Spatial distribution and habitat preferences of co-occurring vertebrate species: Case study of an endangered frog and an introduced toad in Fiji.

Abstract

Wildlife management, particularly the conservation of threatened species, often involves habitat management and an understanding of species preferences. Much ecological data used to establish rare and endangered species distributions and/or habitat associations exists in the form of point counts which often violates the assumptions of commonly used statistical techniques. In this study, the spatial distribution and habitat preferences of an endangered, endemic frog (Platymantis vitianus — Fiji ground frog) and an introduced toad (Rhinella marina — cane toad) were observed and mapped on a 60 ha island using a grid of 232 sampling points with 50 × 20 m spacing. The Spatial Analysis by Distance Indices (SADIE) analytical tool demonstrated that despite the wide range of habitats present on Viwa Island, both ground frogs and cane toads displayed clear and defined macrohabitat preferences. Whilst both species showed a strong preference for Inocarpus fagifer forests, there was little overlap in other habitat types with the ground frogs preferring more densely vegetated forest habitats and cane toads preferring more open habitat types close to both permanent and temporary water sources. Within the I. fagifer forests, there was no evidence of overlap in the distributions of the two species. The spatial pattern evident indicates that cane toad and ground frog populations co-exist, probably due to similar habitat preferences. However, a more detailed study on the interactions between the two species within their natural environment is needed to determine the nature and magnitude of the impact of the cane toad on the ground frog.