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Soil water condition and small mammal spatial distribution in Inner Mongolian steppes, China
Journal of Arid Environments (2003) 54: 729–737 doi:10.1006/jare.2002.1083, available online at http://www.idealibrary.com on
Soil water condition and small mammal spatial distribution in Inner Mongolian steppes, China Guiming Wang%*, Wenqin Zhongw, Qingqiang Zhouw & Zuwang Wangw %
Department of Biological Sciences, Arkansas Tech University, McEver Building, Russellville, AR 72801, U.S.A. wInstitute of Zoology, Chinese Academy of Sciences, 19 Zhongguancun Lu, Beijing 100080, China (Received 6 March 2002, acccepted 11 July 2002) We studied the roles of soil moisture contents and vegetation structure in the spatial distribution of small mammals in the typical steppes of Inner Mongolia, China, using logistic and linear regressions of a data set collected in a 6-year study. Our results indicated that soil moisture contents remained in the most parsimonious models for Spermophilus dauricus, Cricetulus barabensis, Microtus maximowiczii, M. gregalis, and Ochotona daurica. The relative abundance of C. barabensis, M. maximowiczii, and O. daurica was inversely related to soil moisture contents, while that of M. gregalis and S. dauricus was positively related to soil moisture contents in logistic regressions. Linear regression analyses showed that soil moisture contents and the number of small mammal species were inversely related. The negative effects of wet soil were consistent at both small mammal population and community levels in the semi-arid steppes. Above-ground plant biomass and plant coverage also affected the spatial distribution of small mammals in the typical steppe of Inner Mongolia. # 2003 Elsevier Science Ltd. Keywords: small mammals; soil moisture content; spatial distribution; vegetation
Introduction Water is an important limiting factor determining the distribution of organisms in arid and semi-arid areas. Mammals are adapted to various water conditions of habitats by the means of anatomy, physiology, and behavior (Getz, 1968). Some studies have suggested that some small mammals living in mesic habitats had a positive correlation between the water balance of the small mammals and the moisture regime of habitats, whereas small mammal species distributed in xeric habitats had a low requirement of water (Getz, 1968). Moreover, the abundance of volcano rabbits (Romerolagus diazi) was correlated with the environmental conditions of the habitats, including water conditions, along a spatial gradient (Velazquez & Heil, 1996). However, few studies *
Corresponding author. Fax: (479)-964-0837. E-mail: [email protected]
0140-1963/03/040729 + 09 $30.00/0
# 2003 Elsevier Science Ltd.
730
G. WANG ET AL.
investigated the effects of soil moisture conditions of habitats on the relative abundance of small mammals simultaneously at population and community levels. Livestock overgrazing had substantially altered the soil moisture conditions, vegetation, and small mammal communities in the grasslands of Baiyinxile of Inner Mongolia, China, in the past 4 decades (Wang et al., 1997). However, we know little about how the changes in soil moisture contents and the subsequent changes in vegetation under livestock grazing would affect the spatial distribution and community structure of small mammals in a typical steppe at Baiyinxile. Studies of relationships between soil moisture contents and the spatial distribution of small mammals can provide a foundation for understanding the effects of grazing on the community structure of small mammals. The mammal fauna at Baiyinxile was diverse and represented a typical steppe community, including 26 medium and small mammal species (Zhou et al., 1988). The broad spectrum of habitat utilization by small mammals allowed us to study the role of soil water conditions in the spatial distribution of small mammals in semi-arid areas. The Leymus chinensis and Stipa grandis communities were two typical steppe plant communities at Baiyinxile. A sand dune belt, about 40 km long, 10 km wide and 5–20 m tall, stretched across our study area from east to west. Vegetation on the dunes consisted of desert plants on south-facing slopes, shrubs on north-facing slopes, and grasslands in the valleys among sand hills. The sand dune belt provided the suitable habitats for the desert, grassland, and forest rodent species (Zhou et al., 1985). The diverse mammal fauna and heterogeneous landscape at Baiyinxile enabled us to study the effects of soil water conditions on the spatial distribution of small mammals in the semi-arid grasslands of Inner Mongolia. Numerous studies have described and classified the structure and spatial patterns of small mammal communities in Inner Mongolian grasslands (Mi et al., 1990; Wu et al., 1994; Yang, 1989; Zhong et al., 1981; Zhou et al., 1982). Wang (1995) found that vegetation and soil moisture contents were related to the structure of small mammal communities in a principal components analysis (PCA). However, it was difficult to interpret the effects of individual variable on the small mammal spatial distribution in the PCA analysis. Few studies investigated the effects of abiotic and biotic factors on the spatial distribution of small mammals in Inner Mongolia. Objectives of this study were (1) to identify the role of soil moisture contents, plant above-ground biomass, and plant coverage in the spatial distributions of five small mammal species in the typical steppes of Inner Mongolia; and (2) to determine the relationship between the number of small mammal species and soil moisture contents in seven sampling plots from 1980 to 1987.
Study Sites and Methods The study was conducted at Baiyinxile (431260 –441080 N, 1161040 –1171070 E), Inner Mongolia, China. Vegetation at Baiyinxile consisted of two typical steppe communities, the L. chinensis and S. grandis communities. The average annual temperature at Baiyinxile was about 0?11C. Average annual precipitation was about 350 mm, mainly falling in the summer months. Annual precipitation ranged from 182 to 444?4 mm during 1980–1987 (Fig. 1). Snow cover lasted from November to March. Most plant growth occurred from April through August ( Jiang, 1985). Seven permanent sampling plots were established to represent the main plant community types in the Baiyinxile area in 1979. These seven sampling plots were scattered over the study area. The straightline distance between any two plots was from 3 to 30 km. A brief description for each plot is given herein. Plot 1 was located in a valley of rolling hills; the vegetation was dominated by L. chinensis (edificato),
SOIL WATER CONDITION AND SMALL MAMMAL SPATIAL DISTRIBUTION
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Annual precipitation (mm)
500 450 400 350 300 250 200 150 1980 1981 1982 1983 1984 1985 1986 1987
Figure 1. Annual precipitation (mm) at Baiyinxile of Inner Mongolia from 1980 to 1987.
