Visualising continuous intra-landscape isolation for brushtail possums using least-cost modelling

Abstract

Isolation is a fundamental geographical characteristic, affecting processes such as the spread of invasive species. It is therefore important to be able to produce maps that visualise how isolation changes across a landscape in order to enable better invasive species risk assessments to be undertaken. Many methods exist for quantifying isolation using a binary patch-matrix landscape representation. However, when species and landscape characteristics require the use of a continuous landscape representation, there are few, if any, methods currently available to quantify and visualise isolation. Methods using least-cost modelling to measure connectivity are presented that fill this knowledge gap. To begin, I demonstrate that the accumulated-cost of a least-cost path is the only reliable method to measure connectivity using least-cost modelling. Then, a new method is presented that uses graph theory to integrate least-cost modelling approaches for organism dispersal and transportation by humans. This integrated least-cost modelling approach is used to produce catchment areas that can measure and map isolation for a continuous landscape representation. Technical issues relating to processing times and landscape graph construction were identified as challenges to applying the catchment area methodology. These challenges were resolved by developing a new way to construct a landscape graph for continuous landscape representations that allows for a large reduction in computation times, and by developing a general way of determining the appropriate grain for landscape representations. These novel methods were applied to visualise, with uncertainty, the intra-landscape isolation for the brushtail possum (Trichosurus vulpecula), which is a notorious invasive species in New Zealand. Radio-telemetry and landscape genetics data were used to determine the appropriate scale of analysis, and to quantify the effect of landscape features on possum dispersal. The proposed methodologies for quantifying and visualising isolation are a useful approach for aiding invasive species management decisions about intra-regional spread when a continuous landscape representation is required. The key challenge for further development of this approach is the incorporation of landscape dynamics to assess potential future changes in levels of isolation.