... Chihuahuan Desert-U.S. and Mexico. 11, 1986, pp. 1-19 00344

PLEISTOCENE CLIMATES AND ENDEMISM IN THE CHIHUAHUAN DESERT FLORA

THOMAS R. VAN DEVENDER

Arizona-Sonora Desert Museum, Route 9, Box 900, Tucson, Arizona 85743 USA

ABSTRACT

Chronological series of radiocarbon dated plant macrofossil assemblages from small geo· graphic areas are providing excellent developmental sequences of vegetation for the last 35,000 years in the nonh~m Chihuahuan Desert: Midden vegetation chronologies have been completed along a latitudinal gradient from the San Andres (33"11'N) and Sacramento mountains, New Mexico, and the Jueco Mountains (32"N), and the ruo Grande Village area (29"30'N) and Mara villas Canyon (29"33'N) in th.e Big Bend of Texas. Although the chron­ologies demonstrate some differences in composition and the timing of vegetation changes, they a·gree in a stepwise progression with the most mesic pinyon-juniper-oak vegetation in the Late Wisconsin and the most xeric in the last 4000 years. The Middle Holocene (about 8000-4500 years ago) was apparently very warm with strongly developed summer monsoons and a maximum expansion of grasslands. About 4500 years ago, a creosote-descrtscrub corridor between the Chihuahuan and Sonoran deserts was established for the first time since the last interglacial. Deserts are probably at their maximum expansion today with the harshest climates of the present 'interglacial.

The majority of the endemic Cbihuahuan Desert plants have affinities with the grasslands, thomscrub, or woodlands of the Sierra Madre Oriental to the east and south. Isolation during glacial/interglacial fluctuations has provided many opportunities for speciation in both desert and woodland plants. In the Bolson de Mapimi, pluvial lakes alternated with dry playas with exposed halophytic and gypsum substrates. A number of east-west vicariant species pairs whose disjunctions are probably pre-Pleistocene are discussed.

The latitudinal gradient in increasing numbers ofspecieut lower latitudes can be explained in terms oflatitudinal differences in extinction rates. The probability of extinction. of isolated populations at the end of a glacial or an interglacial is greatest in the north and decreases to the south as the probability of speciation increases. Coupled with l 5- 20 glaciaVinterglacial cycles in the l«st 1.8 million years and a mild, equable climate where extremes in cold, heat, or drought have never been great enough to cause wholesale extinctions, the numbers of newly evolved and old relict species have gradually increased in the central and southern Chihuahuan Desert.

RESUMEN

Una serie cronol6gica de colecciones de macrof6siles de plantaS fechadas por radiocarb6n en pequeiias regiones geograncas ofrecen excelentes sucesiones que muestran el desarrollo de Ia vegetaoi6n para los Ultimos 35,000 aiios en el Desierto de Chihuahua nort.eiio. Se han terminado de estudiar las cronologias de vegetaci6n basural a lo largo de un gradiente latitudinal desde Ia Sierra de San Andres (33"ll'N) y la Sierra Sacramento, New Mexico, y Ia Siexta Hueco (32"N), y Ia regi6n del Rio Grande Village (l9"30'N) y Ia Barranca Mara villas (29"33'N) en el Big Bend de Texas. Aunque las cronologias muestran diferencias en com­posici6n y tiempo d.e los cam bios vegetates, estAn, sin embargo, de acuerdo con una progre-

2 Chihuahuan Desert-U.S. and Mexico, II

. de Ia era Tarde Wisconsin a las mas · · • 6 · unipero-encmo si6n gradual desde Ia vegetacJ6n pi.D n-; -~ . (bace unos 8000 a 4500 aiios) era una

· . . 000 - El Medio Holo....,ruco . .6 anda.ae los wumos 4 anos. · de verano y una mh1ma extens1 n epoca aparent~mente muy calid~ COD fuel~~~~~n;~;;rimera vez, desde el UltimO periodo de pradcras. Hace unos 4500 anos sees La ntre los dcsiertos de Chihuahua y Sonora. interglacial, un corredor de gobernadora { rre~~o desarrollo con los climas mas severos Los desiertos hoy posiblemcnt~ estan e~ su m experimentados en el presente mterglaclal. . d ,....._,.,uahua tiene afinidades con las

d · ·cas del Des1erto e \.,llLI.1 La mayoria de las plantas en eml d I s·erra Madre Oriental bacia el este y

· eon los bosques e a I praderas, matorral espmozo o . lac·avinterglacial ha ofrecido muchas opor-el sur. El aistamiento durante las fluc~uacJOn~ g Ide desierto y de bosque. En el Bols6n de tunidades para Ia evoluci6n de especleS de p antas d b'e-rtas con substrates halofiticos

• 1 con playas secas escu 1 Mapirni, Iagunas pluVlales a tcroaron d species vicarias del este y oeste cuyas

de veso. Se discuten un numero de par~ e e Y · b bl 1 del pre-Ple1stoceno. . . separaciones son pro a emen e . . . ero de especies ·en latitudes m(ls baJaS puede

