Facilitators as links between humans biodiversity and ecosystem function

Three case studies illustrate reef turn-off caused by population fluctuations in echinoderm facilitator species. In the first, the turn-off appears to be rapidly reversible without human intervention in human time scales; in the latter two, the situation appears to be irreversible, or at least much slower than expected.

Crown-of-thorns starfish. In the 1960s and 1980s, populations of the crown-of-thorns starfish Acanthaster planci, for reasons unknown, increased by several orders of magnitude and killed much of the coral over large areas throughout the central portion of the Great Barrier Reef (Moran 1986) and elsewhere in the Indo-Paeific. On each occasion, one or more good years for the starfish recruitment combined with abundant palatable coral to allow local starfish populations to explode (Antonelli et al. 1990). Complete consumption of all available coral tissue caused the highly aggregated starfish populations to emigrate to and feed in contiguous areas of high coral cover. Algal turfs and fleshy macro-algae colonised the dead coral skeletons and became the dominant benthic biota. Over a period of a decade or so, corals and coralline algae gradually reestablished their dominance in many shallow reef areas {Done 1992c).

Functionally, the initial change from coral to algal dominance represents a shift from the Figure 15.3a configuration to that of Figure 15.3d, and the recolonisation by coral represents the reverse transition. There was no apparent increase in the abundance of grazing fishes in response to the increased algal biomass (Williams 1986). Thus, the outbreak more likely initiated an increase in the detritus compartment (Figure 15.3e) than in usable protein.

Biodiversity has been implicated in hypotheses about both the cause of the outbreak and the system response. Ormond et al. (1990) proposed that a reduction in fish prédation rates on juvenile starfish, caused by intensification of fishing-pressure since the 1960s, could have led to increased juvenile survival and thus to adult outbreaks. Keesing and Lucas (1992) showed how the level of total coral cover (and hence, presumably, community calcification rate) could be a function of coral biodiversity; viz. the relative abundance of highly palatable versus less-palatable coral species.

Caribbean sea urchin In the Carribean, dense populations of the herbivorous sea urchin Diadema antillarum collapsed throughout the Carribean in 1983 (Lessios 1988). Dramatic changes on some Jamaican reefs subsequent to the collapse have been documented by Hughes (1989, 1994; Figure 15.5). Reef slopes formerly covered by dense coral assemblages became covered by dense beds of benthic algae (Carpenter 1990), and remain in this state today (Hughes 1994).

Like Acanthaster planci outbreaks, the high-density populations of D. antillarum are also believed to be anomalous and related to biodiversity loss (Figure 15.6) Jackson (1994) suggested that in pristine, pre-Columbian times, D. antillarum competed for its share of benthic algae against a diverse assemblage of fish and invertebrates. Corals were beneficiaries, because the

ft t

Hurricane Diadema Hurricane

Allen die off Gilbert ft t

Hurricane Diadema Hurricane

Allen die off Gilbert

Figure 15.5 Changes in the abundance of corals and algae at Discovery Bay, Jamaica, indicating the timing of Hurricanes Alien and Gilbert, and the mortality in Diadema antillarum (redrawn from an unpublished figure with permission of T.P. Hughes)

grazers prevented the algae from overgrowing them. Since Columbus, a relentless increase and diversification in fishing effort and catch has dramatically reduced populations of D. antillarum's competitors and its predators. The release from both competition and predation allowed £>. antillarum populations to increase dramatically (Ogden et at. 1973; Hughes 1994; Jackson 1994), and it alone bceame responsible for grazing benthic algae down to levels at which they were not major competitors of hard corals. However, D. antillarum's high densities, while allowing it to "do the job" of the missing grazers, also made its populations vulnerable to disease. In 1983. perhaps inevitably, a lethal pathogen decimated D. antillarum populations throughout the Caribbean (Lessios 1988). We. may surmise that the pristine situation of smaller, multi-taxa populations of grazers would not have been subject to such an event. Moreover, declines in one grazer population may have been more readily compensated for by increases in others.

Fishes

Pathogenic mass mortality of Diadema _

Fishes

Overfishing

Coral/Algae Diadema Ratio

Overfishing

Pathogenic mass mortality of Diadema _

Coral/Algae Diadema Ratio a.

Figure 15.6 Changes in relative abundance of Caribbean corals as a response to overfishing and mass-mortality in Diadema antillarum (a) Pristine situation prior to overfishing, with grazing role shared among diverse fish and invertebrates; (b) Diadema populations elevated due to overfishing of its competitors and predators; (c) coral to algae ratio depressed due to release of all grazing pressure and death of corals due to algal overgrowth and other causes (after Jackson 1994)

Widely throughout the Caribbean basin, a number of circumstances have acted in synergy with the absence of the urchin to favor the dominance of fleshy benthic macro-algae over corals (Knowlton 1992); the increased availability of nutrients (Hallock 1987), the increased sedimentation associated with deforestation (Cortes and Risk 1985), the physical destruction of large areas of coral by hurricanes (Woodlcy et at. 1981; Hughes 1989), oils spills (Jackson et al. 1989), and mass bleaching of corals (Williams and Bunkley-Williams 1990). Functionally, the changes appear to be equivalent to those caused by A. planci outbreaks - viz. predominance of the trophic pathway over the bioconstructional, and a predominance within the trophic pathway of the detrital branch (Figure 15.3d.e).

East African sea urchin A third variation on the theme has been described in Kenya. East Africa. The density of the herbivorous sea urchin Echino-metra mat thai is 100 times greater on reefs unprotected from fishing than it is on protected reefs (McClanahan and Muthiga 1988, 1989). The difference is correlated with differences in fish predation on the urchin, which is four times greater within than outside protected areas. E. matthai grazes algal turfs, but is also an incidental bio-eroder of reef framework (Figure 15.30-The reefs with high urchin populations are totally devoid of visible macro-algae (it is grazed before biomass accumulates to any appreciable degree) and the coral framework appears to be undermined faster than it can be replaced by the coral growth (T.J. Done, personal observations, 1990). There are poor prospects for coral recolonization. because each square centimetre is grazed too frequently for newly settled corals to survive and grow

(microscopic corals and grazed indiscriminately along with the algal turfs and skeletons).

As with A. planci and D. antillarum, the inference is that human reduction of fish biomass and diversity is a causal link with the abundance of E, matthai. At optimal densities E. matthai facilitates coral recruitment and growth at rates that more than compensate for bioerosion incidental to its grazing. However, like D. antillarum, its dense populations may also be vulnerable to pathogenic disease. Should that happen, and the urchin populations crash, coral to algal transitions similar to those in the Caribbean may follow (see also Done 1992a).

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