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The east-west transition zone of terrestrial vertebrates in central Texas: a biogeographical analysis.

ABSTRACT. -- Two major complexes of native terrestrial vertebrates occupy central Texas. The largest (70 percent of regional species) consists of species of northern and eastern geographic affinities that are known or presumed derivates of Arcto-Tertiary Geofloral environments and the modern eastern deciduous forest. These species probably ranged across the entire region, indeed much of Texas, in full-glacial time. With the onset of warmer-drier climates and consequent emergence of the tallgrass prairie in post-glacial time, forest species retreated and were blocked by the prairie. Some remain undifferentiated across the region, whereas others are distinct subspecies or isolates east and west of the prairie. The tallgrass prairie is such a recent feature that it has few indicator vertebrates compared to the number characteristic of eastern forests and western woodlands. This prairie, not the Balcones Escarpment, is the major barrier to east-west dispersal in central Texas.

The second major group of vertebrates, a southwestern-Mexican contingent (20 percent of species) derives from Madro-Tertiary Geofloral environments and the modern evergreen woodland. It alone defines west-central Texas (Bosque, McLennan, Bell, Williamson, Travis, and the counties westward) as distinct from east-central Texas, because both these subregions house similar fractions of eastern species. Evergreen woodland plants and vertebrates are so condordant that they may be considered a biome, indicative of the Edwards Plateau region. The evergreen woodland assemblage probably arrived in central Texas in postglacial time, when a few representatives also ranged across the developing prairie to the eastern deciduous forest. Some left isolated populations in the east, a few on the prairie, but most were confined to the Edwards Plateau and Balcones Escarpment, as the regional climate dried appreciably in the last 2000 years. Postglacial drying and prairie formation isolated some woodland species east and west of the prairie, and this was reinforced by the uniquely dry environments of the last two millenia.

Central Texas is a transition zone, certainly not a biotic province, between the eastern deciduous forest region (Austroriparian Biotic Province) and central-western evergreen woodland region (Balconian Biotic Province). Two-thirds of the terrestrial vertebrates are at their edge of range here, experience intergradation between partially isolated subspecies, or show a range gap in an otherwise broad distribution. A small assemblage of southern vertebrates (10 percent of all regional species) reaches northern limits in or near the study area but is unimportant to the transitional nature of central Texas. Key words: central Texas; biogeography; mammals; reptiles; songbirds.

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Biogeography, the study of living landscapes on a transcontinental scale, relies heavily on regional subdivisions frequently referred to as biotic provinces. These are terrestrial areas of biotic similarity, determined by particular climates and landforms. But biotic provinces themselves are not uniform. They may contain more than a single plant formation (dominant vegetation of similar structure) and hence more than one biome (animals plus plants determined by vegetation structure), so it is important to utilize these smaller landscape units when discussing transitional biogeography. Of course, landscape patterns are understood best if their temporal as well as spatial dynamics can be inferred or are known.

North American biotic provinces were introduced by Dice (1943) and delineated in Texas by Blair (1950). Unfortunately, Blair included aquatic species and omitted birds, the largest group of vertebrates, in his subjective descriptions, although he did note that central Texas (his Texan Biotic Province) is transitional between eastern forest and western prairie biotas. Webb (1950) also noted this transition zone. Since recent appraisals of transition apply only to herpetofaunas (Owen and Dixon, 1989) or just reptiles (Ward et al., 1990) and not whole landscapes, I shall provide a broader quantitative study of distributional patterns, employing plant formations and all classes of terrestrial vertebrates. Specifically, I wish to measure the faunal interchange across central Texas between the Austroriparian (eastern) and Balconian (western) biotic provinces of the area. Some prehistoric patterns are deduced, but the hard evidence is a synthetic assessment of modern distributions.

PARAMETERS AND METHODS

The study area included Navarro, Freestone, Leon, and Madison counties on the east, bordering the Trinity River, west to Bosque, Hamilton, Lampasas, and Burnet counties near the eastern edge of the Edwards Plateau. The 24 counties sampled range from Bosque, Hill, and Navarro in the north to Grimes, Washington, Lee, Bastrop, and Travis in the south and are transected north to south by the Balcones Escarpment (Fig. 1). This region was chosen, because it is a representative cross-section of the three major plant formations in central Texas--deciduous forest, tallgrass prairie, and evergreen woodland from east to west (Fig. 1).