Artemisia frigida, and Cleistogenes squarrosa. Plot 2 was situated on a super flood land, and the dominant species of vegetation were Iris lactea (edificato) and L. chinensis. Plot 3 was on a chain of sand hills in the sand dune belt; the vegetation was dominated by A. intramongolia (edificato), Agropyron fragile, A. cristatum, L. chinensis, and shrubs. Vegetation on sand dunes consisted of desert plants on south-facing slopes, shrubs on north-facing slopes, and grasslands in valleys. Plot 4 was on a terrace of Xilin River; the vegetation was dominated by A. frigida (edificato), C. squarrosa, Carex duriuscula, and A. cristatum. Plot 5 was in an alluvial fan area of Xilin River, and Stipa krylovii (edificato), L. chinensis, and A. frigida were the dominant species of the vegetation on Plot 5. Plot 6 was on a wide plain of 51 slope on low rolling hills; the vegetation was dominated by L. chinensis (edificato), S. grandis, Koeleria cristata, A. cristatum, and Serratula centauroides. Plot 6 represented the typical L. chinensis steppe in the Eurasian continent. Plot 7 was located on a light rolling plain on a basalt platform, and S. grandis (edificato), A. cristatum, A. commutata, A. frigida, L. chinensis, and K. cristata were the dominant species of the vegetation on Plot 7. Plot 7 represented the typical S. grandis steppe in the study area. We used a trap line method described by Xia (1956) to trap small mammals on the study plots. Three hundred and fifty snap traps were placed in two lines, separated by 50 m. Each line had 175 traps, placed singly at 5 m intervals. One large-size wooden snap trap (6?8 cm wide, 16 cm long) was put at each station. The trap lines were laid across various plant communities (i.e. grasses, shrubs, and desert plants) and transitional areas on the sand dunes. Our trapping method might be biased toward seed-eating small mammals. Non-seed-eating small mammals might not be adequately represented in our capture data. The relative abundance of a species on a sampling plot was measured by the number of individuals caught in 1050 trap days. On each study site, trapping was conducted for three consecutive days in July from 1980 to 1987. Above-ground biomass of plants was sampled with a 100 100 cm frame in the end of August from 1980 to 1987. A total of 15 random quadrats were chosen for each sampling plot. Green plants were cut to the ground and sorted by species. All samples of each plant species were put into envelopes separately and then dried in an oven at 651C for 48 consecutive hours. Dried samples were weighed to the nearest 0?1 g, and total plant biomass in each quadrat (g m2) was recorded. Average above-ground biomass of 15 quadrats was used to estimate above-ground biomass for each plant community. Plant coverage was estimated by the percentage of the area covered by green plant leaves in a quadrat. The mean plant coverage of 15 quadrats was the estimate of plant coverage for each sampling plot.
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Soil moisture contents were measured after the vegetation sampling in 1980– 1983,1985, and 1987. We chose five sites for soil samples, separated by 50 m, along the middle line between the two trap lines on each plot. At each site, we took three soil samples at the depth of 35–40 cm, about 20 g each. Each soil sample was put in a covered aluminum container and was immediately brought back to a laboratory for dehydration. Soil samples were weighed and then dehydrated in an oven at 1001C for 48 h. The difference in weight of each soil sample before and after dehydration was the estimate of water weight for each soil sample. The soil moisture content was expressed as the percentage of water weight in total soil weight before dehydration for each soil sample. Arithmetic mean of moisture contents (%) of 15 soil samples was used to estimate soil moisture content for each plot. One common technique for studying relationships between vertebrate abundance and environmental factors is to fit a regression model or equation to data on animal abundance using environmental variables, e.g. plant biomass and plant coverage, etc., as covariates. We employed an information-theoretic selection approach (Burnham & Anderson, 1998) to select the best model among all possible models, using Akaike information criterion (AIC) (Akaike, 1973). We fit logistic regression equations to capture data, i.e. the number of individuals of each species in 1050 trap days, using PROC GENMOD of SAS software (SAS, 1990). The link was logit (McCullagh & Nelder, 1986). The regression equation had the form: lnð p=1 pÞ ¼ a0 þ a1 bms þ a2 cvr þ a3 swt where p was the relative abundance of a species in 1050 trap days, bms above-ground plant biomass, cvr plant coverage, and swt soil moisture contents. a0, a1, a2, and a3 were regression coefficients. Log likelihood function values (log l) from the SAS output were used to compute the value of AIC. AIC = 2log l + 2K, where K was the number of unknown parameters. We examined all regression models of all possible combinations of three explanatory variables, i.e. plant biomass, plant coverage, and soil moisture contents, to select the most parsimonious model with the smallest AIC value (Burnham & Anderson, 1998). Explanatory variables remained in the most parsimonious models were identified as variables affecting the spatial distribution of small mammals. We chose only three explanatory variables for our analysis because a PCA analysis indicated that these three variables might affect the relative abundance of small mammals on our study site (Wang, 1995). Three variables allowed us to examine all possible alternative regression models. On the other hand, the limited number of predictors might dwindle our power to detect the relationship between an environmental factor and the habitat selection of small mammals. We analyzed the capture data of Spermophilus dauricus, Cricetulus barabensis, Microtus maximowiczii, M. gregalis, and Ochotona daurica because these five species were present on 43 sampling plots, not merely associated with one or two types of habitats. We conducted logistic regressions only for the data of 1980 because the abundance of small mammals in the first trapping year was less affected by the removal trapping. We also performed linear regressions of the number of all caught small mammal species ( y) on soil moisture contents (x) across the seven sampling plots for 1980, 1981, 1982, 1983, 1985, and 1987, respectively, using PROC GLM of SAS (SAS, 1990).
