The Madrean Sky Island Archipelago: A PlanetaryOverview
PeterWarshaltl
Abstract.-Previouswork on biogeographicisolation hAFoncerned itself with oceanicisland chains, islands associated with continents, fringingarchipelagos, and bodiesof watersuch as the Africanlake system which serue as "aquaticislands". This paper reviewsthe "continentalislands" and comparesthem to the Madreansky island archipelago.The geological,hydrological, and climaticcontext for the Afroalpine,Guyana, Paramo, low and high desert of theGreat Basin, etc. archipelagosare comparedfor source areas, number of islands, isolating mechanisms,interactive ecosystems, and evolutionary history. The history of scientificexploration and fieldworkfor the Madrean Archipelagoand its uniquestatus among the planet'sarchipelagos are summarized.
In 1957,Joe Marshall published "Birds of the American Prairies Province of Thkhtajan, 1986) Pine-Oak Woodland in Southern Arizona artd Ad- and western biogeographic provinces, a wealth of jacent Mexico." Never surpassed, this elegant genetically unique cultivars in the Sierra Madre monograph described the stacking of biotic com- Occidental, and a myriad of mysteries concerning munities on each island mountain from the the distribution of disjuncts, species "holes," and Mogollon Rim to the Sierra Madre. He defined the species "outliers" on individual mountains (e.g., Madrean archipelago as those island mountains Ramamoorthy, 7993). The northernmost sky is- with a pine'oak woodland.lnl967, Weldon Heald lands are the only place in North America where (1993), from his home in the Chiricahuas, coined vou can climb from the desert to northern Canada the addictive phrase-"sky islands" for these in- in a matter of hours (Warshall, 1986). The sky is- sular mountains of the North American lands pose numerous puzzles about vertical borderlands. Weldon Heald's catch phrase immor- migration strategies used by plants and animals talized |oe Marshall's meticulous observations. both annually and over glacial time periods. Today's conference is the first solely dedicated to These interests in ecology and evolution mix understanding Madrean sky island biology, with the other citizen interests in skiing, grazing, beauty, and needs for rnanagement and conserva- hunting, fishing, escaping the heat, summer tion. I homes, telescopesand radio towers, bird watch- There are about 40 sky islands (fig. 1) between ing, rock-climbing, military practice maneuvers, the Mogollon Rim and the Sierra Madre Occiden- fuelwood cutting, camping, mining, sacred Native tal (Warshall, in press). Mt. Graham on the American values and ceremonies, archaeological Pinaleno mountains is the tallest peak (10,712 sites,as well as preservation of sky island habitat feet), Relief between valleys and peaks ranges for threatened and endangered species.Underpin- from 1250to 6750f.eet (Mclaughlju;.,7992), bui is ning all these interests is the exceptional beauty of typically between 3000 and 5000 feet. The Ma- the sky islands-their layering of peaks in a dusty drean region has exceptional species richnesi, sunset, lines of vibrant ripariart along arid yellow super-species complexes, unusual neoendemics slopes, the contrast of snow and desert, the baf- and archeoendemics, an exceptional mixture of fling complexity of erratic ridge lines, the power species from the Nearctic and Neotropic regions, of files, and the subtle tones of blue-gray lime- important influences fronn the eastern (North stoner,speckled granite, and pastel volcanics.
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7 WHATARE SI(Y ISTANDS? SI(Y ISI.ANDSOF THE PI-ANET
Sky islands are a type of continerrtal or inland There are about twenty sky island complexes terrain made up of a sequence of valleys and on the planet (table 1 and fig. 3). Atl the conti- mountains. All skv islands have a stack of biotic nents with, perhaps, the exception of Australia, communities that illo* vertical (as well as aspect) harbor sky island complexes. The information on migration arurually or during one of the planet's the sky island complexes of Eurasia, China, and long-term climatic events. The valleys act as barri southeast Asia remains incomplete becauseof the ers or bridges to the colonization by new species difficulty of obtaining English translations (Sus- that attempt to cross the intervening valley. The lov,1967; Aiken, t992). Most of the literature has vallevs become barriers when thev contain an focused on the mourrtains themselves, not the im- ecology alien to the migrating specie's.By analogy portance of the valleys between them. There has with the saltwater seas betweert oceanic islands, been remarkably lithe work comparing the the higher elevation biotic commurrities of sky is- planefs continental island ecosystems (Carlquist, land mountains are isolated by each valley's "sea" 1963), their palaeogeographic history, floristic and of alien vegetation. The mountains, like the Gala- faunal source areas,and vallev barriers. patos or other oceanic islands, act as isolated By creating