Ecology Letters, (2006) 9: 853–869 doi: 10.1111/j.1461-0248.2006.00928.x REVIEW AND SYNTHESIS Ecotypic variation in the context of global climate change: revisiting the rules Abstract Virginie Millien,1* S. Kathleen Patterns of ecotypic variation constitute some of the few ÔrulesÕ known to modern Lyons,2 Link Olson,3 Felisa A. biology. Here, we examine several well-known ecogeographical rules, especially those Smith,4 Anthony B. Wilson5 and pertaining to body size in contemporary, historical and fossil taxa. We review the 6 Yoram Yom-Tov evidence showing that rules of geographical variation in response to variation in the local 1 Redpath Museum, McGill environment can also apply to morphological changes through time in response to University, 859 Sherbrooke climate change. These rules hold at various time scales, ranging from contemporary to Street West, Montreal, QC, geological time scales. Patterns of body size variation in response to climate change at Canada H3A 2K6 2 the individual species level may also be detected at the community level. The patterns National Center for Ecological Analysis and Synthesis, underlying ecotypic variation are complex and highly context-dependent, reducing the University of California – Santa Ôpredictive-powerÕ of ecogeographical rules. This is especially true when considering the Barbara, 735 State St Suite 300, increasing impact of human activities on the environment. Nonetheless, ecogeographical Santa Barbara, CA 93101, USA rules may help interpret the likely influences of anthropogenic climate change on 3University of Alaska Museum, ecosystems. Global climate change has already influenced the body size of several 907 Yukon Drive, Fairbanks, AK contemporary species, and will likely have an even greater impact on animal communities 99775-6960, USA in the future. For this reason, we highlight and emphasise the importance of museum 4 Biology Department, University specimens and the continued need for documenting the earth’s biological diversity. of New Mexico, MSC03 2020, 1 University of New Mexico, Keywords Albuquerque, NM 87131-0001, Bergmann’s rule, body size evolution, climate change, geographical variation, time scale. USA 5Zoological Museum, University of Zu¨ rich, Winterthurerstrasse 190, CH 8057 Zu¨ rich, Switzerland 6Department of Zoology, Tel Aviv University, Tel Aviv 69978, Israel *Correspondence: E-mail: [email protected] Ecology Letters (2006) 9: 853–869 INTRODUCTION life on our planet, major shifts in climate may have led to the entire reorganizations of biota (Sepkoski 1998). For ‘‘The present more or less unstable condition of the example, global warming by 6 °C due to mass volcanism circumstances surrounding organic beings, together may have been the driving factor behind the end Permian with the known mutations of climate our planet has mass extinction, where perhaps as many as 95% of all undergone in past geological ages, points clearly to the species perished (Benton & Twitchett 2003). Unlike these agency of physical conditions as one of the chief earlier events, however, contemporary changes in climate factors in the evolution of new forms of life’’ (Allen are accentuated by anthropogenic inputs and may be 1877, p. 139). occurring at a greatly accelerated pace (Houghton et al. 2001; Climate change is nothing new to the Earth system. At Jones et al. 2001). The past 30 years have been the warmest various times throughout the 3.5 billion years (Ba) history of of the last millennium, and temperature increases have been Ó 2006 Blackwell Publishing Ltd/CNRS 854 V. Millien et al. Review and Synthesis the greatest – c. 0.6 °C – during the 20th century (Houghton is rarely a simple shift to higher latitudes, and the direction et al. 2001). Current estimates suggest that warming of 1.4– and extent of the range shifts may vary across species 5.8 °C is likely over the next 100 years (Houghton et al. (FAUNMAP Working Group 1996; Lyons 2003). In 2001). Recent studies agree that the rate of climate change addition to the spatial scale, the temporal scale is also may be even more critical than its magnitude and duration critical when documenting speciesÕ responses to climate (Davis et al. 2005). In light of the increased rate of global change, and varied responses can be observed with warming, we urgently require a clear understanding of the increasing time intervals (Barnosky et al. 2003) (Fig. 1). potential impacts of these expected climate changes on The challenge of accelerated climate warming in the next living organisms. few decades combined with human-induced habitat deteri- The impacts of climate change on living organisms are oration (McCarty 2001) highlights the importance of already detectable at many levels, from alterations in the adaptation in response to