Dynamics of Rocky Intertidal Communities: Analyses of Long Time Series from South African Shores

Abstract

The abundance of benthic invertebrates in undisturbed and experimentally cleared quadrats on three rocky shores on the east coast of South Africa was monitored quarterly for 13 years. Most of the 11 commonly occurring species exhibited significant seasonal cycles and, although interannual variability was high, there was evidence of biennial cycles in abundance. Long-term trends were evident for many species. Limpets generally decreased in abundance, while periwinkles and rock oysters increased. The two numerically dominant barnacles exhibited an inverse relationship over time, and at some sites one species was replaced by the other. There was a high degree of both inter- and intraspecific correlation at small scales, but this was attenuated at scales of tens to hundreds of kilometres. Many of the time series exhibited significant discontinuities in which changes in abundance of up to three orders of magnitude were evident. Such discontinuities usually marked the start of periods of altered interannual variability lasting for 3–4 years. In some cases, this resulted in time series divided into phases with completely different underlying dynamics. After an initial recovery period of up to 3 years, the cleared areas exhibited many of the same characteristics as the controls, but with a tendency for increased interannual variability. An interesting finding was that areas cleared in spring and summer did not exhibit the biennial cycles characteristic of undisturbed areas, while those cleared in autumn and winter retained this characteristic. Fluctuations in abundance of intertidal invertebrates clearly exhibit complex dynamics, which are discussed in terms of possible external forcing functions.