THE RAREST OF DARWINS FINCHES

The Mangrove Finch (Cactospiza heliobates) is unique among Darwin's fi nches in its specialized habits and naturally fragmented distribution on individual islands It has been reported to occupy six patches of ma ngroves separated to varying degrees on two of the Galapagos islands I sabela and Fernandina. We present the results of a field study in thre e of the patches. The species appears to have become extinct on Fernan dina and to be restricted now to a maximum of four habitat islands on Isabela. in one of them, Playa Tortuga Negra, we estimate the populati on of breeding adults to be 10-20 pairs. At another, west of Villamil, the population size is probably now small due to the cutting of mangr oves. The other two populations, close to each other east of Punta Mor eno, probably hold the largest demes Their status is unknown. We estim ate that the genetically effective size of the combined populations is 30-380 individuals. Because the populations are so small and restrict ed, they are vulnerable to any disturbance of the environment. This ha s probably been so throughout their history because they inhabit a fra gmented habitat in a tectonically active region New anthropogenic thre ats this century are an introduced wasp (Polistes versicolor), which m ay be a predator on a major food item (insect larvae) of the finches, and destruction of the habitat. Protection of the occupied habitat aga inst human exploitation is required for the preservation of the specie s because no alternative habitat is known.