COMMUNITY ECOLOGY OF CHILEAN SANDY BEACHES
Monthly abundances and life history traits of peracarid species (Orchestoidea and Excirolana), and the anomurid crab Emerita analoga were studied during May, 1978 to April, 1980 in an exposed sandy beach of southern Chile (Mehuin Beach, Bahia de Maiquillahue). Beach characteristics differed significantly during these years; i.e. more sand erosion and coarser sand occurred during the first year. The lowest abundances of the total macroinfauna (46-111 ind ${\rm m}\sp{-2})$ occurred during winter 1978, while the highest abundances (896-1251 ind ${\rm m}\sp{-2})$ occurred during winter 1979. The low abundances were a result of severe erosion of the habitat rather than just a coarsening of sand, a conclusion supported by grain-size preference experiments, and macroinfaunal data from adjacent beaches with coarser sand. Differences in beach physical characteristics did not affect the life history traits of peracarids; in contrast, the same physical characteristics may indeed have affected the population structure of E. analoga. Long-term studies (1978-1985) of seasonal abundances at Mehuin Beach, showed a downshore migration of peracarids during the warmer months. Multiple regression analyses were performed with the monthly data of species abundances and sediment characteristics during 1978 to 1980. The analyses showed that fluctuations of the upper beach O. tuberculata and E. braziliensis were primarily explained by the variability in sand desiccation related factors, while fluctuations of the middle beach E. hirsuticauda were explained by mean grain size variability. Three distinct macroinfaunal zonation patterns were distinguished along the beaches of Bahia de Maiquillahue, and adjacent estuarine outlet areas. Differences in sediment texture, wave disturbance and water salinity are believed to be primarily responsible for these patterns. All these data, in addition to those from intra and interspecific coexistence experiments, suggest that the abundances and intertidal distribution of the peracarids O. tuberculata, E. braziliensis and E. hirsuticauda are regulated primarily by the spatial and temporal variability of substrate characteristics. This also holds true for E. analoga, although shorebird predation may be an additional factor regulating its population structure.
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