a parallel typology with recent cradles of evolution. classifications of oceanic islands such as isolated In the Madrean archipelago, the valleys and island "chains" (Hawaii), "continent associates" motrntains are roughly parallel. The stacked biotic (Madagascar,Philippines), and "f ringing archipel- communities (fig. 2) include: montane coniferous agos" (the southern japanese or Sea of Cortez forests; oak-pine (coniferous) woodlartds; tropical island groups), we can approximate a classifica- deciduous forest; oak savanna (deciduous vs. ev- tion for the continental islands. Thble 1 classifies ergreen oaks predominate); short-grass prairie; continental island clusters on the basis of their subtropical thornscrub; and subtropical desert geographic axis, ladtude, whether they are coastal (Brown, 7982). The "heart" of the Madrean archi- or inland, the number per complex, and the con- pelago (its defining characteristic) is the oak-pine figuration of each grouping. Configurations woodlands (Marshall,795n. The barrier "seas" in- include: stepping stone archipelagos (mountains clude the short-grass prairie, the subtropical and valleys spaced between two cordilleras), iso- thomscrub, and subtropical desert. lated massif(s) with outlier sky islands, linear
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OPOSURA ft.'... .t!.-.n'. Flgule 2.-Ex.mple "ttrcklng" of Dlotlc communlderof thc lledrern rrchlpelrgo (llffrhrtl, 1957). Note the podtlon ot thc ork-plne woodlrndr. chainsof outlier mountains/valleys at one end of Table1.-Sky lsland Tvoer a cordillera, and completely isolated groups of rVpe Examples mountains. A surprisi^gly large number of com- Steppingstone archipelago Madreanarchipelago plexes (e.g., the Adamoua, Baja Peninsula, and between two mountainchains Great Basin archipelago arc) contain both oceanicislands and Altai/TienShan Basin East African Meso-Americanmassifs sky islands with both saltwater and vegetative "seas." Like all typologies, there are ambiguously lsolated massif with oulliers Ethiopianhighlands East African arc categorizedgroupings. Saharan massifs Atlas Mountains Jabal Lubnan Adoumoua Mountains PlanetaryExamples Drakenbergs Central European massifE The four descriptive variables used to classify Caucaso-lranianmaasile sky islands act in ioncert. Compare the Madrean Cordillerawith outliers Chaine Annimatique (Vletnam) stepping stone configuration with the isolated ar- Malay peninsula chipelago of the Western Ghats of India (not Baia Galiforniapeninaula Coastal Cordillera(SA) illustrated), the isolated cluster of the Pantepuisof Southern Andes Brazil, Guyana, and Venezuela (fig. 5) and the East African arc and the Ethiopian massif com- lsolatedsky island chains Western Ghats plexes (fig. 6) . The Western Ghats (Manickam, Pantepuig 1992) comprise a north-south cluster of tropical High aliitude sky islands Punas and paramoe (SA) mountains spanning about six degreesof latitude. Himalayas
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Flgure 3.--.Locrdonr of *y lrlrnd complexe. mentlonedIn text on r m.p ot the Florle$cReelmr of thc phnet Only ttc lhdrcrn .]chlpelrgo ltrrddler two; Florlrdc Rcrlmr. (1) llrdreen; (2) Gr.rt B.3ln; (3) Brle Celllomlr (4) ile.o Amerlcen; (5) Cordrl Cordlller|: (€) Prnt pulr: (7) 8ralllrn rhleld; (t) Srhrnn me$l{r; (9) Ehloplrn Hlghlmd.; (1o) E.tt Afrlcrn rlc; (11) Crmcroon Blght; (12) SouthernAfdcrn compler; (13) Weilem chrtr: (14) Ctucro lrenlan mr.rib; (15) Altllmen Snan; (1O)Tnnr-Brlkrl: (17)ChrlneAnnlmrdque; (1S) Malay peninrulr. High valley "lslendf'can be foundIn the nortrernAndcr rnd Himtlayrr. Thev are isolated from the nearest cordillera, the Himalayas, by over 7,200 miles. The Western Ghats ire funtrer isolated to the south and west by the Laccadive Sea. On the north and east, the $ c valley barriers are, at the present, highly human- ^ ized landscapes with pockets of Deccan NOtrlllERtl- thornforests. The Western Ghats contain about five distinct mountain with one major valley (the ROCKY['lOUNfAD.lS Palghat gap) subdividing the archipelago. Some- times, Sri L^ankahas been included as an oceanic ryr island within the sky island archipelago. In part, I '; the high endemism of the montane Malabar rain- NORTtlERl.l forests which cover the Western Ghats can be ROOff traced to its former Gondwanaland corurection. The Pantepuis (Minestero, 1985;Fittkau, 1969; Steyermar k, 7982; deGranvil Le, t982; Haffeg t987) form a scattered, isolated tropical series of 15 og large and 20 smaller sky islands. The Pantepuis sotfiltahl cluster shows no distinct axial direction and relief ROCKT is relatively small (about 2000 feet). In this part of South America, a "tepvi" means a "sky island." The maior "barriers" isolating the individual tepuis are rivers, many with "blackwater" (acid) waters. The southem boundary the gigantic Ama- zon Basin, separates it from the sky island complex of the Brazilian shield, over 1500 miles away. Both the Pantepuis and the shield once were parts of the s:rme plateau and contain nu- merous parallel taxa. On the west, the Orinoco River and, to the east, the Essequibo River act as major barriers. Only to the northeast do a few tepuis approach the outlier mountains of the coastal cordillera, including the offshore islan& along the Caribbean coast. The inter-mountain valley "seas" include savanna, caatinga, and chap arral as well as the'tlackwater" rivers. Both the Ethiopian complex and the 7,anj or East African arc complex (Kingdon, 1990) gained their relief from the great East African rift valley (seebelow). on the Flgur. a.