Effects of habitat fragmentation on Dombeya acutangula (Sterculiaceae), a native tree on La Réunion (Indian Ocean)

Habitat fragmentation on the island of of La Reunion (Mascarene Archipelago, Indian Ocean) has increased over three centuries and has now reached a critical level within forest formations at low altitude. This study was performed to examine the consequences of this fragmentation on the threatened species Dombeya acutangula ssp. acutangula. Seven sites of varying population sizes and levels of disturbance were studied. Richness and abundance of all woody species were quantified and reproductive traits of D. acutangula individuals were measured. The smaller proportion of juveniles observed in small populations of D. acutangula might result from fragmentation of natural populations and/or invasion by introduced species. This study moreover shows that D. acutangula exhibited a highly significant decrease in seed production per fruit with decreasing population size, and that pollination was more limited in small populations. We found no evidence of differences among populations for most other traits, particularly on progenies grown under greenhouse conditions. Results are interpreted as a consequence of different, but not mutually exclusive processes, particularly competition from other species and a decrease in availability of compatible pollen in small populations. We suggest several management measures to slow the decline of this species in its natural habitat. These include control measures for invading species and reinforcement of particularly threatened populations through hand-pollination and introduction of individuals with appropriate alleles.