Native species of terrestrial reptiles, nesting songbirds (passerines), and mammals in each county were ascertained from faunal monographs (Oberholser, 1974; Schmidly, 1983; Dixon, 1987) and five decades of personal field work (1955-present). Extent of the three plant formations in each county was calculated with a plainmeter, using maps by Beaty and Gehlbach (1975) and McMahan et al. (1984), because vertebrate distributions are primarily determined by vegetational patterns (Gehlbach, 1984). All data per county were converted to percentages to avoid species-area effects and because the counties have been unevenly studied.

These data, arcsine-transformed for statistical analysis, were a) percent of each plant formation; b) percent of northern-eastern species in each vertebrate class, those with known or presumed Arcto-Tertiary Geofloral affinities on the bases of geography, habitat, and fossil record, hereafter termed eastern; c) percent of predominantly southwestern-Mexican species per class, those with known or presumed Madro-Tertiary Geofloral affinities, again using geographic, ecologic, and prehistoric guidelines, hereafter termed western; d) percent of all vertebrates with western range limits in the county; e) percent with eastern range limits; and f) percent of vertebrates typifying each of three prominant plant associations (deciduous oak-hickory, evergreen oak-juniper, bluestem-Indiangrass).

Three data-sets of 24 counties each were submitted to principal components analysis (PCA) in order to discern clusters of counties with similar biogeography (see Gehlbach, 1988a). These were a) percentages of eastern and western species among the reptiles, birds, and mammals, six variables per county; b) percentages of each plant formation plus the eastern and western range-limited species, five variables per county; and c) percentages of each plant formation and vertebrates per plant association, six variables per county. Validity of county clusters per PCA was tested by multivariate analysis of variance, followed by univariate analysis of variance of each plant or animal feature to discover significant indicators.

[FIGURE 1 OMITTED]

PCA assembles groups of correlated variables and makes new, linear variables (graph axes) of them. Orthogonal axes are derived from groups of correlated parameters that are relatively uncorrelated among groups. If the first two linear axes account for the most significant variation in the original data, a bidimensional graph adequately describes pattern complexity. Then each original sample (each county in this study) may be plotted in graph space, according to the correlations of its particular features with the synthetic graph axes, and samples (counties) with similar features may cluster together (Fig. 2).

In testing the hypothesis of a major east-west faunal transitional zone, the three PCAs address questions. 1) Which regional influence is strongest, eastern or western, and is that influence similar for each class of vertebrates? 2) Which plant formation is most influential in limiting species? 3) Are the plant formations and vertebrate indicators of their plant associations coincident and hence biomes and are these biomes distinct? After considering the general regional patterns, ancillary evidence of a transition zone afforded by local range gaps, relict populations, and patterns of subspecies intergradation is examined.

BASIC PATTERNS

The first analysis, involving eastern versus western vertebrate assemblages, produces two clusters of counties determined mostly by their fractions of western birds, followed by western reptiles and western mammals (horizonal axis in Fig. 2A). The clusters are quite distinct (Wilks lambda = 0.24, F = 8.6 P < 0.001), based on different percentages of western species in each of the three vertebrate groups (F > 23.1, P < 0.001). However, they have essentially the same eastern species components (F < 1.5, P > 0.05). On a per-county basis, eastern species outnumber western species by a 2.3 (mammals) to 6.3 (birds) margin in central Texas (Table 1). Thus, this region is mostly eastern in biogeographic affinities, but the birds and reptiles are more strongly eastern than mammals (Table 1).

[FIGURE 2 OMITTED]

Eastern birds often have western counterparts in central Texas. Eastern versus western kingbirds, blue (eastern) versus scrub (western) jays, and Carolina (eastern) versus canyon (western) wrens are examples. Among reptiles are a few such pairs (for example, broad-headed versus Great Plains skinks), but most eastern species like the green anole and massasauga lack close western relatives in the area. Characteristic western reptiles are the greater earless lizard and striped whipsnake. Some eastern mammals like the cotton mouse and Baird's pocket gopher have western regional equivalents (white-ankled mouse, plains pocket gopher), but others like the least shrew and swamp rabbit do not. Representative western mammals include the plains harvest mouse, and badger (all scientific names in Appendix).

During the initial assignment of vertebrates to biogeographic groups on the basis of their present ranges, habitats, and fossil histories, it was apparent that a few Neotropical Geofloral (southern) derivates were exceptions to the east-west patterns (Table 1). They reach northern limits in or near the 24-county area. These species were not analyzed further, because they represent significantly low percentages of the regional fauna and are irrelevant to the investigative hypothesis. Examples are the short-lined skink and pygmy mouse (avian examples are problematical).

