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Moisture History and Small Mammal Community Richness during the Latest Pleistocene and Holocene, Northern Bonneville Basin, Utah
QUATERNARY RESEARCH ARTICLE NO.
49, 330–334 (1998)
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Moisture History and Small Mammal Community Richness during the Latest Pleistocene and Holocene, Northern Bonneville Basin, Utah Donald K. Grayson Department of Anthropology and The Burke Museum, University of Washington, Seattle, Washington 98195 Received October 29, 1997
Precipitation and net primary productivity are positively correlated in arid environments. Both variables are, in turn, correlated with mammal species richness, but this relationship is not necessarily positive. With increasing precipitation in arid areas of low to moderate productivity, mammal richness increases linearly; as rainfall and productivity increase beyond this point, mammal richness is known to decline in some areas, producing a relationship that has been termed ‘‘unimodal’’ or ‘‘humped.’’ In the Great Basin of the arid western United States, studies of the relationship between rodent species richness and precipitation have revealed only a positive relationship between these two variables. It has, however, been argued that if areas of higher precipitation were to be sampled within this region, the decline phase would become evident. When latest Pleistocene and Holocene small mammal assemblages from the northern Bonneville Basin (central Utah) are examined across a temporal moisture gradient, species richness declines as moisture declines. Since the Great Basin was significantly moister during the latest Pleistocene and Early Holocene than it has been since that time, the unimodal response model does not appear to apply to this region. q 1998 University of Washington. Key Words: biogeography; mammal history; Great Basin; arid environments; climate change.
INTRODUCTION
The relationships among precipitation, productivity, and mammal species richness in arid environments have been reasonably well studied, even if the causes of some of the observed richness patterns are not yet fully understood (Rosenzweig, 1992, 1995; Rosenzweig and Abramsky, 1993). Precipitation in such environments is positively correlated with net primary productivity (e.g., Brown, 1975; Meserve and Glanz, 1978; Abramsky and Rosenzweig, 1984). Both these variables are, in turn, correlated with mammal species richness (the number of mammal species present), but this relationship is not necessarily positive. With increasing precipitation in arid areas of low to moderate productivity, mammal richness increases linearly (Fig. 1A; Brown, 1973, 1975; Meserve and Glanz, 1978). As rainfall and productivity increase beyond this point, however, mammal richness has been shown to decline in some areas, producing a relationship that has been termed ‘‘uni-
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HOMESTEAD CAVE
Homestead Cave is located a few kilometers west and south of Great Salt Lake on a northwestern spur of the Lakeside Mountains (Fig. 2). This spur, Homestead Knoll, is a low (maximum elevation 1615 m) rocky promontory that is
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modal’’ or ‘‘humped’’ (Fig. 1B; Abramsky and Rosenzweig, 1984; Abramsky et al., 1985; Owen, 1988, 1990; Rosenzweig, 1992, 1995; Wright et al., 1993; on plants, see Tilman, 1982; Tilman and Pacala, 1993). Whereas the initial rise in this relationship is well-understood, the cause or causes of the decline are not. It is also not known whether the unimodal productivity– richness relationship holds for Great Basin small mammals. Brown (1973, 1975; Brown and Gibson, 1993) has shown that the relationship between rainfall and rodent species richness in sandy habitats in the Great Basin is positive and linear up to about 32 cm of annual precipitation (Brown, 1973; Fig. 1A). Rosenzweig (1992, 1995; Rosenzweig and Abramsky, 1993), however, has argued that Brown’s data simply catch the initial, positive part of the full richness relationship. He also argues that close inspection of the uppermost end of the Great Basin curve presented by Brown (1975; Fig. 1A) actually shows ‘‘two data points in decline’’ (Rosenzweig and Abramsky, 1993, p. 52) at ca. 30 cm average annual precipitation. The clear implication is that if we were to sample areas of higher precipitation within the Great Basin, the decline phase would become more evident (see also Abramsky et al., 1985, p. 368). I will refer to these two interpretations of the relationship between precipitation and small mammal species richness in the Great Basin as the ‘‘positive richness-response’’ (e.g., Brown, 1975) and ‘‘unimodal richness-response’’ (e.g., Rosenzweig and Abramsky, 1993) models. Here, I examine which of these two models best fits small mammal assemblages of latest Pleistocene and Holocene age from the northern Bonneville Basin of central Utah. That is, rather than examining small mammal richness responses across a precipitation gradient in space, I examine them across a moisture gradient in time.
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under the direction of D. B. Madsen of the Utah Geological Survey, we stratigraphically excavated a 1 1 1-m column sample from this site, collecting the faunal material in that column by sieving the deposits through a nested series of 1/ 49 (6.4 mm), 1/89 (3.2 mm), and 1/169 (1.6 mm) screens. The extraordinarily rich faunal record provided by this sample resulted primarily from the use of the cave by owls. The deposits contain numerous owl pellets, and many isolated mammal bones and teeth, regardless of their age, still have remains of those pellets adhering to them. To date, I have identified some 170,000 mammal bones and teeth from the 1/49 and 1/89 sample fractions of 15 of the 18 Homestead Cave strata to at least the genus level. These specimens include 25 species of rodents and lagomorphs; carnivores have yet to be fully identified and are not examined here. Work on the identifications continues; of the 18 strata, sufficient work has been completed to allow examination of the taxonomic richness of 13 (Table 1). The details of the depositional history of the sediments in the cave are not relevant to the arguments I develop here, but I do note that 21 radiocarbon dates are available for the Homestead Cave column (Table 2). These dates document that the earliest stratum within the cave was deposited between about 11,300 and 10,200 14C yr B.P., thus encompassing the period that includes the Gilbert phase of Lake Bonneville history (Oviatt, 1997). The available radiocarbon chronology indicates that superincumbent stratum II at Homestead Cave accumulated between 8900 and 8500 14C
FIG. 1. Relationships between rodent species richness and average annual precipitation in selected arid areas. (A) Sandy habitats in the Great Basin (data from Brown, 1973); (B) sand dune habitats of Israel (data from Abramsky et al., 1985).
