Interpretation of the fossil record of Acanthaster planci from the Great Barrier Reef: a reply to criticism

Abstract

The nature and interpretation of the fossil record of Acanthaster planci from the GBR is reviewed in the light of comments from Keesing et al. (1992) and Pandolfi (1992). Skeletal remains of A. planci in reef-top sediment of many reefs has been derived from very large numbers of individuals, indicating substantial, long-term mortality at reef-top locations. The fossil record provides useful perspective on mortality patterns in the absence of substantive ecological data. The incidence of skeletal elements on reefs where they are abundant cannot be adequately accounted for by the mortality of non-outbreak populations as estimated by recent surveys. Analysis of all available data reaffirms a relationship between the incidence of skeletal elements in surface sediment and observed outbreak history. There is no presently identified taphonomic mechanism by which the accumulation of A. planci skeletal elements released on death might be systematically biased relative to other skeletal components of reefal sediment. Because of skeletal degradation, physical transport and extensive bioturbation that applies in shallow-water reefal sediment, reconstructive taphonomic analysis of A. planci skeletal remains is not achievable. Core sediment, on which interpretation of the longterm fossil record of A. planci is based, is homogeneous, unstratified, and has experienced substantial time averaging due to pervasive bioturbation. Extensive bulk sediment dating has shown that the cores have retained a general age structure but fine-scale stratigraphic detail, required for the recognition of outbreak events from the fossil record available in reefal sediment is unlikely. As required by the principle of simplicity, the proposition that abundant A. planci skeletal elements found in sediment from Green Island, John Brewer and other reefs of the GBR represent the time-averaged product of outbreaking populations should be adopted as the favoured working hypothesis. Other alternative explantions have been advanced but all require patterns or processes that have yet to be substantiated.