Climate change and tropical deforestation continue to be the major drivers of forest fragmentation and biodiversity loss. In Western Madagascar, urban demand for charcoal and timber are greatest threats to biodiversity maintenance. If forest removal reduces population sizes of tree species and habitat extent, forest fragmentation and degradation may affect the ability of tree species to persist in a long-term by disrupting key biological processes such as seed dispersal.
In this study, I tested the effects of forest configuration and dispersal traits on the realized dispersal of tree species in a tropical dry forest of Madagascar.
First, I used remote sensing tools to characterize vegetation cover and configuration based on 30m spatial resolution Landsat images. Forest configuration has been stratified by patch size, distance to neighbouring patch, distance to the edge and perimeter area ratio. Second, 114 forest plots of 20m by 20m has been established for each forest type and configuration. Trees had been identified to species and placed into approximate age classes. Dispersal mode (passive, wind, lemurs, birds, birds and lemurs) and dispersal traits (seed size and longevity) had been collected from the field and from specimen collection at the herbarium of Tsimbazaza. Third, I analyzed plot data based on the difference in prevalence between mature and immature of each species and used it as response variable. I used landscape configuration and dispersal mode and traits as covariates and ran species distribution models for each species using Maxent.
Vegetation configuration influenced the realized dispersal of tree species. Area sensitive species showed lower prevalence of immature individuals in small patches. This is ultimately because larger patches support higher densities of seed dispersers. However, patch compactness and isolation had no significant effect on the realized dispersal of any tree species. Only trees with lemur dispersed seeds showed lower prevalence but with no significant difference. Finally, seed traits, especially size and longevity, influenced realized dispersal of trees, potentially mediated by the suitability of the habitat for dispersers of large seeds such as lemurs, birds and fruit bats.
These findings suggest that process-based models for the long-term maintenance of tree species in relation to vegetation configuration and dispersal traits could foster conservation planning.