El gradiente ~atit_udinal en. el C~Jen~e .n~::lnales en porcentajes de e~tinci6n. La ~:oba­ex-plicarse en termmos de dife':ncJas . aut ft es de un periodo glacial o interglacial cs bilidad de extinci6n de poblac!ones 31Sladas a, ~ po en que crece Ia probabilidad en Ia

d. · ha"Cia el sur a uem • . 8 mayor en el norte Y Jsmmuye 20 . los glaciaVinterglacial en los ulumos I. evoluci6n de especies. J_unto co~ los 15 ~form~~onde los extremes de frio, calor, o sequia millones de aiios Y un chma bemgno Y uru grandes ex.tinciones, los nfu:neros de nunca han sido bastlinte fuertes como para c:a~~ar pocO a poco han crecido eo·et·Desierto especies recientemente evolu~ionadas y las vesllgl cs de Chihuahua central y sureno.

11 d plant macrofossil . . . th 1960's the analyses of we -preserve . Begwrung w e . • n dated packrat (Neotorna spp.) rmddens

assemblages from radiocar?ofi . n the history of vegetation and have provided a :-vealth of 10 orma~e~ca (VanDevender et al. 1979,

~~~~~; :th:1.a(~~;:.~e~~::;ed {lantd~a:c~o!~s~:::~::!e~~:: documented widespread expan::;:~o w_: during the last glacial period, into most of the des~rts of No me7 e the packrat midden record for the Wisconsin. In thts paper,_ I sum_ma~gnifiz . cance for the distribution and the Chihuahuan desert and discuss tts st evolution of endemic species.

PACKRAT ECOLOGY

· · d rodents comprizing about 20 Packrats, or wood rats, are medium-slZe elevation in many habitats

species found from s~a level up to 3350 m . various objects to from British Columbia to Gua~emala. p~~~~~f=Finley 1958). Their their houses for use as constructJ.on ~at~~ assortments of plant fragments waste piles ?r ~ddens usually c~ntam ~eo packrats live in rock shelters, collected Within 30-50 m ofthde bou~ middens often serve as uiiriation houses are not well-develope ut e . perches and become :ery hard, dbark, andv~~~;·long as they are dry. The

These indurated mlddens can e preser

VanDevender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES 3

oldest packrat midden discovered to date is more than 50,000 years old (Spaulding et al. 1983). The plant remains in the midden samples are well preserved and give reasonable reflections of the local vegetation near the sites. The plant remains or packrat fecal pellets are excellent for radiocarbon dating, allowing the assemblages to be placed in a time frame­work. Sequential series ofpackrat midden assemblages from a single area provide excellent records of the development of the local vegetation for the last 11 ,000 years or more. Packrat midden chronologies are providing remarkable paleoecological insights from within the deserts themselves.

GEOGRAPHICAL SETTING

The Chihuahuan Desert is unique among the deserts of the American Southwest because it is an interior continental desert in the rainshadows of the Sierra Madre Occidental to .the west, the Sierra Madre Oriental to the east, the· highlands of the Mexican Plateau to the south, and the Rocky Mountains to the north (Johnston 1977). Portions of the Chihuahuan Desert occur in the states of Arizona, New Mexico, Texas, Chihuahua, Durango, Coahuila, Zacatecas, Nuevo !,.eon, and San Luis Potosi along its 1600 km southeast to northwest axis (22°-33°N latitude). Morafka ( 1977) and Henrickson and Straw (1976) have mapped the major biotic provinces of the Chibuahuan Desert. Morafka divided the area from north to south into Trans-Pecos, Mapimian, and Saladan subprovinces based on biotic distributions (mostly of amphibians and reptiles).

The topography of the Chihuahuan Desert is a complex mosaic of mountains and valleys that generally decrease in elevation from the Con­tinental Divide to the Gulf of Mexico in the north and that increase in elevation onto the Mexican Plateau to the south. The climate of the Chibuahuan Desert reflects the topographic complexity in a number of environmental gradients (Schmidt 1979). The hottest, driest, and lowest areas in the Chihuahuan Desert are on the Rio Grande on the United States-Mexican border in the Trans-Pecos (600-1675 m elevation), and in the mutual corners of Chihuahua, Durango, and Coahuila in the Bolson de Mapimi ( 107 5-2000 m elevation). As elevations rise from these central lowlands precipitation increases and temperature deere:rses. The entire Chihuahuan Desert is dominated by a summer monsoonal rainfall pattern, although most areas have winter precipitation as well. The western half of the Chihuahuan Desert has greater percentages of summer precipitation than more easterly areas. Beyond the Chihuahuan Desert to the east both the percent.<}ge of winter precipitation and the total annual precipitation increase as elevation decreases toward the Gulf of Mexico. To the south the percentages of summer precipitation increase with elevation. How­ever, occasional heavy Pacific fronts with winter precipitation reach the

