Breeding Structure of Neotropical
Dry-Forest Tree Species
in Fragmented Landscapes
James L. Hamrick and Victoria J. Apsit
Landscapes that once featured continuously distributed, seasonal dry tropical forests are now characterized in much of Central America by a matrix of pastures and agricultural lands punctuated by occasional patches of remnant forest, secondary forests, and narrow riparian forest corridors. Fragmentation of these once continuous forests could adversely affect several aspects of the biology of tropical dry-forest tree species (Harris 1984; Bierregaard et al. 1992). In particular, changes in pollinator densities and behavior may disrupt or highly modify normal breeding patterns in remnant populations (e.g., Frankie et al. 1997). Such changes in breeding patterns can, in turn, modify levels and distribution of genetic diversity throughout local landscapes (Nason and Hamrick 1997).
Most tropical forest tree species are predominantly outcrossing (Bawa 1974; Opler and Bawa 1978; Loveless 1992; Nason and Hamrick 1997). Population genetic studies have demonstrated that, on average, tropical tree species have quite high levels of allozyme genetic diversity and that the majority (86.5%) of the genetic diversity is found within rather than among populations (Hamrick 1994). Indirect estimates of gene flow among populations (Nm = the number of migrants per generation) of tropical tree species indicate that historical levels of gene flow have been high enough to counteract the effects of genetic drift (i.e., Nm > 1.0). Contemporary measures of gene flow made in relatively undisturbed continuous forests indicate that pollen flow rates above 25 percent often occur over distances of several hundred meters (Hamrick and Murawski 1990; Chase et al. 1996; Stacy et al. 1996).
The question, then, is, Does forest fragmentation change pollinator behavior so that pollen movement is reduced to the extent that over several generations genetic diversity is lost via genetic drift and rates of inbreeding increase within the remaining fragments? In the following sections we first examine theoretical expectations of the effects of fragmentation on the