Using spatially explicit call data of Anhydrophryne ngongoniensis to guide conservation actions

Abstract

It’s been barely 25 years since the Mistbelt Chirping Frog (Anhydrophryne ngongoniensis) was discovered. This secretive amphibian occurs only in the so-called mistbelt grasslands and montane forest patches of south-central KwaZulu-Natal, South Africa and is restricted to an area of occupancy of just 12 square kilometers. This species’ habitat is severely fragmented due to afforestation and agriculture and only two of the remaining populations are formally protected. The species occurs mostly on fragmented grassland patches on forestry land, and any conservation strategies should include the management practices for the landowners. Updated density estimates and insight into habitat utilization are needed to proceed with conservation strategy for the species. Like many other frogs, this species is cryptic in its behaviour, making mark-recapture surveys prohibitively challenging. Audio transects have been used previously, but are dependent on surveyor’s’ experience, hindering standardization. Using automated recorders, in a spatially explicit array with GPS synchronization, one can confidently estimate the density of calling males and reveal the estimated locations of calling males, thus providing insight into their occupancy. We surveyed nine historic sites and detected the species at five of the sites in either isolated grassland patches or indigenous Afromontane forest. We successfully employed the spatially explicit catch recapture (SECR) method at three of the sites using Wildlife Acoustics™ Song Meters with extended microphones in an array. Audio data was processed with Pamguard™ open-source software and analysis done in R using the ascr package. Density estimates of calling males were much higher for the sites than estimated with previous methods. The results also provided insight into calling behaviour and the distribution of the species, which appears to be clumped and localised within a breeding site. The data obtained will be used to update population estimates and guide conservation measures, especially pertaining to land management practices. Recommendations to land owners include the stringent management of road verges and Afromontane forest patches. Even though density estimates were higher using SECR compared to transects, we recommend that the species retain its Endangered listing since the occupation within a breeding site is very limited.