-{-oc.tlon ol tho rrchlpolrgo .r l .t pplng .tdt b} The Ethiopian complex centers tu.on !|| Rocky llountrln cordlllrrr lo Ur. north rnd lro high elevation basaltic plateau that rifted into two Slrnr llrdn to tic lou|h (modlf,rd lrom P..g l9!t). AF massifs (the Simien and Bali mountains). The Fodmrlr boundrrlat ot tho four mrlor tlorldc Fotrlnc.r Indlcdod by rolld llno, Ielor Slrrr lhdrr mountrlnr ftr Ethiopian complex is isolated by the White NiIe to cludo: (1) llrnr-llrdcrn yo|c.nlc Oolt; (2) Sbrn l.dll the west; the Kenyan, Somalian and Saudi deserts Olbrtrl, .nd (3) thr Slorrr llrdrr Occldontrl whhh lr tho as well as the Red and Arabian Seas. It has a norroet Xorlcrn lourer r?oelorth. llrdnrn rrahlpohgo. tho lrdrrn rly lrlendr rhoryn Includo (C) Chlrbehu strong north-south axis covering seventeen de- nountrlnr, p) 9rntr Crtrllnr llountdnr, end llr (t) Pl- grees of latitude with fifteen to twenty peaks n lorpr Uountrlnr. Tho llogollon llon (e) |t lh. rlo|r.tt Focky Xamtrln rou?corr.r tor tfie trd.crn rrchlprlrgo. greater than 9,000 feet in elevation. The relief is Tho slf,n llrdn Orlentrl *y lrlrnd complox Includor ({) typically 5000 feet. The valley barriers include tho Drvlr Tount|lnt rnd (5) Slrrn Elrncr. harsh desert, acacia/commiphora bushland, and dry savanna. The isolated biotic communities in- Asir/Hadramawt sky island complex of Yemen clude the higher elevation iuniper / podocarpus, and Saudi Arabia. montane bamboo, ericaceous tree / shrub and The East African arc massifs, sky islands and afro-alpine belts. In its largest dimensions, the three oceanic islands (Pemba, Mafia, and Ethiopian sky island complex includes the 7-anzibar)create a rough geographic circle involv-
10 EOUATOR
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FlgureS.-Thrco SouthAmerlcrn .ky ldrnd compt.xc..r. lhown: (1) Ibo p.ntcpult; (2)the Bredllrn rhleld; rnd (3) the Co..Ul Cotdllhr.. ing seven African nations and twelve degrees of endemism. For instance, the South American equatorial latitude (fig. 6). The circle comprises p.uamos (the alpine belt) have acted as "popula- older massifs such as the Ruwenzori (Monts Mi- tion traps" during repeated glacial events @rance, tumbe), Kipengere, and Udzungwa ranges; the 1987; Kant, 7989; Haffer, 7987). These high eleva- Nyika and Rukiga plateaus, and the Mau escarp- tion, sky island valleys form a special group of ment combined with uranv more recent isolated continental islands similar to isolated lakes (e.g., mountains including the Usambara, Uluguru, Mt. . Baikal, Malawi) artd caves. Kilimaniaro, Mt. Meru, and Mt. Elgon. Relief ex- ceeds 9,000 feet for the seven highest isolated volcanos. Rift valley lakes (e.g., Lake Victoria and The MadreanArchipelago: Comparisons Lake Tanganyika) and, of course, the Congo River Basin and lndian Ocean act as important barriers The Madrean archipelago is a "stepping stone to gene flow. The valley barrier is predominantly archipelago" between two mountain chains (the savaruvr.The montane communities parallel those Rocky Mountains and its plateaus and the Sierra of the Ethiopian massif. Madre plateau and its mountains) arranged in a Finally, the north Andes and Himalayas con- roughly north-south axis (figs. 1 and 4). Archipel- tain a contrasting variant - high altitude island agos between two cordilleras may have greater valleys.. These cordilleras have "sky island" val- opportunities to increase biological diversity be- leys or plateaus embedded within the mountain cause the configuration provides two source areas ranges. The valleys contain exceptional pockets of instead of one. In general, north-south axes (e.9.,
11 the Madrean archipelago,Baja Peninsula, Western Caucaso-lranian, Central European Highlands, Ghats,Malay Peninsula)also provide for a greater Jabal Lubnan-appear to be so altered by human amount of climatic variation than east-westaxes influences that it is difficult to determine what (e.g., the Himalayas, Eurasian ranges, Coastal biological inJormation remains.) These mid-lati' Cordiliera of. South America) and, perhaps, tude complexes experience greater anrtual and greater potential for evolutionary differentiation. decadal climatic flux. The Madrean is unique, Compare the Madrean archipelago to the Great even amont mid-latitude sky island complexes, Basin.The Great Basin is a sky island cluster two because temperate and subtropical climatic re- cordilleras but its axis, between two major source gioru interfinger with tropical climates found in areas,is predominantly eastto west. the Caribbean. The Madrean archipelago is a mid-latitude sky The tropical sky island complexes include the island complex, along with the Saharan massifs, Meso-American massifs; Ethiopian and Adamoua the South African complex, the Atlas Mountains and East African highlands in Africa; the Coastal (Morocco), the Great Basin, the Baja California Cordillera, the Pantepuis and Brazilian Shield peninsula, and perhaps a range in China. (Other clusters of South America; and the Western Ghats, possible mid'latitude sky islands complexes" the the Malay Peninsula and Chaine Arurimatique of .:i:ii.lTffiffi
