Structure and dynamics of North-Western Mediterranean phytobenthic communities: a conceptual model

Abstract

This paper describes the structural (species richness, species diversity, pattern-diversity, biomass, coverage, specific distribution, homogeneity) and functional (production, PIB ratio) changes observed during the annual cyc1es of eight phytobenthic Mediterranean communities by using PCA analysis. The first axis obtained discriminated between variables related to pattern-diversity and those related to production and coverage (production- diversification or P-D axis). The second axis was mainly related to biomass and homogeneity (biomass-heterogeneity or b-h axis). Shallow-water cornmunities were positioned in the quadrant of the plane associated with high production while deep-water communities were located in the quadrant characterized by high diversification values. The transit of communities through this plane during their annual cyc1es suggests their idealization as geometric figures revolving c10ckwise along the plane defined by the P-D and the b-h axes. These changes allow the distinction of two phases and two stages in the annual cyc1e of the cornmunities, according to the sequence: diversified cornmunity (high heterogeneitY)---7 production phase (high production)---7 developed community (high biomass)---7 diversification phase (high pattern-diversity). Peak production occurs in spring for the mediolittoral cornmunities and is delayed towards summer and autumn at increasing depths. This time lag affects the whole annual cycle described aboye and appears attributable to the adaptation of the different species and communities to the seasonal cyc1e of the main factors limiting their growth (BALLESTEROS, 1 989b). Although production and diversification are inversely related, PIB ratio is not negatively correlated with diversity because of the different mechanisms generating diversity in natural communities: disturbance and evolution. Short-time diversity increases in periods of intense physical disturbance (diversification phases) due to lack of dominance within the cornmunity and the growth of opportunistic species. Thus, short-term diversity and productivity are not inversely correlated. Long-term diversity increases along evolutionary time under environmental stability, explaining the greater diversity of deep-water communities compared to shallow ones, where an inverse correlation between productivity and diversity is observed. An interannual model of the structural and functional performance s of phytobenthic Mediterranean cornmunities can be depicted over the plane defined by the P-D axis and the b-h axis by introducing time as a third axis. In general, the dynamics of persistent communities can be depicted as helicoidal trajectories whose turns represent annual cyc1es. This representation allows the adequate definition of concepts such as persistence, resistance and resilience of cornmunities, and creates an adequate framework for describing successional processes and the effects of all kind of disturbances. The hypothesized existence of multiple stable points in natural communities closely agrees with the model if different degrees of non-punctual, biotic, disturbances are considered. In our opinion this model could also be extended to other temperate seas.