4 Chihuahuan Desert-U.S. and Mexico, II

central Chihuahuan Desert, resulting in an exceptional growth of spring plants (as in the winter-spring of 1978-79). This corresponds to anomaly pattern A ofLaMarche and Fritts (1971) where the entire Southwest and northern Mexico have greater tree ring growth than eastern regions due to exceptional winter precipitation. To the east and south in the central Chihuahuan Desert, milder winter temperatures and increased humidity produce more subtropical climates. The frequency of incursions of con­tinental polar or continental Arctic air decreases to the south but occa­sional frosts will penetrate to the southern edge of the Mexican Plateau.

These precipitation and temperature regimes are reflected in the vege­tation: xeric desertscrub comm.unities in the lowlands are gradually re­placed by desert-grassland to the north, west, south, and southeast. A relatively mesic mesquite scrub community is found on the coastal plain of the Gulf of Mexico and contiguous low areas in Texas, Coahuila, Tamaulipas, and Nuevo Leon (Muller 1939, 1947). The vegetation ofthe southeastern portion of the Chihuahuan Desert is much more subtropical than other areas. In desertscrub communities there is a gradual increase to the south in the numbers of species and in the importance of succulents and other frost-sensitive species. Pine-oak woodland and pine forest are present at higher elevations on montane islands in the Chihuahuan Desert and on the surrounding mountain ranges.

Johnston (1977) reported about 1000 species of plants endemic to the Chihuahuan Desert Region. This number includes many widespread plants such as lechuguilla (Agave lechuguilla) and tarbush (Flourensia cernua) as well as more narrow endemics. At least 65 Chihuahuan endemics are restricted to gypsum substrates (Powell and Turner 1977). The Chihua­huan Desert fauna is not as rich in endemics. Again, some Chihuahuan animals such as Merriam's spiny lizard (Sce/oporus merriamz)~ Trans­Pecos ratsnake (Elaphe subocularis), gray-banded kingsnake (Lampro­peltis mexicana), Chihuahua.n lyre snake (Trimorphodon biscutatus vil­kinsoni) are widespread while bolson tortoise (Gopherusjlavomarginatus), Cuatro Cienegas alligator lizard (Gerrhonotus lugot), bolson fringe-toed lizard ( Uma exsul), Durangan night lizard (Xantusia vigilus extorris) and bolson night lizard (X. bolsonae}have very restricted distributions (Webb 1970, Morafka 1977). In general, the numbers of all species, as well as endemics of both plants and animals, increase to the south.

PLEISTOCENE VEGETATION OF THE CHIHUAHUAN DESERT

In Well's (1966) packrat midden study, he found a pinyon-juniper-oak woodland dominated by Texas pinyon (Pinus remota) at three sites in the Big Bend of Texas that now support Chihuahuan desertscrub vege­tation. Bryant and Larson's (1968) analysis of pollen in sediments from

Van Devender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES 5

caves near Del Rio suggested that this w . early Holocene (ca 9000 Y . oodland was still present in the Meyer's (1973) t d. f ears ago).Just ~ast of the Chihuahuan Desert.

s u Y o the pollen m spnng d · . negas Basin in central Coahuila sh w d r I epostts_ m the Cuatro Cie­the last 20,000 years. o e Itt e change m the vegetation in

In the last 6 years I · th h Worthington, W. G S :~ld:n e help of co-.w?rkers (B. L. Everitt, R. D. Toolt"n R s Tho · P D g, A. M. Phllhps, J. L. BetancoUrt L J

• · · mpson H Ri ki d) h . ' · · middens from a number ~fChihu s n ' ave ?tscovered fossil packrat Andres (33oll'N) and S .ahuan Des:rt sttes ranging from the San

acramento mountams (32oSO'N) New Mexico, along a latitudinal d" . . , south-central Texas, and the Bolson de M . ~a. Ie~t mt~ the Btg Bend (29o12'N), (26oN; Fig. 1). These results a~~~:~ t~n oah~la and !Ju~ango, Mexico from north to south· Van D d e followmg publications, arranged

· even ereta1 (1984)(S Van Devender and Everitt (1977) d .Th acramento Mountains), Cave, Bishop's Cap New Mex· ) a~ Dompson et al. (1980) (Shelter Canyon and Rocky 'Arroyo G lC~ '1 an even~er (1980) (Last Chance Devender et al. (1977, 1979) (~u=d~fu ~ountam~, New Mexico), Van vender and Riskind (1979) (Hueco p ~ountams, Texas), Van De­al. (1978) (Livingston Hills Chinaf~~mntat?s, Texas), VanDevender et der (1978)(Bolson de Ma : . 1. ountams, Texas), and VanDeven­late Wisconsin records a~~m;~:extc~). Many ofthe early Holocene and/ Devender 1977 Van Deve d toe~ on dates are summarized in Van Devender 1981,' and Spauld:;~::l. ~~~~lding 1979, Lanner and Van

