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 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. kXY WORDS: New Zealand. Soft-bottom . Predation. Disturbance Shorebirds. Eagle rays . Recruitment

[1]  P. R. Evans,et al.  Short-term effects of reclamation of part of Seal Sands, Teesmouth, on wintering waders and Shelduck , 1979, Oecologia.

[2]  A. Alldredge,et al.  Effects of moonlight on the vertical migration patterns of demersal zooplankton , 1980 .

[3]  J. Goss-Custard.  The Ecology of the Wash. III. Density-Related Behaviour and the Possible Effects of a Loss of Feeding Grounds on Wading Birds (Charadrii) , 1977 .

[4]  J. Grant,et al.  The control of soft‐shell clam (Mya arenaria) recruitment on intertidal sandflats by bedload sediment transport , 1991 .

[5]  K. R. Clarke,et al.  Statistical Design And Analysis For A Biological Effects Study , 1988 .

[6]  C. Frid.  The role of recolonization processes in benthic communities, with special reference to the interpretation of predator-induced effects , 1989 .

[7]  S. Bell,et al.  Field evidence that shrimp predation regulates meiofauna , 2004, Oecologia.

[8]  J. Goss‐Custard,et al.  Competition for Food and Interference among Waders , 2015 .

[9]  D. Raffaelli,et al.  Aquatic Ecology: Scale, Pattern and Process. , 1995 .

[10]  D. Dean.  Migration of the sandworm Nereis virens during winter nights , 1978 .

[11]  A. J. Belsky.  The Effects of Grazing: Confounding of Ecosystem, Community, and Organism Scales , 1987, The American Naturalist.

[12]  W. Armonies.  Migratory rhythms of drifting juvenile molluscs in tidal waters of the Wadden Sea , 1992 .

[13]  Robert L. Folk,et al.  Petrology of Sedimentary Rocks , 1974 .

[14]  K. R. Clarke,et al.  Non‐parametric multivariate analyses of changes in community structure , 1993 .

[15]  W. Ambrose,et al.  Multiple trophic levels in soft-bottom communities , 1985 .

[16]  S. Carpenter.  Transmission of Variance through Lake Food Webs , 1988 .

[17]  J. T. Curtis,et al.  An Ordination of the Upland Forest Communities of Southern Wisconsin , 1957 .

[18]  M. S. Bartlett,et al.  Nearest Neighbour Models in the Analysis of Field Experiments , 1978 .

[19]  H. Milne,et al.  Utilization by shorebirds of benthic invertebrate production in intertidal areas , 1985 .