Anuran distribution over an elevation gradient on Bioko Island, Equatorial Guinea

Amphibians, especially anurans, represent an important opportunity for ecosystem monitoring, specifically in tropical environments. This pilot study explored potential monitoring methods and generated initial potential baseline results for the distribution of common anurans (frogs and toads) over an elevation gradient in a tropical montane ecosystem on Bioko Island, Equatorial Guinea. In order to collect data to determine distribution, PVC refuges and pitfall traps were tested for use in this area. PVC refuges were found to be an ineffective technique for amphibian sampling in this dense tropical ecosystem. Pitfall traps were found to be effective after modifications, such as increasing leaf litter to encourage approach and adding baby oil gel to discourage escape. PVC refuges were ineffective and pitfall trapping rates were not high enough to determine distribution, therefore a novel approach was developed to determine amphibian distribution using amphibian census. This was possible due to the high amphibian density in this area. Data was collected daily via visual detection along trails that served as transects spanning an elevation gradient of over 900 meters. The data from this study shows that different species have varying altitudinal preferences. Frogs in the genus Arthroleptis were shown to decrease in density with increased elevation. Hyperolius frogs and toads in the genus Bufo did not exhibit a clear density trend based on elevation. Data obtained from this study is preliminary but useful in laying framework for a long term monitoring project on the island. Long term monitoring can expand the knowledge of Bioko Island species composition and range. Monitoring these populations over time can aid in the understanding of both natural and anthropogenic effects on these populations and the ecosystem.