Results We caught 11 species of small mammals on the seven sampling plots in 1980. They were S. dauricus, Allactaga sibirica, C. barabensis, Phodopus sungorus, P. roborovskii, Apodemus peninsulae, Dipus sagitta, M. maximowiczii, M. gregalis, Eothenomys shanseius, and O. daurica. The numbers of caught individuals of these species were presented in Table 1. A. sibirica, D. sagitta, E. shanseius, P. sungorus, P. roborovskii, and A. peninsulae
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were only present on plot 3 or plot 4, we excluded these six species from our logistic regression analyses. The relative abundance of S. dauricus, C. barabensis, M. gregaliss, M. maximowiczii, and O. daurica varied across seven plots (Fig. 2). Soil moisture contents ranged from 1?09% to 5?29% across the seven plots in 1980 (Fig. 3a). The coefficient of variation (CV) of soil moisture contents was 0?485. Above-ground plant biomass varied from 82?82 to 180?4 g m2 in 1980 (Fig. 3b). CV of plant biomass was 0?284. The variation of soil moisture contents was greater than that of above-ground plant biomass across plots. The results of logistic regressions showed that soil moisture contents remained in the most parsimonious models for all the five small mammal species (Table 2). The relative abundance of M. maximowiczii, C. barabensis, and O. daurica was inversely related to soil moisture contents in logistic regressions. Plant biomass remained in the selected models of M. maximowiczii and S. dauricus. Plant coverage was selected for the best model of M. gregalis (Table 2). All the coefficients of plant biomass, plant coverage, and soil moisture contents in the selected models were significant ( po0?05). Linear regression analyses showed that the number of small mammals species present on each plot was inversely related to soil moisture contents in 1983, 1985, and
Table 1. Numbers of individuals of 11 small mammal species caught in 1050 trap days on seven plots at Baiyinxile of Inner Mongolia in 1980
Species
Relative abundance
Spermophilus dauricus Allactaga sibirica Cricetulus barabensis Phodopus sungorus Phodopus roborovskii Apodemus peninsulae Dipus sagitta Microtus maximowiczii Microtus gregalis Eothenomys shanseius Ochotona daurica
Plot 1
Plot 2
Plot 3
Plot 4
Plot 5
Plot 6
Plot 7
7 1 42 0 0 0 0 354 71 0 8
2 0 14 0 0 0 0 120 1 0 1
1 1 65 0 33 1 1 7 1 11 14
15 0 21 18 0 0 13 0 0 0 9
17 0 160 7 0 0 0 0 0 0 0
63 0 26 0 0 0 0 6 3 0 13
77 0 25 0 0 0 0 0 0 0 0
0.33 0.30 0.27 0.24 0.21 0.18 0.15 0.12 0.09 0.06 0.03 0.00
C. baracensis S. dauricus O. daurica M. maximowczii M. gregalis
Plot 1
Plot 2
Plot 3
Plot 4
Plot 5
Plot 6
Plot7
Figure 2. Relative abundance of five small mammal species on seven sampling plots in a typical steppe at Baiyinxile of Inner Mongolia in 1980.
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Soil water contents (%)
6 5 4 3 2 1 0 Plot 1 Plot 2 Plot 3 Plot 4 Plot 5 Plot 6 Plot 7
Aboveground biomass (g/m 2)
(a)
(b)
260 240 220 200 180 160 140 120 100 80 60 40 20 0 Plot 1 Plot 2 Plot 3 Plot 4 Plot 5 Plot 6 Plot 7
Figure 3. Soil water contents (a) and above-ground plant biomass (b) of seven sampling plots in a typical steppe at Baiyinxile of Inner Mongolia in 1980. The vertical line was one standard deviation.
Table 2. Logistic regression analyses of relative abundance of five small mammal species on plant biomass, plant coverage, and soil water contents in a typical steppe at Baiyinxile of Inner Mongolia in 1980. logit (p) = ln[p/(1–p)]; bms is the above-ground plant biomass, swt the soil moisture content (%), and cvr the percentage of plant coverage
Species Microtus maximowiczii Microtus gregalis Cricetulus barabensis Spermophilus dauricus Ochotona daurica
Selected model
AIC value
logit ( p) = 16?897 + 0?14 bms 2?267 swt logit ( p) = 7?053 + 0?037 cvr+0?045 swt logit ( p) = 2?896 0?177 swt logit ( p) = 3?357 0?015 bms + 0?511 swt logit ( p) = 4?126 –2?888 swt
2202?42 1451?34 1990?44 2205?99 580?79
1987 ( po0?05, Fig. 4), and marginally negatively related to soil moisture contents in 1980 ( p = 0?05). All the coefficients of soil moisture contents were negative, including insignificant ones of 1981 and 1982. The greater soil moisture contents, the less number of small mammal species.
SOIL WATER CONDITION AND SMALL MAMMAL SPATIAL DISTRIBUTION 12
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5
1980
10
R2 = 0.565, P = 0.05
8 6 4 2 0
Number of species
0
1
2
3
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5
4
5
6
735
1983 R2 = 0.855, P = 0.003
1
2
3
4
5
6
7
7 1981 2 R = 0.037, P = 0.68
1985
6
R2 = 0.637, P = 0.03
5 4 3 2 1 4
6
9 8 7
8
10
12
14
9 1982
1
2
3
8
R2 = 0.205, P = 0.31
5
6
7
R2 = 0.635, P = 0.03
7
6 5 4
4 1987
6 5 4
3 2
3 1
2
3
4
5
6
2
4
6
8
10
12
14
Soil water content (%) Figure 4. Linear regressions of the number of small mammal species on soil water contents across seven sampling plots in a typical steppe at Baiyinxile of Inner Mongolia in 1980–1983, 1985, and 1987.
