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Page 1: Biogeography,changingclimates,andniche evolution · Biogeography,changingclimates,andniche evolution David B. Wake a,b,1 , Elizabeth A. Hadly c , and David D. Ackerly a,d a Department

Biogeography, changing climates, and niche evolutionDavid B. Wakea,b,1, Elizabeth A. Hadlyc, and David D. Ackerlya,d

aDepartment of Integrative Biology, bMuseum of Vertebrate Zoology, and dJepson Herbarium, University of California, Berkeley,CA 94720; and cDepartment of Biology, Stanford University, Stanford, CA 94305

Edited by James Hemphill Brown, University of New Mexico, Albuquerque, NM, and approved September 29, 2009 (received for review September 25, 2009)

Modern concepts central tocurrent studies of biogeog-raphy, changing climatesand evolution of ecological

niches were born approximately a hun-dred years ago. In 1908, the Regents ofthe University of California establishedthe Museum of Vertebrate Zoology atBerkeley, in accordance with the wishesof Annie M. Alexander (1867–1950).Alexander conceived the institution andselected its first Director, Joseph Grin-nell (1877–1939), a well known natural-ist who had received his Ph.D. at Stan-ford University and was then teaching atthe Throop Institute in Pasadena (laterthe California Institute of Technology).Grinnell, who founded the Museum inlate 1908, meticulously adhered to theprinciples laid down by Alexander (1).Alexander would provide the fundingand inspiration and Grinnell would dothe intellectual and practical work toestablish the institution and set it on atrajectory. The Museum quickly becameknown for its studies of terrestrial verte-brates, conducted in the framework ofwhat we would recognize today as ecol-ogy and evolution. The founders wereself-conscious of their roles from thebeginning. They had no illusions thatthe work they set out to do would beeasy or soon accomplished, if ever.Grinnell (2) set the course:

It will be observed, then, that ourefforts are not merely to accumulateas great a mass of animal remains aspossible. On the contrary, we areexpending even more time thanwould be required for the collectionof the specimens alone, in renderingwhat we do obtain as permanentlyvaluable as we know how, to theecologist as well as the systematist. Itis quite probable that the facts ofdistribution, life history, and eco-nomic status may finally prove to beof more far-reaching value, than what-ever information is obtainable exclu-sively from the specimens themselves.

At this point I wish to emphasizewhat I believe will ultimately proveto be the greatest value of our mu-seum. This value will not, however,be realized until the lapse of manyyears, possibly a century, assumingthat our material is safely preserved.And this is that the student of the

future will have access to the originalrecord of faunal conditions in Cali-fornia and the west wherever we nowwork.

These ideas were prophetic, and havea special poignancy at this time, whenthe impact of global climate change isevident and when we realize how muchwe rely on records from the past, suchas those meticulously kept by Grinnell,his students, coworkers and successors.A century has now elapsed since theMuseum was founded, and we werestimulated by Joseph Grinnell’s words totake stock of our current understandingsof the relationships between geography,climate, and the distribution and ecolog-ical niche dimensions of organisms in anecological and evolutionary context, us-ing modern methods and approaches.

In this issue contributions are pre-sented from an Arthur M. Sackler Col-loquium of the National Academy ofSciences, held in Irvine, CA, December11–13, 2008, in celebration of the Cen-tennial of the Museum of VertebrateZoology at the University of Californiaat Berkeley and with the spirit of Grin-nell’s contributions in mind. The collo-quium was focused on issues central toGrinnell and his colleagues one hundredyears ago: biogeography, niche evolutionand changing climates (or, more gener-ally, environments, for Grinnell focusedmuch attention on human-induced im-pacts on the California environment).Grinnell was a pioneer in studying geo-graphic variation within and betweenspecies, in focusing on the relationshipsbetween geography, ecology and thedistribution of organisms, and especiallyin the formulation of the ecologicalniche concept (3). These are all currentissues in modern science, and recentyears have witnessed the emergence ofnew scientific challenges, conceptualframeworks, and analytic techniques,all of which were on display at thecolloquium.

