The Madrean Sky Island Archipelago: A Planetary Overview, Peter Warshall

13 Pages·2007·1.19 MB·English

The Madrean Sky Island Archipelago:
A PlanetaryO verview
PeterW arshaltl
Abstract.-Previousw ork on biogeographiics olationh AFoncerned
itself with oceanici slandc hains,i slandsa ssociatedw ith continents,
fringinga rchipelagosa, nd bodieso f waters uch as the Africanl ake
system which serue as "aquatici slands".T his paper reviewst he
"continentails lands"a nd comparest hem to the Madreans ky island
archipelagoT. he geologicalh, ydrologicaal, nd climaticc ontextf or the
AfroalpineG, uyanaP, aramol,o wa ndh ighd eserot f theG reatB asine, tc.
archipelagosa re comparedf or source areas, number of islands,
isolating mechanismsin, teractiveec osystemsa,n de volutionarhyi story.
The history of scientifice xplorationa nd fieldworkf or the Madrean
Archipelagoa nd its uniques tatusa mongt he planet'sa rchipelagoasr e
summarized.
In 1957,J oe 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, telescopesa nd 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,a s well as preservation of sky island habitat
feet), Relief between valleys and peaks ranges for threatened and endangered species.U nderpin-
from 1250t o 6750f .eet( Mclaughlju;.,7992),b ui 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,s peckled granite, and pastel volcanics.
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WHATA RE SI(Y ISTANDS? SI(Y ISI.ANDSO F 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 becauseo f 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.B y 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,a nd 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,P hilippines), 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
<-- ioitr SOUrH-> >
alcaa a.ltl.Id 6a ltti
ropoo _-
I,OOO - at,aa aa.aard :- ttaiaaat'r.ar ra.. .*.r.r.a .rta.t.,..
a,ooo-
Jsrt.rrr t.--
WHITg PIIIALENO ltuAcHUcA
llcr alt ana t.arr
OPOSURA ft.'... .t!.-.n'.
Flgule 2.-Ex.mple "ttrcklng" of Dlotlc communldero f thc lledrern rrchlpelrgo (llffrhrtl, 1957). Note the podtlon ot thc ork-plne
woodlrndr.
chainso f 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-
Steppings tonea rchipelago Madreana rchipelago
plexes (e.g., the Adamoua, Baja Peninsula, and between two mountainc hains Great Basin archipelago
East African arc) contain both oceanici slands and Altai/TienS han Basin
Meso-Americanm assifs
sky islands with both saltwater and vegetative
"seas." Like all typologies, there are ambiguously lsolated massif with oulliers Ethiopianh ighlands
categorizedg roupings. East African arc
Saharan massifs
Atlas Mountains
Jabal Lubnan
PlanetarEy xamples Adoumoua Mountains
Drakenbergs
Central European massifE
The four descriptive variables used to classify Caucaso-lranianm aasile
sky islands act in ioncert. Compare the Madrean
Cordilleraw ith outliers Chaine Annimatique (Vletnam)
stepping stone configuration with the isolated ar- Malay peninsula
chipelago of the Western Ghats of India (not Baia Galiforniap eninaula
illustrated), the isolated cluster of the Pantepuiso f Coastal Cordillera( SA)
Southern Andes
Brazil, Guyana, and Venezuela (fig. 5) and the
East African arc and the Ethiopian massif com- lsolateds ky 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 degreeso f latitude. Himalayas
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ANTARCTI
Flgure 3.--.Locrdonr of *y lrlrnd complexe. mentlonedI n text on r m.p ot the Florle$cR eelmr of thc phnet Only ttc lhdrcrn
.]chlpelrgo ltrrddler two; Florlrdc Rcrlmr. (1) llrdreen; (2) Gr.rt B.3ln; (3) Brle Celllomlr (4) ile.o Amerlcen; (5) Cordrl
Cordlller|: (€) Prnt pulr: (7) 8ralllrn rhleld; (t) Srhrnn me$l{r; (9) Ehloplrn Hlghlmd.; (1o) E.tt Afrlcrn rlc; (11) Crmcroon
Blght; (12) SouthernA fdcrn compler; (13) Weilem chrtr: (14) Ctucro lrenlan mr.rib; (15) Altllmen Snan; (1O)T nnr-Brlkrl:
(17)C hrlneAnnlmrdque(1; S)M alayp eninrulr.H ighv alley" lslendf'can be foundI n the nortrernA ndcr 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 ROCrK['YlOy UNrfAD.lS
five distinct mountain with one major valley (the
Palghat gap) subdividing the archipelago. Some-
times, Sri L^ankah as been included as an oceanic
island within the sky island archipelago. In part, NORTtlERI l'.;l
the high endemism of the montane Malabar rain-
ROOff
forests which cover the Western Ghats can be
traced to its former Gondwanaland corurection.
