Patterns, processes and regulatory mechanisms in sandy beach macrofauna: a multi-scale analysis

Physical and biological factors govern community and population features of sandy beach macrofauna. At the macroscale, species richness decreases from tropical to temperate beaches, and from macrotidal dissipative to microtidal reflective beaches. At the species level, life history traits are highly plastic over latitudinal gradients; large-scale variations in environmental variables modulate intraspecific phenotypic differentiation. At the mesoscale, alongshore and across-shore distributions tend to be unimodal, bell-shaped within a beach, with abundance varying from the central region to the boundaries, even though environmental gradients (wave exposure, salinity) can cause asymmetries. Zonation is highly dynamic and not sharply defined. This is attributed to short- (hourly, daily) or medium- (seasonal) term reactions to environmental conditions, passive transport and sorting by the swash (e.g. recruits), active micro-habitat selection (e.g. adults), and intra- and interspecific interactions. Across-shore distribution may become multimodal due to intraspecific segregation by sizes during recruitment. At the microscale (individual neighbourhood or quadrat scale), behavioural factors and intra-/interspecific interactions become more important as density increases. Human induced impacts also generate variability in population demography, structure and dynamics. We identify physical-biological coupling at different temporal and spatial scales, emphasizing the role of life history traits in order to assess alternative regulatory mechanisms and processes. Our synthesis suggests that: (1) biological interactions are more important regulatory agents than previously thought: in benign dissipative beaches or undisturbed sites, intra- and interspecific competition can be more intense than in reflective beaches or disturbed sites, where the populations are physically controlled; (2) supralittoral forms are relatively independent of the swash regime and show no clear response to beach type; (3) marked long-term fluctuations are noticeable in species with planktonic larvae structured as metapopulations, due to environmental disturbances and stochasticity in reproduction and recruitment.