Roles of disturbance, sediment stress, and substratum retention on spatial dominance in algal turf

A common assumption of ecological theories addressing the causes of spatial dominance and stability in natural communities is that organisms have trade-offs between resistance to disturbance, environmental tolerance, and competitive abilities, i.e., that different species or groups of species dominate under different regimes of disturbance and stress. A multifactorial field experiment was conducted from June 1994 to October 1995 in a turf-dominated macroalgal assemblage on a rocky subtidal shore south of Livorno (western Mediterranean, Italy) to test the hypothesis that variable patterns of disturbance and stress influence the differential success of species, and to identify functional characteristics resulting in persistent monopolization of space. The experiment, which included manipulation of the depositional environment, was designed specifically to test whether patterns of recovery following abrasion or removal of the biota differed among patches of different sizes, produced at different times of the year and at different locations and undergoing different rates of sediment deposition. Variable size, intensity, timing and location of disturbance, and variable stress induced by deposition of sediment significantly influenced the success of erect algae that persist by sexual reproduction but did not affect spatial dominance by the filamentous algal turf. The turf regained space very quickly by vegetative reproduction and persisted as the dominant species in all the experimental plots, independent of the disturbance and stress regimes imposed. The impressive rate of vegetative propagation was identified as a major determinant of the structure of the assemblage and the probable basis for the capability of the turf to compete for space and resist disturbance. Patterns of growth may be as crucial as the disturbance regime in determining community structure, and 'packaging' of competitive and resistance abilities into a single species or group of species could be more common than generally thought.