devoid of active springs and permanent streams and that receives approximately 22.5 cm of precipitation per year. The barren playa of Pleistocene Lake Bonneville is located to the immediate west and northwest of this rocky spur, but the knoll itself supports, in addition to a few scattered junipers (Juniperus osteosperma), vegetation that is dominated by shrubs and grasses. Shadscale (Atriplex confertifolia) and horsebrushes (Tetradymia spp.) are the most abundant shrubs, but bud sagebrush (Artemisia spinescens), rabbitbrush (Chrysothamnus sp.), and greasewood (Sarcobatus vermiculatus) are also present, as is big sagebrush (Artemisia tridentata) along seasonally moist drainages. Greasewood becomes increasingly common as the valley bottom is approached, while the invasive Eurasian cheatgrass (Bromus sp.) is abundant on the flats beneath the knoll. Homestead Cave sits on the northwestern edge of Homestead Knoll at an elevation of 1406 m. In 1993 and 1994,
FIG. 2. Map of the northern Bonneville Basin, showing location of Homestead Cave.
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TABLE 1 Numbers of Identified Specimens (NISP) and Numbers of Taxa for the 13 Homestead Cave Strata Analyzed Here Stratum
NISP
N of taxa
XVIII XVII XII XI IX VIII VII VI V IV III II I Total
1047 15,421 22,661 9996 18,043 8215 11,038 18,661 5093 26,200 2774 7756 9906 156,811
9 17 14 14 16 13 15 17 13 19 17 20 19
yr B.P. and thus also suggests that there may have been a depositional gap of about 1000 14C yr between the accumulation of strata I and II. The latest radiocarbon date for the sequence is for Stratum XVII and falls at about 1000 14C yr B.P. The Homestead Cave faunal sequence thus covers the latest Pleistocene and virtually all of the Holocene. The 13 strata examined below include all those dated to between about 11,300 and 6000 14C yr B.P., as well as a discontinuous series of later assemblages (Table 1).
1990, 1992; Grayson, 1993; Rhode and Madsen, 1995). The latest Pleistocene and Early Holocene faunal assemblages from Homestead Cave, which are marked by high proportions of such animals as bushy-tailed woodrats (Neotoma cinerea), yellow-bellied marmots (Marmota flaviventris), and pygmy rabbits (Brachylagus idahoensis), are consistent only with temperatures substantially lower than those that now characterize the area (D. K. Grayson, unpublished data). Accordingly, effective moisture would have been much higher than is implied by the NCAR CCM precipitation estimate alone. It follows that if moisture and small mammal richness have been positively related through time here, then small mammal richness should have been greater during the latest Pleistocene and Early Holocene than it has been subsequently. To address this issue, it is essential to recognize that the greater the number of identified specimens (NISP) in a paleontological fauna, the greater the number of taxa represented by those specimens (e.g., Grayson, 1984). As a result, sample size needs to be taken into account if changing faunal richness through time is to be assessed. Figure 3 provides the relationship between NISP and numbers of taxa across the 13 Homestead Cave assemblages that have been well studied. In constructing this figure, I excluded taxa represented by only a single specimen in any given stratum in order to provide some control over the bias that
TABLE 2 Radiocarbon and Calibrated Ages from Homestead Cave
LOCAL PLEISTOCENE MOISTURE HISTORY AND SMALL MAMMAL SPECIES RICHNESS
Stratum
There is broad agreement, based on a wide variety of paleoclimatic indicators and on quantitative climatic models, that the latest Pleistocene and Early Holocene of the northerly Great Basin was substantially moister than the following interval. Thompson et al. (1993), using the National Center for Atmospheric Research Community Climate Model (NCAR CCM), for instance, estimated that average annual precipitation at 9000 yr B.P. for the area that includes Homestead Cave would have been about 11 cm greater than it is today, or ca. 34 cm. There is some disagreement as to whether this increase in moisture was associated with an increase in both precipitation and temperature, as some quantitative models suggest should have been the case, or by increased precipitation in generally much cooler environments than those that have since characterized the region (e.g., Thompson, 1990; Thompson et al., 1993). However, an impressive variety of empirical data suggests that the latest Pleistocene and Early Holocene of the northerly Great Basin saw both lower temperatures and greater precipitation than today (e.g., Madsen and Currey, 1979; Wigand and Mehringer, 1985; Thompson,
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XVII XVI XIV XIII XII X VII VI V IV II
I (Upper 5 cm) I (General) I (Lower 5 cm)
a
Age (14C yr B.P.) 1020 1200 2850 3480 3400 5330 6160 6185 7120 8230 8195 8520 8790 8830 10,160 10,350 10,910 11,065 11,180 11,265 11,270
{ { { { { { { { { { { { { { { { { { { { {
40 50 50 40 60 65 85 105 70 70 85 80 90 240 85 80 60 105 85 85 135
Lab No.
