of benthic community structure. These fish maintain algal "gardens" (Kaufman 1977; Lobel 1980) that can cover a significant fraction of some reef zones (Klumpp el al. 1987; Done ei al. 1991), sometimes to the detriment of framework builders. Some facilitator abundances are prone to extraordinarily large fluctuations. Examples (discussed in more detail below) are the sea urchins mentioned above, and the sea-star predator on Indo-Pacific corals (the crown-of-thorns starfish Aeanthaster planet), which transforms vast areas from coral to algal dominance, and thus has lasting effects on the rates and nature of framework accretion.
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