Feature

Skink vs Skink

Nick Spencer and Murray Efford

Two species of lizard are helping Landcare Research staff in their enquiries into the factors that maintain and threaten our biodiversity. Both species of skink are in the genus Oligosoma -- possibly our most taxonomically diverse group of terrestrial vertebrates. These skinks are endemic to New Zealand, meaning that they live nowhere else in the world. Unless we conserve them locally they will be lost globally.

As with most of New Zealand's endemic fauna, many species of Oligosoma now occupy only a small fraction of their prehistoric range. Range shrinkage has been driven by a combination of habitat destruction and introduced predators like mice, rats, cats and mustelids. However, the response of skink species to these forces has not been uniform. At one extreme some species have become extinct on the mainland and persist only on predator-free offshore islands. At the other extreme a few species remain common over large areas on the mainland. Why should this be so?

The Landcare Research team has sought answers through an intensive study of two species: the speckled skink Oligosoma infrapunctatum and the green spotted skink Oligosoma lineocellatum. These skinks look much the same, they are the same average size and in some places they can both be found living together under the same rock. However, the speckled skink is known from only a handful of widely-scattered localities on the mainland, from Stephens Island and a few other islands, while the green spotted skink is widespread from Hawkes Bay south possibly as far as Otago, including many islands.

What distinguishes the two species ecologically, and is it possible that one has remained common partly at the other's expense? Perhaps the speckled skink has more narrow habitat requirements or is locally out-competed by the green spotted skink.

A study was begun in 1995 in the Nelson Lakes district. Both species are common on a sunny terrace face in the Upper Buller Valley. However, the local abundance of each species varies in an intricate pattern across the hillside. To map the pattern, skink traps were sunk into the ground on a five metre grid. Traps were the ultimate in low tech: a commercial baked bean tin with its rim flush with the ground. Sugary bait (a cube of tinned pear) tempts a skink into the trap, and they are unable to escape up the vertical wall of the tin. Researchers identify and measure each skink.

Over time, the pattern of capture resolves into a map of the population density. Traps that often catch one or the other species tend to be clustered together. By mapping the habitat on the same five metre grid, the team hoped to find a key to the local peaks and troughs of abundance in each species. Statistical models have been used to predict the occurrence of each species from the characteristics of the habitat at each trap site. The speckled skink is associated with taller vegetation while the green spotted skink seems to have a preference for rock piles.

Microscopical analysis of scats (faeces) has shown that the two species tend to eat different foods. The speckled skink take more fruit and consume prey that are more active during the day compared with the green spotted skink. Prey species like grasshoppers, planthoppers and tiger beetles were common in the speckled skink diet, while millipedes, earthworms and manuka beetles were more common in the green spotted skinks' diet.

Neither species absolutely excludes the other from any area of habitat, but researchers still suspect that there may be behavioural interactions between them. To study these interactions skinks of one or other species have been transplanted between a number of plots. The effects of these experimental manipulations, if there are any, should become apparent once the area is resurveyed when lizard activity resumes again this spring.

Large scale forest clearance, on-going farming practices (eg, annual burning) and introduced predators have had major disruptive effects on New Zealand's fauna, and lizards are no exception. Understanding more about why species respond differently to changes will extend our knowledge of the processes of diversity and enable us to fine tune our conservation approach.

Murray Efford is a researcher with Landcare Research in Dunedin.
Nick Spencer is a researcher with Landcare Research in Dunedin.