Review and Synthesis entire lineages (specialization identified as dead end in 40% vs. 13% of the studies). Studies, in which specialization of at least some of the ancestral species could be directly observed have similar results to those where specialization was reconstructed (Table 2: 1). Results also may depend on whether specialization ⁄ generalization is coded by discrete or continuous variables: emydid turtles ancestors are assigned as aquatic, i.e. habitat specialist using discrete character coding, but are reconstructed as semi-terrestrial, i.e. generalist, using continuous coding (Stephens & Wiens 2003 – but note that treating semi-terrestrials as generalists may not ﬁt our criterion 6 in Table 1). Discrete variables hinder the reconstruction of intermediate values at ancestral nodes, and the number of changes of the trait may be underestimated. Most authors use discrete variables to characterize specialization (which is naturally a continuous phenomenon), resulting in a bias in favour of increased reversibility of specialization (Stephens & Wiens 2003). This is conﬁrmed by our review: binary deﬁnition of specialization corresponds to an overly low tendency of specialists to decline (Table 2: 5). Many phylogenetic studies lack the standardization of niche breadth measures for sample size, but this deﬁciency does not change the assessment of consequences of specialization (Table 2: 2). However, none of the reviewed studies analysed the success of specialists within only a given resource or habitat type (Table 2: 7), hence we do not know whether specialization leads to dead-ends more often under such scenarios. Finally, phylogenetic studies on specialization are largely limited to insects using plant or animal hosts, i.e. a specialization on another taxon (Table 2). Hosts may induce changes in the phytophage or parasite physiology, rendering host shift increasingly difﬁcult, and specialist phytophages or parasites may indeed ﬁnd themselves in a dead end. On the other hand, hosts may diversify, and specialist phyto- phages or parasites may proﬁt from an incumbent advantage and diversify along with their hosts. Such advantages and disadvantages from host-specialization may compensate for each other, and in fact studies on host specialization do not ﬁnd that specialists decline more often than studies on other types of specialization (Table 2). Studies of host specializa- tion often measure host ranges at different taxonomic scales, e.g. from tribes within a plant genus to all Angiosperms. However, we found that the results of studies did not depend on the taxonomic level of the host speciﬁed: specialists were regardless rarely at risk. Summary Are specialists at risk? 857 also question the idea that specialization is an evolutionary dead-end. The different conclusions do not merely reﬂect differences in the taxonomic groups studied (Table 2). In fact, two studies on phytophagous beetles have opposing results (Kelley & Farrel 1998; Morse & Farrell 2005). That specialization may not be evolutionarily disadvantageous seems surprising given the costs of specialization. However, the beneﬁts of increased resource use efﬁciency may outweigh the costs of increased resource specialization, even over long periods (Futuyma 2001). For instance, environmental change leading to the disappearance of one host–plant species may simultaneously permit the appearance of another plant species equally exploitable for the given specialist phytophage (Brooks & McLennan 2002). These specialists, sometimes termed faux generalists specialize on particular biochemical and morphological plant phenotypes rather than strictly on individual plant species. The evolutionary constraint due to specialization needs to be veriﬁed for each study system separately, while accounting for possible methodological biases in ancestral character-state reconstruction. Neverthe- less it seems safe to conclude that, from a phylogenetic perspective, specialization does not, in general, put a species at greater risk in a changing world. W H Y T H E T H R E E A P P R O A C H E S C O M E T O D I F F E R E N T C O N C L U S I O N S : F R O M B I A S E S T O M E C H A N I S M S Specialization appears detrimental for the persistence of species under current environmental change. Many paleo- ecological studies point to similar conclusions although species level studies are rare and proxies for specialization may not accurately reﬂect realized ecological specializa- tion. On the other hand, most phylogenetic studies show that species do not suffer from being specialists; they can avoid extinction and adapt, for instance they respond to the challenges of a new biotic environment when colonizing a new continent. To the best of our knowledge, neoecologists were not aware of these phylogenetic observations when discussing the conserva- tion needs of specialists. Why do three different approaches, studying the same phenomenon, come to different conclusions, and how could they be reconciled to come to pertinent predictions on the future of specialists and generalists under anthro- pogenic change? We will start with biases in data and methods, and then move on explanations invoking different mechanisms. Different biases? Specialization does occasionally limit the capacity of a species to persist across environmental changes, but many studies In neoecological studies, niche breadth measures are seldom standardized such that the tendency of specialists 2009 Blackwell Publishing Ltd/CNRS