The colloquium took place at a propi-tious time, celebrating both the centen-nial of the Museum of Vertebrate Zool-ogy and the sesquicentennial of thepublication of Darwin’s Origin of Spe-cies on November 24, 1859 (Table 1).Moreover, 2009 is also the 150th anni-versary of the death of Alexander vonHumboldt (1769–1859), a monumental

figure in the history of science whowrote extensively about the relationshipsof climate and vegetation. As a result ofhis studies of Volcan Chimborazo inEcuador, von Humboldt appears to havebeen the first well known scientist todiscuss the zonation of vegetation alongan altitudinal transect and to develop aconcept of life zones (4). von Hum-boldt’s work deeply influenced C. HartMerriam who recalled, shortly before hisdeath, his father handing him a volumeof Humboldt’s ‘‘Views of Nature’’,where he found early inspiration for hiswork on species distributions (5). Merri-am’s classic study of the San FranciscoMountains in Northern Arizona and hisbiological survey of Mt. Shasta led tothe development of his highly influentiallife zone concept (6). Merriam recog-nized 12 life zones in the United Statesand mapped them, noting that on altitu-dinal transects (e.g., from the SonoranDesert to Humphrey’s Peak) one wouldpass through as many as six life zones.Groups of species of plants and verte-brates were associated with each zone,and thus the zone itself became predic-tive. Grinnell was strongly influenced bythis approach, and it became a hallmarkof his subsequent research, especiallyevident in his famous studies of Yo-semite National Park (7), and through-out his life he made regular emenda-tions to his life zone map of California.Although life zones became increasinglycontroversial through time, attempts toadapt the life zone approach to modernunderstandings were made by severalauthors, notably Holdridge (8). Hold-ridge’s life zones, based on integrationof biotemperatures, precipitation, andpotential evapotransporation, were usedby many researchers, especially in the

This paper serves as an introduction to the Arthur M. SacklerColloquium of the National Academy of Sciences, ‘‘Bioge-ography, Changing Climates, and Niche Evolution’’ heldDecember 11–13, 2008, at the Arnold and Mabel BeckmanCenter of the National Academies of Sciences and Engineer-ing in Irvine, CA. The complete program and audio files ofmost presentations are available on the NAS web site atwww.nasonline.org/Sackler�Biogeography.

Author contributions: D.B.W., E.A.H., and D.D.A. wrote thepaper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

1To whom correspondence should be addressed. E-mail:[email protected].

www.pnas.org�cgi�doi�10.1073�pnas.0911097106 PNAS � November 17, 2009 � vol. 106 � suppl. 2 � 19631–19636

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New World tropics and life zone mapswere produced for several countries(e.g., refs. 9 and 10).

In parallel with his work on life zones,Grinnell pioneered the analysis of indi-vidual species distributions in relation totheir biotic and abiotic requirements inhis 1917 article ‘‘The niche-relationshipsof the California thrasher’’ (3). Al-though the term ‘‘niche’’ had previouslyappeared in print in its ecological con-text (11), Grinnell’s article is appropri-ately credited with the introduction ofthe niche concept into general use inecology (12). Grinnell apparently firstexplicitly referred to niches in his 1913doctoral dissertation, cited in a 1914monograph dealing with the Lower Col-orado River region (13). Griesemer (14)says that the term ‘‘niche’’ first ap-peared in Grinnell and Swarth (15), apublication dealing with the vertebratefauna of the San Jacinto Mountains. Heguesses that the term probably emergedfrom discussions among active groups ofstudents at Stanford, who were attractedby David Starr Jordan and inspired bythe life zone concept of Merriam. Grin-nell greatly admired Merriam, whoseconcepts were a significant influence onthe development of his work. Grinnell’sidea that each species occupies its ownniche goes back to Darwin as a coreconcept, and Grinnell explicitly dealtwith the issue of competitive exclusionin 1904 (16). By 1917, when Grinnellused the word ‘‘niche’’ in an article title(3), the term was in wide informal use.Grinnell concluded, ‘‘It is, of course,axiomatic than no two species regularlyestablished in a single fauna have pre-cisely the same niche relationships,’’which led Leibold (17, p. 288) toobserve:

Since that was one of the first uses ofthe term niche and one of the firststatements of this principle, one won-ders how much theoretical work wasbeing developed outside of print byGrinnell and his collaborators.