future climate change. Here, we phenology of individuals to physiological, ecological and review the evidence from contemporary, historical and microevolutionary changes in individuals, populations and evolutionary studies to estimate the power of ecogeograph- communities (Hughes 2000; Davis & Shaw 2001; McCarty ical rules in interpreting likely patterns of organismal 2001; Stenseth et al. 2002; Walther et al. 2002; Parmesan & responses to climate change. Yohe 2003; Root et al. 2003). There is accumulating evidence of the detectable results of climate change on a wide spectrum of organisms, including plants, insects, fishes, VARIATION AT THE SPECIES LEVEL reptiles, amphibians and mammals (reviewed in Parmesan & Variation in space: ecogeographical rules Yohe 2003; Root et al. 2003). As might be expected, the response of organisms to ‘‘Ecogeographical rules … are purely empirical gen- environmental change is complex and highly context- eralizations describing parallelism between morpho- dependent, and is shaped by both their physical and logical variation and physiogeographical features.’’ biological environments. The biotic response of species to (Mayr 1956, p. 105). their changing environment may involve modifications to There are a several well-established ecogeographical rules their physiology, phenology, morphology and distribution in animals, reflecting their adaptation to local conditions (Hughes 2000), and can be categorized into three main (Mayr 1956). The widespread existence of these patterns by classes: (i) shifts in the speciesÕ range to track the shifting itself may be evidence for a species’ capacity to adapt to climate; (ii) persistence in situ and local adaptation through fluctuating environmental conditions. Ecogeographical rules phenotypical and/or phenological changes; and (iii) local have been abundantly validated by studies of many species extirpation and at a larger scale, extinction (Fig. 1; McCarty of mammals, birds, other vertebrates, and even some 2001; Walther et al. 2002, 2005; Davis et al. 2005). A invertebrates. Roughly, there are three main patterns of combination of these responses may be observed across geographical phenotypic differentiation between popula- the range of a given species: migration towards the poles at tions within a species: (i) variation in colour; (ii) variation in higher latitudes, morphological and ecological adaptation in the size of appendages; and (iii) variation in body size (Allen the centre of the range and extinction (range contraction) 1877). All these rules were originally formulated for at lower latitudes of the species range (Davis & Shaw endothermic vertebrates and thermal regulation was 2001). For mammals, however, the change in distribution believed to be the chief explanation for their existence Biological (Gloger 1833; Bergmann 1847; Allen 1877). level According to Gloger’s rule, individuals that live in warm and humid areas are darker in colour than those living in Communities Speciation/extinction cold and dry areas (Gloger 1833). The underlying mechan- Range expansion, extirpation ism for Gloger’s rule is that individuals with more cryptic Species colouration are under reduced predation pressure. Gloger’s Phenologic and phenotypic rule also relates to thermoregulation: individuals with darker changes Time pigmentation are found in the tropics, while individuals with 10 1 000 1 000 000 (years) lighter pigmentation predominate towards the poles. This rule was first observed in birds but also applies to mammals, Figure 1 Temporal scales of biological responses to climate and maybe some insects. A classic example is provided by change. Phenotypical and phenological changes occur at the the hares which have white fur in the northern latitudes smaller time scales, from a few years up to several 100 years. Shifts (Lepus arcticus) and a darker, brown fur in temperate habitats in species distributions and extinctions are detectable at larger time (Lepus alleni). scales. Ó 2006 Blackwell Publishing Ltd/CNRS Review and Synthesis Ecotypic variation and climate change 855 According to Allen’s rule, appendages size is reduced in size variation among species within a given genus (Berg- cold climates (Allen 1877). Allen’s rule is based on the mann 1847), but there are many examples of an extension of simple reasoning that the less surface area an organism has Bergmann’s rule describing latitudinal variation of body size relative to its body mass, the more thermally efficient it will among individuals within species. Environmental tempera- be (e.g. the less heat it will lose). One way to reduce heat loss ture is highly correlated with latitude and a positive relation is through the reduction of appendages such as ears, tails, between latitude and body size has