Diet alters interspecific fecundity–size relationships in capital breeding insects

Abstract

Species' life history traits, such as fecundity, and how they co-vary with other traits are central to models in population and community ecology. Within species, increasing fecundity with body size is often driven by nutritional quality of the diet. How and why fecundity varies among species is poorly understood but likely to be related, at least in part, to patterns of resource acquisition and allocation. This study tested for an interspecific, fecundity–size relationship among caddisfly species and tested whether fecundity covaried with larval diet. Data on fecundity and body size were collated for 102 species in 75 genera and 28 families from around the world. Species were assigned to one of four categories of larval diet (algivores, detritivores, filter-feeders, predators) and also two combined diet groups, differentiated by the prevalence of animal versus plant material. A limiting relationship best described the positive association between fecundity and body size of all caddisflies, where size set an upper limit to fecundity. Diet explained variation below the upper limit. Compared to species with plant-based diets, consumers of animal material had higher fecundity and diet-specific fecundity–size relationships with steeper slopes. All relationships were hypoallometric (slopes less than 1), indicating a disproportionate effect of size on fecundity: in each diet group, large-bodied species produced absolutely more, but proportionately fewer eggs than smaller-bodied species, suggesting size-related shifts in resource allocation. The largest species were detritivores, which is consistent with the Jarman–Bell principle that large animals are likely to have nutritionally poor diets. These diet-related patterns in fecundity may lead to diet-related patterns in population dynamics among species within freshwater communities that have not been considered previously