Nevis skink, Oligosoma toka

This work consists of assigning a conservation threat status ranking for the Nevis skink. The Nevis skink is a newly described species from the group of skinks formerly considered to be the cryptic skink, Oligosoma inconspicuum. However, O. toka has very deep genetic divergence from O. inconspicuum (~15%), which is indicative of the conservation value of this species in genetic terms alone.

O. toka is known only from the Nevis Valley in Central Otago, thus it is considered to have one of the most restricted ranges of any mainland skink species. The range of O. toka is likely to be the entire Nevis Valley and adjacent areas. The Nevis area consists of broad plateaus and hills of a moderate altitude (> 600 m to 1500 m asl). The geological composition is complex, but usually consists of schist and greywacke rock formations.

This species can be abundant around man-made rock piles (old gold tailings) alongside the Nevis River and in natural rock formations, and occur in lower abundances on the river flats, around the foothills and up to the Nevis Crossing. O. toka also appear to have important associations with tussocks, rank grasses, Coprosma, Discaria, Muehlenbeckia, Melicytus and Rubus species. The abundance of this species at certain localities does not necessarily imply overall species security, given the apparent restricted range of the species. The Nationally Critical grand and Otago skinks (Oligosoma grande, O. otagense) occupy similar rocky habitat, and are good examples of highly endangered species with locally abundant populations, but a highly uncertain future.

Little is currently known about this species´ range, abundance and population viability. Resolution of this species´ threat category may require some urgency due to the investigations being made by Pioneer Generation Limited into possible hydro generation options for the area around and below the Nevis crossing.

We are planning the following methods for determining the range, abundance and habitat for this species:

• Assessing the range of this species by undertaking extensive rapid presence/abundance surveys outwards from the core population in the Nevis Valley in all accessible directions. These will involve a mixture of detection methods. Minimum convex polygons will be calculated from the areas of detected presence. This method can overcome most issues with land access, by bypassing non-permitted areas.
• Habitat classification and mapping in ArcGIS, using the presence/absence information above against habitat metadata, and assessing changes in habitat type and structure using aerial photography. This habitat mapping will determine (a) the estimated number of populations, range and abundance across the species´ range, and (b) the rate of habitat increases or losses over the past few decades (depending on the availability and resolution of the earliest aerial photographs). This will obtain estimates of current and past habitat sizes and percentage of habitat change over time.
• Detailed local abundance assessments, relating to habitat (using randomization of habitat types and altitudes, and sampling for skinks, using abundance indices). Overall abundance data will be extrapolated against the mapped habitat and range information.