t Mountainousareas ffi Lowlandlorest % Moistsavanna types n Woodlands e Driersavanna types, ffi bushand thicket tl Desenand semi desert mmCapevegetation
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12 South Asia. In recent geological time, the fluctua- _ T.ble 2.-Geology rnd the Sky l.hndr tion of pluvials or inter-pluvials has had a major rYpe Examples _ inlluence on whether the valley vegetation was a Relictualor Subeequent Most Tepuis bridge or a barrier. The Ethiopian region is the (Venezuela),WesternGhats only tropical sky island complex that harbored (India)Brazfi ian ehield signif icant glaciation. Tectonicor Original East Alrican arc volcanoes Distinctly northern complexes, higher than 40 A. Accumulationmountaing Mt. Camoroun (flood degrees latitude, include the Altai/Tien Shan and basalis, volcanice) Dztrngarian basin r.rnges; the heavily altered Py- B. Deformationmountain8 Himalayas rennes and Scottish Highlands and a fewer other Bl. Foldmountaing Andes (altiplano) CIS ranges. These areas lost their biota during the 82. Dislocalionmountains Great Basin glacial periods and have only recently (about 10 to (horsUgraben) MadreanArchipelago years ago) been recolonized. Alrican Rift Valley mountains 15,000 Altai Along with the Pantepuis, the Brazilian shield, Trans-Baikal the Great Basin, and most of the Eurasian r:rnges, the Madrean archipelago is distinctly inland. The 83. Laccolithmountains Madrean has a special featur*the lowest pass or mountain gap in the Rocky Mountain cordillera between northem Canada and the Isthmus of lie- richness and diversity of the Madrean archipel- huantepec. The low elevation pass encouraged a80. east-west movement of animals and plants from valley to valley. Otherwise, the Chihuahuan and Mohave deserts (the desert "seas") and the Gila HOWWERE THE SIff ISI-ANDCOMPLE)GS River geographically isolate the Madrean archi- CONSTRUCTED? pelago. As previously mentioned, the inland configurations are the true sky island complexes. The geologic history of each sky island com- M*y of the others are bordered on one side by * plex provides the stage for its evolutionary and oceErnor large lakes. Some are surrounded on two ecological history. The four most important char- sides by oce.rn. Peninsula sky island complexes acters are: the nature of the bedrock, the general have very limited opportunities for colonization shape of the motrntains and valleys as controlled becausetwo or three sides are ocean compared to by erosive and tectonic forces, the timing of distinctly inland clusters. mountain building, and the creation and destruc- The Madrean Archipelago contains a large tion of barriers and bridges to migration of living number of sky islands per complex (N = 40). Only organisms. Table 2 classifies the sky island com- the Great Basin (N = 25 to 50 depending on defini- plexes by the geologic process which formed the tion), the East African arc (N = two massifs,about highlands and the topographic relief between val- 30 sky islands, and three oceanic islands), and ley and motrntain (Mani and Giddings, 1980). Pantepuis (N = 15 large and about 20 smaller sky The Western Ghats, most of the tepuis, and the islands) have comparable numbers of isolated Brazilian shield complex are relictual or sub- mountains. Depending on how boundaries are sequent mountains, i.e. motrntains that started as drawn, the Ethiopian complex has two major mas- a plateau and were increasingly dissected by river sifs and about 12 smaller skv islands without the erosive forces. For instance, the Pantepui's geo- addition of the Hom of Airica mountains and logic history both constrained and encouraged the those of Saudi Arabia and Yemen. All other rich endemism of its montane flora. The majority groupings are less than 15 isolated mountains of tepuis are hard sandstone with acid lithification with the majority containing only about five. (soil-forming properties). Most tepuis are flat- ln summary, North America's Great Basin and topped motrntains (providing more habitat for Madrean archipelagos appear unique on the colorrization) with steep cliffs and talus slopes planet for the number of mountains and the step (creating diverse habitats). The evolving flora had Pi'lg stone arrangement between roughly parallel to be acidtolerant and, despite the high rainfall, valleys and mountains. Onlv the Madrean archi- xeric in growth form. The few tepuis of granitic pehfo has the distinct nortli-south cormection to comlnsition were not exceptionally differentiated two maior cordilleras and crossesfrom temperate from the sandstone tepuis. Granite also produces to subhopical latitudes. This topography, rclief, acid-soils. Plant colonization between sandstone and location explain a large part of the biological
