Contrasting effects of host tree isolation on population connectedness in two tropical epiphytic bromeliads

Premise of the study Conversion of primary forests to pastures is a major
cause of habitat fragmentation in the tropics. Fragmentation is expected
to impede gene flow for many plant species that are restricted to
remaining forest fragments. Epiphytes may be especially vulnerable to this
effect of forest fragmentation because they depend on host trees. However,
trees that remain in pastures may enhance connectivity across the
landscape for epiphyte species that can thrive on such trees. To
investigate this possibility, we studied the genetic structures of two
such species on isolated pasture trees and surrounding forest, in relation
to their local abundances in different habitat types and aspects of their
reproductive biology including pollen and seed dispersal agents, and
looked for evidence of increased or diminished gene flow. Methods We used
microsatellite markers to assess geographic patterns of genetic diversity
and differentiation in two epiphytic bromeliads, Catopsis nitida and
Werauhia tonduziana, in the Monteverde region of Costa Rica. Key results
About 85% of the FST value for Catopsis nitida was found among pastures
within regions, while for Weruahia tonduziana, about 80% of the FST value
was contributed by differences between regions, indicating much more gene
flow within regions, relative to C. nitida. Conclusions Although there was
substantial genetic differentiation among epiphyte populations, those on
isolated pasture trees were not substantially less diverse than those in
adjacent forests, suggesting that pasture trees may serve as
"stepping stones" that help these species maintain their genetic
connectedness and diversity at larger geographic scales.