Agea (cal yr B.P.)
Beta 101877 Beta 66940 Beta 103692 Beta 101878 Beta 63179 AA 14822 AA 14824 AA 14825 AA 14826 AA 16810 AA 14823 AA 14821 AA 14820 Beta 63438 AA 14819 AA 14818 Beta 72205 AA 14817 AA 16808 AA 16809 AA 14816
933 1078 2948 3707 3632 6105–6171 7017 7028–7148 7912 9210 9047–9193 9484 9694–9852 9873 11,865 12,228 12,832 12,978 13,087 13,172 13,177
Calibrated ages calculated with CALIB 3.0.3c (Stuiver and Reimer, 1993).
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cerning the vegetation of the western Lake Lahontan Basin in far western Nevada. Here, they concluded, latest Pleistocene and Early Holocene vegetation assemblages were far richer than those that followed. Today, average annual precipitation at low-elevation settings in the Great Basin rarely exceeds 30 cm (U.S. Department of Commerce, 1983). During the latest Pleistocene and Early Holocene, the Homestead Cave area appears to have received higher average annual precipitation in a context cooler than today. Given that small mammal richness in this area declined as moisture declined, it would appear likely that the positive richness response model between productivity and small mammal richness applies to all low elevations settings in the Great Basin. It also appears likely that this model has applied to these settings since the latest Pleistocene. CONCLUSIONS
FIG. 3. Relationships between the number of identified specimens (NISP) and taxonomic richness (N of taxa) at Homestead Cave. Note that the faunas of Strata X (ca. 5300 14C yr B.P.) and XIII–XVI (ca. 3500– 1200 14C yr B.P.) have either not been identified or not been substantially identified and are not included in this illustration.
would be introduced by the movement of individual specimens across strata (e.g., Benson, 1975). The protocol discussed by Grayson (1991) was used to prevent counting potentially overlapping taxa. All taxa represented in this figure are either rodents (ranging in size from the little pocket mouse Perognathus longimembris to the yellow-bellied marmot) or lagomorphs (members of the genera Brachylagus, Lepus, and Sylvilagus; Table 3). The lines provided in this figure, fit by least-squares regression, are given simply to indicate the fact that there are two separate relationships present at Homestead Cave between NISP and numbers of taxa; whereas the lower relationship is highly significant (r Å 0.92, P õ 0.001), the upper relationship, drawn through three points, is not (r Å 0.88, P ú 0.10). At any given sample size, the faunas that accumulated between about 11,300 and 8300 14C yr B.P. contain greater numbers of small mammal taxa than those that accumulated after this time. That is, the latest Pleistocene and Early Holocene faunas of the Homestead Knoll area were richer than those that came later, as is predicted by the positive richness response model under conditions of increasing aridity. This conclusion mirrors that reached by Nowak et al. (1994) con-
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Meserve and Glanz (1978) found a positive small mammal richness response across a 1000-km transect of the northern Chilean coastal arid zone that received up to 40 cm of annual precipitation; Brown and Gibson (1983) report a similar response in Sonoran Desert areas that receive up to 30 cm annual precipitation. While Abramsky and Rosenzweig (1984) found that that rodent richness declined above average annual precipitation values of ca. 9 cm (rocky habitats) and 18 cm (sandy habitats) in the Negev Desert, and Abramsky et al. (1985) found that gerbilline rodents reached a richness peak at ca. 12.2 cm average annual precipitation in the sand dune habitats of Israel, Rosenzweig and Abramsky (1993) have suggested that, in general, desert rodent species richness begins to decline after precipitation has reached 35 cm per year. It may well be that even during the latest Pleistocene and Early Holocene, effective moisture in the Great Basin did
TABLE 3 Identified Taxa Represented in the Homestead Cave Faunal Assemblages Examined in This Analysis Ammospermophilus leucurus Brachylagus idahoensis Dipodomys ordii Dipodomys microps Eutamias sp. Lemmiscus curtatus Lepus californicus Lepus townsendii Marmota flaviventris Microdipodops megacephalus Microtus sp. Neotoma cinerea Neotoma lepida
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Ondatra zibethicus Oynchomys leucogaster Perognathus formosus Perognathus longimembris Perognathus parvus Peromyscus sp. Reithrodontomys megalotis Spermophilus townsendii Sylvilagus audubonii Sylvilagus nuttallii Thomomys bottae Thomomys talpoides
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Meserve, P. L., and Glanz, W. E. (1978). Geographical ecology of small
