Reproductive and genetic consequences of forest fragmentation: Two case studies of neotropical canopy trees

Abstract

Large areas of continuous tropical forests are rapidly becoming fragmented as a result of human activities, prompting a growing need for information on the reproductive and genetic responses of tree species to disturbance. Here we review the literature on pollen dispersal in continuous forest stands in order to identity factors expected to enhance or to inhibit interfragment pollen flow. We then describe two empirical studies of the effects of fragmentation on pollen flow in Neotropical populations of Spondias mombin (Anacardiaceae) and five monoecious species of Ficus (Moraceae). In S. mombin, small fragment populations suffered significant reductions in germination rate and fruit production relative to large fragment and continuous forest populations. Nevertheless, 90-100% of the progeny produced in the small fragments were the product of pollen gene flow originating from forest stands located over 80-1000 m away. In contrast to S. mombin, which is relatively abundant and pollinated by a variety of small insects, the strangler fig species studied were relatively rare and pollinated by species-specific wasps that are both small and short-lived. Paternity analyses revealed these highly coevolved pollinators to be effective agents of pollen movement over very long distances, routinely dispersing 6-14 km to receptive host trees. Because of such extensive pollen flow, breeding populations of figs were estimated to consist of hundreds of intermating individuals distributed over 108-632 km2, an area substantially larger than has been documented for any other plant species. The observed patterns of dispersal in S. mombin and Ficus demonstrate the potential for ecologically and evolutionarily significant pollinator and pollen movement among populations occurring in both disturbed and continuous forest environments. Moreover, given the integral relationship of figs, as keystone plant resources, to other plant and animal species, the extensive spatial scale of fig breeding units demonstrates how the conservation of biodiversity in spatially isolated reserves may be dependent on the preservation of forest elements in a surrounding fragmented landscape.