Some insight into this conjecture isderived from the field notes of Grin-nell’s graduate student Walter P. Taylor,who was conducting field research in theYosemite region in early December,1914 (notes on file, Museum of Verte-brate Zoology, University of California,Berkeley, CA). Following routine obser-vations for December 11, 1914, Taylorwrote a separate section headed ‘‘TheEcological Niche.’’ These are musings,obviously stimulated by time in the fieldwith Grinnell since mid-November. Af-ter a brief introduction Taylor observes

It seems to refer, as usage has it, todifferent habitats in the same local-ity, as for instance, in the case ofbirds, different states of foliage, ordifferent kinds of brush.

He continued for 4 pages, stating at onepoint:

But the ordinary sense in which theterm ecological niche is used refersto that critical something which isseized upon by one species, and bywhich it keeps its hold even in a lo-cality where related species areliving.

Taylor concludes:

Put in another way, the continuedexistence of a species in a localitywhere related species are living de-pends upon the critical differences,slight or large, in the totality of re-quirements of each.

These notes, written a decade afterGrinnell’s first formal conceptualizationof niches and competitive exclusion, al-though without using those terms, sug-gest that ‘‘ecological niche’’ was a termalready in circulation and a topic of de-bate and discussion among graduate stu-dents of the day.

An apparently independently devel-oped concept of the ecological niche isattributed to Elton (18), who either wasunaware of Grinnell or chose not to citehim (see ref. 19). Griesemer (14) sum-

marized the similarities and differencesbetween the conceptualizations. Eltonwanted to develop a coherent accountof dynamics of interactions in communi-ties, such as food chains, cycles of abun-dance, and the like; he was less con-cerned with individual species than wasGrinnell. Grinnell’s niche was seen asmore habitat-oriented than Elton’s,which was seen as more function-ori-ented, but both saw the niche as a placeor role that a species occupies in theenvironment rather than as a propertyof the species itself, a view more associ-ated with Hutchinson (ref. 20; see alsorefs. 21 and 22). The main differencebetween Grinnell and Elton identifiedby Griesemer is the issue of whethermore than one species can occupy aniche, the difference stemming ulti-mately from Grinnell’s systematic per-spective and Elton’s more functionalone. In a concise and forceful overview,Udvardy (23) argued that there was noessential difference between Grinnelland Elton in conceptualizing the niche,but that Grinnell’s formulation was ear-lier and broader. Furthermore, he cred-its Grinnell with the concept of compet-itive exclusion, which he calls‘‘Grinnell’s axiom.’’ Hardin (24) agreedthat Grinnell deserved credit, despitethe fact that competitive exclusion waswidely referred to as ‘‘Gause’s princi-ple’’ by that time (1960); he proposedthe term ‘‘competitive exclusion princi-ple’’ to extricate the terminology fromits uncertain historical genesis.

G. Evelyn Hutchinson, in his famous‘‘Concluding Remarks’’ article (25), for-malized the niche concept as a set ofniche axes defining an ‘‘N-dimensionalhypervolume’’ within which a speciescould maintain a viable population.Hutchinson’s concept of the niche as anabstract set of environmental axesclosely mirrors Grinnell’s original pre-sentation, although he did not cite Grin-nell’s article at this time. A critical dis-tinction is that Hutchinson redefined theniche as a property of the species in re-lation to its environment (20). Hutchin-son introduced the distinction betweenthe fundamental niche, reflecting theabiotic requirements of a species, vs. therealized niche, the set of conditions oc-cupied in the presence of competitors.Ellenberg, working independently onvegetation ecology in Central Europe,had also considered how species distri-butions along environmental gradientsshift in response to competitors (26). Itis not known (to us) how the work ofEllenberg and other European ecolo-gists, and that of the ‘‘American school’’of evolutionary ecology in the 50s and60s, may have influenced each other.The subsequent history of the niche

Table 1. Early milestones and anniversaries related to the history of the niche conceptin biogeography, ecology, and evolution

Year Milestone

1805 First publication of von Humboldt’s Volcan Chimborazo vegetation figure1859 May 6: Death of Alexander von Humboldt (1769–1859)

Nov. 24: Publication of Darwin’s Origin of Species1908 Founding of UC Museum of Vertebrate Zoology (Grinnell and Alexander)1910 First published use of �niche� in ecological context (11)1917 Publication of Grinnell’s �The niche-relationships of the California thrasher�1927 Publication of Elton’s �Animal Ecology�

1936 Publication of Gause’s �The Struggle for Existence�

1944 Publication of Simpson’s �Tempo and Mode in Evolution�

1957 Publication of G.E. Hutchinson’s �Concluding Remarks�

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concept, and its central yet at times re-viled role in community ecology, hasbeen discussed in some depth by severalreviewers (refs. 12, 14, 19, 20, 27, and28; see also refs. 21 and 22).

