Completing the circle: stock-recruitment relationships and Acanthaster

Abstract

In the absence of any reliable data, two opposing null hypotheses concerning the relationship between Acanthaster stock size and the level of recruitment tend to be assumed. First, recruitment may be assumed to be independent of stock size. This is appropriate if stocks are considered on a small scale, with most recruitment occurring externally. Second, recruitment may be assumed to be linearly related to stock size. This is appropriate if stock is considered on a large scale, so that larval production occurs from within the stock. If the potential of predators to prevent outbreaks is investigated, these two alternative hypotheses lead to diametrically opposed conclusions as to the importance of the two main parts of the predator functional response. If recruitment is independent of stock size, then the maximum prey-consumption rate of predators per unit of time is the critical factor, whereas if recruitment is linearly related to stock, then it is the searching behaviour of predators when starfish are rare that determines whether outbreaks occur. The extent of internal and external contributions to recruitment may also have profound results for the overall behaviour of Acanthaster populations, irrespective of predation. If a humped stock-recruitment relationship is assumed, together with a very high reproductive potential, then very small changes in the amount of larval interchange between reefs can transform dynamics from regular cycles to chaos and back to cycles. This means that the qualitative behaviour of starfish populations may be greatly affected by very minor variations in the amount of larval interchange between reefs.