Dynamics of grand skink (Oligosoma grande, Gray 1945) metapopulations in indigenous tussock grasslands and exotic pasture grasslands in southern New Zealand

Abstract

A major threat to biodiversity worldwide is human land-use change which increases habitat fragmentation and reduces landscape connectivity leading to species extinctions. For conservation biologists it is essential to understand the underlying factors that influence the population dynamics of species occurring in fragmented and isolated habitat. In this thesis, I investigate the impact of land-use change on the metapopulation dynamics of the endangered New Zealand grand skink (Oligosoma grande)which inhabits schist-rock outcrops scattered through indigenous tussock grasslands and exotic pasture grasslands in central South Island, New Zealand. In particular, I explore the influence of habitat patch characteristics and the type of surrounding grassland type (matrix) on the metapopulation dynamics of the grand skinks by using a spatially explicit metapopulation simulation model.

To develop a spatially explicit simulation model distinct habitat patches have to be defined. Therefore, I attached VHF-radio transmitters to 57 grand skinks to estimate their habitat use and movements. Individual grand skinks inhabited predominantly a single rock-outcrop. However, home ranges were smaller when the matrix surrounding the outcrop was pasture grasslands, and the rare between-outcrop movements were mostly recorded when the matrix was dominated by tussock grasslands. Therefore, individual rock-outcrops were considered habitat patches in the metapopulation models. I also investigated if land-use change impacted on the body condition of grand skinks using a long-term data-set. Especially in autumn, grand skinks on outcrops in pasture grasslands had a better body condition compared to grand skink inhabiting outcrops in tussock grasslands. However, the probability of being pregnant was lower for grand skinks inhabiting rock-outcrops in the pasture matrix than for grand skinks on outcrops in the tussock matrix. Lower pregnancy probabilities could directly affect grand skink population dynamics. At the population-level the analysis of a 3-year data-set of grand skink occupancy of rock-outcrops in both matrix grassland types using methods to account for incomplete detectability of the grand skinks, revealed that habitat patch quality (outcrop structure and vegetation cover on the outcrop), habitat patch size and matrix grassland type strongly influenced colonisation and extinction probabilities. Grand skink populations on rock-outcrops surrounded by a matrix of pasture grasslands had higher probability of extinction and, once extinct, a lower probability of being re-colonised. Simulating the occupancy of outcrops on a landscape scale showed that grand skink populations in pasture grasslands are likely to go extinct while tussock grassland populations were stable or even increasing. Using the simulation model to investigate which outcrop in an un-inhabited landscape provides the most appropriate translocation site, revealed that rock-outcrop density was the most important factor for grand skinks, not outcrop quality. This study showed that indirect and direct effects from the matrix surrounding the habitat patches can have strong influence on metapopulation dynamics. For successful conservation management, these effects need to be taken into consideration in the design of conservation strategies. I demonstrated how simulation models can be used to decide on appropriate translocation sites for grand skinks. Using species-specific information, and integrating habitat-patch and landscape attributes, metapopulation simulations can be useful tools in conservation research.