mammals in the northern Chilean arid zone. Journal of Biogeography 5, 135–148. Nowak, C. L., Nowak, R. S., Tausch, R. J., and Wigand, P. E. (1994). A 30,000 year record of vegetation dynamics at a semi-arid locale in the Great Basin. Journal of Vegetation Science 5, 579–590. Oviatt, C. G. (1997). Lake Bonneville fluctuations and global climate change. Geology 25, 155–128. Owen, J. G. (1988). On productivity as a predictor of rodent and carnivore diversity. Ecology 69, 1161–1165. Owen, J. G. (1990). Patterns of mammalian species richness in relation to temperature, productivity, and variance in elevation. Journal of Mammalogy 71, 1–13. Rhode, D., and Madsen, D. B. (1995). Late Wisconsin/Early Holocene vegetation in the Bonneville Basin. Quaternary Research 44, 246–256. Rosenzweig, M. L. (1992). Species diversity gradients: We know more and less than we thought. Journal of Mammalogy 73, 715–730. Rosenzweig, M. L. (1995). ‘‘Species Diversity in Time and Space.’’ Cambridge Univ. Press, Cambridge. Rosenzweig, M. L., and Abramsky, Z. (1993). How are diversity and productivity related? In ‘‘Species Diversity in Ecological Communities’’ (R. E. Ricklefs and D. Schluter, Eds.), pp. 52–65. Univ. of Chicago Press, Chicago. Stuiver, M., and Reimer, P. J. (1993). Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215–230. Thompson, R. S. (1990). Late Quaternary vegetation and climate in the Great Basin. In ‘‘Packrat Middens: The Last 40,000 Years of Biotic Change’’ (J. L. Betancourt, T. R. Van Devender, and P. S. Martin, Eds.), pp. 200–239. Univ. of Arizona Press, Tucson. Thompson, R. S. (1992). Late Quaternary environments in Ruby Valley, Nevada. Quaternary Research 37, 1–15. Thompson, R. S., Whitlock, C., Bartlein, P. J., Harrison, S. P., and Spaulding, W. G. (1993). Climatic changes in the western United States since 18,000 yr B.P. In ‘‘Global Climate since the Last Glacial Maximum’’ (H. E. Wright, J. E. Kutzbach, T. Webb III, W. F. Ruddiman, F. A. StreetPerrott, and P. J. Bartlein, Eds.), pp. 468–513. Univ. of Minnesota Press, Minneapolis. Tilman, D. (1982). ‘‘Resource Competition and Community Structure.’’ Princeton University Press, Princeton. Tilman, D., and Pacala, S. (1993). The maintainance of species richness in plant communities. In ‘‘Species Diversity in Ecological Communities’’ (R. E. Ricklefs and D. Schluter, Eds.), pp. 13–25. University of Chicago Press, Chicago. U.S. Department of Commerce (1983). ‘‘Climatic Atlas of the United States.’’ Environmental Data Service, U.S. Department of Commerce, Washington, DC. [Reprinted by the National Oceanic and Atmospheric Administration, Washington, DC] Wigand, P. E., and Mehringer, P. J., Jr. (1985). Pollen and seed analyses. In ‘‘The Archaeology of Hidden Cave, Nevada’’ (D. H. Thomas, Ed.), pp. 108–124. Anthropological Papers of the American Museum of Natural History, 61 (1). Wright, D. H., Currie, D. J., and Maurer, B. A. (1993). Energy supply and patterns of species richness on local and regional scales. In ‘‘Species Diversity in Ecological Communities’’ (R. E. Ricklefs and D. Schluter, Eds.), pp. 66–74. Univ. of Chicago Press, Chicago.
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not reach levels sufficient for the decline phase of the curve to be reached. It is also possible that nonanalog combinations of temperature, precipitation, and seasonality during the latest Pleistocene and Early Holocene in this region caused novel relationships between species richness and moisture. Nonetheless, while the unimodal small mammal speciesresponse model clearly applies to some arid areas, the data presented by Brown (1973, 1975) and the paleontological data from Homestead Cave presented here suggest that this model has not applied to the Great Basin since the latest Pleistocene. ACKNOWLEDGMENTS I thank G. J. Kenagy, R. G. Klein, T. E. Lawlor, D. B. Madsen, and R. S. Thompson for extremely helpful comments on an earlier draft of the manuscript; J. M. Broughton, M. D. Cannon, M. A. Etnier, B. Hubler, and L. A. Nagaoka for laboratory assistance; and D. B. Madsen for his assistance, insight, and support throughout this project. The analysis of the Homestead Cave fauna has been supported by grants from the Legacy Project, Department of Defense, and from Hill Air Force Base.
REFERENCES Abramsky, Z., and Rosenzweig, M. L. (1984). Tilman’s predicted productivity–diversity relationship shown by desert rodents. Nature 309, 150– 151. Abramsky, Z., Brand, S., and Rosenzweig, M. L. (1985). Geographical ecology of gerbilline rodents in sand dune habitats of Israel. Journal of Biogeography 12, 363–372. Benson, R. H. (1975). The origin of the psychrosphere as recorded in changes of deep-sea ostracode assemblages. Lethaia 8, 69–83. Brown, J. H. (1973). Species diversity of seed-eating desert rodents in sand dune habitats. Ecology 54, 775–787. Brown, J. H. (1975). Geographical ecology of desert rodents. In ‘‘Ecology and Evolution of Communities’’ (M. L. Cody and J. M. Diamond, Eds.), pp. 315–341. Belknap Press of Harvard Univ. Press, Cambridge, MA. Brown, J. H., and Gibson, A. C. (1983). ‘‘Biogeography.’’ Mosby, St. Louis. Grayson, D. K. (1984). ‘‘Quantitative Zooarchaeology.’’ Academic Press, New York. Grayson, D. K. (1991). Alpine faunas from the White Mountains, California: Adaptive change in the prehistoric Great Basin? Journal of Archaeological Science 18, 483–506. Grayson, D. K. (1993). ‘‘The Desert’s Past: A Natural Prehistory of the Great Basin.’’ Smithsonian Institution Press, Washington, DC. Madsen, D. B., and Currey, D. R. (1979). Late Quaternary glacial and vegetation changes, Little Cottonwood Canyon area, Wasatch Mountains, Utah. Quaternary Research 12, 254–270.