INFLUENCE OF PLANT FORMATIONS

The analysis employing plant formation and range limits data reveals a more complex landscape pattern (Fig. 2B). Nonetheless, the county clusters are distinct (Wilks lambda = 0.003, F = 50.4, P < 0.001), and all five variables differ significantly among the three clusters (F > 5.7, P < 0.01). The incidence of both eastern and western range borders correlates strongly enough with the extent of tallgrass prairie (r = 0.43, 0.48, P < 0.05) that these parameters together account for the most landscape variation in Figure 2B. In fact, eastern and western range limits per county are so coincident (r = 0.81, P < 0.001), that the prairie restricts the movements of certain species regardless of their biogeographic affinity.

In Figure 2B the counties of group two segregate from those of groups one and three on the basis of range limits coincident with prairie. These counties (Group 2 = McLennan, Bell, Williamson, and Travis) form a north-south dividing point in the geographic sample (Fig. 1) and contain the most similar proportions of all three plant formations (Table 2). The group two counties are midway along the east to west, deciduous to evergreen woody plant gradient that separates all three clusters (vertical axis in Fig. 2B). Substantial woody vegetation east and west of the Balcones Scarp (Fig. 1) facilitates the variety of group two vertebrates, many of which are stopped by the prairie barrier because they are not adapted to this environment. Songbirds appear to be especially influenced in this way (Table 1).

Examples of such animals at their western range borders are five-lined skink, timber rattlesnake, evening bat, southern flying squirrel, acadian flycatcher, and yellow-throated vireo. Except for nesting songbirds, this contingent is larger than the fraction of eastern-limited species (Table 1). The latter group is represented by the collared lizard, long-nosed snake, Merriam's pocket mouse, hog-nosed skunk, cactus wren, and rufouscrowned sparrow which are characteristic of open evergreen woodland and hence typical of those counties in group three (Fig. 2B).

The group one counties, characterized by deciduous forest, barely overlap group two toward the positive end of the horizontal graph axis in Figure 2B. Counties involved in the overlap are Hill, Limestone, Falls, Brazos, and Washington from group one and Bell from group two. The group one suite lies in a north-south line immediately east of group two (Fig. 1). Except for Hill County all lack evergreen woodland but have sizeable portions of prairie and deciduous forest (Table 2). Practically no western range-limited species are in this area, but some eastern-limited taxa penetrate across the tallgrass prairie and are stopped by the deciduous forest. Examples are the ground and blind snakes, plains harvest mouse, and Cassin's sparrow.

THE QUESTION OF BIOMES

A limited concordance of vertebrate assemblages and plant formations is shown in the third analysis (Fig. 2C). Three clusters of counties are apparent and valid (Wilks lambda = 0.006, F = 24.7, P < 0.001), based on the statistical reliability of all six variables (F > 5.3, P < 0.02). Yet only the evergreen woodland and evergreen oak-juniper constituents are sufficiently correlated (r = 0.81, P < 0.001) to define a biome, form a graph axis (the horizontal), and distinguish the more western counties of groups two and three from the eastern counties of group one. The separation of group two from three occurs along a gradient toward greater coincidence of the deciduous oak-hickory and bluestem-Indiangrass assemblages on the vertical axis (r = 0.58, P < 0.01), between these two types).

The north to south string of counties in group two is distinguished from group three, immediately to the west, by having more concident oak-hickory and grass associates. This might be predicted from larger fractions of deciduous forest and prairie formations (Table 2), but the vertebrates and plant formations are unrelated geographically (r < 0.29, P > 0.05). The resemblance of prairie and deciduous forest animals is because most prairie species are eastern derivatives that occupy both the central tallgrass prairie and patches of this formation within the eastern forest region. Examples are the slender glass lizard, lined snake, thirteen-lined ground squirrel and dickcissel.

Vertebrates of the bluestem-Indiangrass association are also coincident with those of the evergreen woodland (r = 0.66, P < 0.001), whereas evergreen oak-juniper and deciduous oak-hickory associates are primarily in western or eastern parts of the region (r = -0.81, P < 0.001, cf. Table 2). As noted above, western species penetrate the prairie (for example, the blind snake and Cassin's sparrow), and, like the eastern contigent (lined snake and dickcissel, for example), are patchily distributed in open grassy areas of mostly wooded environments. There is a narrow interdigitation, not broad overlap, of these grass-adapted western and eastern species; so the two geographic complexes remain distinct. True prairie indicators are scarce; those of the bluestem-Indiangrass community average only 29 percent per county compared to 48 percent for all woodland and forest species (Wilcoxon P < 0.001).