13
- and granitic tepuis encountered no immediate The Sierra Madre sourcearea for the archipel- "soil barrier." ago is the largest rhyolite mass on the planet with Tectonic or "original" mountains include 4,000 foot deep sequencesin the Barranca del Co- mountains (1) built-up by volcanic ash and flood bre. The major Rocky Mountain source area for basalts; (2) deformation mountains resulting from the archipelago is the Mogollon Rim and Colo- continental folding or rifting; and (3) laccolith rado Plateau, one of the largest intact sedimentary mountains. I could find no insular laccolithic sequences on the planet. These relatively flat mountains that were also skv islands. source areas have been instrumental in feeding At least six sky island complexesare combina- colonizing speciesto the archipelago. tions of mountain bedrock from volcanic ash The evolutionary and ecological consequences and/or flood basalts with subsequent deforma- of this mix of rock types and horst/graben rifting tion. For instance, the flood basalt mountain are: (1) a confusing array of piedmont, terrace and cluster of the Ethiopian Highlands was frag' valley soils that support a huge diversity of mented and gained relief by the sinking of the grasses,shrubs, and trees (R. McAuliffe and T. East African rift valley. Volcanism stopped three Burgess,this volume); (2) talus slopes which, for to four million years ago, allowing recolonization instance, support a remarkable diversity of snails without further geological disturbance.The Ethio- (McCord, this volume); (3) limestone slopes and pian highlands are relatively flat-topped (vs. outcrops that greatly increase the diversity of peaked) becauseof their plateau origin and age. plants on particular sky islands (Mclaughlin, The flatness provided an extensive area for afro- 1993 and this volume); and (4) altitudinal limits alpine populations to evolve. that extend beyond "typical" climatic zones be- Similarly, the Madrean sky islands are defor- cause of the water holding capacities and heat mation mountains from continental rifting that characteristics of particular rock substrates began about 13 million y€ars ago. The mountains (Shreve,1922). did not rise as much as the vallevs sank. This 'ba- In addition, there are other climatic/geologic sin and rante" or "horst/grab6n" development derived habitats that increase the floral diversity exposed older rocks derived from a highly diverse (e.9., the morainal-related cienagas of the Pi- geologic past: multiple marine invasions, caldera nalenos from the last glaciation (Warshall, 1986), explosions and lava flows, and metamorphic core the seepsof the Galiuros, the relictual clay valleys complexes. "Basin and range" tectonics controlled of the San Rafael and similar unstudied valleys in the parallel valley and mountain sequence so Mexico, the aerosol-derived caliche soils of unique to North America and largely shaped the some lower slopes and valleys, and the remnant large number of "cradles" of evolutionary differ- sand dunes in the Animas, San Simon and other entiation. valleys.) As opposed to the Ethiopian complex,each in- dividual Madrean sky island is a remarkable mixture of rock gpes. Exposed rock types include intrusive igneous rocks (granite), extrusive vol- HOWDID IT HAPPEN? canics (rhyolite, dacite, basalts), metamorphics BARRIERS,BRIDGES, AND SOURCES (gneiss, schists, quartzite), and sedimentary rocks (limestones,shale, conglomerates).For example, The palaeogeographic history of sky islands the Chiricahua mountains are, in large part, an has become an exciting and controversial topic. individual volcano overprinted by Great Basin- Does stabilitv or instabiliW or what combination type dislocation. The Chiricahuas are the planet's of the two fenerate high'levels of biodiversrty? largest recorded volcano whose ash flows contrib- Some biologists like to emphasize proximate uted to parts of five different sky islands in the causes such as the most recent glacial events, the Madrean archipelago. [n contrast, the Pinalenos, mountain's areal size, ruggedness, exposure, as- the Santa Catalinas, the Rincons, and the Dra- pect, altitude, distance to nearest sky island, and goons are metamorphic core complexes. Each core distance to assumed maior source area. Others complex is extensively gneis or granite. Other sky emphasize the long-term historical framework of islands are predominantly limestone or various planetary palaeogeography. It is beyond the scope limestones (e.9., Huachucas) and almost all the of this paper to review sky island speciation theo- sky islands have remnant slopes with limestone ries (see Bush, 1994 for the Amazon region; outcrops. fournal of African Ecology,1981 for EastAfrica).