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QR981970
Moisture History and Small Mammal Community Richness during the Latest Pleistocene and Holocene, Northern Bonneville Basin, Utah Donald K. Grayson Department of Anthropology and The Burke Museum, University of Washington, Seattle, Washington 98195 Received October 29, 1997
Precipitation and net primary productivity are positively correlated in arid environments. Both variables are, in turn, correlated with mammal species richness, but this relationship is not necessarily positive. With increasing precipitation in arid areas of low to moderate productivity, mammal richness increases linearly; as rainfall and productivity increase beyond this point, mammal richness is known to decline in some areas, producing a relationship that has been termed ‘‘unimodal’’ or ‘‘humped.’’ In the Great Basin of the arid western United States, studies of the relationship between rodent species richness and precipitation have revealed only a positive relationship between these two variables. It has, however, been argued that if areas of higher precipitation were to be sampled within this region, the decline phase would become evident. When latest Pleistocene and Holocene small mammal assemblages from the northern Bonneville Basin (central Utah) are examined across a temporal moisture gradient, species richness declines as moisture declines. Since the Great Basin was significantly moister during the latest Pleistocene and Early Holocene than it has been since that time, the unimodal response model does not appear to apply to this region. q 1998 University of Washington. Key Words: biogeography; mammal history; Great Basin; arid environments; climate change.
INTRODUCTION
The relationships among precipitation, productivity, and mammal species richness in arid environments have been reasonably well studied, even if the causes of some of the observed richness patterns are not yet fully understood (Rosenzweig, 1992, 1995; Rosenzweig and Abramsky, 1993). Precipitation in such environments is positively correlated with net primary productivity (e.g., Brown, 1975; Meserve and Glanz, 1978; Abramsky and Rosenzweig, 1984). Both these variables are, in turn, correlated with mammal species richness (the number of mammal species present), but this relationship is not necessarily positive. With increasing precipitation in arid areas of low to moderate productivity, mammal richness increases linearly (Fig. 1A; Brown, 1973, 1975; Meserve and Glanz, 1978). As rainfall and productivity increase beyond this point, however, mammal richness has been shown to decline in some areas, producing a relationship that has been termed ‘‘uni-
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HOMESTEAD CAVE
Homestead Cave is located a few kilometers west and south of Great Salt Lake on a northwestern spur of the Lakeside Mountains (Fig. 2). This spur, Homestead Knoll, is a low (maximum elevation 1615 m) rocky promontory that is
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modal’’ or ‘‘humped’’ (Fig. 1B; Abramsky and Rosenzweig, 1984; Abramsky et al., 1985; Owen, 1988, 1990; Rosenzweig, 1992, 1995; Wright et al., 1993; on plants, see Tilman, 1982; Tilman and Pacala, 1993). Whereas the initial rise in this relationship is well-understood, the cause or causes of the decline are not. It is also not known whether the unimodal productivity– richness relationship holds for Great Basin small mammals. Brown (1973, 1975; Brown and Gibson, 1993) has shown that the relationship between rainfall and rodent species richness in sandy habitats in the Great Basin is positive and linear up to about 32 cm of annual precipitation (Brown, 1973; Fig. 1A). Rosenzweig (1992, 1995; Rosenzweig and Abramsky, 1993), however, has argued that Brown’s data simply catch the initial, positive part of the full richness relationship. He also argues that close inspection of the uppermost end of the Great Basin curve presented by Brown (1975; Fig. 1A) actually shows ‘‘two data points in decline’’ (Rosenzweig and Abramsky, 1993, p. 52) at ca. 30 cm average annual precipitation. The clear implication is that if we were to sample areas of higher precipitation within the Great Basin, the decline phase would become more evident (see also Abramsky et al., 1985, p. 368). I will refer to these two interpretations of the relationship between precipitation and small mammal species richness in the Great Basin as the ‘‘positive richness-response’’ (e.g., Brown, 1975) and ‘‘unimodal richness-response’’ (e.g., Rosenzweig and Abramsky, 1993) models. Here, I examine which of these two models best fits small mammal assemblages of latest Pleistocene and Holocene age from the northern Bonneville Basin of central Utah. That is, rather than examining small mammal richness responses across a precipitation gradient in space, I examine them across a moisture gradient in time.
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under the direction of D. B. Madsen of the Utah Geological Survey, we stratigraphically excavated a 1 1 1-m column sample from this site, collecting the faunal material in that column by sieving the deposits through a nested series of 1/ 49 (6.4 mm), 1/89 (3.2 mm), and 1/169 (1.6 mm) screens. The extraordinarily rich faunal record provided by this sample resulted primarily from the use of the cave by owls. The deposits contain numerous owl pellets, and many isolated mammal bones and teeth, regardless of their age, still have remains of those pellets adhering to them. To date, I have identified some 170,000 mammal bones and teeth from the 1/49 and 1/89 sample fractions of 15 of the 18 Homestead Cave strata to at least the genus level. These specimens include 25 species of rodents and lagomorphs; carnivores have yet to be fully identified and are not examined here. Work on the identifications continues; of the 18 strata, sufficient work has been completed to allow examination of the taxonomic richness of 13 (Table 1). The details of the depositional history of the sediments in the cave are not relevant to the arguments I develop here, but I do note that 21 radiocarbon dates are available for the Homestead Cave column (Table 2). These dates document that the earliest stratum within the cave was deposited between about 11,300 and 10,200 14C yr B.P., thus encompassing the period that includes the Gilbert phase of Lake Bonneville history (Oviatt, 1997). The available radiocarbon chronology indicates that superincumbent stratum II at Homestead Cave accumulated between 8900 and 8500 14C
FIG. 1. Relationships between rodent species richness and average annual precipitation in selected arid areas. (A) Sandy habitats in the Great Basin (data from Brown, 1973); (B) sand dune habitats of Israel (data from Abramsky et al., 1985).