While ecologists were wrestling withthe niche concept and its implications,paleontologists were tackling relatedproblems, although not always using thesame language. George Gaylord Simp-son presaged discussions of niche con-servatism by ecologists through hisconsideration of the ‘‘realized environ-mental hyperspace’’ (29). He viewedeach point in ‘‘environmental hy-perspace’’ as a unique combination ofenvironmental variables, and arguedthat individuals within populations, pop-ulations within species, species withingenera, and so on, will exhibit a slowlydecreasing measure of similarity in thepositions they occupy. Building onHutchinson’s formulations, Valentine(30) wrote:

The species niche is one of the moreimportant concepts in paleobiology.As employed here, the unqualifiedterm niche subsumes the potentialinteractions of a species with all thefactors of the environment, physicaland biological. Since evolution is es-sentially an ecological process operat-ing with genetic machinery, a goodcase can be made that the niche isthe most fundamental single unit ofevolution.

For practical purposes, implicit use ofniche concepts and niche conservatismis the foundation of environmental re-constructions that rely on the fossilrecord. Key traits associated with theniche (distribution, morphology, relativeabundances, etc.) were assumed to beconserved and the veracity of that as-sumption was upheld by practical resultsthat followed. As early as the 1960s, pa-leontologists of the oil industry wereessentially applying the concept in faciesreconstruction based on the assumptionthat certain assemblages of fossil marineorganisms represented particular envi-ronments that were associated with oil-bearing strata. Transfer functions, devel-oped in detail for foraminiferalassemblages (31) and used for treerings, pollen and diatoms, are empiri-cally derived equations used to calculatequantitative estimates of paleoclimate,which are then tested against calibrationdatasets (32). Modern plant distribu-tions and their associated abiotic envi-ronments are fundamental for hindcast-ing the climates of the past (33, 34), andassociations of the environment withpresent mammalian geographic distribu-tions have been used as a technique for

inferring terrestrial paleoclimates andpaleocommunities (35). In the case ofQuaternary plants and mammals, theniches of species have been inferred toremain ‘‘constant’’ over the time periodsstudied (usually the last million yearsor so).

The success of these applications de-pends on the relative constancy of nicherelations over long periods of evolution-ary time. In its strongest formulationthis constancy is manifest as evolution-ary stasis, the prolonged persistence offorms in the fossil record with negligiblemorphological change. The observationof stasis, and its implications for evolu-tionary biology, were the focus ofEldredge and Gould’s theory of punctu-ated equilibria (36, 37). Punctuatedequilibria sparked a heated debate be-tween population geneticists and paleon-tologists, and contributed to the integra-tion of developmental biology andevolution (‘‘evo-devo’’) (38). Less appre-ciated was the role of ecological dynam-ics as a potential mechanism of stasis.As Eldredge wrote later (ref. 39):

[S]pecies tend to change locale—rather than anatomical features–inresponse to environmental change.As long as suitable habitat can befound, a species will move ratherthan stay put and adapt to new envi-ronmental regimes.

[W]e . . . have always emphasizedthat it is habitat tracking—stabilizingnatural selection in the face of envi-ronmental change—that basically un-derlies stasis.

Habitat tracking, together with behav-ioral and phenotypic plasticity and thedynamics of species interactions, offer avariety of ecological factors that mayhelp to resolve the observation of evolu-tionary stasis with the ubiquity of cli-mate change and environmental hetero-geneity (40, 41).

