Defining Landscape Resistance Values in Least-Cost Connectivity Models for the Invasive Grey Squirrel: A Comparison of Approaches Using Expert-Opinion and Habitat Suitability Modelling Claire D. Stevenson-Holt 1 *, Kevin Watts 2 , Chloe C. Bellamy 3 , Owen T. Nevin 4 , Andrew D. Ramsey 5 1 Centre for Wildlife Conservation, University of Cumbria, Ambleside, Cumbria, United Kingdom, 2 Centre for Ecosystems, Society and Biosecurity, Forest Research, Farnham, Surrey, United Kingdom, 3 Centre for Ecosystems, Society and Biosecurity, Forest Research, Roslin, Midlothian, United Kingdom, 4 School of Medical and Applied Sciences, Central Queensland University, Gladstone, Queensland, Australia, 5 School of Biological and Forensic Sciences, University of Derby, Derby, Derbyshire, United Kingdom Abstract Least-cost models are widely used to study the functional connectivity of habitat within a varied landscape matrix. A critical step in the process is identifying resistance values for each land cover based upon the facilitating or impeding impact on species movement. Ideally resistance values would be parameterised with empirical data, but due to a shortage of such information, expert-opinion is often used. However, the use of expert-opinion is seen as subjective, human-centric and unreliable. This study derived resistance values from grey squirrel habitat suitability models (HSM) in order to compare the utility and validity of this approach with more traditional, expert-led methods. Models were built and tested with MaxEnt, using squirrel presence records and a categorical land cover map for Cumbria, UK. Predictions on the likelihood of squirrel occurrence within each land cover type were inverted, providing resistance values which were used to parameterise a least- cost model. The resulting habitat networks were measured and compared to those derived from a least-cost model built with previously collated information from experts. The expert-derived and HSM-inferred least-cost networks differ in precision. The HSM-informed networks were smaller and more fragmented because of the higher resistance values attributed to most habitats. These results are discussed in relation to the applicability of both approaches for conservation and management objectives, providing guidance to researchers and practitioners attempting to apply and interpret a least- cost approach to mapping ecological networks. Citation: Stevenson-Holt CD, Watts K, Bellamy CC, Nevin OT, Ramsey AD (2014) Defining Landscape Resistance Values in Least-Cost Connectivity Models for the Invasive Grey Squirrel: A Comparison of Approaches Using Expert-Opinion and Habitat Suitability Modelling. PLoS ONE 9(11): e112119. doi:10.1371/journal.pone. 0112119 Editor: Benjamin Lee Allen, University of Queensland, Australia Received July 11, 2014; Accepted October 13, 2014; Published November 7, 2014 Copyright: ß 2014 Stevenson-Holt et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. Data on grey squirrel sightings can be obtained from Red Squirrels Northern England http://rsne.org.uk/sightings and Cumbria Biodiversity Data Centre http://www.cbdc.org.uk/. Funding: This project was funded by the Forestry Commission GB and the National School of Forestry at the University of Cumbria. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have the following competing interest: This work was funded by the Forestry Commission GB and National School of Forestry at the University of Cumbria. This does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and materials. * Email: claire.stevenson@cumbria.ac.uk Introduction Effective biodiversity conservation within fragmented land- scapes often requires the modelling of connectivity to define the extent of the problem, target conservation activities and to evaluate the impacts of landscape change [1]. Connectivity is defined as the degree to which the landscape facilitates or impedes species movement among resource patches [2]. A landscape consists of a complex, often dynamic, heterogeneous mixture of habitats and land uses which may impact on important ecological processes, such as species movement, habitat selection and survival, and influence behavioural and physiological responses [2–5]. The study of the impacts of the matrix on species movement, known as functional connectivity [6], is now the subject of much research within modified and fragmented landscapes [7]. Assessing functional connectivity is commonly used to aid conservation strategies by identifying potential movement pathways across fragmented landscapes for species of conservation concern [8–10]. It has also been used to help predict the potential dispersal and movement of invasive species to aid species management by identifying areas to target resources [11,12]. Geographical Information System (GIS), raster-based least-cost analysis techniques are often used to assess functional connectivity by modelling the impact of permeability of the surrounding landscape matrix on species movement [10]. It has been used in conservation [8–10] and invasive species management contexts [11,12]. For example, the population expansion of the grey squirrel (Sciurus carolinensis) in Britain, following its first introduction in 1876 [13], has had negative effects upon the forestry industry and native biodiversity [14–16]. In particular, it has occurred simultaneously with the decline and replacement of PLOS ONE | www.plosone.org 1 November 2014 | Volume 9 | Issue 11 | e112119