Seed dispersal is a critical step in the life cycle of plants, with major implications for individual plant fitness, population dynamics, and community organization. Seed Dispersal Effectiveness (SDE), which considers both the quantity of seeds dispersed and the quality of dispersal provided to each dispersed seed, provides a framework for assessing the success of seed dispersal. As with so much of ecology, most analyses of SDE are based on mean values of quantity and quality, even though we know this obscures tremendous variation among individuals in the components of SDE. Inter-individual variation in the quantity and quality of seed dispersal is driven by both intrinsic traits of individual plants and extrinsic characteristics of the environment. In this talk I present a broad overview of sources of inter-individual variation in seed dispersal. Due to time limitations, I emphasize variation in the quantity component of seed dispersal.
Intrinsic variation in traits associated with seed dispersal occur at the individual fruit/seed level and at the whole plant level. At the fruit/seed level there is extensive within-individual and among-individual variation in fruit and seed size, fruit color, fruit nutritional content, pappus length, presence-absence of dispersal structures, and more, all of which affect seed dispersal. At the whole plant level, inter-individual variation in plant height influences abiotic dispersal while variation in fruit crop size influences plant selection by foraging frugivores. While these sources of trait variation have a genetic component, they also have a very strong environmental component, varying with stress and resource availability. Both the biotic environment and the physical structure of the habitat affect inter-individual variation in seed dispersal. Foraging by frugivores is not based solely on traits of individual plants, but also on the fruiting neighborhood. Co-fruiting conspecifics and heterospecifics can influence dispersal of individual fleshy-fruited plants through competition or facilitation. Variation in seed dispersal can also be due to variation in the abundance and community composition of seed dispersers. Variation in disperser community structure is driven partly by demographic processes of the dispersers and partly by habitat selection based on overall physical structure of the habitat.