Bird Survival in an Isolated Javan Woodland: Island or Mirror?

Differential extinction of forest species following forest fragmentation raises the questions of which populations are most prone to disappear, and why. Hence we studied an 86-hectare woodland in west Java, the Bogor Botanical Garden (BBG), that became isolated when surrounding woodland was destroyed 50 years ago. Out of 62 bird species breeding in the BBG during 1932-1952, 20 had disappeared by 1980-1985, four were close to extinction, and five more had declined noticeably. The two main variables that identify extinction-prone populations in the BBG are 1) small initial population size in the BBG and 2) rareness or absence in the surrounding countryside. Although the BBG retained wooded habitat, it is evidently too small to retain self-sustaining populations of many woodland bird species. Small populations at high risk of extinction for stochastic reasons are doomed to disappear permanently unless subsidized by recolonization from the surroundings. Thus, a too-small reserve cannot function as a distributional island but comes to mirror the species composition of its surroundings. More such case studies documenting species losses from small habitat fragments are required to demonstrate to nonbiologists the need for reserves large enough to support self-sustaining populations.