Discussion Our results demonstrated that the soil moisture content was an important factor determining the relative abundance of the five small mammal species in typical steppes, Inner Mongolia (Table 2). Soil moisture contents remained in the selected best models for all five species. Some studies also suggested that soil moisture contents were important to habitat selection by small mammals. Orrock et al. (2000) reported that red-backed voles (Clethrionomys gapperi) were associated with the mesic soil condition in the Appalachian Mountains, U.S.A. Similarly, Zwank et al. (1997) also found that meadow jumping mice (Zapus hudsonius) in south-central New Mexico selected habitats with moist soils. Dickman (1995) suggested that soil moisture contents could facilitate the burrow construction of shrew Crocidura fuscomurina in arid areas of Namibia, Africa. Velazquez & Heil (1996) also found that volcano rabbits preferred dry habitats. Of the five species, the relative abundance of M. maximowiczii, C. barabensis, and O. daurica was inversely related to soil moisture contents in logistic regressions, indicating that these three small mammal species selected less moist habitats. All of the five small mammal species of our study were subterranean and had burrow systems. Soil moisture contents at the depth of 30–40 cm may be important to the conditions of burrow systems. Alternatively, soil moisture could indirectly affect the spatial distribution of small mammals through altering vegetation. Our results at a community level were consistent with the negative effects of soil moisture contents on the spatial distribution of certain small mammal species in the
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Baiyinxile steppe from 1980 to 1987. The results of the analysis of long-term data showed that the greater soil moisture contents, the less number of small mammal species in the typical steppe communities (Fig. 4). Zhou et al. (1982) also found the Shannon–Wiener diversity index of small mammal communities in our study area was inversely related to soil moisture contents. Dipus sagitta and P. sungorus were found only on the sand dunes (Zhou et al., 1985). Dipus sagitta and P. sungorus were adapted to the xeric conditions. The wet conditions limited the spatial distributions of some small mammal species, e.g. D. sagitta, P. sungorus, M. maximowiczii, C. barabensis, and O. daurica in our study area. Absence or low abundance of these small mammal species may result in the lower species diversity of the small mammal communities in the mesic habitats in some semi-arid areas of Inner Mongolia. Vegetation structure was another important factor affecting the spatial distribution of small mammal species in the steppes. Zhou et al. (1982) detected significant relationships among the Shannon–Wiener diversity index of small mammal communities, plant coverage, and vegetation height in the steppes, Inner Mongolia. However, Wang et al. (2001) did not detect any relationships between small mammal species richness and primary production at Baiyinxile. Microtus maximowiczii selected highly productive habitats under the constraints of soil moisture contents (Table 2, Figs. 2, 3(a,b). On the other hand, the relative abundance of S. dauricus was depressed by high plant production (Table 2, Fig. 3(b)). Tall grasslands usually had high plant biomass production (r = 0?91, p = 0?01, Wang unpubl.). M. maximowiczii and S. dauricus may represent two groups of small mammals at Baiyinxile that selected tall and short vegetation, respectively. Microtus gregalis selected dense vegetation and mesic habitats (Table 2). These results were consistent with previous observations (Zhong et al., 1981). Livestock stocking rate has doubled in our study area in the past two decades. Steppes were overgrazed and became shorter and sparser (Wang et al., 1997). Soil moisture contents of overgrazed steppes were less than those in the steppe excluded from livestock grazing (Wang et al., 1997). Microtus maximowiczii and M. gregalis disappeared from most overgrazed habitats due to short vegetation or dry soil , whereas S. dauricus and C. barabensis became the dominant rodent species in our study area (Wang et al., 1997). Spermophilus dauricus and C. barabensis chose shortgrass or dry-soil habitats. Our study suggested that the changes in the relative abundance and the structure of small mammal communities at Baiyinxile may be the responses to the changes in the structure of plant communities and abiotic environments, e.g. soil water conditions. We thank Guanghe Wang for his assistance in the fieldwork. We also thank Dr William McShea for reviewing the early draft and making helpful comments. Two anonymous reviewers made helpful comments. This study was supported by several grants of the China Natural Science Foundation, the Chinese Academy of Sciences, and the Inner Mongolia Grassland Research Station. The Inner Mongolia Grassland Research Station provided logistic support during this study.
References Akaike, H. (1973). Information theory and an extension of the maximum likelihood principle. In: Petrov, B.N. & Csaki, F. (Eds), Second International Symposium on Information Theory, pp. 267-281. Budapest: Akademiai Kiado. Burnham, K.P. & Anderson, D.R. (1998). Model Selection and Inference: A practical InformationTheoretic Approach. New York: Springer. 353 pp. Dickman, C.R. (1995). Diets and habitat preference of three species of crocidurine shrew in arid southern Africa. Journal of Zoology, London, 237: 499–514.
