Review and Synthesis or day length; Karban 2008). A specialist species may thus not have more physiological or morphological constraints, but fewer search-time constraints, such that its individuals have the option of specializing on the optimal resource or habitat type. They thus have the choice of becoming more generalist or switching to specialization on another resource. Hence, specialization would be much less of a burden in a changing environment. Specialization due to lack of search-time constraints has been considered to be particularly important in host- specialization (Ward 1986). This might explain why many phylogenetic biologists, often studying host specialization, have found that specialization does not put species at risk. In contrast, paleoecologists have often inferred specializa- tion from morphological characters imposing a constraint on resource use, especially food items, and often found specialists to be at risk. Understanding whether a given specialist is specialized due to the lack of search-time constraints or due to physiological or morphological constraints on resource use is thus crucial. Generalization may have different causes, too. A species may be a generalist either because its individuals are generalist or because different individuals are specialized on different resource or habitat types (e.g. Ben-Halima et al. 1985). In the latter case; the specialized individuals would proﬁt from all advantages of being specialized (such as efﬁciency of resource use), and the species as a whole can nevertheless shift between different resources if one declines. While we know of no study on correlates of species decline that has separated individual-level from species-level niche breadth, it is obvious that species-level generalization as the sum of individual-level specializations cannot be detected if a single mean value serves to characterize an entire species. Interestingly, our analyses show that studies inferring generalization of species from mean morphology [i.e. all studies classiﬁed under criterion 1())] nevertheless rank generalist species as equally or more successful than specialist species (Table 2; across-approach analyses, within paleocology analyses) – despite the fact that these generalist species are assumed to have generalist individuals. Also, species-level generalization as the sum of individual-level specializations is more likely in species where each individual encounters only a single-host patch (Futuyma 2001), e.g. in plants or parasites or in phytophages on hosts. However, our analysis provides no evidence that generalist species in these groups are more successful than specialists compared with other groups (Table 2). Different causes of extinction? We broadly deﬁned extinction risk to mean population declines and ⁄ or global disappearances of species or Are specialists at risk? 859 lineages. However, the reduced evolutionary lifespans of specialist species under major environmental change do not necessarily imply the extinction of entire specialist lineages. Vrba (1987) proposed the resource-use hypothesis, stating that specialist taxa exploiting a narrow range of resources will go through periods of population decline and subdivision under environmental change, triggering speci- ation by reproductive isolation. Paleoecological and pop- ulation biological observations seem to lend some evidence to this (Kammer et al. 1998; Zayed et al. 2005). Generalist taxa, in contrast, seldom undergo such drastic reductions in population sizes, that is, conditions that may spur speciation. This explanation critically depends on the assumption that specialists remain specialists and general- ists remain generalists under environmental change, which may not necessarily be true (cf. our above discussion of causes of specialization). Nevertheless, in accordance with this explanation, large mammals with relatively specialized diets have higher speciation rates (Vrba 1987). Overall, even though specialist species may be at greater risk under environmental change than generalists, the increased extinction rates in specialist clades may be offset by increased speciation rates. R E C O M M E N D A T I O N S Our review has revealed multiple open questions, some of more technical nature, others directing to entirely new mechanistic hypotheses to be tested in future. We recom- mend the following for future work to make the study of specialization vs. decline a more interdisciplinary one with results that are more general. Directly quantify specialization in an unbiased way We suggest that specialization should be directly observed instead of being inferred – wherever possible (Table 1; criterion 1). The measure of niche breadth should be independent of the size of the sample (criterion 2). We found analyses where these two criteria are not fulﬁlled are often biased. In addition, we recommend aiming at characterizing habitat specialization based on environmental factors which exist in human-made habitats just as much as in more natural habitats. This reduces the risk of misclas- sifying generalists that use habitat types absent from modern anthropogenic landscapes as specialists. For plants, for instance, light, temperature, soil moisture, soil pH, soil fertility, salt concentrations and grazing have been shown to efﬁciently predict species distribution and assessments of preferences of species along these gradients are available e.g. for central Europe (Ellenberg et al. 1992). These data represent only niche positions, but the niche breadth can then be inferred from the abundant data on composition of 2009 Blackwell Publishing Ltd/CNRS