854 A. Colles, L. H. Liow and A. Prinzing Review and Synthesis Summary Specialists seem to suffer a triple synergistic jeopardy: a usually small initial size of local populations, an often restricted geographic range, and a limited utilization of resources and habitats (McKinney 1997). On a short, neoecological time scale, specialization appears to put species at risk under environmental change (Table 2). Nevertheless, it remains to be veriﬁed whether this pattern holds after accounting for sample size, whether it holds for dietary specialization, and whether specialists differ from generalists in their evolutionary response to environmental change. Looking back into the past, using paleoecological or phylogenetic approaches might facilitate to answer the last question. T H E P A L E O E C O L O G I C A L A P P R O A C H Why paleoecologists study specialization Extinctions and originations shape biodiversity through earth s history (Jablonski 2001). Certain traits may make some taxa more resistant to extinction; hence diversity may be a biased sample of potential taxa. In particular, Simpson s classic Rule of the survival of the relatively unspecialized (1944) postulates that taxa occupying wide adaptive zones are more apt to survive. Taxa which survived mass extinctions (Jablonski 2001; Kiessling & Baron-Sza´bo 2004) and taxa which are more resilient to extinction during background intervals (Johnson et al. 1995) are thought to be non-random with respect to traits such as geographic range, niche breath, body size, complexity, among other traits (McKinney 1997). Here, we review whether specialization correlates with increased survivorship over extinction events or increased lineage longevity, emphasizing insights from publications written after McKinney s (1997). Methods and data Several authors have attempted to directly observe the relative size of the ecological niche occupied by taxa can be directly measured, such as the number of bathymetric zones or habitat types occupied (Kammer et al. 1998; Liow 2007a respectively), or the use of C3 and C4 plants (leaving distinct traces in the teeth of large herbivores; Sanchez et al. 2006; Feranec 2007). Other authors use functional morphology to infer the size of the ecological niche, such as the spectrum of food items that fossil crinoids could have consumed based the size of their ﬁlters (e.g. Baumiller 1993; Kammer et al. 1998). In addition, taxa which are morphologically more complex (Flessa et al. 1975) or those that are outliers in morphospace (Liow 2007b) had been considered special- 2009 Blackwell Publishing Ltd/CNRS ists, although the link between general morphological features and ecological traits is not straightforward. Finally, relative specialization has been inferred from smaller geographic ranges. But because the causal relation between range size and niche breadth is not clear (see The Neoecological Approach), we consider it an untested proxy for ecological specialization and chose not to use it in our literature review. The degree of niche specialization, observed or inferred, is then correlated with total taxon duration or taxon survivorship across previously independently identiﬁed extinction events. Taxa considered are mostly higher-order groups such as genera rather than species, although exceptions exist. Results Specialization has long been thought to increase the extinction risk or decrease survivorship of a given lineage (Cope 1896; Thompson 1994). There is some evidence to substantiate this claim both over extinction events and during background intervals, especially for feeding ecologies. For instance, omnivorous sea urchins preferen- tially survived over herbivorous ones across the Creta- ceous-Tertiary boundary (Smith & Jeffery 1998) as did generalist insects feeding on a wider range of plant species (Labandeira et al. 2002). Planktonic foraminifera species which are trophic generalists (Norris 1992) and coarse- meshed and hence generalist ﬁlter-feeding crinoid genera both have longer taxon longevity (Baumiller 1993) over background intervals . Hypercarnivorous fossil canids have reduced evolutionary lifetimes (Van Valkenburgh et al. 2004) being more susceptible to extinction than more generalized forms (Leonard et al. 2007). Although correlations do not imply causation and exceptions do exist (Munoz-Duran 2002; Feranec 2007), most studies do show a tendency for diet specialization to reduce survivorship (eight of 10 studies in our literature review, Table 2: type of specialization). Specialization has also been studied with respect to the use of habitats or other environmental units. Mammals using a greater number of biomes (Bofarull et al. 2008), crinoids using more habitat types as estimated using the number of facies a given taxon is preserved in (Kammer et al. 1998) and foraminifers able to tolerate a wider range of temperatures, salinities and nutrient settings (Keller et al. 1998) all have increased survivorship. However, multiple exceptions exist e.g. wider bathymetric ranges did not predict greater taxon longevities (Liow 2007a; see also Norris 1992; Jablonski & Raup 1995). In summary, four studies in our literature review suggest that species specialized on particular environments have shorter dura- tions, while four studies did not.