Review and Synthesis Identifying specialization Table 1 Criteria for an ideal identiﬁcation of specialization, decline and for studies relating the former to the latter 1. Use of the environment is observed, not inferred from morphologies 2. The measure of niche breadth of species does not depend on the number of observations available for each species. This prevents abundant species from being ranked as generalists simply because they are found more frequently and, thus, in a larger number of environments. The problem does not apply to studies inferring specialization from morphological characters of species (where all individuals of a given species are usually assumed to have the same character state) 3. Information on specialization is available at the level of the species of interest. That is, specialization is not inferred from higher taxa to which that species belongs, nor from either its descendent or ancestor in a phylogenetic context 4. Niche breadth is quantiﬁed across multiple major niche axes, e.g. habitat and diet, thus approximating a true niche volume, rather than using only isolated information from single niche axes that are analysed separately. Note that this criterion is not met in any of the reviewed articles 5. Specialization is measured on a more than a binary scale; three (or preferably more) ranks are the prerequisite to identify nonlinear relationships between specialization and decline 6. An individual of a generalist species can live on a single resource or habitat type, it does not depend on multiple resource or habitat types. For instance the individual can live all its life in a forest, or it can live all its life on a meadow, it does not need to shift between forest and meadow during its existence. The criterion is obviously fulﬁlled for plants and parasites or phytophage larvae as they hardly move between habitats ⁄ hosts Identifying decline 7. Decline measured within a given type of resource ⁄ habitat and not averaged across all those known. For instance, decline of plant species is measured only on calcareous grasslands, not for the entire region across calcareous grasslands and all other kinds of habitats. The fate of specialists and generalists is thus evaluated within the very same environmental conditions – calcareous grasslands Linking specialization to decline 8. Specialization is inferred independent of decline. Either specialization is known from a period prior to the observed decline, or specialization is studied at a much larger spatial scale than decline. This reduces, but does not eliminate, the risk that the measured specialization is in itself the result of decline. Comparisons of ancestral specialization to the success of descendents were treated as cases where specialization prior to decline is known, even though the ancestral specialization is ultimately reconstructed from the descendant species. Specialization inferred from morphological characters (such as generalized ⁄ specialized mouth parts) was also treated as specialization being inferred independently of decline because such morphological characters are not likely to have changed due to decline 9. The study covers the entire range of species. As the true range is often not known or provided we used the geographic scale of the study as a proxy: the assumption is that studies at continental or global scale will usually cover entire ranges of most species included, and smaller-scale studies only rarely We assigned binary coding to the studies we reviewed. yes if they fulﬁlled the stated criterion and no if they did not. However that ﬁner categorization (e.g. yes, partial and no) lead to very similar results in the analysis of our literature database (Table 2). Are specialists at risk? 851 co-extinction is common where there is mutual specializa- tion between plants and their phytophages ⁄ pollinators (Koh et al. 2004; Rezende et al. 2007). Moving from the terrestrial to the marine realm, over-ﬁshing is considered responsible for the majority of species disappearances and specialized predatory ﬁsh are especially at risk because of the decline of their prey species (Munday 2004). Specialization and the geographic range of distribution Specialist species exploiting narrow ecological niches are often restricted to smaller geographic ranges (Brown 1995), even though other factors such as low dispersal capability may also reduce range size (Williams et al. 2006). A species of narrow geographic range may face increased extinction risk under environmental change because its restricted range may only include areas where environmental conditions deteriorate for the species and not where they are neutral or improve. Butterﬂies of a limited geographic range, for instance, suffer three times higher risks of extinction than cosmopolitan species (Koh et al. 2004). Range limits such as those constrained by a minimum temperature affect specialists and generalists differently. Warren et al. (2001) have shown that among British butterﬂies, mobile-habitat generalists have expanded their range northwardly during recent decades, whereas specialists have declined. Having arrived ﬁrst, the generalists may then proﬁt from an incumbent advantage (Rosenzweig 1995) against specialists that may still arrive in the future. Unfortunately the processes behind these geographic range patterns are not straightforward. While a large ecological niche may increase the number of regions in which a generalist species can establish, the reverse may also 2009 Blackwell Publishing Ltd/CNRS