Recolonization of red sea corals affected by natural catastrophes and man-made perturbations

The recovery patterns of hermatypic corals following an unpredictcd catastrophic low tide were studied on two reef flats in the northern Gulf of Eilat, Red Sea: (a) the nature reserve of Eilat, Israel w'hich is chronically polluted, and (b) a control reef which is pollution-free. The coral community structure and species diversity in both reefs were studied in 1969 and served as a base line for comparing the extent of mortality during the low tide (1970), and the extent of recovery 3 years later (1973). In 1969 no significant differences were found (p > 0.05) between the coral community structure of the nature reserve and the control reef when the average number of species, number of colonies, living coverage, and diversity (H′s) per transect were taken into account. During the low tide both reefs suffered mass mortalities of corals (85% at the nature reserve and 81% at the control reef). Although the extent of mortality in both reefs did not differ significantly (p > 0.05), a marked difference was observed in their recovery 3 years later. The extent of coral recolonization was 23X greater at the control reef, but no significant difference (p > 0.05) was found in the extent of coral community regeneration in both places (15% at the nature reserve and 19.2% at the control reef). While the number of colonies, number of species, living coverage, and H's are drastically decreased at the nature reserve 3 years after the catastrophic low tide, the control reef exhibited an outstanding fast recovery (full recovery is expected 5-6 years from the low tide). The commonest species on the control reef in 1969 showed the highest rates of recruitment in 1973, which might indicate the opportunistic life history of these species. I conclude that one of the differences between man-made polluting activities and natural catastrophes on coral reefs, is the possibility that the human-perturbed environment will not return to its former configuration, while reconstitution of reef areas denuded by natural disturbances is mainly a function of time.The higher coral diversity recorded on the control reef in 1973 (H′s = 2.403) as compared to 1969 (H′s = 2.206) may reflect a succession pattern in which diversity continues to increase in time after a catastrophe, until space becomes limiting and competitive interactions between species cause a decline in diversity. The unpredictable and extremely low tides at the Gulf of Eilat seem to interfere with this scheme and prevent monopolization of the reef flat by competitively superior species. I suggest that catastrophic low tides act as an important diversifying force on the reef flats of Eilat, in a similar way as other biological and physical disturbances affect coral communities (Porter 1974).