14 The Madrean sky islands are uniquely situated migration on each individual mountain, the ge- in this controversy because all kinds of evi- netic/phenotypic plasticity of the taxa, speciation dence-pack rat middens, pollen cores, lake rates,and climatic/vegetation change.Certain sky sediments,tree rings, fossils, geomorphic surface islands have provided "safe sites" for organisms dating, sea temperatures, radiocarbon and other becauseof their latitude and topography. Some isotope dating-
1s
----- floristic realm which partially explains the regions the varied rock substrates and lichens of the indi- outstanding number-of floral endemics (Takhta- vidual sky islands? Why does it appear that many jan,1985). southem mammals have expanded into the north- Reflecting this floristic/faunal straddle and ern sky islands and beyond in the last the relatively recent landbridge between North half-century? How important is vertical migration and South America, the Madrean archipelago and upand-down slope and from the flanks into the its two immediate sources areas contain the bie' valley toward maintaining population stability ? geographic limits of L4 plant families (northern limit); 11 bird families (seven at their southern limit; four at their northem limit), 30 bird, over 35 SUMMARY reptile, and about 15 mammal species.The biore- gion supports the second highest plant endemism From a planetary point of view, the Madrean between the Pacific coast and the Great plains sky island archipelago has these unique features: (Mclaughlin, this volume). Sixty percent or more . It is an archipelago betweert two cordilleras of the plants of the dry forest, oak/pine wood- (the Sierra MadrJOccidental and the Rocky lands, and deserts are probably endemic Mountains); (Rzedowski, 1993). Similarly, the ant fauna (S. . The archipelago contains an extraordinary Cover, this volume), the lichens, the snails, the number of sky islands per complex; reptiles, the birds, and the mammals show excep- . The complexhas a rrorth-south axis spanning tional species richness and/or endemism. The about eight degreesof latitude but contains a bioregion is considered the most diverse sector of distinct east-west valley bridge through the the United Statesfor ants, mammals and reptiles. Deming Gap of the Rocky Mountains; The archipelago harbors many poorly studied o The islands have a mixed geological composi groups (e.9., Buchman, this volume; Carl Olson, tion apparently unrivaled in other areasof the personal communication) and many taxonomi- ptarag cally confusing groups (e.t., McCord, this e The islands have a m,oderate to high relief volume). But, as opposed to other regions, de- (5,000 feet) compared to other valley,/mowr tailed systematic investigations have led to more tain complexes with the obvious exceptions of species or subspecies ("splitting"), not fewer the Himalayas and Andes; ("lumping"). For instance, Erigeron pringlei, ale . The Madrean archipelago is the only sky island cal fleabane, on detailed investigation, was split complex to straddle two maior floristic and into four species, including a new endemic to the two maior faunal realms as well as the conver- Pinalenos (E. heliographus). A new frog was re- tence of three maior climatic zones (hopical, cently discovered in Ramsey Canyon (Rana subtropical, and temperate); subaguavocalis). The Mt. Graham red squirrel Its mid-latitude position placed it at the edge (Tamasciuris hudsonicus grahamensis), pre- of glacial influences without experiencing any of viously considered one of twenty-six subspecies, the mass extinctions that occurred in the sky is- now appears increasingly unique. land complexes of the more northern latitudes; The Madreart archlirelago is the planet's center The Madrean archipelago endemism artd spe- for the long coevolutionary history of several spe- cies richness probably reflects a palaeogeographic cies groups. The coevolution of squirrels, history in which maximal disturbance with a large woodpeckers, jays and one parrot, the woodland's turnover of species (e.9., the Pleistocene mega- mycorrhizal, epigeous and hypogeous mush- fauna) produces great biodiversity, It is not a rooms and truffles (States, 1992) centers in the region of maximal stability. Madrean pine/oak woodlands. The coevolution I have not mentioned one maior influence on of agaves with bats and