Researchers in comparative biology,focused primarily on extant taxa, havealso had a long-standing interest in thesimilarities—and differences—amongclose relatives. Dating back to Darwin(42), comparisons between close rela-tives have played a critical role both toelucidate patterns of evolutionarychange, and in mechanistic studies to‘‘control’’ for the many features sharedby relatives and isolate the importanceof traits of interest. Clutton-Brock andHarvey (43), studying behavioral organi-zation in primates, recognized that thesimilarities of close relatives also pose astatistical problem for the analysis ofcomparative data, as closely related andphenotypically similar species may notrepresent ‘‘independent data points’’

with respect to adaptive hypotheses. Al-though this view has generated consider-able debate, their work did represent akey step in the development of statisti-cal comparative methods in phylogenet-ics. Harvey and Pagel (44), in their 1991monograph, coined the term ‘‘phyloge-netic niche conservatism’’, referring tothe role of ecological sorting processesas factors that would promote stabilizingselection and minimize niche evolution.They drew their concept from a briefpassage by Grafen that outlined thesame argument, acknowledging that itwas ‘‘so natural that it cannot be origi-nal’’ (ref. 45, p. 143). There has subse-quently been a lively debate regardingthe definition, significance and mecha-nisms of niche conservatism, with noimmediate resolution in sight (40, 46–48). Currently, the integration of phylo-genetic comparative methods and statis-tical niche models is opening up a newera of insight into niche evolution and itsrelationship to diversification and bioge-ography (49–51).

In recent years, the niche concept hascome full circle and returned to play animportant role in biogeography, particu-larly in regards to the impacts of climatechange (52–54). Geographical distribu-tions of species in relation to underlyingclimate gradients have received renewedattention, particularly as the foundationfor what are variously termed ‘‘speciesdistribution models’’, ‘‘climatic nichemodels’’, or ‘‘statistical niche models’’(55). These models draw on a variety ofstatistical methods to model species geo-graphic distributions in relation to large-scale climatic and topographic variables(56). Predictions of species responses toclimate change are then derived by com-bining the models with outputs fromglobal circulation models that projectchanges in spatial patterns of tempera-ture, precipitation, and associated cli-mate variables. A critical and unresolvedissue in the use of predictive models isthe extent to which dispersal limitationwill constrain short-term (i.e., century-scale) range shifts (57). However thisproblem is resolved, the models still of-fer one of the most powerful approachesto project potential impacts of climatechange on biodiversity.

In this Sackler Colloquium, we soughta wide array of perspectives on theniche concept, and its role in ecology,evolution, and climate change biology.We intentionally intermixed topics, jux-taposing paleontology with conserva-tion, and conceptual reviews with casestudies. Most of the talks from the col-loquium are represented by papers inthis Special Issue. The result, we hope,is an intellectually diverse yet inter-

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twined series of papers that will illumi-nate current advances at the intersectionof ecology, evolution and biogeography.

The Colloquium opened with a contri-bution from the ‘‘Grinnell Project,’’ a se-ries of studies by personnel associatedwith the Museum of Vertebrate Zoologyto resurvey original study sites laid out byGrinnell in the early part of the last cen-tury. The ambitious project is stimulatedby Grinnell’s prediction that the value ofhis team’s research might not be fully ap-preciated for a century. The first publica-tion from the project focused on mam-mals (58), and showed that although somespecies in the Sierra Nevada (YosemiteNational Park) had similar elevationaldistributional patterns as in the past, oth-ers had undergone major shifts in distri-bution. Tingley et al. (59) found thatmany species of birds have also experi-enced changes in distributional patterns.Bird communities were studied along fourelevational gradients in the Sierra Nevada,including 82 species and 53 separate local-ities. Species tracked changes in the geo-graphical distribution of their climaticniches through time. These results suggestthat climatic niche modeling may proveuseful in predicting the distribution ofbirds under different models of climatechange.

Several authors addressed issues relatedto conceptualization of niches and somehistorical perspectives. An advantage ofthe flurry of activity in discussions ofniche conservatism is the multiple per-spectives afforded by workers from manyfields. The disadvantage is that conceptsof the niche may not always align, whichconfuses the discussion. Soberon and Na-kamura (22) focus specifically on discrimi-nating between fundamental, potential,and realized niches to clarify exactly whatis conserved. The distinctions between theHutchinsonian ‘‘response niche’’ and theGrinnellian ‘‘impact niche’’ are timely be-cause they have received relatively littleattention and figure prominently inwhether we view niches as conserved ornot. Using a mathematical approach, So-beron and Nakamura also examine thefundamental differences in niche model-ing algorithms that use presence data ver-sus those that also rely on absence data.They conclude that clearly defined termi-nology and explicit treatment of the un-derlying variables in niche conservatismwill do much to integrate this field. Col-well and Rangel (21) explore howHutchinson’s niche concept differed fromearlier conceptions. Whereas Grinnell andElton saw niches as elements of environ-ments, Hutchinson attributed them to spe-cies. They identify Hutchinson’s termbiotope with a formal separation of physi-cal place from environment and argue

