850 A. Colles, L. H. Liow and A. Prinzing deﬁnition of extinction ranging from population declines (potentially leading to local or global extinction) to global disappearances of species or entire lineages. The link between specialization and response to environ- mental change has been investigated by neoecologists studying extant species in their current environments, and by paleoecologists investigating fossil taxa in the geological past where both milder environmental ﬂuctuations and mass extinction conditions had occurred. In a similar vein, phylogenetic biologists ask whether specialization is a dead- end leading to extinction and the hindrance of diversiﬁcation. These three approaches – neoecology, paleoecology and phylogenetic biology – frequently involve disparate temporal and spatial scales, often use inherently different data types and analytical tools, and might thus provide complementary insights into the question of whether (and why) specialists are particularly at risk under environmental change. To our best knowledge, insights from these three approaches have thus far not been synthesized. Our goal is to give a short overview of studies on the effect of specialization on the response of species to environmental change in each of these disciplines. For each discipline, we review underlying conceptual motivations, methods, data, results and limita- tions. We compare studies with respect to a set of criteria characterizing an ideal study on specialization and its link to decline (Table 1). We show that neoecologists, paleoecolo- gists and phylogenetic biologists came to partly different conclusions. We discuss possible reasons for these differ- ences, and make recommendations for future research on the link between specialization and decline. T H E N E O E C O L O G I C A L A P P R O A C H Why neoecologists study specialization We deﬁne neoecology broadly as the study of ecological phenomena in the Holocene or 10 000 BP till today. We reserve the term ecology to refer to the sum of neo- and paleoecology. Species extinctions have been common during the Holocene and are increasingly attributable to both over-hunting by humans and our drastic modiﬁcations of the environment (McKinney 1997). To ameliorate species declines, neoecologists attempt to understand how partic- ular life-histories or interspeciﬁc interactions put species at risk of extinction under anthropogenic impact. Specialists have long been considered to be particularly susceptible to population declines; hence they have been targets of conservation efforts (Thompson 1994; Julliard et al. 2003). On the neoecological time scale, species may respond to environmental change via phenotypically plastic responses, changes in the relative frequency of different phenotypes within populations or redistribution of species in space, and the tracking of habitats and climates (e.g. Ackerly 2003). We 2009 Blackwell Publishing Ltd/CNRS Review and Synthesis reserve the discussion of response via the establishment of new heritable traits to the The Phylogenetic Approach. Phenotypic plasticity and changes in the relative frequency of phenotypes may be relatively easy to accomplish for generalist species given their often large range of pheno- types (Spitze & Sadler 1996), while specialists might more strongly depend on spatial redistribution to track environ- ments. Redistribution, however, is challenging for many species given the destruction of habitat corridors across landscapes; and anthropogenic large-scale transport may facilitate disperse generalists more than specialists (Prinzing et al. 2002a). Thus, environmental change may compel specialists to redistribute and simultaneously prevent them from doing so. Only some species already specialized on anthropogenic habitats may proﬁt from environmental change (Munday 2004). Methods and data Neoecologists usually directly observe a species specializa- tion (dietary or habitat use) in its natural environment (Warren et al. 2001; Julliard et al. 2003; Koh et al. 2004). Inference of specialization from other traits, such as morphology, is rare (e.g. Saﬁ & Kerth 2004 used wing morphology to infer habitat use in bats). Specialization is then compared with population trends across one to multi- ple decades, for which precise records on environmental changes are often available (see below). Neoecological studies cover a wide range of taxa, including insects, vertebrates, plants (see below). Results Specialization and the risk of extinction The correlation between habitat niche breadth, or less commonly dietary niche breadth, and the risk of extinction, is usually studied at local to regional scales [Table 2: 9())]. Habitat specialization is usually found to be correlated to increased extinction risk, e.g. among birds (Julliard et al. 2003), bats (Saﬁ & Kerth 2004), bumblebees (Williams 2005) and plants (Walker & Preston 2006). The link between diet specialization and extinction risk is less unambiguous (Table 2 type of specialistation ). On the one hand, some studies found no relationship between dietary specialization and decline (e.g. Saﬁ & Kerth 2004; Williams 2005 on bats and bumblebees respectively). Possibly, humans directly affect the quality and abundance of habitats, but inﬂuence dietary resources only indirectly. On the other hand, multiple studies on insect–plant interactions (phytophages, pollinators and plant-hosts) found that specialists seem to be at a higher risk of local extinction [13 of 15 cases (Appendix S1), e.g. Biesmeijer et al. 2006, see also Brooks & McLennan 2002]. In fact,