Determining the spatial associations of the Fiji Ground Frog and the Cane Toad on Viwa Island, Fiji using SADIE: Implications for conservation and management
The negative impacts that invasive cane toads (CT) Bufo marinus have had on native fauna in other parts of the world have raised concerns on the nature of its relationship with the endangered Fiji ground frog (FGF), Platymantis vitianus. The aim of this study was to measure the spatial associations between the two amphibian species and with identified habitat types on Viwa Island (a proposed site in the Fiji Islands for the eradication of CTs) using the Spatial Analysis by Distance Indices (SADIE) program. The SADIE program was initially developed for the spatial analysis of count data in insects and plants permitting visual representation and comparisons of species distributions and densities. We collected count data for both FGFs and CTs at spatially referenced locations on Viwa and used SADIE to produce maps of spatial distribution and association for the two species. Despite the abundance of CTs being much higher than FGFs both species were widely distributed over Viwa Island. Although their distributions were positively associated with each other and with the forested landscape, the two species had significant clustering of high counts in exclusive locations on Viwa. The results of this study highlight the need for a more detailed study on the interactions of the FGFs and CTs at the microhabitat level. The SADIE maps suggest that the spatial distribution of the two species is strongly influenced by human and/or agricultural activities on Viwa Island. In addition, SADIE has provided an accurate visual depiction of target areas for CT removal and FGF conservation. This is the first time SADIE has been used for herpetofauna data and it has proved successful in demonstrating spatial associations between FGFs and CTs and the landscape on Viwa Island making it a potentially useful tool in future terrestrial vertebrate ecology studies.