that his duality (niche vs. biotope) estab-lished many elements of the modern ap-proach to classifying and mapping envi-ronments, modeling species distributionsunder different climate models, and ingeneral contributed greatly to broaderdimensions of modern studies of niches.Holt (55) follows with an investigation ofthe demographic basis of the Hutchinso-nian niche, emphasizing that under cer-tain circumstances, possibly more wide-spread than currently recognized,conditions allowing population growthfrom low density can differ from the con-ditions under which an established popu-lation can maintain itself. Holt’s work alsoprovides an important link between mi-croevolutionary processes and larger mac-roevolutionary patterns.

Few workers use physical principles tomodel persistence in environments. Porterand Kearney (60) apply an ecophysiologi-cal approach to explicitly model the ther-mal niche of a endothermic ellipsoid or-ganism. From first principles of physics,they quantify how shape, size and bodyinsulation (e.g., mammalian fur) affectmetabolism in variable environments andtest this against field and laboratory data.Their data show the power of biophysicalmodels for combining functional traitswith environmental data to recapitulateobserved gradients in body size and sug-gest this as a fruitful area of further re-search. For example, recent exciting re-search combines ecophysiological modelswith population dynamics (see ref. 61).

From the outset, niche theory hasplayed a central role in communityecology. The mechanisms underlyingcommunity composition and phyloge-netic diversity are the subject of thestudy by Graham et al. (62) comparing189 hummingbird communities in Ec-uador. A robust phylogenetic hypothe-sis is used to assess how species andphylogenetic lineages change along en-vironmental (e.g., elevational) gradi-ents and biogeographic barriers. Atlower elevations there is less phyloge-netic clustering than at higher eleva-tions, in accord with ideas that compe-tition is more important in thelowlands and environmental filtering inthe uplands, where coexistence of closerelatives is found. Their study providesinsight into the pattern of faunalbuildup in a biotically rich and com-plex region. Community assemblage isalso the focus of Okie and Brown (63),who examine the effects of rising sealevel and concomitant creation of is-lands in the Sunda Shelf region. Theyshow how the original mammal faunaof this region has been disassembledon islands of differing size and com-plexity following the late Pleistocene

events. Diversity is inferred to havedropped thoughout the islands. Okieand Brown propose that unique mam-malian traits, including body size andniche characteristics such as habitatand food requirements, played a rolein extinction probability.

Jackson et al. (64) apply niche con-cepts to long-term ecological dynamics,emphasizing some simple yet surprisingimplications of climate variability nestedacross temporal scales. They illustratethat contrasting conditions for establish-ment and persistence can generate an‘‘ecological ratchet,’’ with episodic dy-namics of range expansion and contrac-tion (see related discussion in ref. 55).They further discuss several sources ofuncertainty in the application of correla-tive niche models to the problem of fu-ture climate change, including the com-plex dynamics of climate change, theimportance of the regeneration niche,and historical contingencies that canalter dynamic response of ecologicalsystems.

Several papers address aspects ofniche evolution, from small scale popu-lation level dynamics at range edges, tomacroevolutionary patterns evident incomparative data and the fossil record.Processes operating at the margin of thegeographic ranges will attract increasingattention as the magnitude of climatechange becomes more evident to biolo-gists. Angert (65) conducted field stud-ies of the demography of two species ofclosely related monkey-flowers (Mimu-lus) in the Sierra Nevada of California,focusing especially on populations at themargins of geographic ranges. Centraland marginal populations showed strik-ing differences within species but therewere also differences between species.One species had less productive popula-tions at lower elevational marginal re-gions whereas both displayed higher fe-cundity and population growth in theupper elevational marginal regions. Re-cent climate changes are thought tohave shifted climatic envelopes in thedirection of higher elevations.

The study of niche evolution is animportant focus of comparative biology,integrating ecological diversity and phy-logenetic history. Ackerly (48) presentsa simple extension of the theory of inde-pendent contrasts to measure rates oftrait diversification, focusing on planttraits that are associated with growthand regeneration strategies. Compari-sons among clades reveal �100-fold dif-ferences in rates of trait diversification,with higher rates on Hawaiian islandclades compared with continental cladesfrom California and the North Temper-ate flora. This approach could be ap-

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plied widely across other groups, ascomparative data and time-calibratedphylogenies become available.

