858 A. Colles, L. H. Liow and A. Prinzing to decline is probably over-estimated. Because neoecolog- ical studies are often restricted to local or regional scales, specialists may appear to be more often on the decline than they may be if global data were available (analysis across approaches, Table 2: 9). Paleoecological studies may also over-estimate the tendency of specialists to decline due to the indirect inference of specialization of species from proxies such as morphology (analyses within paleoecology, Table 2: 1). Finally, phylogenetic approaches possibly underestimate the tendency of spe- cialists to decline due to the binary categorization of specialization (analysis within phylogeny, Table 2: 5), the inference of character states of ancestral species from descendents, the lack of information on decline within habitat or resource types (analysis across approaches, Table 2: 1 and 7), or the numerical dominance of specialist species (potentially biasing the reconstruction of ancestral states, see above). Taking into account only the (few) studies that do not suffer from the listed biases renders the effects of specialization more similar across different approaches. But proportions are still different: specialists decline according to 73% of the unbiased neoecological approaches, but only according 45% and 43% of the unbiased paleoecological and phylogenetic approaches respectively. Different study systems? The three approaches often considered very different taxa. However, our analysis showed that this does not bias the results: conclusions are overall consistent across different taxa (Table 2; but note low sample sizes in some taxa). For instance, paleoecological studies largely focus on marine invertebrates because of their higher preservation rates and neoecological studies often focus on insects or vertebrates, but the two approaches reach similar conclu- sions nevertheless (Table 2). The type of specialization, in contrast, may be a major contributor to different conclusions among approaches. Habitat specialists found to be more at risk than dietary specialists in neoecological studies (Table 2). Dietary specialists may be specialized on distinct traits of their food-species (e.g. their prey or host) rather than on the food-species per se. Hence the disappearance of a food species may be more easily compensated by the usage of newly appearing species with the same traits ( faux generalists , see Brooks & McLennan 2002). Habitats, in contrast, may be more complex than food items, and disappearance of one habitat type may be more rarely compensated by appearance of another. However, habitat specialization is not always correlated with extinction risk in paleontolog- ical datasets. 2009 Blackwell Publishing Ltd/CNRS Review and Synthesis Differences in study systems among the three ap- proaches seem to explain in part why they come to different conclusions on the relationship between special- ization and decline. However, even for the same study systems neoecological studies tend to indicate strongest declines in specialists, phylogenetic studies weakest declines (Table 2; but note lower sample sizes). Differences between results from neoecological, paleoecological and phylogenetic studies might thus also be due to different temporal and spatial scales of the studies and in the processes responsible for specialization, generalization and extinction. Different temporal scales and amplitudes of environmental change? Neoecologists typically study environmental impacts on a short-time scale (102 years), aiming to identify species facing risk of extinction under current anthropogenic impacts. Many paleoecologists are concerned with survivorship of taxa on a geological time scale (c. 105–108 years) over both long background time intervals but also shorter periods of mass extinctions. Finally, phylogenetic biologists study diversiﬁcation throughout the lifetime of entire lineages (c. 106–108 years). Neoecological and some paleoecological studies often focus on catastrophic or severe environmental changes, while phylogenetic studies tend to integrate over the entire life time of species. However, as outlined above, many paleoecological studies, studying long-term back- ground extinctions between major catastrophes still con- cluded that specialists are at greater extinction risk than generalists, suggesting that differences in temporal scale in themselves do not confound results. Different causes of specialization or of generalization? Some species are specialized due to physiological or morphological constraints that prevent them from being more generalist (for instance dietary specialization in phytophages, Bernays 1998). However, behavioural ecol- ogists have long recognized that individuals make decisions throughout their lives: an individual capable of using many resource types may often decide to specialize on one type of resource (Ward 1986; Prinzing 2003). Generalists may use suboptimal resources because they cannot afford to wait till they ﬁnd their preferred, optimal resource type (Ward 1986). Such individual decision making processes may also drive species-level specializa- tion: species with life-histories restricting search time cannot be choosy and are hence generalists (Prinzing 2003; note that decision making is not restricted to animals – plants do so too, for instance seeds germinate or not as a function of changes in ambient temperature