Inequalities in fruit‐removal and seed dispersal: consequences of bird behaviour, neighbourhood density and landscape aggregation

Abstract

Summary Frugivores disperse the seeds of the majority of woody plant species world‐wide. Thus, insights on how frugivores influence the dispersal of plants and the variability of this process are crucial for understanding plant population dynamics in a rapidly changing world. We used a spatially explicit, stochastic, individual‐based model that simulates fruit‐removal and seed dispersal by birds to assess bird density, landscape and neighbourhood effects on the inequalities of within‐population fruit‐removal rates and seed dispersal. We also compared model predictions with spatially‐explicit field data. In our simulations, bird density had a strong effect on the distribution of fruit‐removal rates creating large inequalities among plants. Also, for equal bird densities, inequalities increased with the landscape level aggregation of plants. Fruit removal increased with increasing plant neighbourhood density although there was a tendency to decline at the highest densities. Neighbourhood density also changed average dispersal distances, but with shorter distances at higher densities. Plants with few neighbours not only had longer distance dispersal but also a larger variance in seed rain across distances than plants with ten or more neighbours. These relationships between neighbourhood density and fruit removal and dispersal distance were scale‐dependent with a peak in correlations at 150‐m radius. Similar to model predictions, field data shows an inverse relationship between dispersal distances (inferred from bird movements) and fruiting neighbourhood density. Also, fruit‐removal rates observed in the field show large numbers of plants receiving little or zero fruit‐removal. Fruit‐removal rate distributions were statistically indistinguishable between the simulation and field data. But, distributions were strikingly different from two alternative models that lacked spatial effects. Synthesis. Our model and field data show that as fruiting plants become aggregated, inequalities in fruit‐removal rates increase and seed dispersal distance decreases. Both of these processes could help create and maintain plant aggregation and affect genetic structuring. The model also predicts that small‐scale neighbourhood effects can be controlled by large‐scale processes such as overall frugivore abundance and landscape‐level plant aggregation. Most importantly, both simulations and field data shows an interaction between plant spatial pattern and bird foraging, which results in neighbourhood‐specific dispersal and rates of fruit removal.