Application of new techniques for the accurate analysis of choice of prey

Many studies have demonstrated non-random associations between consumers and particular types of food. These patterns have frequently been inferred to demonstrate an active behavioural choice for that type of food, but did not elucidate the mechanisms responsible for the observed pattern. The designs of experiments to demonstrate preference for a particular prey are complex, because they must show that selection of prey when a choice of food is available is different from what happens when there is no choice. Procedures have recently been developed to account for errors associated with estimating diets in the absence of choices and to calculate the correct probability that observed patterns of choice of prey represent a deviation from random selection, i.e. that there is a preference. This study applies these techniques to experimental data on consumption of prey. Laboratory experiments were done to determine whether recognised patterns of consumption by Ozius deplanatus (a reef crab) were the result of active choice. We predicted that patterns of selection would be similar to those found in a congeneric species, where crabs of different sizes are associated with prey of different sizes. Despite the presence of strong associations between sizes of crabs and prey, little evidence of preference for prey-size could be shown. This reinforces the notion that association is not the same as preference and that careful experiments and analysis must be done before preference can be inferred. Knowledge of whether or not observed non-random patterns of association are caused by preference is of value to ecologists for predicting and understanding responses to changes in availability of resources.