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Getz, L.L. (1968). Influence of water balance and microclimate on the local distribution of the redback vole and white-footed mouse. Ecology, 49: 276–286. Jiang, S. (1985). An introduction to the Inner Mongolia Grassland Ecosystem Research Station, Academia Sinica. In: Inner Mongolia Grassland Ecosystem Research Station (Ed.), Research on Grassland Ecosystem, No. 1, pp. 1–11. Beijing: Science Press. 251 pp. McCullagh, P. & Nelder, J.A. (1986). Generalized Linear Models. New York: Chapman and Hall. 532 pp. Mi, J., Wang, G. & Wang, C. (1990). Cluster analysis on rodent communities in the eastern desert part of desert-steppe. Acta Theriologica Sinica, 10: 145–150. Orrock, J.L., Pagels, J.E., McShea, W.J. & Harper, E.K. (2000). Predicting presence and abundance of a small mammal species: the effect of scale and resolution. Ecological Applications, 10: 1356–1336. SAS. (1990). SAS/STAT User’s Guide. Cary: SAS Institute Inc. Velazquez, A. & Heil, G.W. (1996). Habitat suitability study for the conservation of the volcano rabbit (Romerolagus diazi). Journal of Applied Ecology, 33: 534–554. Wang, G. (1995). Resource Utilization and Community Organization of Small Mammals in Typical Steppes, Inner Mongolia. Beijing: the Institute of Zoology, the Chinese Academy of Sciences. 126 pp. Wang, G., Zhou, Q., Zhong, Q., Sun, C. & Wang, Z. (2001). Species richness-primary productivity relationship of plants and small mammals in the Inner Mongolian steppes, China. Journal of Arid Environments, 49: 477–484. Wang, M., Zhong, W., Wang, G. & Wang, G. (1997). Cooperative changes of plant-rodent communities in steppe along a grazing gradient. In: Inner Mongolia Grassland Ecosystem Research Station (Ed.), Research on Grassland Ecosystem, No. 5, pp. 32–42. Beijing: Science Press. 291 pp. Wu, X., Shi, D., Liu, Y. & Gao, J. (1994). An analysis of rodent communities in Kubu Qi desert and adjacent region. Acta Theriologica Sinica, 14: 43–50. Xia, W. (1956). On the method for census of rodents. Bulletin of Biology, 6: 16–18. Yang, G. (1989). Preliminary study on structures of rodent communities in steppe desert region in the north of the Yin Mountain, Nei Mongol. Acta Sciebtiarum Universiratis Intramongolicae, 20: 251–239. Zhong, W., Zhou, Q. & Sun, C. (1981). Study on structure and spatial pattern of rodent communities in Baiyinxile typical steppe, Inner Mongolia. Acta Ecologica Sinica, 1: 12–21. Zhou, Q., Zhong, W. & Sun, C. (1982). Study on species diversity of rodent communities in Baiyinxile typical steppe, Inner Mongolia. Acta Theriologica Sinica, 2: 89–94. Zhou, Q., Zhong, W. & Sun, C. (1988). Characteristics of mammalian fauna in Baiyinxile area. In: Inner Mongolia Grassland Ecosystem Research Station (Ed.), Research on Grassland Ecosystem, No. 3, pp. 269-275. Beijing: Science Press. 290 pp. Zwank, P.J., Najera, S.R. & Cardenas, M. (1997). Life history and habitat affinities of meadow jumping mice (Zapus hudsonius) in the middle Rio Grande Valley of New Mexico. The Southwestern Naturalist, 42: 318–322.
Soil water condition and small mammal spatial distribution in Inner Mongolian steppes, China Guiming Wang%*, Wenqin Zhongw, Qingqiang Zhouw & Zuwang Wangw %
Department of Biological Sciences, Arkansas Tech University, McEver Building, Russellville, AR 72801, U.S.A. wInstitute of Zoology, Chinese Academy of Sciences, 19 Zhongguancun Lu, Beijing 100080, China (Received 6 March 2002, acccepted 11 July 2002) We studied the roles of soil moisture contents and vegetation structure in the spatial distribution of small mammals in the typical steppes of Inner Mongolia, China, using logistic and linear regressions of a data set collected in a 6-year study. Our results indicated that soil moisture contents remained in the most parsimonious models for Spermophilus dauricus, Cricetulus barabensis, Microtus maximowiczii, M. gregalis, and Ochotona daurica. The relative abundance of C. barabensis, M. maximowiczii, and O. daurica was inversely related to soil moisture contents, while that of M. gregalis and S. dauricus was positively related to soil moisture contents in logistic regressions. Linear regression analyses showed that soil moisture contents and the number of small mammal species were inversely related. The negative effects of wet soil were consistent at both small mammal population and community levels in the semi-arid steppes. Above-ground plant biomass and plant coverage also affected the spatial distribution of small mammals in the typical steppe of Inner Mongolia. # 2003 Elsevier Science Ltd. Keywords: small mammals; soil moisture content; spatial distribution; vegetation
Introduction Water is an important limiting factor determining the distribution of organisms in arid and semi-arid areas. Mammals are adapted to various water conditions of habitats by the means of anatomy, physiology, and behavior (Getz, 1968). Some studies have suggested that some small mammals living in mesic habitats had a positive correlation between the water balance of the small mammals and the moisture regime of habitats, whereas small mammal species distributed in xeric habitats had a low requirement of water (Getz, 1968). Moreover, the abundance of volcano rabbits (Romerolagus diazi) was correlated with the environmental conditions of the habitats, including water conditions, along a spatial gradient (Velazquez & Heil, 1996). However, few studies *
Corresponding author. Fax: (479)-964-0837. E-mail: [email protected]
0140-1963/03/040729 + 09 $30.00/0
# 2003 Elsevier Science Ltd.
730
G. WANG ET AL.
investigated the effects of soil moisture conditions of habitats on the relative abundance of small mammals simultaneously at population and community levels. Livestock overgrazing had substantially altered the soil moisture conditions, vegetation, and small mammal communities in the grasslands of Baiyinxile of Inner Mongolia, China, in the past 4 decades (Wang et al., 1997). However, we know little about how the changes in soil moisture contents and the subsequent changes in vegetation under livestock grazing would affect the spatial distribution and community structure of small mammals in a typical steppe at Baiyinxile. Studies of relationships between soil moisture contents and the spatial distribution of small mammals can provide a foundation for understanding the effects of grazing on the community structure of small mammals. The mammal fauna at Baiyinxile was diverse and represented a typical steppe community, including 26 medium and small mammal species (Zhou et al., 1988). The broad spectrum of habitat utilization by small mammals allowed us to study the role of soil water conditions in the spatial distribution of small mammals in semi-arid areas. The Leymus chinensis and Stipa grandis communities were two typical steppe plant communities at Baiyinxile. A sand dune belt, about 40 km long, 10 km wide and 5–20 m tall, stretched across our study area from east to west. Vegetation on the dunes consisted of desert plants on south-facing slopes, shrubs on north-facing slopes, and grasslands in the valleys among sand hills. The sand dune belt provided the suitable habitats for the desert, grassland, and forest rodent species (Zhou et al., 1985). The diverse mammal fauna and heterogeneous landscape at Baiyinxile enabled us to study the effects of soil water conditions on the spatial distribution of small mammals in the semi-arid grasslands of Inner Mongolia. Numerous studies have described and classified the structure and spatial patterns of small mammal communities in Inner Mongolian grasslands (Mi et al., 1990; Wu et al., 1994; Yang, 1989; Zhong et al., 1981; Zhou et al., 1982). Wang (1995) found that vegetation and soil moisture contents were related to the structure of small mammal communities in a principal components analysis (PCA). However, it was difficult to interpret the effects of individual variable on the small mammal spatial distribution in the PCA analysis. Few studies investigated the effects of abiotic and biotic factors on the spatial distribution of small mammals in Inner Mongolia. Objectives of this study were (1) to identify the role of soil moisture contents, plant above-ground biomass, and plant coverage in the spatial distributions of five small mammal species in the typical steppes of Inner Mongolia; and (2) to determine the relationship between the number of small mammal species and soil moisture contents in seven sampling plots from 1980 to 1987.