BIOCHRONOLOGIES

The current emphasis is to btrlld d .ail . ~long a latitudinal gradient through ~~e ~~i~ca~ vegetatiOn chr?nologies m composition and the timin f h ua uan ~esert. Differences circulation patterns through t" g o ~ ~ge can help mfer atmospheric opment of the vegetation Th~m~ an It e role of dispersal in the devel­tains is published (Van D. de rono ogy from the Sacramento Moun-

even er et al 1984) Oth been completed for the San A d M . er chronologies have Mountains (32oN· Fig 2) Ma n"llres Canountains, New Mexico, Hueco

' . , ravt as yon (29o33'N p· Grande Village area (29ol 2 'N· F" 4) T ; 1g. 3), and Rio

All f h • tg. , exas o t e vegetation sequences are s. "I . . .

in vegetation with the most m . . 1~ ar 10 sho~ng s~ep-wise changes

xeric in the last 5000 years in t~stc m e late Wtsconsm and the most the late Wisconsin is marked b e lated Ho~ocene. In all areas the end of ~c~uding Colorado pinyon (Pin;{ e~ ~~ u~wn of the more mesic plants J~Iper (Juniperus scopulorum) abo~/?i O·;;mota, and Ro~ky Mountain twn of the Rio Grande V"ll , years ago. With the excep-

t age sequence, the early Holocene vegetation

il . . ~ ,, .

..

6 U S and Mexico, II

Chihuahuan Desert- · ·

6

. . . dland. The middle Holocene was was a transitional xenc oak-Jumper woo dl d and many important

· d 1 k:i g both woo an a desert-grassland peno tc n The development of the modern com-Chihuahuan desertscrub Pants. t CertainlY creosote bush munities was in all case~ a late Ho~o~n;l:=~) and lechuguilla (Agave (Larrea divaricata), ocotillo (Fouquzena srth ,._'!~.uahuan Desert. The

. als in the no em vllU-1 . lechuguilla) are late amv th b daries between the early, rnlddle, vegetation changes that mark e oun

VanDevender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES 7

and late Holocene have some temporal variability but were completed by 8000 and 4000 years ago, respectively.

The Rio Grande Village chronology differs from the other records, even the one from Mara villas Canyon, 40 km to the north, in several important ways. Important Cbihuahuan plants, including Agave lechuguilla and cru­cifixion thorn (Koeber./inia spinosa), were ~mrnon in the late Wisconsin pinyon-juniper-oak woodland. The Big Bend was probably the Ice Age refugium for some of the plants that are important in Chihuahuan des­ertscrub communities to the north. Samples dated at 10,390 ± ~30 BP (average of A-2937 on Koeber/inia spinosa and A-3,129 on Prosopis glan­dulosa; Tunnel View #58) and 8980 ± 130 BP (average of A·2936 on Lycium puberulum and A-3128 on Opuntia phaeacantha; Tunnel View #SA) document the early development of honey mequite (Prosopis glan­dulosa) dominated desert-grassland in the early Holocene instead-of a . xeric woodland. Although this is the earliest record of a shift to desert­grassland in the entire Southwest, the community was very different from the modem one. Prominent plants such as blind prickly pear (Opuntia rufida), false agave (Hechtia scariosa), candelilla (Euphorbia antisyphilii­ica), and resurrection plant (Se/aginella /epidophyllum) probably migrated into the Big Bend in the late Holocene.

The two late Wisconsin midden assemblages from the Bolson de Map-. imi contained remains of woodland plants in assemblages rich in suc­culents (VanDevender 1978). The sample from Sierra de la Misericordia, near Bermejillo, Durango, was dated at 12,280 ± 345 BP (average of A-1895 and A-1896 on midden debris) and contained Pinus remota and Juniperus sp. The sample from Puerto de Ventanillas ( 43 km NE San Pedro de las Colonias), Coahuilla, at 12,700 ± 165 BP (Wk-165 on Juniperus sp.) only contained Juniperus sp. (Fig. 5). These sites on the dry end of the gradient from Cuatro Cienegas to the center of the Bolson de Mapimi suggest that the Cuatro Cienegas pollen record (Meyer 1973) is much too conservative and may have been dominated by the local hygric vegetation near the permanent springs. Although woodland plants expanded into desert lowlands as they did to the north, the Ice Age climates were not harsh enough or so different from today's that the wealth of endemic succulents, including Opuntia ru.fida, were displaced southward. The Bolson de Mapimi was probably the refugium for any Chihuahuan Desert plants displaced out of the Big Bend during the late Wisconsin.