The Pantepuis (Minestero, 1985;F ittkau, 1969;
Steyer mar k, 7982; deGranvil Le, t982; Haffeg t987)
form a scattered, isolated tropical series of 15 og
sotfiltahl
large and 20 smaller sky islands. The Pantepuis
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
(seeb elow). The Ethiopian complex centers on the Flgur. a.-{-oc.tlon ol tho rrchlpolrgo .r l .t pplng .tdt b}
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
Ethiopian complex is isolated by the White NiIe to Icnluddlcod:o d(1 )b yll rrnorl-lldlr dllcnron, Ieyloor| c.nSllcrr r Oolhltd; r(r2 ) mSborunn trlln.dr llftr
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?oel orth. 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
greater than 9,000 feet in elevation. The relief is Focky Xamtrln rou?cor r.r tor tfie trd.crn rrchlprlrgo.
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)c reate a rough geographic circle involv-
10
EOUATOR
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FlgureS .-Thrco SouthA merlcrn .ky ldrnd compt.xc..r. lhown: (1) Ibo p.ntcpult; (2)t he 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 MadreanA rchipelago:C omparisons
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.T he 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,B ajaP eninsula,W estern Caucaso-lranian, Central European Highlands,
Ghats,M alay Peninsula)a lso provide for a greater Jabal Lubnan-appear to be so altered by human
amount of climatic variation than east-westa xes 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.T he 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,i s predominantly eastt o 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
Mountainouasr eas
ffi
Lowlandl orest
%
Moists avannaty pes
n
Woodlands e
ffi Driers avanntay pes,
busha ndt hicket
tl
Desena nds emid esert
mm
Capev egetation
Flgu?c B.-four Alrlcan rky lrlend complerer arc rhown: (1) The Adoumour chaln In Grmeroonr; (2) the Ethlopltn Hlghlrnd comple& (3)
the Etil Afrlcrn rrc compler or Zanl; ad (4) the South Alrlcrn compler.
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
Relictualo r Subeequent Most Tepuis
bridge or a barrier. The Ethiopian region is the (Venezuela),WesternG hats
only tropical sky island complex that harbored (I ndia)Brazfiia n ehi eld
signif icant glaciation. Tectonico r Original East Alrican arc volcanoes
Distinctly northern complexes, higher than 40 A. Accumulationm ountaing Mt. Camoroun
degrees latitude, include the Altai/Tien Shan and (flood basalis, volcanice)
Dztrngarian basin r.rnges; the heavily altered Py- B. Deformationm ountain8 Himalayas
rennes and Scottish Highlands and a fewer other Bl. Foldm ountaing Andes (altiplano)
CIS ranges. These areas lost their biota during the
82. Dislocalionm ountains Great Basin
glacial periods and have only recently (about 10 to (horsUgraben) MadreanA rchipelago
15,000y ears ago) been recolonized. Alrican RiftV alley mountains
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. Laccolithm ountains
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 HOWW ERET HES Iff ISI-ANDC OMPLE)GS
CONSTRUCTED?
River geographically isolate the Madrean archi-
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
oceErno r 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
becauset wo 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,a bout 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 crossesf rom 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 sourcea rea for the archipel-
"soil barrier." ago is the largest rhyolite mass on the planet with
Tectonic or "original" mountains include 4,000 foot deep sequencesi n 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 complexesa re combina- colonizing speciest o 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,s hrubs, and trees (R. McAuliffe and T.
East African rift valley. Volcanism stopped three Burgess,t his volume); (2) talus slopes which, for
to four million years ago, allowing recolonization instance, support a remarkable diversity of snails
without further geological disturbance.T he Ethio- (McCord, this volume); (3) limestone slopes and
pian highlands are relatively flat-topped (vs. outcrops that greatly increase the diversity of
peaked) becauseo f 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).
'ba-
did not rise as much as the vallevs sank. This 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 seepso f 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,e ach in-
dividual Madrean sky island is a remarkable
mixture of rock gpes. Exposed rock types include
intrusive igneous rocks (granite), extrusive vol- HOWD IDI T HAPPEN?
canics (rhyolite, dacite, basalts), metamorphics BARRIERSB,R IDGESA, NDS OURCES
(gneiss, schists, quartzite), and sedimentary rocks
(limestones,s hale, conglomerates).F or 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,1 981f or EastA frica).