Study Sites and Methods The study was conducted at Baiyinxile (431260 –441080 N, 1161040 –1171070 E), Inner Mongolia, China. Vegetation at Baiyinxile consisted of two typical steppe communities, the L. chinensis and S. grandis communities. The average annual temperature at Baiyinxile was about 0?11C. Average annual precipitation was about 350 mm, mainly falling in the summer months. Annual precipitation ranged from 182 to 444?4 mm during 1980–1987 (Fig. 1). Snow cover lasted from November to March. Most plant growth occurred from April through August ( Jiang, 1985). Seven permanent sampling plots were established to represent the main plant community types in the Baiyinxile area in 1979. These seven sampling plots were scattered over the study area. The straightline distance between any two plots was from 3 to 30 km. A brief description for each plot is given herein. Plot 1 was located in a valley of rolling hills; the vegetation was dominated by L. chinensis (edificato),
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Annual precipitation (mm)
500 450 400 350 300 250 200 150 1980 1981 1982 1983 1984 1985 1986 1987
Figure 1. Annual precipitation (mm) at Baiyinxile of Inner Mongolia from 1980 to 1987.
Artemisia frigida, and Cleistogenes squarrosa. Plot 2 was situated on a super flood land, and the dominant species of vegetation were Iris lactea (edificato) and L. chinensis. Plot 3 was on a chain of sand hills in the sand dune belt; the vegetation was dominated by A. intramongolia (edificato), Agropyron fragile, A. cristatum, L. chinensis, and shrubs. Vegetation on sand dunes consisted of desert plants on south-facing slopes, shrubs on north-facing slopes, and grasslands in valleys. Plot 4 was on a terrace of Xilin River; the vegetation was dominated by A. frigida (edificato), C. squarrosa, Carex duriuscula, and A. cristatum. Plot 5 was in an alluvial fan area of Xilin River, and Stipa krylovii (edificato), L. chinensis, and A. frigida were the dominant species of the vegetation on Plot 5. Plot 6 was on a wide plain of 51 slope on low rolling hills; the vegetation was dominated by L. chinensis (edificato), S. grandis, Koeleria cristata, A. cristatum, and Serratula centauroides. Plot 6 represented the typical L. chinensis steppe in the Eurasian continent. Plot 7 was located on a light rolling plain on a basalt platform, and S. grandis (edificato), A. cristatum, A. commutata, A. frigida, L. chinensis, and K. cristata were the dominant species of the vegetation on Plot 7. Plot 7 represented the typical S. grandis steppe in the study area. We used a trap line method described by Xia (1956) to trap small mammals on the study plots. Three hundred and fifty snap traps were placed in two lines, separated by 50 m. Each line had 175 traps, placed singly at 5 m intervals. One large-size wooden snap trap (6?8 cm wide, 16 cm long) was put at each station. The trap lines were laid across various plant communities (i.e. grasses, shrubs, and desert plants) and transitional areas on the sand dunes. Our trapping method might be biased toward seed-eating small mammals. Non-seed-eating small mammals might not be adequately represented in our capture data. The relative abundance of a species on a sampling plot was measured by the number of individuals caught in 1050 trap days. On each study site, trapping was conducted for three consecutive days in July from 1980 to 1987. Above-ground biomass of plants was sampled with a 100 100 cm frame in the end of August from 1980 to 1987. A total of 15 random quadrats were chosen for each sampling plot. Green plants were cut to the ground and sorted by species. All samples of each plant species were put into envelopes separately and then dried in an oven at 651C for 48 consecutive hours. Dried samples were weighed to the nearest 0?1 g, and total plant biomass in each quadrat (g m2) was recorded. Average above-ground biomass of 15 quadrats was used to estimate above-ground biomass for each plant community. Plant coverage was estimated by the percentage of the area covered by green plant leaves in a quadrat. The mean plant coverage of 15 quadrats was the estimate of plant coverage for each sampling plot.
