Consequences of intraspecific variation in seed dispersal for plant demography, communities, evolution and global change.

As the single opportunity for plants to move, seed dispersal has an important impact on plant fitness, species distributions and patterns of biodiversity. However, models that predict dynamics such as risk of extinction, range shifts and biodiversity loss tend to rely on the mean value of parameters and rarely incorporate realistic dispersal mechanisms. By focusing on the mean population value, variation among individuals or variability caused by complex spatial and temporal dynamics is ignored. This calls for increased efforts to understand individual variation in dispersal and integrate it more explicitly into population and community models involving dispersal. However, the sources, magnitude and outcomes of intraspecific variation in dispersal are poorly characterized, limiting our understanding of the role of dispersal in mediating the dynamics of communities and their response to global change. In this manuscript, we synthesize recent research that examines the sources of individual variation in dispersal and emphasize its implications for plant fitness, populations and communities. We argue that this intraspecific variation in seed dispersal does not simply add noise to systems, but, in fact, alters dispersal processes and patterns with consequences for demography, communities, evolution and response to anthropogenic changes. We conclude with recommendations for moving this field of research forward.

Seed dispersal and movement patterns in two species of Ceratogymna hornbills in a West African tropical lowland forest.

We studied two species of Ceratogymna hornbills, the black-casqued hornbill, C. atrata, and the white-thighed hornbill, C. cylindricus, in the tropical forests of Cameroon, to understand their movement patterns and evaluate their effectiveness as seed dispersers. To estimate hornbill contribution to a particular tree species' seed shadow we combined data from movements, determined by radio-tracking, with data from seed passage trials. For 13 individuals tracked over 12 months, home range varied between 925 and 4,472 ha, a much larger area than reported for other African avian frugivores. Seed passage times ranged from 51 to 765 min, with C. atrata showing longer passage times than C. cylindricus, and larger seeds having longer gut retention times than smaller seeds. Combining these data, we estimated that seed shadows were extensive for the eight tree species examined, with approximately 80% of seeds moved more than 500 m from the parent plant. Maximum estimated dispersal distances for larger seeds were 6,919 and 3,558 m for C. atrata and C. cylindricus, respectively. The extent of hornbill seed shadows suggests that their influence in determining forest structure will likely increase as other larger mammalian dispersers are exterminated.