The total number of naturalized species can be a reliable predictor of the number of alien pest species

Abstract

In her recent paper, Shawna J. Dark (2004) concluded that the distributions of ‘invasive alien plants’ and ‘non-invasive alien plants’ in California were significantly correlated. She found the highest numbers of both ‘invasive’ and ‘non-invasive’ species in regions with low elevation, high road density, and high native species richness. Dark actually uses two definitions of ‘invasive’ in the paper; we discuss the implications of this on her conclusions. We point out that the most important message of Dark’s paper is that the total number of naturalized species can be, at least at some spatial scales, a reliable predictor of the numbers of exotic pest species. We also provide another example to support this conclusion. We follow the definitions of native, non-native, naturalized, casual, pest, and weed in Pysek et al . (2004) in this note unless otherwise specified. It is necessary to examine how Dark defined ‘invasive’ in her paper because the data she used include only a small number of plant species (78) classified as ‘invasive’ while the remaining 1097 naturalized species listed in the most recent statewide flora and online updates (total 1175; Hickman, 1993; http:// ucjeps.berkeley.edu/interchange.html) were classified as ‘non-invasive alien plants’. Dark first defines ‘invasive alien plants’ in the paper’s second sentence as ‘alien plants that produce offspring in very large numbers and at large distances from parent plants and thus have potential to spread over a considerable area (Richardson et al ., 2000)’. Later, however, we learn that species actually classified as ‘invasive’ for data analysis in this study are only those rated as ‘A’ and ‘B’ on the California Exotic Pest Plant Council’s (1999) list of ‘Exotic Pest Plants of Greatest Ecological Concern’ (Dark, 2004; text under Fig. 1; CalEPPC, 1999). CalEPPC (1999) in turn defines its A and B categories as covering species that