  The large variation in seed mass among species inspired a vast array of
theoretical and empirical research attempting to explain this variation. So
far, seed mass variation was investigated by two classes of studies: one class
focuses on species varying in seed mass within communities, while the second
focuses on variation between communities, most often with respect to resource
gradients. Here, we develop a model capable of simultaneously explaining
variation in seed mass within and between communities. The model describes
resource competition (for both soil and light resources) in annual communities
and incorporates two fundamental aspects: light asymmetry (higher light
acquisition per unit biomass for larger individuals) and growth allometry
(negative dependency of relative growth rate on plant biomass). Results show
that both factors are critical in determining patterns of seed mass variation.
In general, growth allometry increases the reproductive success of small-seeded
species while light asymmetry increases the reproductive success of
large-seeded species. Increasing availability of soil resources increases light
competition, thereby increasing the reproductive success of large-seeded
species and ultimately the community (weighted) mean seed mass. An unexpected
prediction of the model is that maximum variation in community seed mass (a
measure of functional diversity) occurs under intermediate levels of soil
resources. Extensions of the model incorporating size-dependent seed survival
and disturbance also show patterns consistent with empirical observations.
These overall results suggest that the mechanisms captured by the model are
important in determining patterns of species and functional diversity.