PALEOCLIMATES

·'the Pleistocene has traditionally been diVided into four glacial periods. However, the results of studies of oxygen isotope ratios from deep sea

r 1 8

i.

Chihuahuan Desert-U.S. and Mexico, II

FIG. 2. A. Limestone ridge with rock shelters in western Hueco Mountains, El Paso Co., Texas; Navar Ranch midden series, 1340 m elevation. B. Close-up of Tank TraP Wash midden site. C. Chronological sequence of relative abundances of selected plants from packrat m.iddens from the shellers. Tandem linear accelerator radiocarbon date of 4080 ± 1500 BP (AA-382) verifies the full-glacial record of Prosopis glandulosa.

Van Devender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES

10 Chihuahuan Desert-U.S. and Mexico, II

F1G. 3. A. Limestone ridge in Mara villas Canyon, Black Gap Wildlife Refuge, Brewster Co., Texas. B. Cave at 635 m elevation. C. Chronological sequ.ence of relative abundances of selected plants from packrat middens from the cave (modified results of Wells (1966]

included).

Van Devender • ENDEMIC FLORA AND PLErs;rocENE CLIMATES

12 Chihuahuan Desert-U.S. and Mexico, II

FIG. 4. A. View of the Sierra del Carmen, Coahuila, from jusi above the tunnel near Rio Grande Village, Big Bend National Park, Brewster Co., Texas. The lowest elevations in the Chihuahuan Desert (5 50-600 m) are along the Rio Grande in the center of photo. B. Packrat midden site at 680 m on limestone near tunnel. C. Chronological sequence of relative abundances of selected plants from packrat middens from several sites in the area. Tandem linear accelerator radiocarbon date of 22,600 ± 1500 BP (AA-382) verifies the full-glacial

record of Prosopis g/andulosa.

Van Devender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES 13

.· 14 Chihuahuan Desert-U.S. and Mexico, II

Fto. s. A. Puerto de Ventanillas, 43 km northeast of San Pedro de las Colonias ~n road to Cualro Cienegas, Bolson de Mapimi, Coahuila, Mexi.co. Playa surface was _Piuvtal lake in late Wisconsin. B. Qose-up of succulent dcsertscrub on limestone. Hecht La: montana. Coryphantha strobiliformis. and Thelocactus bicolor are visible .. C. ~ose-~~ ~f succule~t desertsorub. Opuntia bradsoniana. Agave lechuguilla. Euphorbia an/lsyphtlmca, Opu~tla rufida and sotol (Dasylirion sp.) are visible. D. Packrat midden shelter at 1190 m elevauon

(lower right).

cores have demonstrated that there have been many more glacial/inter­glacial cycles, perhaps as many as 1 5-20 in the 1.8 million years includ~d in the Pleistocene. These studies have also demonstrated that each glacial period has been on the order of 80,000-100,000 ye~rs while in~erglacial periods have been only 10,000-20,000 years (Imbne and Im?ne 1979). The packrat midden chronologies have documented that re~atively mod­em conditions were only established in the last 4000 years m the present interglacial. Thus, the modem climate and biotic relationships that .we know and consider as .. pristine" a few centuries ago, may be representative of as little as 4% of the time since the Pliocene. The glacial periods with woodlands widespread ~t the expense of the deserts is clearly the more typical situation. With these kinds of major and minor environmental

Van Devender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES 15

fluctuations throughout the Quaternary, stable equilibria in the biota may never be achieved, or not for long. In the late Wisconsin, fairly uniform woodland vegetation appears to have been relatively stable for 10,000-20,000 years in some records and are the best cases for community equi­libria.

The paleoclimate inferred for the northern Chihuahuan Desert (32-330N) during the maximum expansion of the Wisconsin continental gla­cier (18,000 BP) included mild winters, greatly cooled summers, and somewhat greater annual precipitation strongly shifted to the winter from Pacific frontal sources at the expense of the summer monsoons from the Gulf of Mexico (Lanner and VanDevender 1979). The warmth of the summers and the proportion of summer rainfall increased to the south as it does today. In general, full-glacial and modem climates were diiferent at all latitudes with the greatest in the north and the least in the south.

ENDEMISM IN THE CHIHUAHUAN DESERT

The uplift of the Rocky Mountains and Sierras Madre in the middle Tertiary, roughly 20-25 million years ago, had a profound effect on the circulation of the atmosphere and led to regional climatic differentiation and the segregation of plants into r~Iatively simple new formations. By

