Size-Number Trade-Offs and the Variation of Seed Size with Plant Resource Status

Abstract

I develop a general treatment of the effects of parental resource status on optimal offspring size. The model shows that even when there is a resource trade-off between size and number of offspring within individuals, positive correlations between size and number may occur among individuals due to individual variation in resources. Such positive correlations imply that parental resource status affects the fitness-maximizing offspring size, in contrast to the predictions of the standard Smith-Fretwell model. I show that parental resource status affects the fitness-maximizing offspring size whenever the size-number fitness function is nonhomogeneous in offspring number. This condition implies some sort of density-dependent interactions among offspring, although it is possible to have either positive or negative sib interactions in fitness functions that are homogeneous in offspring number. In the latter case offspring size should be insensitive to parental resource status. I explore several cases including linear (Smith-Fretwell), nonlinear but homogeneous, and nonhomogeneous seed-number functions with independent size and number effects, as well as models with interacting offspring size-number effects. I briefly review the plant literature on offspring density dependence and seed-size variation with parental resource status. Some suggestions for testing the ideas with plants and some alternative explanations of positive seed-size/plant-size and seed-size/seed-number correlations are discussed.