Data sets matter, but so do evolution and ecology

In a recent paper, Peterson & Nakazawa (2008) (hereafter PN) contest key findings in our study (Fitzpatrick et al ., 2007) that suggest that Solenopsis invicta (hereafter the fire ant) underwent a niche shift upon its invasion of North America. Using niche-based models, we proposed that the fire ant established in environments similar to those found in its native range but subsequently spread into environments unlike those found within its native range – a pattern strikingly similar to that suggested by Broennimann et al . (2007) for spotted knapweed ( Centaurea maculosa ). PN counter that our findings are simply an artefact of the environmental variables we used to model the fire ant’s distributions and suggest instead that selection of alternative variables can produce a more correct prediction of the fire ant’s invasion. PN conclude that the biological explanations offered in Fitzpatrick et al . (2007) for the non-predictivity between the fire ant’s native and invaded distributions, namely enemy release, genetic founder effects and hybridization, are not necessary. Here we respond to PN’s criticisms. We disagree with the contentions outlined in PN on the grounds that the authors (1) subjectively consider what represents a ‘correct’ prediction of the fire ant’s niche, (2) do not discuss the potential for niches to be conserved along some environmental axes but not others and, most significantly, (3) do not adequately represent our original analyses in Fitzpatrick et al . (2007) by not testing the ability of the fire ant’s invaded distribution to predict its native range using their alternative data sets. We demonstrate, using the procedures outlined in Fitzpatrick et al . (2007) and the set of environmental variables in PN that represents a subset of the variables used in Fitzpatrick et al . (2007), that the results from our original study stand. Issue 1: subjective consideration of what constitutes a ‘correct’ prediction