Review and Synthesis be true: a large range size (resulting, for instance, from efﬁcient dispersal) exposes a specialist species to many types of resources and habitats and thus drives it to become more generalist. Specialization and niche conservatism Environmental changes can modify the distribution of species including the local disappearance of predators or superior competitors of a given species. Prey species or inferior competitors may proﬁt by expanding into these vacated localities, possibly by increasing their repertoire of habitat types and hence become more generalist (Rosen- zweig 1995). However, Losos et al. (1994) have shown that Caribbean lizard species did not shift to a more generalist state in the absence of a superior competitor. Similarly, Prinzing et al. (2002b) found that among plants poor competitors do not respond to regional shifts of stronger competitors by changing their habitat use. These results can be explained by niche conservatism, the tendency of species to retain ancestral ecological characteristics over time (Wiens & Graham 2005). Losos et al. (1994) concludes that most niche changes are to be expected only when the currently used niche disappears, not when a new one becomes available. Across short, ecological time scales, niche conservatism is thought to render niche expansion difﬁcult (Wiens & Graham 2005). Species instead respond to environmental variation by shifts in physiology or life-history (Rapoport 1982), that is, specialists do not turn into generalists (but see Fontaine et al. 2008). Even across macroevolutionary time scales, specialized ancestors tend to have specialist descendents (Bra¨ndle et al. 2002 for birds; Colles, A., Ozinga W., Hennekens, S. Schamine´e, J., Bartish I., Prinzing A. unpublished data for plants, but see Sargent & Vamosi 2008 for pollinators). Limitations Most studies involve populations of a given species within a single smaller region, e.g. a country [Table 2: 9())], and comparisons of the same species among different localities or regions in various studies are sometimes hampered by methodological differences (Julliard et al. 2003). Where comparisons can be made, extinction risks of populations of a species in one country may correlate only weakly to those in a neighbouring country (e.g. R2 = 0.16 for population trends of birds in France and the Netherlands; Julliard et al. 2003). Moreover, the species of different biogeographic origins may show different degrees of specialization (Dyer et al. 2007; but see Novotny et al. 2006). Nevertheless, the restriction of many studies to regional scales does not seem to introduce a bias as global- scale studies also tend to conclude that specialist species are declining [Table 2: 9(+)]. Note also that global scale studies Are specialists at risk? 853 may have their own drawbacks: they often rely on heterogeneous and possibly biased literature information on the distribution of species across habitats or resources. In the majority of the studies, we do not know whether (i) individuals of generalist species actually depend on multiple resource or habitat types (which can be considered as an extreme form of specialization and not generalization); (ii) specialists decline within their preferred habitat type; or (iii) specialization may have resulted from decline. But again these do not seem to bias the observed correlation between specialization and decline (Table 2: 6, 7 and 8). What seems more worrisome is the lack of standardiza- tion of niche breadth for sample size in many studies [Table 2: 2())]. If rare species are erroneously inferred to be specialists because the few observations available inevitably come from only few resource ⁄ habitat types, this may result in a pseudocorrelation between specialization and decline. Neoecological studies are largely based on insects and vertebrates, and on specialization with respect to hosts and habitats. While these taxonomic limitations did not affect the general conclusion, the focus on hosts and habitats may have lead to an overrepresentation of studies reporting a decline of specialists (Table 2: type of specialization). Evolutionary processes are rarely accounted for in neoecological studies on declines in generalist vs. specialist species. Evolutionary responses to environmental change, including the establishment of novel, heritable phenotypes, such as those by migratory species responding to climate warming (e.g. Berthold et al. 1992), might take place over only a few generations. Moreover, neoecological studies might be biased towards evolutionarily derived species (species derived from the root by many phylogenetic ramiﬁcations). Existing studies primarily reﬂect patterns among derived species, as they make up the majority of a given extant species pool while basal species are underrep- resented (Prinzing et al. 2004).This may introduce bias, for instance, because basal species with wide global distributions are not necessarily widely distributed within the regions they occupy, explaining the high extinction risks in these species (Prinzing et al. 2004, Vamosi & Wilson 2008; see also Williams et al. 2006). Finally, care should be taken when habitat specialization is inferred from the number of habitat types used across largely anthropogenic landscapes, such as in much of Europe. In such landscapes, fewer habitat types are (semi- )natural, and species using these habitat types will thus likely be labelled specialists , even though in a less human-altered environment, these species may actually behave as general- ists (i.e. faux specialists sensu Brooks & McLennan 2002). A further decline of natural habitat types in anthropogenic landscapes will result in population declines in specialists , leading to an apparent relationship between specialization and decline. 2009 Blackwell Publishing Ltd/CNRS