Coral and sea urchin assemblage structure and interrelationships in Kenyan reef lagoons

Abstract

Patterns of hard coral and sea urchin assemblage structure (species richness, diversity, and abundance) were studied in Kenyan coral reef lagoons which experienced different types of human resource use. Two protected reefs (Malindi and Watamu Marine National Parks) were protected from fishing and coral collection, but exposed to heavy tourist use. One reef (Mombasa MNP) received protection from fishermen for one year and was exploited for fish and corals prior to protection and was defined as a 'transitional reef'. Three reefs (Vipingo, Kanamai, and Diani) were unprotected and experienced heavy fishing and some coral collection. Protected and unprotected reefs were distinct in terms of their assemblage structure with the transitional reef grouping with unprotected reefs based on relative and absolute abundance of coral genera. Protected reefs had slightly higher (p < 0.01) coral cover (23.6 plus or minus 8.3% plus or minus S.D.) than unprotected reefs (16.7 plus or minus 8.5), but the transitional reef had the highest coral cover (30.8 plus or minus 6.4) which increased by 250% since measured in 1987: largely attributable to a large increase in Porites nigrescens cover. Protected reefs had higher coral species richness and diversity and a greater relative abundance of Acropora, Montipora and Galaxea than unprotected reefs. The transitional reef had high species richness, but lower diversity due to the high dominance of Porites. Sea urchins showed the opposite pattern with highest diversity in most unprotected reefs. Coral cover, species richness, and diversity were negatively associated with sea urchin abundance, but the relative abundance of Porites increased with sea urchin abundance to the point where Porites composed > 90% of the coral cover at sites with the highest sea urchin abundance. Effects of coral overcollection was only likely for the genus Acropora (staghorn corals). A combination of direct and indirect effects of human resource use may reduce diversity, species richness, and abundance of corals while increasing the absolute abundance of sea urchins and the relative cover of Porites.