Keystone species: The concept and its relevance for conservation management in New Zealand

Abstract

The keystone species concept has proved both promising and elusive in theoretical and applied ecology. The term has its origins in Robert Paine's studies of rocky shore communities in California. When the top predator (a starfish) was removed the species assemblage collapsed, prompting the architectural analogy with the keystone of an arch. By definition keystone species are those whose effect is large, and disproportionately large relative to their abundance. They include organisms that (i) control potential dominants, (ii) provide critical resources, (iii) act as mutualists, and (iv) modify the environment. Identifying keystone species can be problematic. Approaches used include experimental manipulations, comparative studies, natural history observations, and 'natural experiments', but no robust methodologies have been developed. Our inability to monitor and manage all aspects of biodiversity has led to the development of paradigms that focus on either single-species (e.g. indicators, umbrellas or flagships) or whole ecosystems (ecological processes and habitats). Not surprisingly, both have their advocates and detractors. The keystone species concept, which retains a species focus while avoiding the need to examine every species, and emphasises processes that directly (e.g. predation, competition) rather than indirectly (e.g. nutrient cycling) control biodiversity, may allow managers to combine the best features of both these paradigms. By itself, however, the concept is unlikely to provide a panacea for biodiversity managers.