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,a nd climatic/vegetation change.C ertain sky
sediments,t ree rings, fossils, geomorphic surface islands have provided "safe sites" for organisms
dating, sea temperatures, radiocarbon and other becauseo f their latitude and topography. Some
isotope dating-<an be brought forward to argue sky islands have more frequent colonization at-
for various driving forces toward increased spe- tempts becauseo f riparian corridors or valley
ciation. In the southwest, the history of winter vs. conliguration. Still other sky islands have main-
summer rainfall and the decoupled influences of tained and added diversity becauset he invading
temperature vs. rainfall can help explain when the flora and/or flora had great phenotypic or geno-
valley vegetation was a barrier that restricted typic plasticity (e.9., Erigeron). These complex
gene flow or when the valleys were bridges that interactions await researchers.
encouraged gene flow. Nevertheless,t he Madrean archipelago indi'
From a partial review of the literature of the cates that regions of maximal disturbance (vs.
planet's sky islands available in English, I would maximal stability) can encourages peciesr ichness,
like to summarize a few lessons.F irst, sky island if not endemism, in particular taxonomic groups.
species richness can have complex sources and The continental position of the archipelago and its
migration routes. For instance, Bursera r3an Afri- relation to three major storm fronts can only have
can tenus that migrated through l.aurasia and created highly erratic rainfall and temperature re-
across North America to the present southwest in gimes. In the past two million years, 15 to 20
the upper Cretaceous/Eocene (Sousa,1 993).O ther glacial fluxes (eacha bout 90,000y ears) with short
plants colonized the archipelago after the Panama "reprieves" lasting 10 to 15,000y ears have influ-
landbridge connection (Wendt, 1993). A simple enced the biodiversity of the archipelago. The
model based on changing precipitation md/or Madrean archipelago has been spared the "wipe
temperature for the last glacial period can only out" experiencedb y sky island complexes of more
explain a small part of the Madrean archipelago's northern latitudes (e.9., Eurasian) where the gla-
diversitv and richness. ciers covered the land's surface. Only the Pinaleno
Second, the richness of certain Madrean taxa mountains appears to have experienced a small
(e.g., flightless beetles, talus snails) may be better montane glacier.
explained by the down-falling of the valleys with
subsequent vicariant allopatry than by cli-
matic/vegetation flux, Third, other types of
barriers and bridges besides valley vegetation oc- LIFEO N THEM ADREANS I(YI SLANDS
cur (Table 3). These include the Pleistocene lakes
such as Cochise Lake in the Wilcox playa, the an- Sky island complexes tend to have greater spe-
cient Gila River which was connectei t6 the Yaqui cies richness, greater endemism, more clinal
River, and the presence of extensive alluvial fans variation, more biogeographical specialties, and
and roaring rivers in the Pleistocene. These unique cultivars compared to other inland ter-
river/lake barriers and bridg-efiss ha re essential in de- rains. A comparison of planetwide sky island
scribing the diversity of in the Madrean diversity is beyond the scope of this paper. In-
archipelago (Hendrickson, 1980). Fourth, there is stead, I will briefly describe the "big picture" of
a complexipteraction between vertical and aspect biodiversity in the Madrean archipelago.
McLaughlin (this volume) provides more detail
T.blc a.-Sky ltlrnd Brldger rnd Brrrlerr. on the flora.
The Sierra Madre of Mexico, the Madrean ar-
chipelago and other outlier sky islands have been
Rivera ndf ivors ystems Riparians ysiemsin aridl ands
identified as one of the three "megadiversit5l" cen-
Lakes Stepping stones (hills,m is.) ters of the planet. It is the only Eroup of sky
islands straddling two major floristic (the Neot-
Coastlines Wind. water.a nd animal
dispersal ropic/Holarctic) and two faunal realms
(Neotropic/Nearctic) on the planet (Waltea 1979).
High altitudec ordilleras Valleys/gapsw ith lavorable
The Ethiopian complex borders two faunal prov-
soils climat or ecology
ince boundaries but lies within one floristic realm.
Valleysw ith alien climate,s oils Alluvialt ans (?) in arid lands The Drakenbergs and other South African sky is-
or ocology Phenotypic/genotypicp lasticity
lands are associated with the unique Capensis
1s
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