Temporal variability in the isotopic niches of rocky shore grazers and suspension-feeders

Carbon and nitrogen stable isotope ratios were measured in rocky inter-tidal suspension-feeders (brown mussels and cape reef worms) and grazers (goat's eye limpets and cape sea urchins) to determine the influences of lifestyle and time on the diets of consumers. Niche partitioning between consumer species within the same feeding guild was assessed using isotopic niche area (a proxy for trophic niche). Specimens were collected monthly at a single site in South-eastern South Africa from July 2010 to June 2011. Temporal variations in isotopic signatures were generally greater in the suspension-feeders compared with the grazers, isotopic niche widths were smaller in the suspension-feeders and intra-population variations in isotope signatures were larger in the grazers. No inter-specific niche overlap (according to standard ellipse areas) was observed within either feeding guild unless standardization calculations were used. Temporal variations in the diets of all the consumers appeared uncoupled from temporal variability in the isotopic signatures of basal resources in the region; as such, shifts in the consumer diets most likely pertained to feeding behaviour and food preferences. Our data provide new insights into how syntopic rocky shore consumers coexist by partitioning their temporally variable food environment.