RANGE GAPS, INTERGRADE ZONES, AND DISJUNCT POPULATIONS

Among all terrestrial vertebrates that range across central Texas, one-fifth show an east-west hiatus in distribution, indicating a restriction on movements of species in the region. The three vertebrate groups do not differ in this regard ([X.sup.2] = 0.3, P > 0.05, cf. Table 1). Regionally, the percentages of species showing any kind of east-west limitation (edge of range, gap in range, intergrade zone) are 57 (mammals), 67 (songbirds), and 78 (reptiles). Inter-group differences in numbers, relative to the numbers of unlimited species, are insignificant ([X.sup.2] = 1.1, P > 0.05). Clearly, the central Texas landscape is an east-west barrier to the majority ([bar.x] 67.3 percent) of its constituent vertebrates.

Perhaps the most intriguing examples of range gaps are those of transcontinental and hence broadly adapted species like the ringneck and gopher snakes and big brown bat (Manning et al., 1989). Both snakes are absent from local tallgrass prairie but live in prairies elsewhere. Many intergrade species are also prairie dwellers that must experience partial isolation by the juxtaposition of this habitat between forest and woodland in central Texas. Examples are the Great Plains ratsnake, coachwhip, fulvous harvest mouse, eastern meadowlark, and horned lark.

The range gaps and intergrade zones of woodland and forest species are easier to understand, because the prairie is an obvious barrier, and deciduous forests to the east are quite different in structure from evergreen woodlands to the west. Most involved species require trees if only in a savanna. Some examples are the eastern fence lizard, copperhead, fox squirrel, eastern woodrat, tufted titmouse, and Carolina wren. In each of these an eastern forest subspecies blends rather abruptly with a western woodland race along the narrow, trans-prairie corridors formed by riparian deciduous forest. For certain species, like the copperhead and tufted titmouse, the zone of gene exchange is displaced toward the west, where rivers cut through the Balcones Escarpment (Dixon, 1989; Gloyd and Conant, 1990).

The Balcones Escarpment has been considered a major barrier to east-west dispersal (Smith and Buechner, 1947), but there is biotic evidence to the contrary. The eastern deciduous forest not only penetrates the scarp via rivers and creeks but blends with evergreen woodland on its slopes (Gehlbach, 1988b). Tallgrass prairie terminates abruptly at the scarp edge, but among prairie animals, only the thirteen-lined ground squirrel does likewise. Most grassland species venture across the scarp in both directions as noted previously, and deciduous forest species do also (for example, black ratsnake, white-footed mouse, Carolina chickadee). Vertebrates restricted by the scarp are mostly those confined to its easternmost evergreen woodland, such as the Texas alligator lizard, Merriam's pocket mouse and brown towhee, which substantiates the unique nature of this biome in central Texas.

Some species have widely separated populations in and adjoining the study area, suggesting that they are remnants of former broad distributions. Such patterns, the inverse of range gaps, also reveal the transitional nature of the central Texas landscape. The sugar maple, for instance, is widespread in eastern Texas but a relict in the study area and southwestward (Gehlbach and Gardner, 1983). Similarly, the black and white warbler and woodland vole are deciduous forest affiliates represented by disjunct populations in the study area and adjacent Edwards Plateau. The spotted night snake is of western affinities and restricted to evergreen woodland in the study area but also isolated east of the Trinity River, and the hog-nosed skunk ranges eastward to the limit of evergreen woodland with a disjunct population in the deciduous forest region.

TRANSITIONS IN TIME

Fossil pollen deposits in central Texas furnish strong evidence of environmental change. Sphagnum bogs in Leon, Robertson, Milam, and Lee counties present a combined vegetational-climatic record dating back to the last full-glacial period (Bryant and Holloway, 1985). Spruce and birch accompany basswood, maple, alder, and similar deciduous forest indicators in reflecting a cooler, moister climate suitable for the northern coniferous forest trees about 15,000 years ago. At this time such eastern regional elements as the sugar maple and woodland vole probably ranged across the area, which lacked continuous tallgrass prairie and evergreen woodland (Martin and Harrell, 1957; Blair, 1958). The eastern or Arcto-Tertiary-derived element dominated central Texas.

During late-glacial time, 14,000 to 10,000 years ago, coniferous forest trees disappeared and herbaceous plants, especially grasses, began to replace woody plants. The tallgrass prairie must have originated from selected forest plants here, as elsewhere (Wells, 1970), but did not attain its broad, historic expanse until later. Thus, eastern forest species inhabited the entire 24-county area, although some with present isolates like the black and white warbler or subspecies like the copperhead may have had patchier distributions in accord with the warmer, drier climate and diversifying plant formations. But prairie and evergreen woodland species were mostly if not entirely absent, and transcontinental species ranged across the region.

The emergence of tallgrass prairie as a distinct formation must date no earlier than postglacial time, beginning about 10,000 years ago. The prairie lacks a large array of indicators and is a barrier to so many eastern and western species that comparative recency is indicated. An older prairie would have allowed more time for vertebrates to adapt and colonize it. But pollen evidence of any prairie is lacking (Bryant and Holloway, 1985). Similarly, pollen data on the evergreen woodland do not exist. It is likely that evergreen oaks and junipers began to replace deciduous forest in the Edwards Plateau uplands no earlier than about 10,000 years ago, because the climate was not sufficiently warm and dry earlier, and the substitution process is incomplete. Deciduous forest continues on many lowland sites (Amos and Gehlbach, 1988), supporting a considerable eastern element, including isolates such as the black and white warbler and distinctive subspecies as in the copperhead.

The partial replacement of eastern by western mammals on the Edwards Plateau, 6000-11,000 years ago, is clearly evident (Table 3). That replacement also is incomplete, because postglacial mammalian assemblages do not differ with respect to biogeographic make-up. Only an archaic fauna, dominated by extinct and extirpated species and prevalent until about 12,000 years ago, is different. Thus fossil mammal assemblages corroborate my postulate, based on living vertebrates, that eastern species prevailed across central Texas until Holocene time when an incursion of warm-dry adapted, western species occurred (coincident with a limited increase in tropical species, Table 3).

Apparently there was no warm-moist or dry Altithermal interval in central Texas but rather a pronounced period of drier and possibly warmer conditions starting about 2000 years ago (Holloway et al., 1987). At this time, western portions of the deciduous forest became open woodland or savanna, and hence the last aspect of the historic landscape emerged. Range fragmentation of deciduous forest species that began with the onset of drying in late-glacial time could have been complete before this latest dry episode, whereas the isolated populations of evergreen woodland species in east Texas are more likely to have originated then. None of the isolates, either western or eastern in origin, seems to be morphologically distinct, suggesting that they are quite recent products.

APPENDIX I. -- Scientific names of species arranged alphabetically by common name.

Acadian Flycatcher, Empidonax virescens

Alder, Alnus sp.

Baird's pocket gopher, Geomys breviceps

Badger, Taxidea taxus

Basswood, Tilia sp.

Big brown bat, Eptesicus fuscus

Birch, Betula sp.

Black ratsnake, Elaphe obsoleta

Black-and-white warbler, Mniotilta varia

Blind snake, Leptotyphlops dulcis

Bluejay, Cyanocitta cristata

Bluestem, Andropopon (sensu lat.)

Broad-headed skink, Eumeces laticeps

Brown towhee, Pipilo fuscus

Cactus wren, Campylorhynchus brunneicapillus

Canyon wren, Catherpes mexicanus

Carolina chickagee, Parus carolinensis

Carolina wren, Thryothorus ludovicianus

Cassin's sparrow, Aimophila cassinii

Coachwhip, Masticophis flagellum

Collared lizard, Crotaphytus collaris

Copperhead, Agkistrodon contortrix

Cotton mouse, Peromyscus gossypinus

Dickcissel, Spiza americana

Eastern fence lizard, Sceloporus undulatus

Eastern kingbird, Tyrannus tyrannus

Eastern meadowlark, Sturnella magna

Eastern woodrat, Neotoma floridana

Evening bat, Nycticeius humeralis

Five-lined skink, Eumeces fasciatus

Fox squirrel, Sciurus niger

Fulvous harvest mouse, Reithrodontomys fulvescens

Gopher snake, Pituophis melanoleucus

Great Plains ratsnake, Elaphe guttata

Great Plains skink, Eumeces obsoletus

Greater earless lizard, Cophosaurus texana

Green anole, Anolis carolinensis

Ground snake, Sonora semiannulata

Hickory, Carya sp.

Hog-nosed skunk, Conepatus mesoleucus

Horned lark, Eremophila alpestris

Indiangrass, Sorgastrum nutans

Juniper, Juniperus sp.

Least shrew, Cryptotis parva

Lined snake, Tropidoclonion lineatum

Long-nosed snake, Rhinocheilus lecontei

Maple, Acer sp.

Massasauga, Sistrurus catenatus

Merriam's pocket mouse, Perognathus merriami

Oak, Quercus sp.

Plains harvest mouse, Reithrodontomys montanus

Plains pocket gopher, Geomys bursarius

Pygmy mouse, Baiomys taylori

Ringneck snake, Diadophis punctatus

Rufous-crowned sparrow, Aimophila ruficeps

Scrub jay, Aphelocoma coerulescens

Short-lined skink, Eumeces tetragrammus

Slender glass lizard, Ophisaurus attenuatus

Southern flying squirrel, Glaucomys volans

Spotted nightsnake, Hypsiglena torquata

Spruce, Picea sp.

Striped whipsnake, Masticophis taeniatus

Sugar maple, Acer saccharum

Swamp rabbit, Sylvilagus aquaticus

Texas alligator lizard, Gerrhonotus liocephalus

Thirteen-lined ground squirrel, Spermophilus tridecemlineatus

Timber rattlesnake, Crotalus horridus

Tufted titmouse, Parus bicolor

Western kingbird, Tyrannus verticalis

White-ankled mouse, Peromyscus pectoralis

White-footed mouse, Peromyscus leucopus

Woodland vole, Microtus pinetorum

Yellow-throated vireo, Vireo flavifrons
TABLE 1. Biogeographic components of the terrestrial vertebrate fauna in
central Texas. Regional-affinity and range-limit values are average
percentages computed from a 24-county sample; range-gap and intergrade-
zone values are percentages calculated from all species regionwide.

 Mammals Reptiles Songbirds
 (48 species) (59 species) (73 species)
Regional affinities (1)
 Eastern 56 71 83
 Western 24 23 13
 Southern 20 6 4
Range limits (1)
 Western 56 41 49
 Eastern 17 25 47
 Northern 27 34 4
Range gaps 4 9 3
Intergrade zones 14 14 18

(1) Columns = 100 percent (transcontinental species omitted). [X.sup.2]
> 14.3 (P < 0.006) for the intergroup comparisons (affinities and range
limits tested separately). In Wilcoxon tests (N = 24), significantly
different percentages (P < 0.05) obtain among all eastern and eastern
range-limited groups, birds versus reptiles and mammals in the western
and western range-limited groups, and eastern plus western versus
southern affinity groups.

TABLE 2. Ecological features of groups of similar counties in central
Texas. Groups were determined by principal components analysis (Fig. 2),
verified by multivariate analysis of variance, and are arranged on the
regional gradient of more deciduous forest in the eastern part of the
area, through tallgrass prairie, to more evergreen woodland on the west.
Values are average percentages calculated from the counties in each
group.

 Group one Group two Group three
Ecological features (15 counties) (four counties) (five counties)

Plant formations
 Deciduous forest 70 10 4
 Tallgrass prairie 30 51 7
 Evergreen woodland 1 39 89
Vertebrates in plant
 associations
 Deciduous oak-hickory 69 40 38
 Bluestem-Indiangrass 30 31 27
 Evergreen oak-juniper 1 29 35

TABLE 3. The Pleistocene (Wisconsin)--Holocene transition of mammal
faunas on the Edwards Plateau, Texas, reanalyzed from Graham (1987).
Data are percentages of species per category per time interval.

 Thousand years before present
Biogeographic affinity 25-12 12-11 11-8 8-6 6-3 <3 Historic

Northern-eastern (1) 16 44 32 26 26 21 20
Southwestern-Mexican (2) 27 20 33 41 46 44 42
Tropical 3 ? 6 3 8 12 11
Extinct-extirpated 41 12 6 5 0 0 0
 Total species 37 (3) 25 66 39 46 34 66

(1) Arcto-Tertiary Geofloral derivates.
(2) Madro-Tertiary Geofloral derivates (that is, the Mexican element is
upland, not lowland tropical).
(3) Only this fauna differs in frequency of groups from the others
([X.sup.2] = 10.3, P = 0.03); note that widespread species are included
in the totals but not as a separate category.


ACKNOWLEDGMENTS

I thank Robert S. Baldridge, Keith L. Dixon, Neil B. Ford, and Kenneth T. Wilkins for their constructive comments on this study, and I dedicate it to the memory of my esteemed colleague, W. Frank Blair, who made natural history research respectable in the age of modern science.

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FREDERICK R. GEHLBACH

Department of Biology, Baylor University, Waco, Texas 76798
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Author:Gehlbach, Frederick R.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Nov 1, 1991
Words:5043
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