Pollen movement and the genetic prospects for fragmented forests in tropical and temperate systems

Much concern has been raised over dire genetic prospects for fragmented forests. Population genetic theory predicts that conservation and restoration efforts will be thwarted by loss of genetic variability, genetic isolation, and increased inbreeding due to lack of pollen and seed dispersal among fragments. However, the reality of genetic isolation will be determined by many factors, including species abundances, mating systems, and agents of pollen and seed dispersal. These additional ecological complexities may explain why predicted genetic declines have often not been supported by empirical data. In diverse tropical forests, most tree species occur at low densities, and forest fragments will contain very few individuals of each species, thus predicting that tropical forests will be more susceptible to genetic degradation. On the other hand, tropical forest trees and their pollinators have evolved in the context of low densities and longer distances between conspecifics, so their pollination systems may be better adapted for long-distance pollination than are temperate forest trees. Another oversimplification is the assumption that fragment boundaries delineate populations. Recent studies of effective pollination have revealed that long distance pollination may be quite common in both temperate and tropical forests, and dispersal curves may have fat tails that extend well beyond the fragment edge. Diverse ecological processes will complicate and often supersede simple genetic predictions in both tropical and temperate systems.