Inequalities in frugivory and seed dispersal: consequences of bird behaviour, neighbourhood density and landscape aggregation

CARLO, T. A.; MORALES, J. M.

Journal of ecology (Print)

British Ecological Society

Año: 2008 vol. 96 p. 609 - 618

0022-0477

Frugivores disperse the seeds of the majority of woody plant species worldwide. Thus, insights on how frugivores influence plant distribution and fitness are crucial for understanding plant population dynamics in a rapidly changing world. We used a spatially-explicit, stochastic, individual-based model to simulate frugivory and seed dispersal by birds. To assess bird, landscape, and neighbourhood effects on inequalities in distributions of frugivory rates and seed dispersal kernels we used two simulation models and an analytical binomial model. We also compared model predictions with spatially-explicit field data on frugivory rates and seed dispersal. In simulations, landscape aggregation and neighbourhood density strongly affected the distribution of frugivory rates, increasing or decreasing inequalities. Landscape aggregation increased inequalities in frugivory rates, especially when birds where in short supply.  Seed dispersal distances had an inverse relationship with neighbourhood density in simulations, despite that neighbourhood density increased frugivory rates under most model factor combinations. Thus, the simulation model predicts a prevalence of plant-plant facilitation in frugivory rates, although the strength of facilitation is conditional on the landscape aggregation pattern of plants and on bird abundance.  Similar to model output, field data shows an inverse relationship between dispersal distanes (i.e., bird movements) and fruiting neighbourhood density. Frugivory rates observed in the field and produced by simulations show large numbers of plants receiving few or cero frugivory.  Frugivory rate distributions were statistically indistinguishable between the simulation model and field data. But, distributions were strikingly different from the two alternative models that lacked spatial effects. This model shows that as fruiting plants become aggregated, inequalities in frugivory rates increase and seed dispersal decreases. Both of these processes could help create and maintain plant aggregation and affect genetic structuring since some plants disperse much more man and affect genetic structuring since few plants disperse many seeds and most plants disperse few. This model also predicts that small-scale neighbourhood effects can be controlled by large-scale processes such as frugivore abundance and landscape-level plant aggregation. Most importantly, both simulations and the field data shows feedback between plant patterning and bird foraging, which results in neighbourhood-specific dispersal patterns and rates of fruit removal.