Geological evidence for recurring outbreaks of the crown-of-thorns starfish

Abstract

The abundance and distribution of Acanthaster planci skeletal elements in reef sediments have been presented as evidence that population outbreaks of the crown-of-thorns starfish on the Great Barrier Reef are not a new occurrence, but have been an integral part of the ecosystem for at least 7000 years on some reefs (Walbran et al. 1989a). Reassessment of the evidence shows that these claims are not justified and challenges the validity of several assumptions that are crucial to their thesis that outbreaks have been a recurrent phenomenon on the Great Barrier Reef. These are: (i) that the majority of starfish from outbreak populations remain and die on the host reef and that their skeletal elements add to the reef sediment, (ii) that reefs which have had recent A. planci outbreaks can be discriminated from those which have not by the abundance of starfish skeletal remains in recent sediments, (iii) that outbreaks will significantly increase the number of skeletal elements in reef sediments above normal background levels and, (iv) that the age of individual skeletal elements can be predicted from the age of their surrounding sediment or their depth in the sediment pile. We conclude that Walbran et al. do not have sufficient data to infer the outbreak history of A. planci from the sediment recored and that there are alternative interpretations of their findings. The possibility cannot be discounted that destructive population outbreaks of A. planci witnessed on the Great Barrier Reef since 1960 are unprecedented. The question of whether A. planci outbreaks are a naturally recurring phenomena or a novel, more recent development remains unanswered. Show more