Functional Types: A Comment on Steneck and Dethier (1994)

Abstract

May I echo the plea by Steneck and Dethier (1994) for deployment of functional types in analyses of community and ecosystem structure. In particular, I support their attempt to recognise functional types by relating the spatial distribution of organisms to independent measurements of potential productivity and disturbance. This approach is an advance on methods which rely upon subjective assessments and complements the experimental approach in which communities and ecosystems are allowed to assemble under strictly controlled conditions of productivity and disturbance (Heal and Grime 1991, Campbell and Grime 1992). Despite broad agreement in method and conclusions, an interesting difference is apparent between the classification of algal functional types of Steneck and Dethier (1994: Fig. 7c) and the general model of plant and animal functional types in Grime (1989: Fig. 1). Both models propose that only half of the rectangular matrix corresponding to orthoganally opposed gradients in productivity and disturbance is tenable but there is a clear difference with respect to the functional type said to be associated with conditions of high productivity and severe disturbance. In Grime (1989), fast-growing ephemerals of high fecundity (ruderals) occupy the high productivity-high disturbance corner of the matrix whereas Steneck and Dethier (1994) allocate this position to disturbance-tolerant perennial organisms and propose that ephemeral algae occupy habitats of moderate productivity and with intermediate levels of disturbance. There are several possible explanations for these divergent interpretations: (1) Marine algae may exhibit a fourth primary strategy not represented in terrestrial vegetation. (2) C-S-R theory may have failed to recognise a fourth primary strategy (disturbance-tolerance) occurring in both aquatic and terrestrial environments. (3) Steneck and Dethier (1994) may have misinterpreted the selection forces associated with disturbance-tolerance. In order to consider these three hypotheses it is necessary first to add a dimension omitted from the discussion of Steneck and Dethier (1994). This is the integral role of disturbance-tolerance in stress-tolerance (Whittaker 1975, Grime 1977). The argument that defence against disturbance achieves maximum expression in the plants of unproductive habitats has been stated as follows: