Macrofossil analysis of wood rat (Neotoma) middens as a key to the quaternary vegetational history of arid America

QUATERNARY

RESEARCH

Macrofossil

6, 223-248

Analysis

(1976)

of Wood

to the Quaternary

Rat (Neotoma)

Vegetational PHILIP

Departments

of Botany

History

Middens

of Arid America

V. WELLS

and of Systematics and Ecology, Lawrence, Kansas 66045 Received

as a Key

University

of Kansas,

May 28,1975

Wood rat (Neotoma) deposits preserved in dry rock shelters have radiocarbon ages extending from close to the present to >40,000 BP, thus providing elaborate samples of changing vegetation -matic shifts oTthe late Pleistocene and Holocene. The established record extends geographically from Orzgzd WyomigA(atTN) (G-T-l, Sonora, and to Tehuacan at 18 N).,!_n_S.Mexico. mmns have been ‘i&%vered, and over 130 have been ,. , 3: -A\,‘&-dated. ,. There are about 20 extant species of Neotoma with a combiied range extending from the Yukon and New England to Nicaragua and Florida, Hence, a wider application for the method seems likely, wherever dry caves favor preservation. The acquisitive rats accumulate an incredibly detailed inventory of the local flora and fauna within a small home range, measured at about 100 m or less in radius. The biomass spectrum of a modern wood rat deposit was compared with associated pollen spectra and source vegetation. The most dominant arborescent species, as determined by a quantitative study of the local vegetation, were proportionately represented in the midden, but not in the pollen rain, Less dominant species were variable in proportionality. A novel approach to the comparative ecological physiology of long-dead plants has been demonstrated with macrofossils from ancient wood rat deposits. The ratio of the stable isotopes of carbon (* 3 C/l 2 C) is more altered from the atmospheric proportion during CO2 fixation by C3 plants than it is by the different (PEP) carboxylating enzyme of heliophile or xerophytic, C4 or CAM plants. Mass spectrometry of the carbon of macrofossils enables the distinction to be made in the past. The desiccated, allelochemic urine, or amberat, indurates and preserves the middens by cementing the loose debris of macrofossils into a tough, coherent mass of surprising strength and rigidity, that adheres tenaciously to rock surfaces. Water softens the most indurated middens by dissolving the crystallized urine, causing them to dislodge and to fall apart; it also removes the osmotic and allelochemic deterrent to decay by fungi, or to consumption by termites, crickets, or other decomposing herbivores. A secure position in a dry rock shelter is therefore essential to preservation of the macrowith time is seen in the fossils for time periods on the order of lo4 years. Attrition frequency distribution of radiocarbon dates obtained on 132 Neotoma middens, The distribution is skewed toward the end of the Wisconsin glacial and the Holocene, and there is a decline in frequency with age. Cave erosion, including destructive rockfalls and new crevices that admit seepage and cause deterioration and dislodgement of the middens, is probably the main factor in the progressive attrition with increasing age.

INTRODUCTION Ancient wood rat deposits, sheltered in dry caves, rock crevices, or overhangs, were radiocarbon-dated and developed as a paleoecological method soon after the discovery of middens containing evidence of coniferous woodland at what 223 Convright

0 1976 bv the Universitv

of Washington

are now desert elevations in the arid Mohave Desert region of southern Nevada (We&, 1961; Fergusson and Libby, 1962). The abundant and beautifully preserved plant and animal macrofossils in the Neotoma middens demonstrated in an elegant way the postglacial

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vegetational change from late Pleistocene, Juniperus-dominated woodland to Holocene, Larreu-dominated desert scrub. The initial find of an ancient Neotoma midden was made in 1960 at Indian Springs ranch, about 65 km northwest of Las Vegas, Nevada. In one of the numerous rock shelters on the face of the limestone escarpment above the ranch, I found a strange, indurated organic deposit, veneered with a dark, lustrous material, that also cemented it to the ceiling of the shelter in the fashion of bat guano. The deposit contained fragments of cacti and other desert plants, but these were sealed inside and there was no indication of recent accumulation. Wider observation in other rock shelters in the region uncovered more of these evidently subfossil deposits. Clive D. Jorgensen confirmed a Neotoma origin on the basis of the numerous rat scats present in a fragment that I had collected near our field headquarters at Mercury. The first realization that the deposits were more than possibly ancient coprolitic curiosities came on a serendipitous field trip to Aysees Peak, the highest mountain in the extremely arid Frenchman Flat area. At 1906 m, it seemed high enough to support a relict stand of pinyon-juniper woodland, although none was visible on the western side of the mountain, facing the Frenchman Flat basin. Mainly to satisfy our curiosity on this point, Jorgensen and I climbed the peak in 1961, but to our disappointment found that desert shrubs ascended to the summit from all sides. Coming down by a different route, we dropped into a narrow canyon cut in limestone and came upon a rock shelter enclosing one of the subfossil middens. Unlike the others I had been finding, this deposit contained abundant leafy twigs of the woodland conifer, Juniperus osteosperma. It should be pointed out that we realized this on the spot because

Jorgensen, a trained zoologist, was curious to examine his first ancient Neotoma midden, and fortunately exposed the critical juniper twigs, which would not have been noticed otherwise. Because we had just established the absence of juniper or other conifers, even on the summit or in the canyons heading in the highest divides of the mountain range, we knew immediately that the deposit had great paleoecological significance. Although the elevation of the site (1525 m) was not greatly below the lowest juniper woodlands on some of the highest mountain ranges at this latitude, where they descend to about 1700 m, nevertheless, the nearest wooded mountain was 30 km from Aysees Peak. Thus, in the midst of a vast sweep of sparse, creosote-bush scrub, we had uncovered dramatically conclusive evidence as to the magnitude of vegetational and climatic change on a barren, rocky peak in the Mohave Desert. A radiocarbon date on the Aysees midden gave an age of 9320 + 300 BP Other (Fergusson and Libby, 1964). late Wisconsin Neotoma deposits at elevations extending from 1830 m down to 1100 m in the other arid, limestone ranges around Frenchman Flat established a paleozonation of pluvial, pinyonjuniper woodlands with juniper segregating from pinyon pine at lower elevations. Juniper displaced a maximal 600 m downward into the now extremely arid, desert lowlands, over a time span including the Wisconsin glacial maximum, at about 17,500 BP, and older Wisconsin dates, ranging from 27,000 to >40,000 BP (Wells and Jorgensen, 1964). The downward displacement of the pinyonjuniper woodland zone into what is now desert scrub proved to be a recurrent theme throughout the lowlands of the Mohave and Sonoran Deserts (Wells and Berger, 1967; Wells, 1969, 1976; Van Devender and King, 1971), and indeed, throughout southwestern North America.

VEGETATIONAL

HISTORY

A wider geographic range for the method materialized with the uncovering in 1964 of Ice Age Neotoma middens containing macrofossils of an entirely different pinyon-juniper-oak woodland flora in the Chihuahuan Desert of Texas, about 1500 km to the southeast of the original finds in the Mohave Desert (Wells, 1966, 1976). A wider range in time developed with the dating of a series of Holocene wood rat deposits containing remains of Pinus ponderosa and Juniper-us scopulorum at 1100 to 4000 BP, which gave evidence of postglacial climatic change in the Laramie Basin and adjacent Great Plains of southeastern Wyoming (Wells, 1970a). The method has been extended recently to northwestern Wyoming, southern Idaho, and eastern Oregon, and in a southward direction in Mexico, to the southern Sonoran Desert in Baja California and Sonora, and in the tropics, to the valley of Tehuacan, Puebla at 18”N (Wells, unpublished data). Several hundred more or less ancient middens have been uncovered, and over 130 have been radiocarbon-dated. GEOGRAPHIC AND ECOLOGICAL DISTRIBUTION OF WOOD RATS The genus Neotoma includes about 20 species of rat-like, cricetine rodents with a wide distribution in North and Middle America, where the genus is endemic. Wood rats are neat, furry, large-eyed herbivores, varying in length from about 25 to 45 cm (including the long hairy tail), and in weight from about 95 to 585 g. At night, they are exceedingly active and curious animals, but during much of the day, they are secluded within barricades, usually built of plant materials collected at night. Most of the species of Neotoma are in western North America, west of the Great Plains, where they range from the Yukon, close to the Alaska boundary, south to Nicaragua (Fig. 1). One species, N. floridana, ex-

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tends east to the Appalachian region and Florida, and N. micropus ranges north from Tamaulipas to the Arkansas River on the southern Plains. The known Quaternary record of Neotoma middens may be attributed to the four wide-ranging species that inhabit rock shelters in western North America: N. cinerea, N. lepida, N. albigula, and N. mexicana. The very large, bushytailed wood rat (N. cinerea) is the only boreal species, and it alone inhabits the coniferous spire forests from the Yukon south to central Arizona. In the Great Basin, Colorado Plateau, and Rocky Mountains, this species also inhabits xerophilous woodlands dominated by pinyons, junipers, mountain mahogany, etc. Because N. cinerea is the most scansorial and rock-sheltered species in the genus, being an almost obligate cliffdweller and troglodyte, it has left an outstanding record of Quaternary vegetation in its abundant well-preserved middens. In Nevada and Wyoming, skulls of this species have been identified in ancient Neotoma deposits. The small desert wood rat (N. lepida) overlaps in montane woodland habitat with N. cinerea, but extends downward into desert lowlands from Oregon to Baja California. Where the two woodland species are sympatric, the much smaller size of N. lepida (25-34 cm and 95-160 g) enables it to occupy crevices too small for N. cinerea (to 45 cm and 585 g), thus partitioning the rock-shelter niche (Finley, 1958). Also, if rock shelters are unavailable, N. lepida builds massive stick-houses or lodges. A skull of N. lepida has been identified in a late Pleistocene midden from southern Nevada (Wells and Jorgensen, 1964). wood rat The large, white-throated (N. albigula) inhabits the cactus-rich deserts and woodlands mainly to the southeast of the ranges of N. cinerea and N. lepida. It is the principal modern species of the Sonoran and Chihuahuan

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FIG. 1. Map of North and Central America, showing the modern distribution of the genus Neotoma. The dotted line indicates the range of the eastern wood rat (N. floridana) and the continuous line delimits the combined ranges of the 19 western species, extending from the Yukon to Nicaragua. Localities where Quaternary Neotoma middens have been collected are marked by large dots. A wider application for the method seems likely wherever dry caves favor preservation.

VEGETATIONAL

HISTORY

Deserts and of the woodlands on their bordering mountains, ranging south to Hidalgo . The resourceful N. albigula also builds large stick-houses where rock shelter is unavailable, but to a greater degree than with N. lepida, and it employs much greater quantities of cacti and other armed plants in the construction of its houses and rock-sheltered middens (Vorhies and Taylor, 1940). Possibly N. albigula was the species responsible for the ancient middens in the lowlands of the Sonoran Desert, and also in the Chihuahuan. Lastly, the small Mexican wood rat (N. mexicana) is a montane, pine or pine-oak woodland species that resembles N. cinerea in its rock-climbing and crevice-dwelling proclivities, rarely ranging away from rocky habitat, and not building lodges. It extends from Colorado south in the mountains of Mexico to Guatemala and Honduras. It is possible that N. mexicuna contributed to the Quaternary Neotoma records in the Chihuahuan Desert and in the Tehuacan Desert. About half of the 20 species currently recognized in Neotoma (Hall and Kelson, 1959; Hall and Genoways, 1970) account for most of the distributional range of the genus. The remaining species in the genus are more or less narrowly endemic, and nine of them are closely related to the polymorphic N. lepida and N. albigula, occurring as peripheral isolates. In fact, five of the narrow endemics are restricted to small islands along the Pacific coast of northern Mexico, and are perhaps best regarded as insular subspecies of N. lepida and N. albigula. The combined ranges of four of the most abundant, rock-dwelling species of Neotoma extend over most of western North and Middle America, excluding the humid, tropical forest belts and the suitable, but isolated, Yucatan peninsula (Fig. 1). Other, more tropical species are

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which ranges from Honduras to Nicaragua, and N. alleni, which extends through Mexico from Sinaloa to Oaxaca. Little is known about the ecology of these species, but by analogy with the rest of the genus, they may inhabit thorn woodland in relatively arid lowlands (N. alleni) or pine-oak woodland of the cool, tierra templada belt in the mountains. The Pacific slope from Oregon to northern Baja California is occupied by the large, tree-dwelling, dusky-footed wood rat, which specializes in building enormous stick-houses, but may have left some deposits in rock shelters also (Linsdale and Tevis, 1951). Hence, likely candidates for preserving records of Quaternary vegetation are available throughout much of the range of the genus, and an even wider application of the Neotoma paleoecological method seems inevitable (cf. Fig. 1). N. chrysomelas,

BEHAVIORAL ECOLOGY OF WOOD RATS The members of the genus Neotoma share the most peculiar trait of accumulating large amounts of plant material, either in massive stick-houses or in less structured middens within rock shelters. The wood rats appear to be unique among North American animals in having a behavior pattern that favors the preservation of elaborate plant macrofossil records. The beaver (Castor) and the muskrat (Ondatra) build lodges of wood, cattail, or other plant debris only in or near water, which ensures rapid decay; and the mountain beaver (Aplodontia) and pika (Ochotona) merely store cured hay, most of which is eaten, especially during the long, snowy winters of high-montane habitat. In contrast, not only do wood rats collect an incredibly detailed inventory of the local flora and fauna in the form of twigs, leaves, fruits, seeds, even flowers, and bones, exoskeletons, shells, etc., but they also deposit them in dry caves, and

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often impregnate them with their possibly allelochemic, viscous urine. The latter appears to play a key role in the preservation of Neotoma middens, as discussed below. The unique behavior pattern of wood rats is so effective as a fossilizing agent that beautifully intact, delicate, plant structures are preserved in their middens for time periods exceeding 40,000 radiocarbon years. Thus, records of late Pleistocene and Holocene vegetation are accessible within the present or former range of wood rats possessing the appropriate rock-dwelling habits. The outstanding behavioral trait of wood rats, one that relates to their contribution to the paleoecological record, is reflected in the vernacular names pack rat, trade rat, and cave rat. The “pack” and “trade” appellations refer to the strongly acquisitive behavior pattern of the entire genus, and the third epithet to the preferred depository of some species. Mainly under the cover of darkness, the herbivorous wood rats make many short forays over a very limited range (usually
stick-house or lodge, is built on sites lacking adequate rock shelter, but usually with a large cactus, shrub, tree, or rock that provides some initial refuge (Vorhies and Taylor, 1940; Linsdale and Tevis, 1951; Finley, 1958; Stones and Hayward, 1968; Cameron and Rainey, 1972). Since the lodge is more highly organized than the deposits restricted to rock shelters, it gives insights into the character of the latter, which contain the TypiQuaternary macrofossil record. cally, the lodge is dome-shaped, often more than a meter in diameter, with an outer covering of woody branches and twigs, the stick-lattice. The lattice is expandable, as sticks are added on the outside to accommodate increases in volume due to the steady accumulation of food debris and fecal pellets within the interior. The exterior plant material is often fiercely armed with spines, choice materials being provided by various Agavaceae and Cactaceae, especially the horridly spined joints of cholla (cylindrical Opuntia), if available. The sharp tips of conifer needles are substituted where armed xerophytes are lacking. The spiny armament is often strategically concentrated at the many small entrances around the basal perimeter of the lodge. Obviously, there is a deterrent effect on potential predators. Indeed, the fortification aspect of wood rat architecture is evidently a prime selective advantage, inasmuch as protective barricades are retained to some degree even inside caves, where there is excellent shelter from exterior climatic stresses as, for example, wetting rains, winds, and seasonally extreme temperature fluctuations. Underneath the loosely interwoven stick-lattice is the coherent, or even indurated, lodge-matrix, composed of finer plant debris that is consolidated and structured by rat activity, including tunneling, excretion, and compaction. This central core of the lodge contains chambers and passages with well-tamped

VEGETATIONAL

HISTORY

floors and rounded ceilings (Stones and Hayward, 1968). The narrow passages form a network connecting the multiple, comparatively spacious, chambers with the exterior entrances. A large, superficial chamber located just underneath the stick-lattice, at the apex of the domelike roof, is the most frequent site of the principal food cache in the lodge, containing stores of seeds or other seasonally available plant food, e.g., pinyon nuts. More securely positioned within the indurated lodge-matrix are one or more chambers harboring the nest, which is a globular or gourd-shaped mass of shredded, fibrous bark (of juniper, where available), or other fine, soft, plant material. The nest contains a snug, hollow pocket in which the rat sleeps or rests for long time periods, especially during cold or inclement weather, but the nest is rarely, if ever, contaminated with excrement. It should be pointed out that almost any structure built by rats is loosely designated a “rat’s nest” in the vernacular, but for Neotoma the term nest has the special meaning assigned here (Finley, 1958). A small food cache of fresh leaf cuttings or the like is kept close by the nest, as wood rats are incessant feeders, even when resting (Linsdale and Tevis, 1951). Also in the nest chamber, and in other chambers used exclusively for this function, are interior middens or refuse deposits of food waste and fecal pellets, more or less indurated by the desiccated, viscous urine. Although wood rats do not “foul their own nest,” they do foul the midden chambers and even the nest chamber, as most excretion takes place inside the lodge (Linsdale and Tevis, 1951). The long periods of confinement that cause poor sanitation apparently have the overriding selective advantage of reduced exposure to predation and adverse weather, while the animal draws on its ample food stores inside the lodge. Wood rats do at least some house cleaning at intervals, pushing fecal pel-

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lets, food refuse, and other loose debris out through the entrances to form large exterior middens. Nevertheless, a net surplus of food waste and excrement builds up within the lodge-matrix, which grows in volume as new layers (with new chambers and passages) accumulate inside the expanding stick-lattice, and the initially loose debris becomes indurated into a cohesive mass as a result of cementation by the impregnating, quick-drying urine. Leaching, softening and rotting from soaking rains may counteract the enlargement process. Outside the lodge, there is usually at least one perch on a rock, a stump, or other vantage point where feces and urine are voided, but perches are usually too exposed to weathering to permit the accumulation of middens. A system of runways or trails extends from the lodge to all feeding areas within the limited home range of the wood rat. In some species dwelling in cactus-rich habitat (e.g., N. albigulu), the runways may be lined with spiny areoles, thus extending the armament tactic of the stick-lattice throughout the territory of the individual (Vorhies and Taylor, 1940). Cave deposits. Under the protection of caves, crevices, overhangs, or other rock shelter, wood rat architecture degenerates to the point that the well-structured lodge is not built. However, fragmentary elements of lodge structure are adapted to fit the microrelief available in the rock shelter, with the result that different lodge elements may be segregated among the ledges, nooks, and crannies of a cave. Thus, the stick-lattice may be present as a barricade in front of a secure rock cavity that harbors the fibrous nest, while middens and food caches accumulate on a nearby ledge. The middens of plant fragments and fecal pellets are the most massive deposits laid down by wood rats in rock shelters. On the horizontal surfaces of floors or ledges, middens may build up to a depth of a meter or more, some-

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times filling all available space to the ceiling of the shelter, and therefore terminating that episode of deposition. On sheer rock walls, accumulation may be nil, unless below a perch, where the viscous urine quickly dries and crystallizes to form a glossy, dark brown or blackish veneer known as amberat (On-, Amberat commonly stains the 1957). rock walls in vertical streaks or curtains below crevices or ledges, but also may permeate and coat the middens. Because the excrement of cave-inhabiting wood rats is usually better segregated from the nesting cavity, sanitation is more advanced than in the stick-house. Rock shelter is the preferred habitat of most species of Neotoma, wherever it is available, and it is almost the sole habitat of some species (Finley, 1958). Some of the reasons for this preference may be greater security from predation, together with energy economy in the gathering of less building material, and a more equable microenvironment with more or less complete protection from wetting rains and desiccating or chilling winds. NATURE OF PLEISTOCENE NEOTOMA DEPOSITS Ancient wood rat deposits are confined to rock shelters that afford adequate protection from weathering. The shelters range in size from huge limestone caverns to small cavities or crevices with a capacity of only a few liters. In the former, the rats usually place their main deposits in relatively secluded microsites not far from the entrance chamber (sometimes deep within the cave), but their litter may be strewn quite widely over the floor. Radiocarbon dates on inactive Neotoma middens demonstrate a range in age from a few centuries to the limits of the usual method at >40,000 years. Active deposits with fresh plant materials extend the Neotoma record up to the current year. However, a continuous strati-

graphic record at any one site has not been established, because episodic deposition undoubtedly was as commonplace in the past as it is today, inasmuch as the favored crevices and cavities can be filled to capacity very quickly. The industrious wood rats are avidly acquisitive and prodigiously excretory. An experimental study on caged individuals of the relatively tiny desert wood rat (N. lepida) documents the ability of a single rat to deposit as many as 359 pieces of plant material in one night, and to construct an entire stick-house, more than a meter in diameter, in a week (Bonaccorso and Brown, 1972). Also, caged individuals of N. fuscipes have been shown to excrete up to 247 fecal pellets within 24 hr, and averaged 124 pellets per day over a 24-day observation period (Linsdale and Tevis, 1951, p. 261). At the latter rate, there would be an accumulation of more than a thousand liters of feces within 200 years. The concomitant plant litter would increase the volume to a level greater than that of the largest known Quaternary Neotoma midden. Because very large deposits are rare, interfering forces such as erosion, decomposition, or disturbance of the site by other animals must prevent the complete filling of larger rock shelters that have suitable geometry for the accumulation of thick deposits. The elaborate Neotoma lodges, built where rock shelter is unavailable, can be assembled very quickly, and are apparently ephemeral structures, vis-a-vis the time scale for persistence of the cave deposits. Although stick-houses constructed in favorable feeding habitat may persist as focal points of deposition for some decades, as has been established by direct observation, there is considerable turnover in the contained materials (Linsdale and Tevis, 1951). Weathering and decay are promoted by penetrating rains that soak the porous mass of the lodge, and the rotting materials are

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periodically dragged out and replenished. Even temporary abandonment of an unsheltered stick-house results in rapid deterioration. Ancient cave deposits. As in modern cave deposits, the older Quaternary Neotoma structures are mainly middens of plant refuse, fecal pellets, and desiccated urine (amberat), with only occasional or fragmentary inclusions of stick-lattice, food-cache, and (very rarely) nest materials. It is extremely fortunate for paleoecology that Neotoma middens contain a great wealth of beautifully preserved, readily identifiable, plant and animal material, ideal for macrofossil analysis, as well as abundant pollen, epidermal fragments and other microfossils. The latter may be observed in the eolian fines trapped in the matrix of the midden, or within the fecal pellets, thus affording a contrast between random entrapment and dietary preference. Consistent associations of two or more single fecal pellets is species within firmer evidence of their former coexistence within one feeding range. The eolian pollen, if present, may provide significant supplemental information on the regional pollen rain (King and Van Devender, 1976) from dominant elements of the vegetation that happen to be anemophilous in their mode of pollination; but the main reliance, perforce, should be placed on the direct evidence of firmly identified macrofossils. The Neotoma macrofossil method benefits from some unique attributes of wood rat deposits: preservation of morphological detail in the most delicate structures, and the great species diversity of plant and animal materials collected within the limited home range of the acquisitive rats. Excellence of preservation. Most of the plant and insect macrofossils are mummified materials that retain their original gross morphology and fine, cellular tissues. The plant macrofossils, for

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example, consist of readily identifiable leaves, twigs with leaf-scars and buds, fruits and seeds, and sometimes perfectly preserved flowers; wood specimens, which are usually very abundant, can be identified by microscopic study of transverse, radial and tangential sections. Needless to say, accurate identification of species is a comparatively simple matter with the wealth of material representing the dominant constituents of a Neotoma deposit. By way of contrast, generic identification of pollen is a problem in the Cupressaceae (Juniperus, Cupressus, Thuja, and others), whereas, the leafy twigs, berrycones, and seeds in ancient wood rat middens often permit accurate determiThus, the nation at the species level. Pleistocene juniper of the Mohave Desert was J. osteosperma (Torr.) Little; in the Sonoran Desert in Baja California, it was J. californica Carr., and in Sonora it was J. monosperma Sarg.; in the Chihuahuan Desert it was J. deppeana Steud. and J. pinchotii Sudw.; and in Holocene deposits in the Laramie Basin, Wyoming, it was J. scopulorum Sarg. (Wells, 1969, 1970,1976). The reasons for preservation of Neotoma deposits for time periods of as much as several tens of thousands of years are fairly obvious. Without special protective features in the deposits themselves, the dryness of the caves or rock shelters would preserve much of the contained material if it were laid down in a position secure from decay, dislodgePlant and animal ment, and removal. macrofossils have been obtained from ordinary sedimentary deposits in rock shelters; e.g., witness the good preservation of plant structure in the Holocene record of maize and many other species from stratified floor sediments in the dry cavelets of Tehuacan (Byers, 1967). What is unique about Neotoma deposits, aside from the spiny armament against disruption by other animals, is the dual

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role of amberat (the crystallized urine of the rats). Amberat solidifies and hardens many deposits into a coherent, almost concrete-like mass, and also may act as an allelochemic substance that retards decay. Desiccation of the viscous urine forms a smooth, hard shell on the outer surfaces of many deposits and confers rigidity and surprising mechanical strength, as well as containment for loose plant material within. Frequently, the urine has permeated the entire midden, cementing the plant materials, fecal pellets, and other debris into a tough, indurated mass which is difficult to take apart, except by splitting with a chisel on fissile, ieafy, stratigraphic planes. Furthermore, the amberat adheres to the rock, thereby anchoring the midden in place, even in extremely precarious situations on steep rock walls. In this manner, large, very heavy, ancient middens may cling to vertical surfaces where modern middens could no longer accumulate, because of erosional changes in the configuration of the rock shelter (see illustration of >40,000 BP midden adhering to ceiling in Wells and Jorgensen, 1964: Fig. 2). Thus, the strengthening and cementing action of the amberat plays a critical role in the secure positioning of some cave deposits that are destined to persist for thousands of years, by retarding mechanical disintegration and dislodgement. This role is readily confirmed by treating an amberatimpregnated, ancient midden with water, which dissolves the crystallized urine and causes the entire mass to soften and to fall apart. Hence, the exclusion of water is a sine qua non for the preservation of many Neotoma deposits, just considering the mechanical aspects of erosion-resistance and persistence in situ. Another aspect of the amberat is its allelochemic role in preserving delicate, easily decayed plant structures (for ex-

ample, membranous or succulent tissues, trichomes, flowers) from the enzymatic attacks of fungi and bacteria during humid weather or from direct consumption by herbivorous invertebrates such as termites, ants, crickets, millipedes, land snails, etc. Most of the remarkably intact, fine structures are stained with the amber tint of the Neotoma urine. The osmotic concentration of the crystallized urine is a powerful deterrent per se; but furthermore, the amberat is rich in diversely toxic secondary plant products that are actively excreted by the herbivorous rats. For example, twodimensional paper chromatography of Pleistocene amberat from Nevada reveals a striking array of acid-soluble phenolics that fluoresce under ultraviolet light (Wells, unpublished data). Also, the many kinds of toxic compounds (terpenes f modified to amber, tannins, alkaloids, etc.) that are natively present in the preserved plant materials should not be overlooked. Because the subject of allelochemic interactions among higher plants and their diverse consumers, the herbivores and decomposers, has been so extensively discussed in recent literature (e.g., Whittaker and Feeny, 1971; Rice, 1974), it is unnecessary to enlarge here on the implications for amberat, and the allelochemic substances in the plant macrofossils themselves, as deterrents to the consumption or disruption of nutrient-rich Neotoma middens, during time spans measuring tens of thousands of years. Again, protection from the dissolving action of water is critical for the persistence of Neotoma deposits. Species diversity of macrofossils. The middens accumulated by Neotoma inside rock shelters contain a rich sample of biological materials from the communities living in the neighboring vicinity. Existing wood rats are not only diversely generalist in their herbivorous habits, but also widely indiscriminate in their compulsive collection of transportable ob-

VEGETATIONAL

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jects that are used in building barricades, or that are merely deposited at random in their middens within the cave. The legendary collecting habits, as distinguished from the dietary preferences (which show a somewhat lesser catholicity of taste), have been observed extensively in many species of Neotoma (Finley, 1958; Rainey, 1956; Linsdale and Tevis, 1951; Vorhies and Taylor, 1940; Stones and Hayward, 1968). The principal bias is toward spiny plant material, where available; otherwise, collection of constructional material appears to be at random. The net result is an extremely detailed inventory of the local flora and fauna with some quantitative bias toward favorite food plants and spiny armament, fortunately including the acicular or pungently scaly leaves of conifers. The six species of Neotoma extant in Colorado, which have been studied exhaustively by Finley (1958), actually eat at least 12 of the 14 species of conifers present in the state. A grand total of 219 species of trees, shrubs, lianes, cacti, grasses, and forbs were found in the food litter of living wood rats in Colorado by Finley. Practically all of the dominant species of woody plants in the Rocky Mountains (and in much of the Colorado Plateau and Great Basin) are represented in a total of 176 modern wood rat deposits examined at 70 localities by this astute observer. Quaternary Neo toma middens preserve a comparably rich species diversity. Late Pleistocene deposits from several different sectors of the Mohave Desert, and the northwestern fringe of the Sonoran Desert, yielded an abundance of macrofossils of pinyon-juniper woodland plants, enabling the identification of 45 species (Wells and Jorgensen, 1964; Wells and Berger, 1967). A smaller, but comparable sample of Neotoma deposits from a restricted part of the northern Chihuahuan Desert contained an entirely

FROM

WOOD

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233

different woodland assemblage (e.g., Pinus cembroides instead of P. monophylla) in which 33 species were identified (Wells, 1966). Late-Wisconsin and Holocene wood rat middens from the floristically rich Sonoran Desert have yielded over 100 species of plants (Van Devender, 1973; Wells, 1969, 1976, and unpublished), including many species previously identified in the Mohavean deposits. The paleoecological and biogeographic aspects of these findings have been discussed in the papers cited above. Animal remains are also preserved in ancient Neotoma middens with varying degrees of abundance and diversity. Bones of small vertebrates, such as lizards, snakes, birds, and rodents are especially frequent, but the long bones of larger mammals have been collected occasionally, and plastrons of a possibly extinct tortoise (Gopherus) were obtained from two late Pleistocene middens in western Texas (Wells, unpublished). Remains of invertebrates are consistently present in the deposits, with a wide variety of groups represented, including scorpions, spiders, snails, millipedes, ticks, and many insects (especially beetles). Van Devender (1973) has identified a toad, 6 lizards, 9 snakes, and 12 small mammals from late-Wisconsin middens in western Arizona. A similar array of mammals was recorded from deposits of similar age in southern Nevada, but the latter included a skull of the significantly extralocal marmot (Marmo ta flaviventris), which now survives only at much higher elevations in central Nevada (Wells and Jorgensen, 1964). BIOMASS SPECTRUM OF A MODERN NEOTOMA MIDDEN COMPARED TO POLLEN SPECTRA AND SOURCE VEGETATION The quantification of Neotoma macrofossil analysis is relevant to paleoecological interpretation only to the extent that the relative proportions of different

234

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V. WELLS

species represented in analogous, modern, midden samples are correlated with relative proportions in the source vegetation. The great detail accessible by purely qualitative analysis of species present in wood rat deposits could provide quite accurate reconstructions of the source communities. Unquestionably firm identifications of an almost incredible diversity of species has been demonstrated repeatedly in Neotoma middens from many different sectors of western North America (Wells, 1969, 1970, 1976; Mehringer and Ferguson, 1969; Leskinen, 1970; Van Devender, 1973; Phillips and Van Devender, 1974). In all of these studies, at least a semiquantitative estimate of relative abundance of most species has been attempted from the immediately obvious evidence of the identified macrofossils, sorted as to species. An outstanding generalization that emerges is the consistently large relative abundance of arborescent species in wood rat deposits, invariably the dominant species of trees in the analogous modern communities. On the other hand, species that are sparsely represented by macrofossils in the middens tend to be subordinate also in equivalent modern vegetation. However, critical studies on quantitative proportionality are lacking in the literature. A comparison of the “macrofossil” spectrum from a modern or very recent Neotoma deposit with closely associated pollen spectra and the source vegetation is available from an archeological site in Pratt Cave, McKittrick canyon, in the Guadalupe Mountains, Texas. A team investigation of the ‘Neotoma midden (Wells), the pollen (J. Schoenwetter), and the modern, local vegetation (P. Sanchez) at Pratt Cave was carried out under the coordination of A. Schroeder of the National Park Service. An independent, quantitative analysis of the vegetation was made by Gehlbach (1967) as part of his dissertation study of the vegetation of the Guadalupe escarpment.

Pratt Cave is in the limestone on the north-facing side of the canyon at an elevation of 1610 m and lies about 100 m above the intermittent streambed. The local vegetation is an open, xerophilous, evergreen woodland of broad sclerophylls and conifers, dominated by shrubby live-oaks and junipers, with a few Pinus ponderosu in the canyon bottom. The shrubs include a number of xerophytes characteristic of the Chihuahuan Desert and its bordering woodlands and desert grasslands: the catclaw (Mimosa biunciferu), goldeneye (Viguieru sacahuista (Nolinu microstenolobu), curpu), datil (Yucca buccutu), sotol (Dusylirion leiophyllum) and prickly pear (Opuntiu engelmunnii), with an abundance of the invasive composite, Brickelliu luciniutu, on the open streambed. Although Gehlbach (1967) sampled extensively in McKittrick canyon and recorded the presence of most of the perennial species, only a handful of species were sufficiently abundant in his samples from the vicinity of Pratt Cave to provide numerical estimates. The wood rat deposit collected in Pratt Cave is a loose, unconsolidated midden without amberat, chiefly composed of the abundant leaves and branchlets of liveoak, juniper, et al., and the inevitable fecal pellets. Presence of chlorophyllous leaves was a sure indication of a very limited antiquity. The sample submitted to me weighed about 3 kg, including several limestone pebbles and a large proportion of fecal pellets; a grand total of about 210 g of securely identifiable plant material was painstakingly sorted from the mass of loose debris, which spread out to occupy the entire area of a standard laboratory bench-top. To give an idea of the time required for midden analysis, the operation of sorting, identification, and weighing of each species took a full month. The results are presented in Table 1. As may be seen in this summation, the wood rat midden is a better source of information about the

VEGETATIONAL

HISTORY

local flora in the Pratt Cave area of McKittrick canyon than the necessarily restricted samplings of field botanists. The combined observations of Gehlbach and Sanchez yielded a total of 21 species (eliminating duplication), while the industrious wood rats gathered in 28 taxa, 25 being readily identifiable to species. In fairness to the botanists, they did record the missed species in other sectors of the canyon (Gehlbach, 1967, and unpublished checklist). On the other hand, the wood rats did not miss any species recorded by the botanists in the Pratt Cave area! However, it is the method of pollen analysis that suffers most by comparison with the gleanings of wood rats and trained botanists. The pollen spectrum taken from cave sediment closest to the level of the Neotoma midden comprised only 10 taxa, none of which could be identified to species and but half to genus; the other half could be placed to family or tribe only (Table 1). Even if data from all eight additional pollen spectra, taken at various levels in cave sediment of undetermined age, are pooled, there is a gain of only four taxa identified to genus and two to family; but most of these additional taxa were represented by only a few pollen grains and were not graphed. Besides the poverty of taxa represented and the problems of identification to a meaning ful taxonomic level, there is a crucial difficulty in that long-distance transport of pollen cannot be excluded for any of the anemophilous taxa, except oak. Oak pollen at 81% so dominates the pollen rain at Pratt Cave that the provenance of the other species, represented in relatively small proportion, is open to question. Thus, juniper (cupressaceous pollen) at 1% and pine at 1% are in doubt, even though they occur at or near the site; and the usual interpretation of Artemisia (at 4%) as sagebrush (A. tridentatu) would be in error because the latter species does not occur in Texas

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or southern New Mexico, nor was any Artemisicz recorded by the botanists or the wood rats. Similarly, the Ambrosieae, constituting about 4% of the pollen rain, was otherwise unrecorded near Pratt Cave. In summary, the pollen record failed to identify 21 of 26 genera of plants actually present at the site and also in the Neotoma midden; furthermore, 8% of the pollen identified was from taxa of indicator significance for pollen analysis, but apparently lacking at the site. Although the pollen spectra correctly identified oak as the principal dominant of the vegetation, the level of oak estimated from the pollen was twofold greater than the relative percentage of oak calculated from samples of the vegetation itself by Gehlbach (1967). The quantitative macrofossil spectrum of the Pratt Cave Neotoma midden (Table l), expressed as relative percentages of the total biomass of identifiable plant materials, is almost incomparably superior in all respects to t,he pollen record at this site. The point needs emphasis because of the implicit faith of the general scientific public in the superiority of pollen analysis within the field of paleoecology. Palynology has its place as a stratigraphic record of the regional pollen rain, but it paints a picture of local vegetation with a very coarse brush indeed and cannot compete with the meticulous detail of a superb collection of macrofossils. A combination of pollen and macrofossils can be complementary (King and Van Devender, 1976), but Neotoma deposits are often so replete with information as to make a local pollen record redundant. The leading dominants of the vegetation at the Pratt Cave site are proportionally represented as to relative biomass in the wood rat midden, with 54% oak and 5.2% juniper in the deposit, compared to relative importance values of 42% and 12.7% in the source vegetation, as estimated from Gehlbach’s samples. However, the hackberry (Celtis), at nearly

236

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19% biomass, is clearly overrepresented in the midden, probably because the heavy endocarps are concentrated by a Neotoma dietary preference for the sweet drupes. Although many of the 25 relatively minor taxa in the wood rat deposit may be somewhat disproportionately represented, they convey a very faithful, generalized description of the surrounding vegetation, which is itself far from being homogeneous. Because of the frequently patchy distribution of colonies of the less dominant species,

Comparison

their relative abundances fluctuate drastically from spot to spot, making a for each precise, overall quantification species somewhat meaningless. The mere presence of the 25 minor species near one site is the most powerful evidence for deducing the nature of the local environment. Home range of Neotoma. From the discussion of the behavioral ecology of wood rats, it is clear that their mode of existence centers on a secure shelter, often partly or wholly of their own mak-

TABLE 1 of Biomass Spectrum from Neotoma Deposit and Associated Pollen Spectrum, Data on Source Vegetation near Pratt Cave, Guadalupe Mountains, Texas Neotoma Plant Taxa

Trees Quercus grisead (gray oak) Quercus pungens (sandpaper oak) Celtis reticulata (hackberry) Juniperus monosperma (oneseed juniper) Juniperus deppeana (alligator juniper) Juniperus spp. (undifferentiated) Ungnadia speciosa (Mexican buckeye) Mows microphylla (mulberry) Arbutus xalapensis (madrone) Pinus ponderosa (yellow pine) Shrubs, liana Rhus trilobata (squaw bush) Choisya dumosa (star-leaf) Gutierrezia sarothrae (snakeweed) Mimosa biuncifera (catclaw) Vitis arizonica (wild grape) Garrya ouata (silk-tassel) Ribes sp. (gooseberry) Berberis trifoliolata (algerita) Brickellia laciniata (cutleaf brickellia) Dalea argyraea (silver dalea) Viguiera stenolo ba (goldeneye)

midden

Pollen

with

Source vegetation

Biomass (%I

Relative (%I”

Quantitativeb (I.V. %)

QualitativeC

Leaves, acorns

54.1

81

42.0

Abundant

Leaves Endocarps

Trace 18.9

Trace

Present

Common -

-

Present

Present

12.7

Present

Structures

Seeds

0.6

Seeds

0.4

Leafy twigs

4.2

1

-

-

Seed

0.3

-

-

Common

Twigs

0.2

-

-

Common

Fruits

0.2

-

Present

Present

Leaf

Trace

1

-

Present

Seeds Leaves, twigs

0.4 0.3

-

Present -

Abundant Abundant

Capitula Legumes Twigs Leaves Twigs

0.2 0.2 0.2 0.2 0.1

-

Present Present -

Uncommon Uncommon -

Leaves

0.1

-

-

Uncommon

Involucres Legumes

0.1 0.1

-

Present -

-

Trace

-

Present

Present

Capitula

-

1=

(Continued)

VEGETATIONAL

HISTORY TABLE

FROM

Agavaceae, Cactaceae Nolina microcarpa (sacahuist a) Yucca baccata (datil) Opuntia engelmannii (prickly pear) Dasylirion leiophyllum (sotol) Grasses and Forbs Andropogon scoparius (little bluestem) Lithospermum incisum (puccoon) Malvaceae (mallows) Salvia sp. (a mint) Artemisia sp. (wormwood, herb)h Ambrosieae (ragweeds, etc.) Compositae (high-spine) Species Diversity Identified taxa Identified species

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Pollen

Structures

Biomass (%I

Leaves Leaves, seeds

11.4 4.5

Areoles,

RAT

1 (Continued)

Neo toma midden Plant Taxa

WOOD

seeds

Leaves, seeds

Relative (%)”

2f

Source vegetation Quantitativeb (I.V. %)

Present -

Qualitativec

Abundant Uncommon

1.8

-

4.2

Common

1.6

-

1.5

Abundant

Spikelets

Trace

5g

-

-

Nutlets Fruits Calyx

Trace Trace Trace

Trace -

-

-

-

-

4 4 2

-

-

-

28 25

13 13

18 18

11 0

a Pollen analysis by J. Schoenwetter (unpublished). bImportance values (I.V.) by F. Gehlbach (1967). ‘Observations of P. Sanchez (National Park Service). dIntrogression by Q. mohriana suspected. eReported as Acacia. ‘Reported as Liliaceae. gReported as grass. ‘Sagebrush definitely absent.

ing. The wide use of horridly spined or pungently tipped plant materials strongly suggests that fortification against predation is the principal strategy of wood rat architecture. Therefore, it seems obvious that fitness in Neotoma will be greatest with the minimal home range capable of supporting the individual. This should follow not only from the standpoint of minimizing predation, but also from that of maximizing efficiency of energy expenditure in the gathering of food and building materials. The rather scanty field measurements of home range in Neotoma, based on repeated trapping, do confirm a very limited foraging distance from the home shelter. The average maximal distance travelled by the desert wood rat (N. Zepida) within a 24-hr

period was 77 m, with an extreme range for males of 125 m (Stones and Hayward, 1968). However, these movements were between different stick-houses, and it is likely that foraging for plant materials that are brought back to a given shelter is more limited in scope. The intensive studies of N. fuscipes also record a maximal home range of 122 m, but it was estimated that 90% of this species’ activity is restricted within a radius of 15 m from any one home shelter (Linsdale and Tevis, 1951, p. 599). Migrations of up to 670 m over longer time periods have been observed by the latter authors, but seasonal dispersal movements of this nature should not be confused with the long-term foraging ranges that determine the rela-

238

PHILIP

V.

tive quantities of different plant materials accumulated in the Neotoma middens. Hence, there is a strong probability that plant species represented in wood rat deposits grew in the immediate vicinity of the shelter, and that this is especially true for the dominant conIn deep stituents of the middens. canyons, cliff-dwelling species of Neotoma (e.g., A? cinerea) may be capable of a maximal foraging range of 100 m or more in a vertical sense, and this possibility should be taken into account in estimating maximal elevational displacements of vegetation zones from Neo toma records on steep gradients in anomalously exaggerated topography, as, for example, in the Grand Canyon (Phillips and Van Devender, 1974). PHOTOSYNTHETIC MECHANISMS AND PALEOECOLOGY DEDUCED FROM 13C/12C RATIOS A novel approach to the comparative physiology and ecology of long-dead plants has been demonstrated with macrofossils of Atriplex and Opuntia from late Pleistocene Neotoma middens obtained in Nevada (Troughton, Wells, and Mooney, 1974). Photosynthetically specialized plants, like Atriplex and Opuntia, that initially fix CO2 with phosphoenolpyruvate (C,) to form the C4 oxaloacetic and malic acids (C, plants and many succulents) have been shown recently to have nearly twofold less negative 6 13C values (ca. - 13)* compared to C3 plants (ca. -27) that fix CO2 with ribulose diphosphate (Cs), which then splits to form 2 moles of the C3 phosphoglyceric acid (Bender, 1968; Hatch et al., 1971). The enzyme that fixes CO2 in phosphoenolpyruvate (PEP

13C/12C *613C The PDB belemnite

(per

mil)

=

sample

13C/12C PDB standard-is carbon from in the Cretaceous Pee Dee

-1

x 103. 1 the fossil formation.

WELLS

carboxylase) to form oxaloacetic acid causes a preferential uptake (relative to C3 plants) of the heavier 13C isotope of carbon, but still discriminates against it slightly (relative to the atmosphere). The 13C isotope constitutes about 3.3 ppm of the 300 ppm of CO2 normally present in the modern atmosphere, a level only about 0.6 to 2.0% greater than that of either C4 or C3 plants, respectively. Hence, the l3 C/12C ratio provides a readily measurable and very sensitive means of distinguishing the two ecologically divergent photosynthetic strategies in living or fossil plants; very small samples (milligram range) suffice for mass spectrometry of these comparatively abundant isotopes of carbon, much smaller than the samples required for radiometric age measurements on the very low levels of the 14C isotope. The peculiar plants specializing in the fixation of CO2 in the C4 oxaloacetic acid (and by reduction, malic acid, or by amination, aspartic acid) are also specialized ecologically, being strongly heliophile and often xerophytic. Although the xerophytic advantages of the C4 (HatchSlack) photosynthetic pathway have been stressed for desert plants and halophytes (Bjiirkman and Berry, 1973), and rightly so, the C4 strategy is also highly adaptive among less xerophytic plants playing pioneer roles in open, sunny, early-successional habitats in humid, forested regions. Many pioneering, tropical and temperate (warm-season) grasses (Gramineae), sedges (Cyperaceae), amaranths or pigweeds (Amaranthaceae), chenopods (Chenopodiaceae), spurges (Euphorbiaceae) et al. follow the C4 strategy, even though they have the advantage of ample soil-moisture generated by abundant precipitation (or irrigation in more arid regions). Among the C4 grasses, for example, consider the familiar bluestems (Andropogon) with about 30 species in humid, southeastern North America, a mainly forested region.

VEGETATIONAL

HISTORY

Some species (especially A. uirginicus and A. scoparius) play a conspicuous and well-known role in early, old-field succession (the “broom sedge” stage; cf. Oosting, 1942), and all of the species occur in relatively open, seral habitats. A similar ecological role is apparent in the C4 Indian grasses (Sorghastrum) and switch grass (Panicum virgatum). A select few of the same species of C4 grasses that follow a pioneering way of life in the forested Southeast have extensive distributions in North America, ranging from the Rocky Mountains throughout the Great Plains to the Atlantic coast (Wells, 1970b). These very species are also the familiar “big four” dominant grasses of the tall-grass prairie: big and little bluestems (Andropogon gerardi and A. scoparius), Indian grass (Sorghastrum nutuns), and switch grass (Panicurn virga turn). In the Central Plains region, the C4 grasses are growing under a much more arid environment than in the eastern part of their ranges, and the great advantages accruing to the C4 mode of COz fixation, especially the efficient stomata1 control of transpiration and high rates of photosynthesis under moisture stress (Bjorkman and Berry, 1973), are highly adaptive here. The C4 tall-grasses of the Plains region are known to be high-polyploid endmembers of a series of infraspecific chromosomal races, with the ancestral, diploid end-members concentrated in southeastern North America (Stebbins, 1975). Thus, independent lines of evidence, including floristic analysis and paleoecology (Wells, 1970a,b) and cytology (Stebbins, 1975), suggest an ancestral origin in the relatively humid Southeast for the C4 tall-grasses of the Great Plains. Hence, it is now apparent that the Hatch-Slack photosynthetic strategy may evolve in open, sunny, relatively xeric (compared to shady) habitats in a humid, forested region, and that the C4 plants may subsequently migrate to an

FROM

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239

arid region, where the water-conserving aspects of the C4 fixation pathway confer an even greater relative fitness. Under conditions of high light intensity, high heat load, high concentration of Oz, and low concentration of COz in the intercellular space inside the leaf (all characteristics of the photosynthetic milieu of pioneer plants of humid regions), the C4 or Hatch-Slack carbonfixation pathway is much more efficient than the Cs or Calvin-Benson cycle. The latter is specifically inhibited by high O2 at high temperatures and is only about half-saturated with COz at the full 300 ppm level, whereas the C4 pathway is much less sensitive to Oz and is already nearly saturated with COz at 200 ppm. Moreover, C4 plants are such efficient scavengers of COz that they can remove almost all of it from a closed system, but Cs plants cannot fix carbon if the level of COs falls below 50 ppm. In full sunlight, C4 plants photosynthesize much more rapidly than C, plants. Thus, C4 plants are well equipped to compete with Cs plants in unshaded habitats of forested regions, but can do so to greater advantage under arid conditions, where their much greater efficiency in fixing COz also permits them to conserve water by greater stomata1 closure than is possible in Cs plants (Bjdrkman and Berry, Therefore, the geographic and 1973). ecological scope of the C4 carbon fixation pathway is very broad, and the distinction between the two photosynthetic strategies has wider application in the field of paleoecology than is indicated in Troughton et al. (1974). The great diversity of systematic groups that fix COz in oxaloacetic (and then malic) acid fall into two distinctive physiognomic-anatomical classes: 1. Nonsucculent plants (mostly herbaceous or subligneous), with chloroplasts concentrated in a double-layered sheath of cells forming concentric cylinders around the vascular bundles in the

240

PHILIP

V. WELLS

finer leaf-veins (“Kranz anatomy”). The outer layer of the sheath consists of special mesophyll cells containing the PEP carboxylase enzyme that fixes COz (ultimately) in malic acid. The inner layer (also chlorophyllous) is the bundle sheath that carries the normal, CalvinBenson (Cs) photosynthetic cycle, using CO1 decarboxylated from malic acid, which is translocated in from the outer sheath. Therefore, the more efficient COz fixation of the outer layer is coupled with the complete photosynthesis of the sequestered inner layer. The compartmentalized arrangement is a beautiful structural mechanism, enhancing COs uptake, and retarding the loss of water by ensheathing the source itself: the vascular bundles carrying water up from the roots. Members of this class are arbitrarily designated C4 plants (the next group also uses the C4 pathway). 2. Succulent plants with very watery, mucilaginous tissues enveloped by a thick, waxy cuticle that retards water loss; without the Kranz arrangement of chloroplasts; includes many large or gigantic species with woody skeletons in the stems. Many, but not all, succulents have strong PEP carboxylase activity and accumulate large concentrations of malic (C,) and other organic acids, especially in the dark, when the stomata are open. During the day, when the stomata remain closed, the COz previously fixed in the carboxyl groups of organic acids at night is available for photosynthetic reduction to carbohydrate. Thus, the segregation of COz fixation, which is a “dark reaction,” from the light reactions of photosynthesis (generation of the reducing and phosphorylating agents, NADPH and ATP), permits a very efficient timing for stomata1 opening, coinciding with minimal transpiration. Hence, many succulents have a complicated photosynthetic strategy correlated with their strikingly adaptive growth forms, a combination that su-

perbly equips them for the xerophytic way of life. The general biochemical pathway of carbon fixation in succulents, which was first elucidated in the Crassulaceae, is known as crassulacean acid metabolism (CAM). Paleophysiology. The presence in Pleistocene Neotoma deposits of the same species of plants that now have C4 or CAM photosynthetic pathways raised the question of whether they were operating the PEP carboxylase enzyme under different environmental conditions during the past 40,000 or more years. The 13C!/12 C ratio tests for the operation of PEP carboxylase in either C4 or CAM plants today, and the ratio should be stable in the carbon of fossil plants (J. H. Troughton, in Hatch et al., 1971). Measurement of 613C of a living saltbush (A triplex confertifolia, the shadscale), growing in the northern Mohave Desert, gave a value of - 14.8, and thus confirms the presence of the C4 pathway in this species, as in other xerophytic species of Atriplex. It happens that the shadscale is represented by macrofossils in many late Pleistocene Neotoma middens from the northern Mohave Desert (Wells and Berger, 1967). Measurement of 613C values for macrofossils of shadscale from deposits with radiocarbon ages of 10,000 and >40,000 BP yielded typical C4 numbers of -16.2 and -13.4, respectively. Evidently, A triplex confertifolia has been fixing carbon dioxide with PEP carboxylase as long ago as 10,000 to >40,000 BP, and therefore, was operating its C4 or Hatch-Slack photosynthetic cycle during the cooler, pluvial climate of the last glacial of the Pleistocene. However, not until after about 17,000 BP did the shadscale become an abundant constituent of the ancient Neotoma middens, suggesting drier conditions during the waning phase of the Wisconsin glacial, despite the continued presence of juniper woodlands. Measurements of the 13C isotope level

VEGETATIONAL

HISTORY

in a succulent with photosynthesis of the CAM type was also made, on macrofossils of Opuntiu from the same Neotoma deposits that yielded the Atriplex specimens. The 6 l3 C value for Opuntia at 10,000 BP was - 13.9, again indicating the operation of PEP carboxylase, which in succulents initiates the CAM cycle. However, the Opuntiu specimen from the >40,000 BP deposit yielded a C, -like number, - 21.9. Since CAM plants do not have the structured Kranz anatomy of the C4 plants, which compartmentalizes the Hatch-Slack and Calvin-Benson pathways (with the latter ensheathed by, and obligately dependent upon, the former), they are capable of fixing carbon dioxide directly from the atmosphere by the C3, as well as by the C4 mechanism. The enzyme fixing COz in the Calvin-Benson cycle (ribulose diphosphate carboxylase) causes the maximal isotope fractionation (discriminating against 13C) characteristic of most plants (C,). Hence, the result obtained for the >40,000 BP Opuntia may indicate more mesic conditions, that permitted the normal C3 pathway to operate at that indefinite time period. However, more should be learned about the present-day photosynthetic behavior of different species of Opuntia, and a substantial survey should be made of 6 l3 C values of fossil CAM plants, before any conclusions can be drawn about succulents as paleoecological indicators. RADIOCARBON CHRONOLOGY OF QUATERNARY NEOTOMA MIDDENS Another great advantage to macrofossil analysis of wood rat deposits is the abundance of carboniferous material available for radiocarbon-dating. In contrast to pollen analysis of sediments, where stratigraphic control is calibrated by getting ‘*C dates on any organic matter that happens to be present in the profile, it is usually possible to date directly the very

FROM

WOOD

RAT

MIDDENS

241

macrofossil layers of an ancient Neotoma midden that are most significant paleoecologically. Many compact (<30 cm thick) wood rat deposits that completely fill secure cavities in the wall of a rock shelter are uniform in macrofossil composition, and multiple dating of this type of deposit has indicated a uniform age. For example, a rather large (50 cm), homogeneous, securely placed midden from central Baja California was sampled at different levels in a three-dimensional sense, including the top, bottom, front, and rear of the deposit. The radiocarbon dates on three widely separated positions in the midden were not significantly different: 10,000 + 125, 10,100 + 160, and 10,150 + 130 BP (UCLA 1367, Evi1366, and 1365, respectively). dently, this deposit recorded only a brief episode in time. A much larger Neo toma deposit from the Chihuahuan Desert filled a tunnel-like cave to the ceiling with several thousand liters of debris from a pluvial, pinyon-juniper-oak woodland. Samples from the top, middle and basal horizons of a vertical section, measuring 74 cm in height, yielded radiocarbon ages of 11,560 f 140, 12,000 t 150, and 12,550 t 140 BP, respectively (Wells, 1966); and the two younger horizons had a distinctly different, more xerophytic composition, suggesting a shift to drier climate after about 12,000 years ago (Wells, 1976). Hence, there was an orderly filling of the low-roofed tunnel with midden layers in normal superposition until the millennium-long episode, or sequence of episodes, was terminated by the simple fact that all available space had been occupied by consolidated Neo toma debris. In general, the length of an episode of deposition by wood rats is often limited by the size and configuration of the cavity being filled. The very rapid rates of deposition of both plant material and fecal pellets (discussed under wood rat deposits) ensure that any

242

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small, secure cavity that can be filled completely also will be filled quickly. On the other hand, there are many situations in caves and smaller rock shelters where complete filling with Neotoma deposits is prevented by an unsuitable geometry, or exposure to erosional hazards such as rockfalls or disturbance by larger animals. An episode of filling may be followed after a long interlude of stability by partial removal of the ancient deposit, thus making room for renewed deposition of much younger material by a succeeding generation of wood rats on the erosional platform cut in the old midden. Also, rockfalls at long, erratic intervals may initiate new episodes of wood rat deposition by exposing a new crop of favorable cavities, crevices or shelves. Complicated sequences of Neotoma middens are sometimes seen in large rock shelters with steep walls and steeply sloping floors on which only shelf-like deposits, cemented by amberat, can accumulate. Problems of this nature were encountered very early in the development of the Neotoma method, because two of the rock shelters in the Frenchman Flat series had a complex array of middens. One small cave in the Ranger Mountains contained a large, only partly discontinuous mass of middens (superficially, it looked like one midden) that yielded radiocarbon dates varying between 10,100 + 160 and 28,900 f 1200 BP, a range of almost 19,000 years from four discrete samples. This was pointed out in the first paper reporting the existence of Pleistocene wood rat middens (Wells and Jorgensen, 1964, p. 1172). A more segregated sequence of middens in close proximity within one cave in the Spotted Range gave dates of 9450 f 90 and >40,000 BP on two discrete samples. Despite the great range in age, most of the Ice Age Neotoma deposits from low, desert elevations in the Mohave Desert region preserve a monotonous record of juniper

WELLS

woodland, and therefore macrofossil analysis did not permit a more refined segregation than the obvious one of the rare Pleistocene woodland deposits and the abundant Holocene desert deposits. However, detection of strata can sometimes be guided by differences in macrofossil composition (cf. Wells, 1976). Clearly, the solution to the problems of stratigraphic complexity within heterogeneous Neotoma middens, or among more or less discrete, individually homogeneous middens of different age, is to obtain as many radiocarbon dates on as many samples as possible. Because of economic considerations, this has rarely been done, but the Neotoma method itself should not be faulted on this account. If the Neotoma record were not so rich, if there were a paucity instead of a plethora of macrofossil layers in an average deposit, and if the chronology were spotty and restricted instead of extending throughout the range of the radiocarbon method, we would have fewer problems but also less information! Radiocarbon chronology. During the past 15 years, more than 130 Quaternary Neotoma macrofossil records have been dated. The deposits were uncovered at about 80 sites in 20 subregions of western North America (Fig. 1), extending from northern Wyoming south to central Baja California, central Sonora, southwestern Texas, and to Tehuacan in southern Mexico (Wells, 1966, 1969, 1970, 1976; Wells and Berger, 1967, and unpublished; Mehringer and Ferguson, 1969; Van Devender, 1973, and unpublished; Madsen, 1973; Long and Martin, 1974; Phillips, unpublished). The number of deposits that have been dated at least once is much smaller than the number actually observed in the field because a deliberate search has been made for older deposits containing macrofossil evidence of extralocal plant species. Although Holocene or very recent wood rat deposits are everywhere

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-DESERTS

IN

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RAT

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LOWLANDS

IN

02 z P P I

WOODLANDS

FROM

30.‘;

,LOWLANDS\

N=l32

14C

YEARS

BP

FIG. 2. Frequency distribution plot of 132 radiocarbon dates obtained on subfossil Neotoma middens that have been preserved in dry rock shelters in western North America. The dates are grouped by time intervals of one millennium. The apparent modal peak at 9000 to 12,000 BP is an artifact of under-representation of the ubiquitously abundant Holocene deposits. An extrapolated projection, based on the late-glacial trend, is added as a dotted line to give a more realistic plot for the Holocene. The vegetational change from pluvial woodland to desert scrub in the lowlands of the Southwest is time transgressive, depending on latitude and elevation of site.

vastly more abundant than the paleoecologically interesting Pleistocene middens, nevertheless, more than a hundred of the dated macrofossil records fall in the rarer, Ice Age category. There remains untapped an exceedingly rich Neotoma macrofossil record of Holocene vegetation throughout western North America. Studies in the Laramie and Bighorn Basins of Wyoming (Wells, 1970, and unpublished) indicate that the Holocene has a few surprises to offer. A frequency distribution plot of 132 radiocarbon dates2 obtained on wood rat deposits from 20 subregions of western North America is presented in Fig. 2. An extrapolated projection for the Holocene, based on the late-glacial trend, is added to give a more realistic view of the great abundance of postglacial deposits, 2The dates are can years, which calendar years.

expressed in radiocarbon be converted readily to

which have been set aside deliberately (for later study) by most workers in this field. The early phase of paleoecological research on Neotoma middens during the past 15 years has been devoted to establishing the magnitude of vegetational change during the transition period extending from the last great pluvial episode of the Pleistocene at the close of the Wisconsin glacial to the onset of Holocene, desert conditions in the lowlands (a time-transgressive, cliseral shift, varying from about 12,000 to about 8000 BP, depending on the latitude and elevation of the site). Pluvial-age deposits are readily identified by their content of one or more extralocal species, presently surviving only at higher elevations. In the existing deserts of the Southwest, the common pluvial indicators in ancient Neotoma deposits are various species of Juniperus, and sometimes pinyon pines and oaks; but the oak-rich middens are chiefly confined to

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the regions of the present Chihuahuan and Sonoran Deserts (Wells, 1976). The use of these indicator species to select middens for dating has proven to be an almost infallible method for obtaining radiocarbon dates in the range from 9,000 to greater than 40,000 BP. In fact, 101 of 103 dates based on extralocal species in the Southwest are >9000 BP. The scattering of postglacial dates (younger than about 8000 BP) is the result of deliberately sampling a few deposits containing records of existing vegetation and lacking extralocal species. The Holocene record of xerophytic conifers from the Laramie Basin in southeastern Wyoming is an interesting exception (Wells, 1970a). The frequency distribution of 14C dates in Fig. 2 is clearly skewed toward the end of the Wisconsin glacial and the Holocene. The false peak at about 9000 to 12,000 BP is an artifact of bypassing most of the ubiquitous Holocene or modern records (which are monotonously dominated by local species still growing in the vicinity of the middens) for the relatively rare records of pluvial vegetation, usually confined to the most secure rock shelters. The rarity of Pleistocene (relative to Holocene and currently active) Neotoma deposits is a major difficulty of the method, one that can be overcome only by diligent searching through innumerable rock shelters. In my experience, Pleistocene middens are at best an order of magnitude less frequent than deposits containing records of essentially modern vegetation, but more often they are exceedingly rare in any one area. The skewness of the l4 C-date distribution (extrapolated through the Holocene by the dotted lines in Fig. 2) reflects the inverse relation between abundance and age of middens. Causes of attrition with time. Deterioration in situ or actual removal are constant hazards for all but the most

securely placed middens. Deterioration is caused by erosion of the rock shelter itself, leading to expansion of openings that admit wind and rain, or to the development of cracks that allow seepage to wet the ancient deposits. Catastrophic removal of middens by rockfall, or through the activities of larger mammals, are other possibilities. An example of the interference of a large, cave-inhabiting mammal with deposition by wood rats is seen in the very unusual sequence of sediments on the floor of Rampart Cave in the Grand Canyon of Arizona (Long and Martin, 1974; Phillips and Van Devender, 1974). A single, narrow seam of Neotoma litter about 15 cm thick divides a late Pleistocene deposit about 140 cm thick, laid down on the floor of the cave. The whole deposit, which has been exhaustively dated between 10,780 + 200 BP at the top and >40,000 BP near the bottom, is dominated by the dung of the extinct ground sloth (Nothrotheriops shastense), except for the wood rat seam (Long and Martin, 1974). The latter was deposited during the long, full-glacial hiatus of ground sloth activity in Rampart Cave, extending from about 14,000 to 24,000 BP. High pluvial levels of the Colorado River may have prevented the sloths from entering the narrow canyon through the Grand Wash Cliffs, which is the only feasible access route into Rampart Cave (Long et al., 1974). Evidently, the presence of the copiously defecating ground sloths had an inhibiting effect on Neotoma deposition on the floor of the cave, where rat litter is lacking except during the full-glacial hiatus of sloth activity, and again after the extinction of the sloths about 11,000 years ago. The principal agent of destruction of ancient wood rat deposits is water. The crystallized urine, which is readily soluble, cements the loose materials of the middens into a tough, coherent mass, and frequently causes the deposits to

VEGETATIONAL

HISTORY

adhere to precarious positions on rock walls. Water softens the most indurated middens, causing them to fall apart and to dislodge. Rapid weathering usually causes gross deterioration of the plant macrofossils that are leached by water. Thus, accidental dislodgement of any kind from a secure cavity in a rock shelter will lead to a quick demise, if the midden is exposed thereby to the action of water. Ancient Neo toma middens that are removed from caves and discarded in the open by vandals are leached and crumbled to fine dust in a short time. Damage to middens from seepage within a cave has been observed also. Because of the crucial role of the rock shelter in protecting ancient Neotoma middens from weathering, the nature of the bedrock in which the cave has formed might be expected to have some effect on preservation. However, Pleistocene deposits have been uncovered in limestones or dolomites, various kinds of sandstones, tuffs, rhyolites, basalts, granites, and metamorphic rocks. Although limestone and dolomite have yielded the greatest number of secure shelters containing ancient middens, the other rocks mentioned also harbor many well-preserved Neotoma deposits of equally great age. On the other hand, there are some types of geological substratum in which cavities suitable for Neotoma deposition are too ephemeral, and therefore too recent, to contain very ancient deposits. Unconsolidated alluvial sediments, even weakly cemented fanglomerate, or soft, crumbly sandstones and shales, apparently are too quickly erodable to retain secure caves or shelters on their exposed surfaces for the time periods under consideration here. At any rate, soft rocks tending to produce finely dissected or badland terrain usually yield only Holocene Neotoma records, if any at all. Mehringer (1969) reports a midden with a radiocarbon age

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of nearly 1000 BP from alluvial gravels on the floor of Death Valley. Dating problems. Contamination by redeposition of unconsolidated older Neotoma material is a possibility with wood rat middens, as it is in other kinds of sedimentation. However, the dense, indurated middens usually have a smooth, impervious veneer of the lustrous amberat, which prevents intrusion of younger material after it hardens. There may be later deposition on the indurated midden, but the old, outer rind of amberat marks the interface between the two episodes. Complicated sequences of this kind have been detected by detailed radiocarbon dating, as discussed above for some of the pioneering efforts (Wells and Jorgensen, 1964); similar complexes of ancient midden layers of less widely disparate ages have been analyzed stratigraphically and I4 Cdated in more recent work on Pleistocene middens from Nevada (Madsen, 1973, Fig. 8). The loose, unconsolidated litter, sometimes found on the floors of large caves, as in Rampart Cave (Long and Martin, 1974), offers much greater possibilities for serious contamination by reworking of the older layers. Nevertheless, in many instances the problem can be resolved by multiple dating of different layers, or by directly dating the wood of an extralocal indicator species that has special paleoecological significance. The method of monospecific dating should not, however, supersede a painstakingly stratigraphic approach to analyzing the contents of middens. Since radiocarbon dating has documented abrupt, episodic shifts in Neotoma deposition, with an intervening hiatus of 10,000 years or more between immediately adjacent layers, it is hazardous to pool many small macrofossils of the same species from a large mass of midden material in order to obtain a sufficient sample for a monospecific 14C date.

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Thus, the advantages of routinely processing massive sections of middens up to 25 cm thick, and weighing as much as 1400 g, in order to obtain yields of 13,000 or more tiny macrofossils (Van Devender, 1973, Table l), are offset by the risk that the macrofossils may differ in age. It is convenient to soak a midden in water for a few days, which causes the whole mass to disintegrate for easy screening and sorting of macrofossils; a large number of very light, leafy twigs of Juniper-us (the most common extralocal component of pluvial-age Neotoma deposits) can be extracted for a 14C date (Van Devender, 1973). But Juniperus occurs throughout the time span of the late Pleistocene accessible to radiocarbon dating (Wells, 1969). Although dating juniper from a Pleistocene midden at desert elevations is a useful method of eliminating contamination by any Holocene intrusion of local, desert species into the deposit, it is also true that younger juniper macrofossils may contaminate much older ones. The latter contamination is especially likely because of the frequency distribution of ages among late Pleistocene wood rat deposits (Fig. 2). The probability of a macrofossil mix weighted for younger materials is greatly enhanced by the strongly peaked frequency of middens in the younger range from 9000 to 16,000 BP. By suspending and homogenizing the contents of stratigraphically complex middens in water, and then melding the sorted materials by species, the smaller macrofossil yield of relatively rare, very old, midden layers may be combined with the much more abundant remains of the same species from the more frequent younger layers. Thus, a striking feature of the series of radiocarbon dates recently produced in this manner is the anomalous scarcity of dates older than 15,000 BP (only about lo%), with the other 90% skewed into the late-glacial time range (<15,000

These results deviate strikingly the distribution of comparable Neotoma dates obtained from carefully separated stratigraphic layers (Wells and Jorgensen, 1964; Wells, 1966, 1969, 1976; Wells and Berger, 1967, and unpublished). The latter method has produced a much wider spread of 14C dates, with about 60% older than 15,000 BP, a percentage sixfold greater than in the series based on monospecific dating. The deficiency of old dates in the singlespecies technique is seen especially near the older limit of the radiocarbon method at about 40,000 BP, which has been barely approached in a sample fully as large as in the more precise stratigraphic technique. Again, there is a sixfold greater number of older radiocarbon dates in the indefinitely old range from 30,000 to >40,000 BP for the method of dating thin strata. The deviation is too large to be attributed to chance, and there is a considerable geographic and ecological overlap of the midden localities. As the Neotoma macrofossil method was developing, I became aware of the very wide range of radiocarbon ages obtainable from very similar layers of midden material in close proximity within the same rock shelter, sometimes aggregated into a single, complex mass (cf. original paper, Wells and Jorgensen, 1964, p. 1172). The greatest care must be employed in collecting samples from a wood rat deposit in situ in the rock shelter in order to secure relatively simple stratigraphic entities. In the laboratory, the field samples are further split along stratigraphic planes of cleavage rich in plant macrofossils. These fissile, leafy and twiggy layers are intercalated at intervals between indurated strata of fecal pellets cemented in a matrix of crystallized urine. The leafy strata (often predominantly juniper) are usually very thin (
VEGETATIONAL

HISTORY

feces undoubtedly represent a very short time period of accumulation (cf. Bonaccorso and Brown, 1972; Linsdale and Tevis, 1951; discussed above). Hence, a radiocarbon date on a thin, plant-rich, stratigraphic layer of a midden, after treatment with HCl to remove possible acid-soluble contaminants (e.g., carbonates; Neotoma urine permeating from other layers), maximizes the probability of obtaining an accurate age for the plant assemblage in the layer, regardless of the complexity of midden stratigraphy. ACKNOWLEDGMENTS My research on ancient Neotoma middens has been supported by the National Science Foundation, currently under Grant GB-40306 to the University of Kansas. I thank R. S. Hoffman, H. E. Wright, and eight unidentified reviewers for comments on the manuscript.

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Hall, E. R., and Genoways, H. H. (1970). Taxonomy of the Neotoma albigula group of wood rats in central Mexico. Journal of Mammalogy 51,504-516. Hall, E. R., and Kelson, K. R. (1959). “The Mammals of North America.” Ronald Press, New York, 2 volumes. Hatch, M. D., Osmond, C. B., and Slatyer, R. A., Eds. (1971). “Photosynthesis and Photorespiration.” Wiley, New York. King, J. E., and Van Devender, T. (1976). Pollen analysis of fossil packrat middens from the Sonoran Desert. Quaternary Research. Leskinen, P. H. (1970). “Late Pleistocene Vegetation Change in the Christmas Tree Pass Area, Newberry, Mountains, Nevada.” M.S. thesis, Department of Geography, University of Arizona, Tucson. Linsdale, J. M., and Tevis, L. P., Jr. (1951). “The Dusky-footed Wood Rat.” University of California Press, Berkeley/Los Angeles. Long, A., Hansen, R. M., and Martin P. S. Extinction of the Shasta ground (1974). sloth. Geological Society of America Bulletin 85,1843-1848. Long, A., and Martin, P. S. (1974). Death of American ground sloths. Science 186, 638640. Madsen, D. B. (1973). “Late Quaternary in the Southeastern Great Paleoecology Basin.” Ph.D. dissertation, University of Missouri, Columbia. Mehringer, P. J. (1969). Isotopes’ radiocarbon measurements. VII. Radiocarbon 11,53-105. Mehringer, P. J., and Ferguson, C. W. (1969). “Pluvial Occurrence of Bristlecone Pine (Pinus aristata) in a Mohave Desert Mountain University of Arizona, Range,” pp. l-16. Department of Geochronology Interim Research Report 14. Oosting, H. J. (1942). An ecological analysis of the plant communities of Piedmont, North American Midland Naturalist 28, Carolina. 1-126. On the occurrence and Orr, P. C. (1957). Observations, Western nature of “amberat.” Speleological

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complexes in the evolution of North American grasslands. Taxon 24,91-106. Stones, R. C., and Hayward, C. L. (1968). Natural history of the desert wood rat, Neotoma lepida. American Midland Naturalist 80,458-476. Troughton, J. H., Wells, P. V., and Mooney, H. A. (1974). Photosynthetic mechanisms and paleoecology from carbon isotope ratios in ancient specimens of Cd and CAM plants. Science 185,610-612. Van Devender, T. R. (1973). “Late Pleistocene Plants and Animals of the Sonoran Desert: A Survey of Ancient Packrat Middens in SouthPh.D. dissertation, Univerwestern Arizona.” sity of Arizona, Tucson. Van Devender, T. R., and King, J. E. (1971). Late Pleistocene vegetational records in western Arizona. Journal, Arizona Academy of Science 6, 240-244. Vorhies, C. T., and Taylor, W. P. (1940). Life history and ecology of the white-throated wood-rat, Neotoma albigula Hartley, in relation to grazing in Arizona. University of College of Agriculture Technical Arizona, Bulletin 86,455-529. Wells, P. V. (1961). An investigation of vegetational and climatic change in the Mohave Desert by means of plant remains preserved in subfossil packrat middens. Unpublished research proposal to the National Science Foundation, September 14,196l. Wells, P. V. (1966). Late Pleistocene vegetation

and degree of pluvial climatic change in the Chihuahuan Desert. Science 153,970-975. Wells, P. V. (1969). Preuves paleontologiques d’une vegetation tardiPleistocene (datee par le 14C) dans les regions aujourd’hui desertiques d’Am&ique du Nord. Revue de Giographie Physique et de Gkologie Dynamique 11,335-340. Wells, P. V. (1970a). Postglacial vegetational history of the Great Plains. Science 167, 1574-1582. Wells, P. V. (1970b). Historical factors controlling vegetation patterns and floristic distributions in the Central Plains region of North America. In “Pleistocene and Recent Environments of the Central Great Plains” (W.Dort, and J. K. Jones, Eds.), pp. 211-221. University of Kansas Press, Lawrence. Wells, P. V. (1976). Postglacial origin of the Chihuahuan Desert less than 11,500 years ago. In “Symposium on the Biological Resources of the Chihuahuan Desert Region (R. H. Wauer and D. H. Riskind, Eds.). U.S. Govt. Printing Office, Washington, D.C. Wells, P. V., and Berger, R. (1967). Late Pleistocene history of coniferous woodland in the Mohave Desert. Science 155,1640-1647. Wells, P. V., and Jorgensen, C. D. (1964). Pleistocene wood rat middens and climatic change in Mohave Desert: A record of juniper woodlands. Science 143,1171-1174. Whittaker, R. H., and Feeny, P. P. (1971). Allelochemics: chemical interactions between species. Science 171,757-770.