Since the initial publications regarding niche conservatism, the idea has attracted quite a bit of attention, and the body of relevant literature has grown considerably. As such, after a decade, it is useful to step back and ask where the field stands. The key aspect of this review is that I have organized available evidence by temporal scale (Table 1). I cover a broader range of literature than previous reviews, including evidence from studies on topics as diverse as species invasions, geographically structured distributional predictions, longitudinal studies (i.e. studies examining a single species or lineage through time) and phylogenetic studies that consider explicitly the evolutionary history of ecological characteristics. In choosing studies for inclusion in this overview, I searched Google Scholar (http://scholar.google.com/) using the keywords ‘niche’ and ‘distribution model’, as well as (separately from keyword searches) the names of leading researchers in this field (e.g. R. Anderson, M. Araújo, C. Graham, A. Guisan, J. Lobo, E. Martínez-Meyer, R. Pearson, J. Soberón, W. Thuiller). I included only studies that met the following basic requirements: (1) ecological requirements were evaluated across significant portions of ranges of species (i.e. excluding studies that evaluated niches based on small and potentially non-representative subsets of species distributional areas, such as Vanreusel et al., 2007); (2) spatial or temporal stratification of training and testing data sets was employed, thus avoiding problems with autocorrelation and non-independence of training and testing samples (eliminating, for example, Elith et al., 2006); and (3) the focus was on coarse-resolution variables (i.e. the ‘scenopoetic’ variables of Hutchinson; see Soberón, 2007), rather than on more micro-scale environmental features that tend to incorporate elements of biotic interactions (eliminating, for example, Losos et al., 2003). More generally, any such broad review, and particularly one in which re-analysis of actual original data is not feasible, will necessarily smooth over many methodological differences and inconsistencies among studies – for example, in some summary statistics, I consider the relatively few studies testing niche identity together with studies testing niche similarity (Warren et al., 2008), but checked the effects of their inclusion. I undertook the literature search in December 2008. While I have made every effort to make the literature review comprehensive, without doubt several relevant publications have escaped me, particularly given the burgeoning literature in the field. As such, this review serves as an overview of broad patterns only.
My review of the evidence for or against ecological niche conservatism covers a broader swathe of literature (76 publications) than previous reviews, and covers 299 species or higher taxa, again a much greater diversity of examples than previously analysed (see Appendix S1 in Supporting Information). On the most basic level, the results confirm the conclusions of all previous reviews: that evidence for niche conservatism is mixed. However, when sorting the evidence by time-scale, some degree of structure is evident in the results: recent and short-term events (e.g. species invasions, distributional shifts at the end of the Pleistocene) show a considerable tendency towards conservatism. Longer-term events, on the other hand, such as differentiation across phylogenies, show increasing degrees of breakdown of conservatism (Fig. 1). This trend in the prevalence of niche conservatism makes the mixed and possibly confusing literature on the subject more intelligible: quite simply, and to many not surprisingly, niche conservatism breaks down over time.
A more important question is the rate at which this breakdown occurs. It is worthwhile to return to the issue on which the original niche conservatism analysis focused (Peterson et al., 1999) – that of possible associations between speciation events and ecological innovation. What we can see in the results of this review, however, is that niche conservatism extends deeper in time than has generally been appreciated. That is, on time-scales more or less comparable to those of speciation events, almost all lineages show overall niche conservatism (Fig. 1). No ecological ‘signal’ associated with speciation is discernible. Indeed, if ecological innovation is at all involved in the speciation process, it must be manifested in fine-scale dimensions only: coarse-grained, range-wide-extent studies such as those reviewed herein do not show a signal of ecological differentiation associated with speciation events. Note that these patterns of overall conservatism on short-to-medium time-scales actually became stronger when six studies that used a distinct means of testing for conservatism (niche-identity tests, discussed in the next section) were removed from analysis, as all six found non-conservatism, and all were at the deeper-time end of the spectrum (see Appendix S1). As a consequence, my results should be robust to controlling for the diverse methods considered for testing conservatism.