Colonization of shade coffee agroecosystems: Genetic insights into historical seed dispersal processes

Background/Question/Methods

Seed dispersal is essential for the re-colonization of disturbed landscapes and the regeneration of tropical forests. Molecular tools can provide useful insights into colonization processes within fragmented tropical forests. Previous studies have examined seed dispersal processes for only large overstory tropical trees, and specifically those located in forest fragments and in adjacent pasture. Surprisingly, these studies have found that seed dispersal can be extensive and that isolated trees in non-forested habitats often contribute heavily to the paternity of seedlings within the forest. However, colonization and seed dispersal patterns may be completely different in habitats with greater levels of existing vegetation, especially for understory tree species, whose seeds are often dispersed by small birds and bats. Using microsatellite markers, we examined the spatial genetic structure and historical colonization processes of the understory tropical tree Miconia affinis (Melastomataceae) within a forest fragment and two adjacent coffee plantations which differ in vegetation structure (overstory tree density and diversity). We predicted that M. afffinis would exhibit greater clustering of related individuals in the newly colonized coffee farms due to limited seed dispersal during initial colonization events. We also predicted that habitat vegetation structure within the farms would influence historical patterns of colonization.

Results/Conclusions

We found that across the entire landscape, genetic relatedness is significantly higher than normal at inter-individual distances of less than 100m (r_ij =0.059), indicative of strong limitations in gene flow across the landscape. Within the intact forest fragment, individuals of M. affinis exhibit normal levels of relatedness across all geographic distances (r_ij =0.033), likely the result of extensive historical seed dispersal and overlapping seed shadows of ancestral trees. Within the colonized coffee farms, individuals were significantly more related to their neighbors (within 200m) than to the average individual in each habitat, indicating that the seeds of colonizing trees are being dispersed within the farms, but that these dispersal distances were limited. Patterns of relatedness across distance, however, were distinct for each coffee farm, suggesting that seed dispersal processes are markedly different in agroecosystems with differing vegetation management styles. These results support previous observational findings that the movement of frugivore seed dispersers depends heavily on the vegetative structure within a habitat.