Matrix matters: differences of grand skink metapopulation parameters in native tussock grasslands and exotic pasture grasslands

Abstract

Modelling metapopulation dynamics is a potentially very powerful tool for conservation biologists. In recent years, scientists have broadened the range of variables incorporated into metapopulation modelling from using almost exclusively habitat patch size and isolation, to the inclusion of attributes of the matrix and habitat patch quality. We investigated the influence of habitat patch and matrix characteristics on the metapopulation parameters of a highly endangered lizard species, the New Zealand endemic grand skink (Oligosoma grande) taking into account incomplete detectability. The predictive ability of the developed zxmetapopulation model was assessed through cross-validation of the data and with an independent data-set. Grand skinks occur on scattered rock-outcrops surrounded by indigenous tussock (bunch) and pasture grasslands therefore implying a metapopulation structure. We found that the type of matrix surrounding the habitat patch was equally as important as the size of habitat patch for estimating occupancy, colonisation and extinction probabilities. Additionally, the type of matrix was more important than the physical distance between habitat patches for colonisation probabilities. Detection probability differed between habitat patches in the two matrix types and between habitat patches with different attributes such as habitat patch composition and abundance of vegetation on the outcrop. The developed metapopulation models can now be used for management decisions on area protection, monitoring, and the selection of translocation sites for the grand skink. Our study showed that it is important to incorporate not only habitat patch size and distance between habitat patches, but also those matrix type and habitat patch attributes which are vital in the ecology of the target species.

Figure 1. Location of study sites with all monitored habitat patches (black marks).

The 2006-08 data was collected in study sites A and B comprising of exotic pasture grasslands and indigenous tussock grasslands, respectively. Study site C (tussock grasslands) and D (pasture grasslands) were used for collection of evaluation data in 2010.

Figure 2. Influence of habitat patch characteristics on the mean estimated occupancy, colonisation, extinction and detection probability.

Displayed are habitat patch characteristics which were included in the final model for each metapopulation parameter. Occupancy and detection probabilities were estimated for 2006 (black circles), 2007 (dark gray circles) and 2008 (light gray circles). Colonisation and extinction probabilities were estimated from occupancy differences between 2006 and 2007 (black triangles), and 2007 and 2008 (gray triangles).