The community structure of a tropical intertidal mudflat under human exploitation

The impact of human exploitation on the community structure of intertidal mudflats was investigated in an exploited and unexploited control area at Inhaca Island, Mozambique. An increase in the species richness in the exploited area, as expected by the intermediate disturbance hypothesis, was not confirmed. The log body weight-log abundance regression lines were not steeper nor lower, as would be expected if larger species were taken relatively more often. Species diversity was similar in both areas but species composition was significantly different. When the species composition of the five different substrates in each of the two different areas was compared a negative correlation between the total biotic and abiotic stress load and the species diversity appeared. Substrates under a larger total stress load had a lower species diversity, a lower evenness and a higher dominance. A decrease in evenness in exploited areas was expected because of the disappearance of the larger prey species that are exploited by people. Evenness was indeed reduced but this was caused more by the increase of opportunistic species than by the decrease in the abundance of prey species. These findings suggest that intertidal mudflats are more structured through environmental stress factors than through top-down processes, such as competitive exclusion or predation. Human exploitation increased the total stress load of the substrates and thereby affected the species diversity negatively. ? 2002 International Council for the Exploration of the Sea