The importance of predators on a sandflat: interplay between seasonal changes in prey densities and predator effects

Abstract

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 year by removing individual prey items, and eagle rays Myliobatis tenuicaudatus which are only present during the summer and disturb large volumes of sediment when extracting prey. The experiment consisted of bird 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 were high following recruitment, and 8 mo later when rays were present and bivalve population structure was not dominated by new recruits. At the end of the experiment analysis of surficial sediment features did not indicate the experiment was confounded by localised modifications of 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 predators. However, the 6 mo results indicated that bird predation resulted in indirect effects due to adult/juvenile interactions amongst the dominant bivalve Macomona liliana. At the end of the experiment, 14 mo after its initiation, analysis of common taxa generally revealed direct negative effects of predation, with significantly high densities in the ray + bird exclusion treatment. Infaunal density changes in response to the exclusion of shorebirds and rays did not indicate the presence of multiple trophic levels in this infaunal assemblage. Differences between the results obtained from the bird exclusion and the ray + bird exclusion treatments on the first sampling occasion were attributed to an edge effect around the bird exclusion plots which effectively increased their area. This edge effect emphasises the importance of infaunal mobility and its potential to swamp predator effects. The results of this experiment highlight the importance of considering the role of predators within an appropriate spatial and temporal context.

Temporal and spatial variation in macrofauna community composition imposed by ghost shrimp Callianassa filholi bioturbation

“…Therefore, other factors must also be playing an important role. For example, predation has also been shown to impose small-scale differences in macrofaunal community measures for intertidal habitats (Thrush et al 1994). In addition, the temporal changes in sediment sorting observed at the site hint at the role of physical parameters (e.g.…”

Temporal variations in microbenthic metabolism and inorganic nitrogen fluxes in sandy and muddy sediments of a tidally dominated bay in the northern Wadden Sea

“…mussel beds (Andersen & Kristensen, 1988;Asmus & Asmus, 1996), and physical removal (and deposition) of organic carbon caused by, for example, winds, tides and currents (Bayerl et al, 1996), which can be important and probably balance the budgets. Furthermore, export of carbon via crab, fish and bird predation should also be considered (Raffaeli & Milne, 1987;Hall et al, 1990;Thrush et al, 1994).…”

Assemblage structure, spatial patterns, recruitment, and post‐settlement mortality of subtidal bivalve molluscs in a large harbour in north‐eastern New Zealand

“…obs. ), but at much lower densities than in Manukau Harbour (Thrush et al 1994;Hine:» et al 1997), and we doubt that they play an importai it role. Putt's (1996) study of three invertebrate predators of bivalves present across Centre Bank, (the sea stars Coscinasterias calamaria and Patiriella regularis and the whelk Cominella adspersa) suggested that abundances of these predators are generally no : high, and that the low movement rates of those preiators in patches of an abundant resource were a result of satiation.…”

“…Vertebrate predators of bivalves such as eagle rays and wading birds influence distributions of intertidal bivalves in Manukau Harbour (Thrush et al 1994). Wading birds seldom have access to bivalve populations of a sub tidal area such as Centre Bank.…”