· ·the latest Miocene, about 5-8 million years ago, such new formations as tundra, coniferous forests, grasslands, and deserts were well-developed (Axelrod 1979). Most of the groups of plants and animals that are suc­cessful today have their evolutionary roots in this Mio-Pliocene revolu­tion. The evolution of most of the Chihuahuan Desert endemic species occurred at this time or later.

The advent of glacial climates in the Pleistocene has had profound effects on the Chihuahuan Desert biota in many ways, including changes in distribution of species, composition of communities, extinction, spe­ciation, and in species richness. During each glacial, woodland plants ·expanded greatly into the lowlands while many desertscrub species were · displaced southward. During each interglacial, the 'woodland plants re­treated to the north or onto mountain islands as first desert-grassland, then desertscrub, plants expanded to higher elevations and latitudes. Con­sidering the number and frequency of glacial/interglacial cycles, the mag­nitude of the changes in distribution. and difference in dispersal rates, community composition has been very dynamic for most of the Pleis­tocene.

The role of Pleistocene climatic fluctuations in evolving new species can be seen in the distribution and floristic affinities of narrow endemics in the Chihuahuan Desert. On the northern periphery of the Chihuahuan

16 Chihuahuan Desert-U.S. and Mexico, II

Desert in south-central New Mexico and adjacent Trans-Pecos Texas there are few narrow endemics. A few examples include high rolls rabbitbush (Chrysothamnus spathulatus) (Anderson 1964) and Sibara grisea (Rollins 1982) from the Sacramento and Guadalupe mountains, Rhodes Canyon pennyroyal (Hedeoma todsenil) (Irving 1979) in the San Andres Moun­tains, and Navar Ranch rock daisy (Perityle huecoensis) (Powell 1983).

To the south the number of both narrow and widespread endemics increases dramatically. The affinities of most of these plants are with the Sierra Madre Oriental of northeastern Mexico. The narrow endemics represent both old relicts and younger species in groups that are rapidly evolving. Several of the older species have closely related vicariant species in the Sonoran Desert in Arizona and California. These include crucifixion thorns (Caste/a stewarti!C. emoryi), crucifixion thorns (Canotia wendtil C. holacantha), cliff roses (Cowania ericaefolia!C. subintegra) and dia­mond chollas (Opuntia anteojoensis/0. ramosissima) (Pinkava 1976). The night lizard (Xantusia bolsonae and X. v. extorris) and the fringe­toed lizard ( Urna exsul) are similar old disjuncts with western vicariants (i.e., Xantusia vigilus ssp. and Uma notata). These species pairs are prob­ably by the result of middle Pliocene dispersals and separation at the onset of Pleistocene climates. None of the interglacial climates in the Pleisto-cene was xeric enough to allow subsequent gene flow between them. None of the glacial climates has been severe enough to cause extinction of any of the species-pairs species.

In other groups, new species have probably evolved rapidly during the glacial/interglacial fluctuations in the Pleistocene. Examples of rapidly evolving groups can be found in cacti (Coryphantha spp. and Mammil­laria spp.), composites, grasses, and cricetine rodents. Often the genera and some ofthe species evolved much earlier but have given rise to more dynamic descendants.

LATITUDINAL SPECIES GRADIENTS

The total number of species and often the number of species in a single genus increases to the south. This latitudinal gradient in species richness was explained in terms of climate by Fischer ( 1960) but has subsequent ly been viewed as the result of the biotic interactions involved in compe­tition, species packing, co-evolution, and resource partitioning (Mac­Arthur 1972, Brown and Gibson 1983). The detailed paleoecological re­cord from the last glacia:l/interlgacial cycle of the Chihua:huao Desert suggests that the distribution patterns can be best interpreted as historical, environmental phenomena.

Speciation is most likely to occur when a population becomes isolated. In the north, the differences between modem and glacial clim~t~ were

VanDevender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES 17

