Variability of species' roles in marine communities: change of paradigms for conservation priorities

The structure and organisation of aquatic communities, moulded in each environment by combinations of abiotic factors, recruitment and productivity rates, rely upon a network of both pairwise and transitive interactions among organisms. In many cases, a few strong interactors drive basic ecological processes by playing a leading role in channelling the available resources. Among these, keystone species may control the outputs of local biodiversity through large indirect effects, disproportionately large relative to their abundance. Functional roles are not fixed labels, and species interactions have variable outputs in both time and space: also, in spite of a growing literature on species interactions, terminology is often poorly applied. This leads to the loss of the informative value of concepts, like the keystone species, which might represent useful trade-offs between science and environmental politics. Species' roles are often used to set taxonomic conservation priorities, although this might even be regarded as unethical, ecologically wrong, or in disregard of the evolutionary meaning of species coexistence and interaction. A re-assessment of species' roles is given here, attempting to highlight their limits and applicability. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-001-0769-2.