Hadly and colleagues (41) examinepatterns in the range size of mammalgenera in North America from the latePleistocene through the late Holocene,comparing distributions in the fossiland modern record. They find remark-able stability in range size within gen-era through time, despite the glacial-deglacial transition, extinction ofspecies within the genus or how manyspecies are included in the genus. Theysuggest that different processes mayinf luence niche conservatism at highertaxonomic levels, positing that intrinsictraits are more important at higher lev-els and environmental controls mayplay more of a role at the species level.Using range size as a proxy for the re-alized niche, they propose a ‘‘genusniche’’ and underscore the importanceof maintaining a genus pool for con-servation of North American mamma-lian communities.

Across the broadest spatiotmporalperspective, Vieites et al. (51) apply aninnovative use of the niche to explorehow environmental tolerances ofsalamanders, which are thought to beprime examples of organisms with phy-logenetically conserved niches, haveevolved since mid-Tertiary times. Theycalculate the environmental niche spaceof species in the family Salamandridaein the northern hemisphere and usethese data to propose the environmentalniches of the lineage. The study exam-ines methods and shows the promise ofcurrent approaches while at the sametime indicates some of the dauntingproblems remaining.

The final set of papers examines ap-plications of niche theory in climatechange and conservation biology. Zim-merman and colleagues (66) considerthe importance of climatic extremes inshaping distributions of tree speciesalong climate gradients in Switzerland.Incorporation of climatic extremes(i.e., interannual variability) offers amodest but significant improvement infitting distributions to climate data,and especially helps to correct mar-ginal areas where over and under-pre-diction problems arise. Given the clearphysiological impacts of of climate ex-tremes, they suggest several reasonswhy niche models based on climateaverages may perform as well as theydo. Wiens et al. (67) implement twospecies distribution models on a finegeographic scale to predict the futuredistribution of bird species in Califor-nia by 2070 (also see 68). Of the 60species they model, a majority show areduction in geographic distributionwithin California. As a practical exer-cise, Wiens et al. compare and contrastthese models and discuss implicationsto managers of the differences betweenthem. They are mindful of the uncer-tainties and assumptions inherent inspecies distribution models and futureprojections, but stress the urgency ofaction despite them. The world ischanging at a pace perhaps faster thanour ability to model biotic responsewith our traditional standards of statis-tical rigor.

Niche theory has a variety of applica-tions in conservation and restorationbiology, beyond the current focus onclimate change. Using patterns of abun-dance and distribution in AustralianWet Tropic vertebrate species, Williams

et al. (69) investigate the relative size ofthe niche. Counter to prevailing wisdom(70), they find that narrowly distributedspecies confined to the rainforest havehigh local abundances. They posit thatpersistence of restricted species is possi-ble when intrinsic demographic traitspermit high local abundance. Their re-sults challenge the paradigm that geo-graphically restricted species have agreater extinction probability than dowidely distributed species.

In the following pages, a rich array ofcontributions to our modern under-standing of the relationship betweenorganisms and species, their ecologicalniches, and biogeography is presented.Although the integration of these topicshas been a century-long goal, we have asense of rapid progress during the lastdecade, and especially at the presenttime. In this era of rapid climatechange, new ways of thinking are essen-tial as we confront the reality of impactson organisms and their distributionthrough space and time. If even a frac-tion of the projected changes are real-ized, the changes we face are trulyfrightening and the world of 2100 maybe drastically transformed from itspresent state.

ACKNOWLEDGMENTS. We thank the colloquiumpresenters and authors of the accompanying pa-pers for their thoughtful and insightful contribu-tions; S. Marty and her staff at the Beckman Cen-ter for hosting the colloquium; J. Patton, whofound the Walter Taylor field notes and broughtthem to our attention; A. Prinzing for pointingout Ellenberg’s early contributions; G. Clausingfor assistance with translation; J. H. Brown on theProceedings of the National Academy of Sci-ences Editorial Board; and D. Stopak and K. Du-gan in the Proceedings of the National Academyof Sciences office for their guidance and over-sight of the editorial process for this special issue.

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