devoid of active springs and permanent streams and that receives approximately 22.5 cm of precipitation per year. The barren playa of Pleistocene Lake Bonneville is located to the immediate west and northwest of this rocky spur, but the knoll itself supports, in addition to a few scattered junipers (Juniperus osteosperma), vegetation that is dominated by shrubs and grasses. Shadscale (Atriplex confertifolia) and horsebrushes (Tetradymia spp.) are the most abundant shrubs, but bud sagebrush (Artemisia spinescens), rabbitbrush (Chrysothamnus sp.), and greasewood (Sarcobatus vermiculatus) are also present, as is big sagebrush (Artemisia tridentata) along seasonally moist drainages. Greasewood becomes increasingly common as the valley bottom is approached, while the invasive Eurasian cheatgrass (Bromus sp.) is abundant on the flats beneath the knoll. Homestead Cave sits on the northwestern edge of Homestead Knoll at an elevation of 1406 m. In 1993 and 1994,
FIG. 2. Map of the northern Bonneville Basin, showing location of Homestead Cave.
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TABLE 1 Numbers of Identified Specimens (NISP) and Numbers of Taxa for the 13 Homestead Cave Strata Analyzed Here Stratum
NISP
N of taxa
XVIII XVII XII XI IX VIII VII VI V IV III II I Total
1047 15,421 22,661 9996 18,043 8215 11,038 18,661 5093 26,200 2774 7756 9906 156,811
9 17 14 14 16 13 15 17 13 19 17 20 19
yr B.P. and thus also suggests that there may have been a depositional gap of about 1000 14C yr between the accumulation of strata I and II. The latest radiocarbon date for the sequence is for Stratum XVII and falls at about 1000 14C yr B.P. The Homestead Cave faunal sequence thus covers the latest Pleistocene and virtually all of the Holocene. The 13 strata examined below include all those dated to between about 11,300 and 6000 14C yr B.P., as well as a discontinuous series of later assemblages (Table 1).
1990, 1992; Grayson, 1993; Rhode and Madsen, 1995). The latest Pleistocene and Early Holocene faunal assemblages from Homestead Cave, which are marked by high proportions of such animals as bushy-tailed woodrats (Neotoma cinerea), yellow-bellied marmots (Marmota flaviventris), and pygmy rabbits (Brachylagus idahoensis), are consistent only with temperatures substantially lower than those that now characterize the area (D. K. Grayson, unpublished data). Accordingly, effective moisture would have been much higher than is implied by the NCAR CCM precipitation estimate alone. It follows that if moisture and small mammal richness have been positively related through time here, then small mammal richness should have been greater during the latest Pleistocene and Early Holocene than it has been subsequently. To address this issue, it is essential to recognize that the greater the number of identified specimens (NISP) in a paleontological fauna, the greater the number of taxa represented by those specimens (e.g., Grayson, 1984). As a result, sample size needs to be taken into account if changing faunal richness through time is to be assessed. Figure 3 provides the relationship between NISP and numbers of taxa across the 13 Homestead Cave assemblages that have been well studied. In constructing this figure, I excluded taxa represented by only a single specimen in any given stratum in order to provide some control over the bias that
TABLE 2 Radiocarbon and Calibrated Ages from Homestead Cave
LOCAL PLEISTOCENE MOISTURE HISTORY AND SMALL MAMMAL SPECIES RICHNESS
Stratum
There is broad agreement, based on a wide variety of paleoclimatic indicators and on quantitative climatic models, that the latest Pleistocene and Early Holocene of the northerly Great Basin was substantially moister than the following interval. Thompson et al. (1993), using the National Center for Atmospheric Research Community Climate Model (NCAR CCM), for instance, estimated that average annual precipitation at 9000 yr B.P. for the area that includes Homestead Cave would have been about 11 cm greater than it is today, or ca. 34 cm. There is some disagreement as to whether this increase in moisture was associated with an increase in both precipitation and temperature, as some quantitative models suggest should have been the case, or by increased precipitation in generally much cooler environments than those that have since characterized the region (e.g., Thompson, 1990; Thompson et al., 1993). However, an impressive variety of empirical data suggests that the latest Pleistocene and Early Holocene of the northerly Great Basin saw both lower temperatures and greater precipitation than today (e.g., Madsen and Currey, 1979; Wigand and Mehringer, 1985; Thompson,
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XVII XVI XIV XIII XII X VII VI V IV II
I (Upper 5 cm) I (General) I (Lower 5 cm)
a
Age (14C yr B.P.) 1020 1200 2850 3480 3400 5330 6160 6185 7120 8230 8195 8520 8790 8830 10,160 10,350 10,910 11,065 11,180 11,265 11,270
{ { { { { { { { { { { { { { { { { { { { {
40 50 50 40 60 65 85 105 70 70 85 80 90 240 85 80 60 105 85 85 135
Lab No.
Agea (cal yr B.P.)
