Estimating spread rates of non-native species: the gypsy moth as a case study.

Abstract

Abstract Estimating rates of spread and generating projections of future range expansion for invasive alien species is a key process in the development of management guidelines and policy. Critical needs to estimate spread rates include the availability of surveys to characterize the spatial distribution of an invading species and the application of analytical methods to interpret survey data. In this chapter, we demonstrate the use of three methods, (i) square-root area regression, (ii) distance regression and (iii) boundary displacement, to estimate the rate of spread in the gypsy moth, Lymantria dispar, in the USA. The gypsy moth is a non-native species currently invading North America. An extensive amount of spatial and temporal distributional data exists for this invader. Consequently, it provides an ideal case study to demonstrate the use of methods to estimate spread rates. We rely on two sources of data: (i) polygonal data obtained from county quarantine records describing the geographical extent of gypsy moth establishment; and (ii) point data consisting of counts of male gypsy moths captured in pheromone-baited traps used to detect and monitor newly established gypsy moth populations. Both data sources were compiled during the gypsy moth's invasion of the Lower Peninsula of Michigan, USA. We show that even with spatially crude county records of infestation, spread rates can still be estimated using relatively simple mathematical approaches. We also demon strate how the boundary displacement method can be used to characterize the spatial and temporal dynamics of spread.