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Micro-algae (the principal food of intertidal gastropods) were quantified in areas surrounding mid-shore refuges of the intertidal snail Nerita atramentosa to test the hypothesis that removal of important grazers would lead to more micro-algae. Digital colour-infrared imagery provided independent quantitative measurements of amounts of chlorophyll (as an index of amounts of micro-algae) over small areas through time. To determine the effect of grazing on micro-algal abundance, cages were used to exclude animals from refuges. Other refuges were left uncaged or had partial, control cages. Contrary to predictions, amounts of micro-algae in areas close to refuges from which N. atramentosa had been excluded were smaller than in controls, suggesting facilitatory effects of N. atramentosa on micro-algal growth or survivorship. Facilitation may occur because of decreased grazing by the limpet Cellana tramoserica where N. atramentosa (a competitive dominant) are present or to facilitatory effects of N. atramentosa directly on the micro-algae. To unravel complex ecological interactions involving grazing around refuges and the impact of grazing on ecological structure and function it is necessary to make repeated, independent measurements through time. Remote-sensing provided information on distribution of food that would be difficult or impossible to achieve using conventional methods. This research identified complexities and interactions that have implications for the study of grazing effects across many terrestrial, intertidal and marine habitats.

Unexpected patterns of facilitatory grazing revealed by quantitative imaging