Beta 101877 Beta 66940 Beta 103692 Beta 101878 Beta 63179 AA 14822 AA 14824 AA 14825 AA 14826 AA 16810 AA 14823 AA 14821 AA 14820 Beta 63438 AA 14819 AA 14818 Beta 72205 AA 14817 AA 16808 AA 16809 AA 14816
933 1078 2948 3707 3632 6105–6171 7017 7028–7148 7912 9210 9047–9193 9484 9694–9852 9873 11,865 12,228 12,832 12,978 13,087 13,172 13,177
Calibrated ages calculated with CALIB 3.0.3c (Stuiver and Reimer, 1993).
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SMALL MAMMALS AND CLIMATE IN CENTRAL UTAH
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cerning the vegetation of the western Lake Lahontan Basin in far western Nevada. Here, they concluded, latest Pleistocene and Early Holocene vegetation assemblages were far richer than those that followed. Today, average annual precipitation at low-elevation settings in the Great Basin rarely exceeds 30 cm (U.S. Department of Commerce, 1983). During the latest Pleistocene and Early Holocene, the Homestead Cave area appears to have received higher average annual precipitation in a context cooler than today. Given that small mammal richness in this area declined as moisture declined, it would appear likely that the positive richness response model between productivity and small mammal richness applies to all low elevations settings in the Great Basin. It also appears likely that this model has applied to these settings since the latest Pleistocene. CONCLUSIONS
FIG. 3. Relationships between the number of identified specimens (NISP) and taxonomic richness (N of taxa) at Homestead Cave. Note that the faunas of Strata X (ca. 5300 14C yr B.P.) and XIII–XVI (ca. 3500– 1200 14C yr B.P.) have either not been identified or not been substantially identified and are not included in this illustration.
would be introduced by the movement of individual specimens across strata (e.g., Benson, 1975). The protocol discussed by Grayson (1991) was used to prevent counting potentially overlapping taxa. All taxa represented in this figure are either rodents (ranging in size from the little pocket mouse Perognathus longimembris to the yellow-bellied marmot) or lagomorphs (members of the genera Brachylagus, Lepus, and Sylvilagus; Table 3). The lines provided in this figure, fit by least-squares regression, are given simply to indicate the fact that there are two separate relationships present at Homestead Cave between NISP and numbers of taxa; whereas the lower relationship is highly significant (r Å 0.92, P õ 0.001), the upper relationship, drawn through three points, is not (r Å 0.88, P ú 0.10). At any given sample size, the faunas that accumulated between about 11,300 and 8300 14C yr B.P. contain greater numbers of small mammal taxa than those that accumulated after this time. That is, the latest Pleistocene and Early Holocene faunas of the Homestead Knoll area were richer than those that came later, as is predicted by the positive richness response model under conditions of increasing aridity. This conclusion mirrors that reached by Nowak et al. (1994) con-
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Meserve and Glanz (1978) found a positive small mammal richness response across a 1000-km transect of the northern Chilean coastal arid zone that received up to 40 cm of annual precipitation; Brown and Gibson (1983) report a similar response in Sonoran Desert areas that receive up to 30 cm annual precipitation. While Abramsky and Rosenzweig (1984) found that that rodent richness declined above average annual precipitation values of ca. 9 cm (rocky habitats) and 18 cm (sandy habitats) in the Negev Desert, and Abramsky et al. (1985) found that gerbilline rodents reached a richness peak at ca. 12.2 cm average annual precipitation in the sand dune habitats of Israel, Rosenzweig and Abramsky (1993) have suggested that, in general, desert rodent species richness begins to decline after precipitation has reached 35 cm per year. It may well be that even during the latest Pleistocene and Early Holocene, effective moisture in the Great Basin did
TABLE 3 Identified Taxa Represented in the Homestead Cave Faunal Assemblages Examined in This Analysis Ammospermophilus leucurus Brachylagus idahoensis Dipodomys ordii Dipodomys microps Eutamias sp. Lemmiscus curtatus Lepus californicus Lepus townsendii Marmota flaviventris Microdipodops megacephalus Microtus sp. Neotoma cinerea Neotoma lepida
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Ondatra zibethicus Oynchomys leucogaster Perognathus formosus Perognathus longimembris Perognathus parvus Peromyscus sp. Reithrodontomys megalotis Spermophilus townsendii Sylvilagus audubonii Sylvilagus nuttallii Thomomys bottae Thomomys talpoides
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Meserve, P. L., and Glanz, W. E. (1978). Geographical ecology of small