so great that the probability of extinction of isolated populations at the end of ei~her a ~acial ~r interglacial was very high. Speciation has only occun:ed m special hab1tats such as cliff faces, cold air sinks, and gypsum d:posits. To the south, as the difference between modern and glacial climates becomes less, the probability of extinction decreases as that of speciation increases. The climate was stable only in the sense that the environmental extremes of cold, heat, and drought have never been great enough to cause major extinctions. The repeated climatic fluctuations in the Pleisto~ene and the resulting changes in the distributions of organisms have provided a great many opportunities for speciation in isolation. Gypsum beds and playa surfaces have been repeatedly exposed and lime­stone sierras have repeatedly become islands as pluvial lakes filled. The combination of repeated opportunities for speciation in isolation in a fluctuating climate and of low extinction rates in an equable climate readily explains the accumulation of species at lower latitudes. Biotic inter~ctive processes may be important in structuring communities at any o~e .time, but the latitudinal d~v~rsity gradient appears to be a legacy of chmates of the past and present.

ACKNOWLEDGMENTS

The Chihuahuan Desert packrat midden research has been supported by NSF grants #76-19784 toT. R. VanDevender. Additional funds were provided to study the Hueco Mountains Holocene middens by Ft. Bliss Army Base contract #DABTS 1-82-M-3459EA through Glen de Ganno and K. von ~inger. L. J. Toolin and R. S. Thompson helped with the analyses. Tandem accelerator radiOcarbon dates were provided by D. J. Danohue, A. J. T. Jull, T. Zabell, and A. Long, National Science Foundation for Radioisotope Analysis, Univ. Arizona. Ellen Jesse typed the manuscript.

LITERATURE CITED

ANDERSON, L. C. 1964. Taxonomic notes on the Chrysothamnus viscidiflorus complex (Asterae, Compositae). Madroiio 17:222-227.

AXELROD, D. I. 1979. Age and origin of the Sonoran Desert vegetation. Occ. Pap. Cal. Acad. Sci. 132:1-74.

BROWN, J. H. AND A. C. GIBSON. 1983. Biogeography. C. V. Mosby Co., St. Louis, Missouri. BRYANT, V. AND D. A. LARSON. 1968. Pollen analysis of the Devil's Mouth Site, Val Verde

County, Texas. Pp. 57-70 in The Devil's Mouth Site, the third season-1967 (W. M. Sorrow, ed.). Pap. Texas Arch. Salvage Proj. No. 14.

FINLEY, R. B. 1958. The wood rats of Colorado: distribution and ecology. Univ. Kans. Publ. Mus. Nat. Hist. 10:213-552.

FISCHER, A. G. 1960. Latitudinal variations in organic diversity. Evolution 14:64-81. HENRICKSON, J. R. AND R. M .. STRAW. 1976. A gazetteer of the Chihuahuan Desert region.

A supplement to the Ch1huahuan Desert Flora, Calif. State Univ., Los Angeles, pp. 273, plus maps.

IMBRIE, J. AND K. P. IMBRIE. 1979. Ice Ages, solving the mystery. Enslow Publ., Hillside, New Jersey:

18 Chihuahuan Desert-U.S. and Mexico, II

IRVING, R. S. 1979. Hedeoma todsenii (Labiatae), a new and rare species from New Mexico. Madroiio 26:184-187.

JoHNSTON, M. C. 1977. Brief resume of botanical, including vegetational, features of the Chihuahuan Desert region with special emphasis on their uniqueness. Pp. 335-362 in Trans. symposium on the biological resources of the Chihuahuan Desert region, United States and Mexico (R. H. Wauer and D. H. Riskind, eds.). Nat!. Park Serv. Trans. & Proc. Ser., No.3, Washington, D.C.

LAMARCHE, V. C. AND H. C. FRITTS. 1971. Anomaly patterns of climate over the western United States, 1700-1930, derived from principal component analysis of tree-ring data. Monthly Weather Rev. 99:138-142.

LANNER, R. M. AND T. R. VANDEVENDER. 1981. Late Pleistocene piiion pines in the Chihuahuan Desert. Qu(lt, Resear. 15:278-290.

MACARTHUR, R. H. 1982. Geographical ecology. Harper & Row, Pub!., New York, New York.

MEYER, E. R. 1973. Late-Quaternary paleoecology of the Cuatro Cienagas Basin, Coahuila, Mexico. Ecology 54:982-995.

MORAFKA, D. J. 1977. A biogeographical analysis of the Chihuahuan Desert through its herpetofauna. Pp. 1-313. Dr. W. Junk, The Hague, The Netherlands.

MULLER, C. H. 1947. Vegetation and climate of Coahuila, Mexico. Madroiio 9:33-47. PINKA VA, D. J. 1976. A new species of cholla (Caetaceae: Opuntia)·from Coahuila, Mexico.

Madroiio 23:292-294. POWELL, A. M. 1983. Perity/e (Asteraceae), new species and notes. Madroii.o 30:217-225. --- AND B. L. TURNER. 1977. Aspects of the plant biology of the gypsum outcrops of

the Chihuahuan Desert. Pp. 315-325 in Trans. symposium on the biological resources of the Chihuahuan Desert region, United States and Mexico (R. H . Wauer and D. H. Riskind, eds.). Nat!. Park Serv. Trans. & Proc. Ser. No. 3, Washington, D.C.

RoLLINS, R. C. 1982. Species of Draba, Lesquere/la and Sibara (Cruciferae). Contr. Gray Herb. Harvard Univ. 211:129-133.

