Distribution data
Global distribution data for GAS was downloaded from GBIF (ww.gbif.org). Only records which were labelled as “human observation” or “occurrence” were retained. These records were then filtered to remove any coordinates with high levels of uncertainty. Data on the presence of GAS collected during this current study were then added to the cleaned GBIF records. This represented our overall dataset, which consisted of 3219 records. This dataset was filtered so that only one presence was recorded in each climatic grid-cell, resulting in a working dataset of 730 distributional records.
Statistical Species Distribution Models (SDMs) require information on where a species is absent. Often there is insufficient verified data, and thus “pseudo-absences” must be used. To allow us to use, and test the predictive accuracy, of statistical SDM methods, ten sets of pseudo-absences were sampled. Each of the sets of pseudo-absences were restricted so that they were always within 500km of a verified GAS location, but were outside of a grid cell occupied by a presence location. An upper distance for the pseudoabsences was specified as this has been shown to prevent models from contrasting completely different climate conditions, e.g. temperate vs. tropical (VanDerWal et al., 2009). The prevalence of the pseudo-absences was always equal to the number of presence points (i.e. 730)