mammals in the northern Chilean arid zone. Journal of Biogeography 5, 135–148. Nowak, C. L., Nowak, R. S., Tausch, R. J., and Wigand, P. E. (1994). A 30,000 year record of vegetation dynamics at a semi-arid locale in the Great Basin. Journal of Vegetation Science 5, 579–590. Oviatt, C. G. (1997). Lake Bonneville fluctuations and global climate change. Geology 25, 155–128. Owen, J. G. (1988). On productivity as a predictor of rodent and carnivore diversity. Ecology 69, 1161–1165. Owen, J. G. (1990). Patterns of mammalian species richness in relation to temperature, productivity, and variance in elevation. Journal of Mammalogy 71, 1–13. Rhode, D., and Madsen, D. B. (1995). Late Wisconsin/Early Holocene vegetation in the Bonneville Basin. Quaternary Research 44, 246–256. Rosenzweig, M. L. (1992). Species diversity gradients: We know more and less than we thought. Journal of Mammalogy 73, 715–730. Rosenzweig, M. L. (1995). ‘‘Species Diversity in Time and Space.’’ Cambridge Univ. Press, Cambridge. Rosenzweig, M. L., and Abramsky, Z. (1993). How are diversity and productivity related? In ‘‘Species Diversity in Ecological Communities’’ (R. E. Ricklefs and D. Schluter, Eds.), pp. 52–65. Univ. of Chicago Press, Chicago. Stuiver, M., and Reimer, P. J. (1993). Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215–230. Thompson, R. S. (1990). Late Quaternary vegetation and climate in the Great Basin. In ‘‘Packrat Middens: The Last 40,000 Years of Biotic Change’’ (J. L. Betancourt, T. R. Van Devender, and P. S. Martin, Eds.), pp. 200–239. Univ. of Arizona Press, Tucson. Thompson, R. S. (1992). Late Quaternary environments in Ruby Valley, Nevada. Quaternary Research 37, 1–15. Thompson, R. S., Whitlock, C., Bartlein, P. J., Harrison, S. P., and Spaulding, W. G. (1993). Climatic changes in the western United States since 18,000 yr B.P. In ‘‘Global Climate since the Last Glacial Maximum’’ (H. E. Wright, J. E. Kutzbach, T. Webb III, W. F. Ruddiman, F. A. StreetPerrott, and P. J. Bartlein, Eds.), pp. 468–513. Univ. of Minnesota Press, Minneapolis. Tilman, D. (1982). ‘‘Resource Competition and Community Structure.’’ Princeton University Press, Princeton. Tilman, D., and Pacala, S. (1993). The maintainance of species richness in plant communities. In ‘‘Species Diversity in Ecological Communities’’ (R. E. Ricklefs and D. Schluter, Eds.), pp. 13–25. University of Chicago Press, Chicago. U.S. Department of Commerce (1983). ‘‘Climatic Atlas of the United States.’’ Environmental Data Service, U.S. Department of Commerce, Washington, DC. [Reprinted by the National Oceanic and Atmospheric Administration, Washington, DC] Wigand, P. E., and Mehringer, P. J., Jr. (1985). Pollen and seed analyses. In ‘‘The Archaeology of Hidden Cave, Nevada’’ (D. H. Thomas, Ed.), pp. 108–124. Anthropological Papers of the American Museum of Natural History, 61 (1). Wright, D. H., Currie, D. J., and Maurer, B. A. (1993). Energy supply and patterns of species richness on local and regional scales. In ‘‘Species Diversity in Ecological Communities’’ (R. E. Ricklefs and D. Schluter, Eds.), pp. 66–74. Univ. of Chicago Press, Chicago.
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not reach levels sufficient for the decline phase of the curve to be reached. It is also possible that nonanalog combinations of temperature, precipitation, and seasonality during the latest Pleistocene and Early Holocene in this region caused novel relationships between species richness and moisture. Nonetheless, while the unimodal small mammal speciesresponse model clearly applies to some arid areas, the data presented by Brown (1973, 1975) and the paleontological data from Homestead Cave presented here suggest that this model has not applied to the Great Basin since the latest Pleistocene. ACKNOWLEDGMENTS I thank G. J. Kenagy, R. G. Klein, T. E. Lawlor, D. B. Madsen, and R. S. Thompson for extremely helpful comments on an earlier draft of the manuscript; J. M. Broughton, M. D. Cannon, M. A. Etnier, B. Hubler, and L. A. Nagaoka for laboratory assistance; and D. B. Madsen for his assistance, insight, and support throughout this project. The analysis of the Homestead Cave fauna has been supported by grants from the Legacy Project, Department of Defense, and from Hill Air Force Base.
REFERENCES Abramsky, Z., and Rosenzweig, M. L. (1984). Tilman’s predicted productivity–diversity relationship shown by desert rodents. Nature 309, 150– 151. Abramsky, Z., Brand, S., and Rosenzweig, M. L. (1985). Geographical ecology of gerbilline rodents in sand dune habitats of Israel. Journal of Biogeography 12, 363–372. Benson, R. H. (1975). The origin of the psychrosphere as recorded in changes of deep-sea ostracode assemblages. Lethaia 8, 69–83. Brown, J. H. (1973). Species diversity of seed-eating desert rodents in sand dune habitats. Ecology 54, 775–787. Brown, J. H. (1975). Geographical ecology of desert rodents. In ‘‘Ecology and Evolution of Communities’’ (M. L. Cody and J. M. Diamond, Eds.), pp. 315–341. Belknap Press of Harvard Univ. Press, Cambridge, MA. Brown, J. H., and Gibson, A. C. (1983). ‘‘Biogeography.’’ Mosby, St. Louis. Grayson, D. K. (1984). ‘‘Quantitative Zooarchaeology.’’ Academic Press, New York. Grayson, D. K. (1991). Alpine faunas from the White Mountains, California: Adaptive change in the prehistoric Great Basin? Journal of Archaeological Science 18, 483–506. Grayson, D. K. (1993). ‘‘The Desert’s Past: A Natural Prehistory of the Great Basin.’’ Smithsonian Institution Press, Washington, DC. Madsen, D. B., and Currey, D. R. (1979). Late Quaternary glacial and vegetation changes, Little Cottonwood Canyon area, Wasatch Mountains, Utah. Quaternary Research 12, 254–270.
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