Abundance of the Fijian crested iguana ( Brachylophus vitiensis) on two islands

Abstract

The Fijian crested iguana ( Brachylophus vitiensis) has a severely restricted distribution and is secure on only one island, the Iguana Sanctuary island of Yadua Taba. Line transect surveys of sleeping iguanas at night on Yadua Taba were used to assess the feasibility of this technique for population surveys of other islands, and for monitoring iguana numbers during the recovery process on islands selected for habitat restoration. Our surveys on Yadua Taba revealed population densities of 196 iguanas per hectare in forest habitat. On the smaller island of Monuriki, where crested iguana sightings were frequent in the early 1980s, the population density estimate was nine iguanas per hectare for the largest stand of forest. The total population estimate for Yadua Taba is > 6000 iguanas, while for Monuriki it is <100. Vegetation surveys of these islands suggest that the abundance of the herbivorous crested iguana reflects the relative abundance of food trees present, with 63% of forest trees on Yadua Taba being edible species, compared to only 2% on Monuriki. Although both islands are uninhabited and have no introduced predators, Monuriki has been subjected to over three decades of intensive goat grazing and regular dry season burning. The combination of goats and fire may have reduced the survivorship and recruitment of iguana food tree species, several of which are known to be fire sensitive. Selective browsing by goats on the seedlings of palatable tree species on Monuriki has resulted in the competitive release of unpalatable species, inedible to both goats and crested iguanas.