ScHMIDT, R. H., JR. 1979. A climatic delineation of the 'real' Chihuahuan Desert region. Phyto1ogia 44:129-133. . .

SPAULDING, W. G., E. B. LEOPOLD, AND T. R. VANDEVENDER. 1983. Late Wisconsin palaeoecology of the American Southwest. Pp. 259-293 in The Late Pleistocene en­vironments of the United States (S. C. Porter, ed.). Vol. 1. Univ. Minn. Press, Min­neapolis, Minnesota.

THOMPSON, R. S., T. R. VANDEVENDER, P. s. MARTIN, T. FOPPE, AND A. LoNG. 1980. Shasta ground sloth (Nothrotheriops shastense Hoffstetter) at Shelter Cave, New Mexico: environment and extinction. Quat. Resear. 14:360-376.

VANDEVENDER, T. R. 1977. Holocene woodlands in the southwestern deserts. Science 198:189-192.

---. 1978. Glaciopluvia1 woodlands in the Bolson de Mapimi, Durango and Coahuila, Mexico. Am. Quat. Assoc. Abstracts 1978:234.

---. 1980. Holocene plant remains from Rocky Arroyo and Last Chance Canyon, Eddy County, New Mexico. Southwest. Nat. 25:361-372.

--- AND B. L. EvERITT. 1977. The latest Pleistocene and recent vegetation ofBishop's Cap, south-central New Mexico. Southwest. Nat. 22:22-38.

--- AND D. H. RISKIND. 1979. Late Pleistocene and early Holocene plant remains from Hueco Tanks State Historical Park: the development of a refugium. Southwest. Nat. 23:127-140.

--- AND W. G. SPAULDING. 1979. Development of vegetation and climate in the southwestern United States. Science 204:701-710.

Van Devender • ENDEMIC FLORA AND PLEISTOCENE CLIMATES 19

---, J. L. BETANCOURT, AND M. WIMliERLY. 1984. Biogeographical implications of a · packrat midden sequence from the Sacramento Mountains, south-central New Mexico.

Quat. Resear. 22:344-360. · ---,C. E. fREEMAN, AND R. D. WORTHINGTON. 1978. Full-glacial and recent vegetation

of the Livingston Hills, Presidio County, Texas. Southwest Nat. 23:289-302. --, W. G. SPAULDING, AND A.M. PHtWPS m. 1979. Late Pleistocene communities

in the Guadalupe Mountains, Culberson County, Texas. Pp. 13-30 in Biological in­vestigations in the Guadalupe Mountains Nation.al Park, Texas (H. H. Genoways and R. J. Baker, eds.). Nat!. Park Serv. Proc. & Trans. Ser. No. 4, Washington, D.C.

--. P. s. MARTIN, A.M. PHrwPS m. AND W. G. SPAULDING. 1977. Late Pleistocene Biotic communities from th.e Guadalupe Mountains, Culberson County, Texas. Pp. 107-114 in Trans. symposium on the biological resources of the Chihuahuan Desert region, United States and Mexico (R. H . Wauer and D." H. Riskind, eds.). Nat!. Park Serv. Trans .. & Proc. Ser. No. 3, Washington, D.C.

WEBB, R. G. 1970. Another new night lizard (Xantusia) from Durango, Mexico. Contr. Sci. Los Angeles Co. Mus. Nat. Hist. 194:1-100.

WELLS, P. V. 1966. Late Pleistocene vegetation and degree of pluvial climatic change in the Chihuahuan Desert. Science 153:970-975.

~OBERT_P: ~~~"':1~~-~~~~~~~ {QWAJU) G . . API~LECiA

, . ._.=_•...,.,,.-_ ...

viEGAN' BJESEiE, PH.D. ~QUE CAMPOS-Lcm:if-Pif.D. f!ROINIA CociAN, PH.D. J;l , . ·~ . "' • • {.:~r::~~~~~~~m ~OOER CoNA.N:.r, PHD •. · • .'.'.". ~ - : .. .. ,llll~~~t iAI.VADOR CoNTRAR.As·BALDEAAS, ~ :iL.ENNA DEAN, PH.D . .'; ..... ~ ..... . ACK D.El.OACH, PH:D .. : .·. : . ...... . toBERT DILLON, PH.D ..• , ...... · .. AMES R; DIXON', PH.D.·'·;' ); . ·'·· ....• \RTHUJt H. DUNHAM, PH.D ..••.• . . . \R.THUR_,H. HARRIS, PH.D •...• ~ .. ~ AMES~RICKSON, PH.D.'.-... : . --~ IJCHARO HlLsENBECK, PH.D . . -... . : . .iAR.sHALL C. JoHNSTON; PH.D: . • ~ . -~;el''-l\ '~~~~~ ~OM J. ~RY, PH.D.. • .. : . . ~'. ORGE S. MAR~OQUIN, PH.D. ~ .. ... , · . ·;;;;,:."'~""'~·•

V. BRUCE McG!LWV'R.Av, PH.D. : ;. >;· . ·~:r.•.M.u:si~~J~i~~ )ENNJS.N.El.SON, PH.D ... ~: .. t-. ••• ~··. tOBERT~Ii ScHMIDT, PH.D ... "; . ·. AMES'f.,~DDAY; fH.D. ; . :...: , . --_, . .,,,.,.~~

). ]. SIBLEY, JR., M.D. >:: .... · ·..: .. \USTIN SlFTON, PH.Q .. ~. . . ~

. . • . • .• , .. , . TEXAS ~c.M::~9;1UC~~~~ IARBARAN·. TIMMERMAN~. t'HII .• ,,,.

'EDRO JAMIE v ARGAS, . ENd. ~ou.ND~:A:, W AUER, ~

.. -

, f I __ ... ~t

.. · ... j )

•.•

.•

SECOND SYMPOSIUM ON . · pE$OURCEs ;OF-·THF · - - ·

dinmJAHUAN DESERT REGION r~

i.JNITED STATES AND MEXICO 20-21 OCTOBER 1983

CHIHUAHUAN DESERT RESEARCH INSTITUTE

Box 1334 sur. Ross STATE· UNIVERSITY

;._... ALPINE, TEXAS 798j}, U.S.A.

Edited by ]ON C. BARLOW·_

A. MICHAEL PoWELL

BARBARA N. TIMMERMANN

SPONSORED BY:

'

CHIHUAHUAN DESERT RESEARcH ·INsTITUTE AND suc::R'OsS STATE

UNIVERsiTY

t -Co-SPONSORS: ·

\ BIG BEND NATURAL HISTORY AssociATION; CENTER FOR

INTER-AMERICAN STUDIES-EL ·PAso; ]AMES TRUSTEE PROFESSOR OF

BIOLOGY, SOUTHERN METHODIST UNIVERSITY-DALLAS

PUBLISHED BY THE CHIHUAHUAN DEsERT RESEARCH INSTITUTE