Community-level attributes of exploited and non-exploited rocky infratidal macrofaunal assemblages in Transkei

Abstract

The rocky intertidal biota along the Transkei coast of southern Africa is subject to intense exploitation by the indigenous coastal inhabitants. To assess the putative ecological impact of such unconstrained foraging activities the infratidal macrofaunal assemblages at three exploited sites were contrasted with those at two adjacent non-exploited sites. Comparisons of community-level attributes were based on a variety of univariate, graphical and multivariate techniques. Two-way nested anovas revealed that trends in species richness and diversity were not consistent with the presence/absence of exploitation. The apparent conflict between these results and expectations based on Connell's intermediate disturbance hypothesis appear to be related to differences in the competitive abilities of “early” and “late” successional species on these shores. Macrofaunal biomass was the only univariate parameter to show significant treatment effects. Although the repeated pulse disturbance regime associated with exploitation differs from the sustained press regime imposed on pollution-impacted communities, their respective abundance/biomass curves show similar trends. Multivariate analyses, in the form of dendrograms, multidimensional scaling (MDS) ordinations and SIMPER routines, derived from Bray-Curtis measures of similarity based on abundance and biomass data, provided the most clear-cut discrimination between exploited and non-exploited sites. The difference in community structure appears to result from a reduction in the abundance of sessile filter-feeders, increase in abundance of certain seaweed-associated species, and decrease in biomass of exploited species and their associated fauna. Differences in community structure amongst sites within treatments were attributed to additional environmental variations. The putative effects of exploitation are also contrasted with those resulting from other forms of anthropogenic disturbance elsewhere.