Seed dispersal by pulp consumers, not 'legitimate' seed disperses, increases Guettarda viburnoides population growth

摘要

We examined the effect of seed dispersal by Purplish Jays (Cyanocorax cyanomelas; pulp consumers) and the Chestnut-eared Ara?ari (Pteroglossus castanotis; "legitimate" seed dispersere) on population growth of the small tree Guettarda vibumoides (Rubiaceae) in northeastern Bolivian savannas. Because each bird species differs with respect to feeding and post-feeding behavior, we hypothesized that seed dispersal by each species will contribute differently to the rate of increase of G. vibumoides, but that seed dispersal, by either species will increase population growth when compared to a scenario with no seed dispersal. To examine the effects of individual dispersere on the future population size of G. vibumoides, we projected population growth rate using demographic models for G. vibumoides that explicitly incorporate data on quantitative and qualitative aspects of seed dispersal by each frugivore species. Our model suggests that seed dispersal by C. cyanomelas leads to positive population growth of G. vibumoides, whereas seed dispersal by P. castanotis has a detrimental effect on the population growth of this species. To our knowledge, this is the first study to report negative effects of a "legitimate" seed disperser on the population dynamics of the plant it consumes. Our results stress the importance of incorporating frugivore effects into population projection matrices, to allow a comprehensive analysis of the effectiveness of different dispersere for plant population dynamics.