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Soil moisture contents were measured after the vegetation sampling in 1980– 1983,1985, and 1987. We chose five sites for soil samples, separated by 50 m, along the middle line between the two trap lines on each plot. At each site, we took three soil samples at the depth of 35–40 cm, about 20 g each. Each soil sample was put in a covered aluminum container and was immediately brought back to a laboratory for dehydration. Soil samples were weighed and then dehydrated in an oven at 1001C for 48 h. The difference in weight of each soil sample before and after dehydration was the estimate of water weight for each soil sample. The soil moisture content was expressed as the percentage of water weight in total soil weight before dehydration for each soil sample. Arithmetic mean of moisture contents (%) of 15 soil samples was used to estimate soil moisture content for each plot. One common technique for studying relationships between vertebrate abundance and environmental factors is to fit a regression model or equation to data on animal abundance using environmental variables, e.g. plant biomass and plant coverage, etc., as covariates. We employed an information-theoretic selection approach (Burnham & Anderson, 1998) to select the best model among all possible models, using Akaike information criterion (AIC) (Akaike, 1973). We fit logistic regression equations to capture data, i.e. the number of individuals of each species in 1050 trap days, using PROC GENMOD of SAS software (SAS, 1990). The link was logit (McCullagh & Nelder, 1986). The regression equation had the form: lnð p=1 pÞ ¼ a0 þ a1 bms þ a2 cvr þ a3 swt where p was the relative abundance of a species in 1050 trap days, bms above-ground plant biomass, cvr plant coverage, and swt soil moisture contents. a0, a1, a2, and a3 were regression coefficients. Log likelihood function values (log l) from the SAS output were used to compute the value of AIC. AIC = 2log l + 2K, where K was the number of unknown parameters. We examined all regression models of all possible combinations of three explanatory variables, i.e. plant biomass, plant coverage, and soil moisture contents, to select the most parsimonious model with the smallest AIC value (Burnham & Anderson, 1998). Explanatory variables remained in the most parsimonious models were identified as variables affecting the spatial distribution of small mammals. We chose only three explanatory variables for our analysis because a PCA analysis indicated that these three variables might affect the relative abundance of small mammals on our study site (Wang, 1995). Three variables allowed us to examine all possible alternative regression models. On the other hand, the limited number of predictors might dwindle our power to detect the relationship between an environmental factor and the habitat selection of small mammals. We analyzed the capture data of Spermophilus dauricus, Cricetulus barabensis, Microtus maximowiczii, M. gregalis, and Ochotona daurica because these five species were present on 43 sampling plots, not merely associated with one or two types of habitats. We conducted logistic regressions only for the data of 1980 because the abundance of small mammals in the first trapping year was less affected by the removal trapping. We also performed linear regressions of the number of all caught small mammal species ( y) on soil moisture contents (x) across the seven sampling plots for 1980, 1981, 1982, 1983, 1985, and 1987, respectively, using PROC GLM of SAS (SAS, 1990).
Results We caught 11 species of small mammals on the seven sampling plots in 1980. They were S. dauricus, Allactaga sibirica, C. barabensis, Phodopus sungorus, P. roborovskii, Apodemus peninsulae, Dipus sagitta, M. maximowiczii, M. gregalis, Eothenomys shanseius, and O. daurica. The numbers of caught individuals of these species were presented in Table 1. A. sibirica, D. sagitta, E. shanseius, P. sungorus, P. roborovskii, and A. peninsulae
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were only present on plot 3 or plot 4, we excluded these six species from our logistic regression analyses. The relative abundance of S. dauricus, C. barabensis, M. gregaliss, M. maximowiczii, and O. daurica varied across seven plots (Fig. 2). Soil moisture contents ranged from 1?09% to 5?29% across the seven plots in 1980 (Fig. 3a). The coefficient of variation (CV) of soil moisture contents was 0?485. Above-ground plant biomass varied from 82?82 to 180?4 g m2 in 1980 (Fig. 3b). CV of plant biomass was 0?284. The variation of soil moisture contents was greater than that of above-ground plant biomass across plots. The results of logistic regressions showed that soil moisture contents remained in the most parsimonious models for all the five small mammal species (Table 2). The relative abundance of M. maximowiczii, C. barabensis, and O. daurica was inversely related to soil moisture contents in logistic regressions. Plant biomass remained in the selected models of M. maximowiczii and S. dauricus. Plant coverage was selected for the best model of M. gregalis (Table 2). All the coefficients of plant biomass, plant coverage, and soil moisture contents in the selected models were significant ( po0?05). Linear regression analyses showed that the number of small mammals species present on each plot was inversely related to soil moisture contents in 1983, 1985, and
Table 1. Numbers of individuals of 11 small mammal species caught in 1050 trap days on seven plots at Baiyinxile of Inner Mongolia in 1980
Species
Relative abundance
Spermophilus dauricus Allactaga sibirica Cricetulus barabensis Phodopus sungorus Phodopus roborovskii Apodemus peninsulae Dipus sagitta Microtus maximowiczii Microtus gregalis Eothenomys shanseius Ochotona daurica
Plot 1
Plot 2
Plot 3
Plot 4
Plot 5
Plot 6
Plot 7
7 1 42 0 0 0 0 354 71 0 8
2 0 14 0 0 0 0 120 1 0 1
1 1 65 0 33 1 1 7 1 11 14
15 0 21 18 0 0 13 0 0 0 9
17 0 160 7 0 0 0 0 0 0 0
63 0 26 0 0 0 0 6 3 0 13
77 0 25 0 0 0 0 0 0 0 0
0.33 0.30 0.27 0.24 0.21 0.18 0.15 0.12 0.09 0.06 0.03 0.00
C. baracensis S. dauricus O. daurica M. maximowczii M. gregalis
Plot 1
Plot 2
Plot 3
Plot 4
Plot 5
Plot 6
Plot7
Figure 2. Relative abundance of five small mammal species on seven sampling plots in a typical steppe at Baiyinxile of Inner Mongolia in 1980.
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Soil water contents (%)
6 5 4 3 2 1 0 Plot 1 Plot 2 Plot 3 Plot 4 Plot 5 Plot 6 Plot 7
Aboveground biomass (g/m 2)
(a)
(b)
260 240 220 200 180 160 140 120 100 80 60 40 20 0 Plot 1 Plot 2 Plot 3 Plot 4 Plot 5 Plot 6 Plot 7
Figure 3. Soil water contents (a) and above-ground plant biomass (b) of seven sampling plots in a typical steppe at Baiyinxile of Inner Mongolia in 1980. The vertical line was one standard deviation.
