Abundance–suitability relationships for invasive species: Epiphyas postvittana as a case study

Prediction of the geographic distribution of an exotic species in a new environment is an important element of risk assessment as it provides an assessment of the spatial scale of potential impacts and the range of ecosystems affected by the establishment of an exotic species. For a more complete risk assessment, however, information on how abundant the species is likely to become at specific locations within the predicted geographic distribution would allow a more detailed analysis of potential environmental and economic impacts. In this study an improved species distribution model for Epiphyas postvittana , an Australian leafroller, is developed and used to test for an abundance–suitability relationship based on survey data on its relative abundance in coastal California where it has become established as an exotic species. To better assess the current status of this exotic leafroller as a quarantine pest in California, the potential geographic distribution and its relative abundance are predicted for the conterminous United States. These analyses predict that E. postvittana would primarily be limited to the Pacific coastline with small pockets of less suitable environment supporting lower relative abundance on the east coast and in the southwest. This study also highlights the potential value of the core distribution from the CLIMEX species distribution model as a more informative metric of environmental suitability with respect to relative abundance, based on surveys of larval populations of E. postvittana at 30 different locations in coastal California.