Effects of habitat fragmentation on Dombeya acutangula (Sterculiaceae), a native tree on La Reunion (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 cr itical level within forest formations at low altitude. This study was perfo rmed to examine the consequences of this fragmentation on the threatened sp ecies Dombeya acutangula ssp. acutangula. Seven sites of varying population sizes and levels of disturbance were studied. Richness and abundance of al l woody species were quantified and reproductive traits of D. acutangula in dividuals were measured. The smaller proportion of juveniles observed in sm all populations of D. acutangula might result from fragmentation of natural populations and/or invasion by introduced species. This study moreover sho ws that D. acutangula exhibited a highly significant decrease in seed produ ction per fruit with decreasing population size, and that pollination was m ore limited in small populations. We found no evidence of differences among populations for most other traits, particularly on progenies grown under g reenhouse 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 popul ations. 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 h and-pollination and introduction of individuals with appropriate alleles. ( C) 1999 Elsevier Science Ltd. All rights reserved.