Selectivity and Growth of the Generalist Herbivore Dolabella Auricularia Feeding Upon Complementary Resources

Abstract

An assumption of most optimal diet theory is that different resources are substitutable, that is, that they are identical in all relevant aspects and so can be ranked in value using a single currency, such as energy. However, this assumption is probably not valid in many cases. The sea hare Dolabella auricularia (Gastropoda: Opisthobranchia: Anaspidea) grew far better upon an ad libitum mixture of four species of algae than upon any of the algal species alone, suggesting that algae were complementary resources. When offered three pairs of algae in 4:1 and 1:9 ratios, Dolabella changed its foraging behavior so that the rarer alga was preferred. Consequently, the mixtures of algae consumed were more similar between treatments (ratios) than the mixtures of algae offered. Replicate Dolabella were maintained on six single algal diets for 6—7 d and then offered the maintenance alga and another alga at equal abundances. Preference for a species of algae was always higher when it was not the maintenance diet compared to when it was. These results suggest that Dolabella actively maintains a mixed diet because it is nutritionally superior to any single diet. An alternative hypothesis (the nonadditive toxin hypothesis) suggests that herbivores might consume a mixed diet because of mixture of plant secondary metabolites is less toxic than any one metabolite alone; consequently, herbivores can consume more total biomass of a mixed than of a single diet. However, sea hares fed single— and mixed—species diets did not consume more of mixed diets than of single—species diets, and animals fed a mixed diet grew better than those fed single diets even when the biomass of algae offered was held constant. If resources are complementary, as suggested here, several important implications follow: (1) consumers will not rank resources in any absolute way because the value of each will depend upon what else is consumed, (2) consumer specialization will be opposed by the benefits of a mixed diet, and (3) consumer foraging will tend to destabilize the resource community, because rarer resources will be disproportionately consumed.