hummingbirds centers in the planet's sky islands - humans. The influence the semi-desert grasslands and thomscrub. of Iron Age deforestation and the spread of do- There are irurumerable mvsteries: Whv do mestic livestock have permanently changed many some localities have only piries or only iraks, sky island valleys and mountains. The Horn of while others have pine/oak woodland? Why are Africa, for instance, has had livestock since 5,000 there no chipmtrnks on the Huachucas?Why does BC. The Mediterranean mountains have experi- the Mexican chickadee (Parts sclateri) stop at the enced livestock and significant treecutting since Chiricahuas and not cross the 35 miles to the Pi- about 1,000 BC. The loss of diversity cannot be nalenos where the Mt. Chickadee (P. gambeli) retrir:ved or fully known in sky island troups starts? How have the talus snails coevolved with such as the Pyrerutes, the Atlas Mountains of Mo-
16 rocco,or the Cabal Lubnan of Lebanon. The Ma- ACKNOWLEDGMENTS drean archipelago and its admirers are lucky. Malor Iron Age and domestic livestock influences I would like to thank Robert Scarboroughfor on the Madrean archipelago started less than 200 immense help with finding sky island ecosystems 'I'ears ago. on world maps and discussing all aspectsof their However, the pace and intensity of human in- geology.Tony Burgess,Paul Martin, Charles Cole, fluence has been accelerating. 150 species of Steve Mclaughlin and Dale Turner helped this concern-species listed as vulnerable-dwell talk bv wav of referencesand conversation.Diana within the sky island archipelago, excluding bats Hadldy hai been,as usual, a great editor. (Warshall,in press;Sky Island Alliance, 1992).The jaguar, grizzly, ocelot and graywolf have been ex- tirpated from the United States portion of the LITERATURECITED archipelago and are probably extirpated from the Mexican portion. Genetic swamping or replace- Aiken, S.R.and C.H. Leigh.1992. Vanishing Rain Forests: ment from translocated species (e.9., The EcologicalTransition in Malaysia.Oxford Mono- hybridization of rainbow and Apache trout, graphs in Biogeography No. 4. Clarendon Press. gairdneri X Oncorhynchus apache;); inva- Oxford,England. SaLmo (ed.). by over sixty non-native plants which have Barton,A.and S.Sloane 1992.ChiricahuaMountains sion ResearchSymposiurn Proceedings. Southwest parks naturalized and degraded valley and desert vete- andMonuments Association. Tucson, Arizona. tation (E. Pierson and ]. McAuliffe, this volume); Brown,D.E. (ed.).1982.Biotic Cornmunities of theAmeri- long-term declines of some game species, espe- canSouthwestUnited States and Mexico.DesertPlants cially band-tailed pigeons (Columba fasciata); and 4:1-342. urban pressure that has fragmented, if not elimi- Bush,M. L994.Amazonian speciation: a necessarilycom- nated, the corridors between various sky islands plexmodel.Journal of Biogeography2l:5 -17. are clear and present harms. Carlquist,S. 1974.IslandBiology. Columbia University Policy and attitudes concerning livestock rais- Press.NewYork,NY. ing and fire management are in flux with de Granville,I.19.82.RainForest and XericFlora Refuges a state of - attempts to understand "more natural" or "bet- in FrenchGuiana. Pp. 159 182.In G. T.Prance ( 1982). ter" or "more balanced" mafttgement practices. Fa,J.and L.M. Morales.Patternsof Mallunalian Diversity 355.InRamaworthy, T.P, et. al (ed.), Piecemeal in Mexico.Pp.319 destruction of both the remaining forest 1e93. and aquatic habitats is a major concern in both the Fittkau,E J., J. Illies, H . Klinge,G .H.Schwabe and H. Sioli Coronado Forest and in Mexico. Increasingly, (eds).1969. Biogeography and ecologyin SouthAmer- Mexico and the United States realize that some ica.Monographs on Biology 19.Junk Publishers.The problems and some solutions are international. Bi- Hague.The Netherlands. national inventories of the sky island flora and Flores-Villela,O. 1993.Herpetofauna of Mexico:Distribu- fauna will require close coope.itiott among scien- tion and Endemism. Pp. 253 - 287. In T.P. tists and meetings like the present conference. Ramamoorthy,et al. (ed.),1993. Mexico remains the best source for some animals Haffer,J.1987. Quaternary History of Tropical America. - that are now near extinction in the United States Pp.1 19.In T'C.Whitmore and G.T. Prance (1987). (e.9., black-tailed prairie dog, Cynomys ludovi- Haffer,J.1987. Biogeography of NeotropicalBi rds. Pp . 