Reproductive success, genetic diversity, and gene flow in fragmented populations of two understory tree species in Costa Rica

Habitat loss and fragmentation is causing widespread declines in biodiversity by negatively affecting survival, reproduction and dispersal in many species. I investigated the reproductive and genetic consequences of forest fragmentation on two species of understory trees, Symphonia globulifera and Matisia ochrocalyx, in lowland rainforest of northeastern Costa Rica. I estimated fruit set, seedling density, genetic variation, and gene flow for several populations by combining field studies with analyses of genetic markers (AFLP). I examined genetic change by fingerprinting individuals in three cohorts: adults, seedlings and seeds, reflecting successively more recent population genetic processes. Populations of trees in small forest fragments did not suffer a consistent reduction in fruit set, although individual flowers of M. ochrocalyx were less likely to set fruit in forest fragments than in intact forest. Densities of small seedlings were elevated in forest fragments, but only near parent trees, suggesting that fragments lack mammalian dispersers. In contrast, large seedlings were scarcer in fragments than in continuous forest, indicating reduced survival. Fragmentation had a negative impact on genetic variation only in the seedling cohort; there was no affect on adults and some positive effects on variation in seeds. S. globulifera populations exhibited increasing differentiation between fragment and intact forest populations through time. Substantial pollen flow into fragmented populations, estimated directly by paternity analysis of seeds, suggests that the mobility of hummingbirds and bats that pollinate S. globulifera and M. ochrocalyx, respectively, is not ill-affected by fragmentation, at least up to the maximum distance of 360 m separating study populations from other forest-like vegetation. For S. globulifera, pollen flow from outside of study populations was positively correlated with distance to continuous forest. This trend was absent in M. ochrocalyx, for which external pollen flow decreased as tree density increased. Observed patterns of pollen movement are consistent with each species' population structure and pollination ecology. My results confirm that pollen flow can be maintained in fragmented landscapes in Costa Rica, but that fragmentation has negative impacts on the seedling stage through reduced dispersal, lower survival and strong selection.