THE IMPORTANCE OF PREDATORS ON A SANDFLAT - INTERPLAY BETWEEN SEASONAL-CHANGES IN PREY DENSITIES AND PREDATOR EFFECTS

An experiment was designed to assess the role of 2 different predators in determining the macrobenthic community structure of an intertidal sandflat, The 2 predators were: shorebirds which feed throughout the y ear by removing individual prey items, and eagle rays Myliobatis tenui caudatus which are only present during the summer and disturb large vo lumes of sediment where extracting prey. The experiment consisted of b ird exclusion, ray + bird exclusion and reference plots. Samples were collected from each plot on 2 occasions: 6 mo after the initiation of the experiment, when rays were absent and common bivalve densities wer e high following recruitment, and 8 mo later when rays were present an d bivalve population structure was not dominated by new recruits. At t he end of the experiment analysis of surficial sediment features did n ot indicate the experiment was confounded by localised modifications o f sediment or hydrodynamic conditions. Community level differences on both occasions were driven by effects on common taxa. The seasonality of effects in our experiment precluded direct comparison of the 2 pred ators. However, the 6 mo results indicated that bird predation resulte d in indirect effects due to adult/juvenile interactions amongst the d ominant bivalve Macomona liliana. At the end of the experiment, 14 mo after its initiation, analysis of common taxa generally revealed direc t negative effects of predation, with significantly high densities in the ray + bird exclusion treatment. Infaunal density changes in respon se to the exclusion of shorebirds and rays did not indicate the presen ce of multiple trophic levels in this infaunal assemblage. Differences between the results obtained from the bird exclusion and the ray + bi rd exclusion treatments on the first sampling occasion were attributed to an edge effect around the bird exclusion plots which effectively i ncreased their area. This edge effect emphasises the importance of inf aunal mobility and its potential to swamp predator effects. The result s of this experiment highlight the importance of considering the role of predators within an appropriate spatial and temporal context.