105 -l C.Whitmoreand G J. Prance(f 987). cianus, or thick-billed 45.InT. parrot, Rhynchopsitta Heald, W.F. 1993.The ChiricahuasSky Island. Bantlin p achyrhynch a) and cross-border cooperation may Pub.,Tucson,Az. be necessarv to reintroduce the Mexican wolf, Hendrickson,D.A., W.L. Minckley, R.B. Miller, DJ. Siebert Canis lupuls baileyi, manage various races of and P.H.Minckley. 1980. Fishes of the Rio YaquiBasin, pronghorn as well as migratory birds, bats, and Mexico and the United States.Vol. 15, Number 3. butterflies. In short, to conserve both nations' Journalof theArizona-Nevada Academy of Science. unique natural heritage, a deep commitment to Journal of African Ecology.1981. Special Issue on East preserving minimal viable habitats for resident Africanbiogeography. and migratory speciesis paramount. Part of this Kant, S. 1989.Phytogeography of the North-WesternHi- malayas. AEB Monograph-l, The Academy of effort is understanding the unique-Earth's position of the sky island archipelago in the family of Environmental Biology, India. University continental island Kingdon, J. 1990. Island Africa. Princeton ecosystems. Press. Mani,M S. and L.E.Giddings. 1 980. Ecology of Highlands. MonographiaeBiologicae, Volume 40. Dr W Junk bv Publishers.The Hague, Netherlands,
17 Manickam,VS. and V.Iruday araj.1992.PteridophyteFlora States,I.S. 1992.Distribution of Mushrooms, Myco' of the WesternGhats " South India. B.I. Publications. phagists and Conifers on Sky Islands of the NewDehli,lndia. Southwest.InA. Barton.and S. Sloane (ed.),1992. Marshall,l.L957. The Birds of the PineBakWoodland in Steyermark,I.A. L982.Relationships of SomeVenezue- southern Arizona and adjacent Mexico. Pacific Coast lan ForestRefuges with Lowland TropicalFloras. Pp. AvifaunaNo.32. L82-22l.InG.T.Prance 1982 . Mctaughlin, S. P. 1993.Notes on the Botany of the "Sky Sousa,M.S. and A. Delgado. 1993.Mexican Legumi- Islands"Region of SoutheasternArizona. Pp.42. Mns. nosae:Phytogeography, Endemism, and Origins.Pp. Minestero del Ambiente y de los Recursos Naturales 459- 513. In Ramamoorthy,et al (ed.), 1993 . Renovables.1985.Atlas de Ia Vegetacionde Venezuela, Suslov,S.P. 1951. Physical Geography of Asiatic Russia. Republicasde Venezuela. W.H. Freeman.San Francisco, Ca. Peet,R. K. 1988.Forests of the Rocky Mountains. [n North Takhtajan,A. 1985.Floristic Regionsof the World. Uni- American Terrestrial Vegetation,edited by MG. Bar- versityof California Press.Berkeley, Ca. bour and W. D. Billings.Cambridge University Press. Trimble, S. 1989.The SagebrushOcean; A Natural His- Pliego, P. 8., A.G. Navarro Siguenza, and A. Townsend tory of theGreatBasin.Universityof NevadaPress. Petercon.1993. A Geographic, Ecological,and Histori- Walter,H.1979. Vegetation of the Earth.Secondedition. cal Analysis of Land Bird Diversity in Mexico. Pp. 253 Springer-Verlag.New York. -281.In Ramamoorthy,et. al (ed.).193. Warshall,P. 1985. Biogeography of the high peaksof the Prance,G.T. 1982.Biological Diversiffcation in the Tropics. Pinalenos.Reprinted from the EnvironmentalData Columbia University Press.New York. Book,US ForestService, Coronado National Forest, Prance,Gf. 1987.Biogeography of Neotropical Plants.Pp. by the Maricopa Audubon Society,Phoenix, Ari- 116-65. In T.C.Whitmore and G f. Prance(1987). zona. Ramamoorthy, T.A., R. Bye, A. Lot, and J. Fa (ed.) 193. Warshall,P. In press.Status and Trendsof Southwestern Bological Diversity of Mexico: Origins and Distribu- Sky Island Ecosystems.InStatus and TrendsReports, tion.Oxford University Prcss.NewYork. National BiologicalSurvey. Rzedowski,J. 1903.Diversity and origins of the phanero- Wendt, T. 1993.Composition, Florsitic Affinities, and gamic flora of Mexico. Pp. L29-L4. ln TP. Origins of the Canopy Tree Flora of the Mexican Ramamoorthy,et al, (ed), 1993. Atlantic Slope Rain Forests.pp. 595 - 581. In Rama- Shteve, F. 1922. Conditions indirectly affecting vertical worthy(1993). distribution on desertmountains.Ecologya269 -2R. Whitrnore,T.C.and G.T.Prance.1987. Biogeography and Sky tsland Alliance.1992.Sky lsland National Biodiversity Quaternary History in Tropical America. Oxford Conservation Area, A Proposal. Sky Island Alliance, Monographs in Biogeography No. 3. Clarendon Tucson,Az. Press.Oxford, England.
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