Table 2. Logistic regression analyses of relative abundance of five small mammal species on plant biomass, plant coverage, and soil water contents in a typical steppe at Baiyinxile of Inner Mongolia in 1980. logit (p) = ln[p/(1–p)]; bms is the above-ground plant biomass, swt the soil moisture content (%), and cvr the percentage of plant coverage
Species Microtus maximowiczii Microtus gregalis Cricetulus barabensis Spermophilus dauricus Ochotona daurica
Selected model
AIC value
logit ( p) = 16?897 + 0?14 bms 2?267 swt logit ( p) = 7?053 + 0?037 cvr+0?045 swt logit ( p) = 2?896 0?177 swt logit ( p) = 3?357 0?015 bms + 0?511 swt logit ( p) = 4?126 –2?888 swt
2202?42 1451?34 1990?44 2205?99 580?79
1987 ( po0?05, Fig. 4), and marginally negatively related to soil moisture contents in 1980 ( p = 0?05). All the coefficients of soil moisture contents were negative, including insignificant ones of 1981 and 1982. The greater soil moisture contents, the less number of small mammal species.
SOIL WATER CONDITION AND SMALL MAMMAL SPATIAL DISTRIBUTION 12
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5
1980
10
R2 = 0.565, P = 0.05
8 6 4 2 0
Number of species
0
1
2
3
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5
4
5
6
735
1983 R2 = 0.855, P = 0.003
1
2
3
4
5
6
7
7 1981 2 R = 0.037, P = 0.68
1985
6
R2 = 0.637, P = 0.03
5 4 3 2 1 4
6
9 8 7
8
10
12
14
9 1982
1
2
3
8
R2 = 0.205, P = 0.31
5
6
7
R2 = 0.635, P = 0.03
7
6 5 4
4 1987
6 5 4
3 2
3 1
2
3
4
5
6
2
4
6
8
10
12
14
Soil water content (%) Figure 4. Linear regressions of the number of small mammal species on soil water contents across seven sampling plots in a typical steppe at Baiyinxile of Inner Mongolia in 1980–1983, 1985, and 1987.
Discussion Our results demonstrated that the soil moisture content was an important factor determining the relative abundance of the five small mammal species in typical steppes, Inner Mongolia (Table 2). Soil moisture contents remained in the selected best models for all five species. Some studies also suggested that soil moisture contents were important to habitat selection by small mammals. Orrock et al. (2000) reported that red-backed voles (Clethrionomys gapperi) were associated with the mesic soil condition in the Appalachian Mountains, U.S.A. Similarly, Zwank et al. (1997) also found that meadow jumping mice (Zapus hudsonius) in south-central New Mexico selected habitats with moist soils. Dickman (1995) suggested that soil moisture contents could facilitate the burrow construction of shrew Crocidura fuscomurina in arid areas of Namibia, Africa. Velazquez & Heil (1996) also found that volcano rabbits preferred dry habitats. Of the five species, the relative abundance of M. maximowiczii, C. barabensis, and O. daurica was inversely related to soil moisture contents in logistic regressions, indicating that these three small mammal species selected less moist habitats. All of the five small mammal species of our study were subterranean and had burrow systems. Soil moisture contents at the depth of 30–40 cm may be important to the conditions of burrow systems. Alternatively, soil moisture could indirectly affect the spatial distribution of small mammals through altering vegetation. Our results at a community level were consistent with the negative effects of soil moisture contents on the spatial distribution of certain small mammal species in the
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Baiyinxile steppe from 1980 to 1987. The results of the analysis of long-term data showed that the greater soil moisture contents, the less number of small mammal species in the typical steppe communities (Fig. 4). Zhou et al. (1982) also found the Shannon–Wiener diversity index of small mammal communities in our study area was inversely related to soil moisture contents. Dipus sagitta and P. sungorus were found only on the sand dunes (Zhou et al., 1985). Dipus sagitta and P. sungorus were adapted to the xeric conditions. The wet conditions limited the spatial distributions of some small mammal species, e.g. D. sagitta, P. sungorus, M. maximowiczii, C. barabensis, and O. daurica in our study area. Absence or low abundance of these small mammal species may result in the lower species diversity of the small mammal communities in the mesic habitats in some semi-arid areas of Inner Mongolia. Vegetation structure was another important factor affecting the spatial distribution of small mammal species in the steppes. Zhou et al. (1982) detected significant relationships among the Shannon–Wiener diversity index of small mammal communities, plant coverage, and vegetation height in the steppes, Inner Mongolia. However, Wang et al. (2001) did not detect any relationships between small mammal species richness and primary production at Baiyinxile. Microtus maximowiczii selected highly productive habitats under the constraints of soil moisture contents (Table 2, Figs. 2, 3(a,b). On the other hand, the relative abundance of S. dauricus was depressed by high plant production (Table 2, Fig. 3(b)). Tall grasslands usually had high plant biomass production (r = 0?91, p = 0?01, Wang unpubl.). M. maximowiczii and S. dauricus may represent two groups of small mammals at Baiyinxile that selected tall and short vegetation, respectively. Microtus gregalis selected dense vegetation and mesic habitats (Table 2). These results were consistent with previous observations (Zhong et al., 1981). Livestock stocking rate has doubled in our study area in the past two decades. Steppes were overgrazed and became shorter and sparser (Wang et al., 1997). Soil moisture contents of overgrazed steppes were less than those in the steppe excluded from livestock grazing (Wang et al., 1997). Microtus maximowiczii and M. gregalis disappeared from most overgrazed habitats due to short vegetation or dry soil , whereas S. dauricus and C. barabensis became the dominant rodent species in our study area (Wang et al., 1997). Spermophilus dauricus and C. barabensis chose shortgrass or dry-soil habitats. Our study suggested that the changes in the relative abundance and the structure of small mammal communities at Baiyinxile may be the responses to the changes in the structure of plant communities and abiotic environments, e.g. soil water conditions. We thank Guanghe Wang for his assistance in the fieldwork. We also thank Dr William McShea for reviewing the early draft and making helpful comments. Two anonymous reviewers made helpful comments. This study was supported by several grants of the China Natural Science Foundation, the Chinese Academy of Sciences, and the Inner Mongolia Grassland Research Station. The Inner Mongolia Grassland Research Station provided logistic support during this study.
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