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Changes in vegetation patterns and their effect on Texas kangaroo rats (Dipodomys elator).
Article Type:
Report
Subject:
Kangaroo rats (Food and nutrition)
Kangaroo rats (Protection and preservation)
Wildlife habitat improvement (Research)
Grazing (Environmental aspects)
Habitat destruction (Research)
Authors:
Nelson, Allan D.
Goetze, Jim R.
Watson, Elizabeth
Nelson, Mark
Pub Date:
05/01/2009
Publication:
Name: The Texas Journal of Science Publisher: Texas Academy of Science Audience: Academic; General Format: Magazine/Journal Subject: Science and technology Copyright: COPYRIGHT 2009 Texas Academy of Science ISSN: 0040-4403
Issue:
Date: May, 2009 Source Volume: 61 Source Issue: 2
Topic:
Event Code: 310 Science & research
Geographic:
Geographic Scope: United States Geographic Name: Texas Geographic Code: 1U7TX Texas; 1USA United States

Accession Number:
208630425
Full Text:
Abstract.--Investigations of vegetation in Wichita County, Texas indicate that changes in patterns of grazing and the introduction of non-native plant species may affect populations of the Texas kangaroo rat. Intensely and moderately grazed areas were compared to each other and to a previous investigation involving an ungrazed pasture dominated by introduced Japanese brome (Bromus japonicus). Thirty Dipodomys elator were trapped at the intensely and moderately grazed sites, whereas only two animals were caught on the periphery of the ungrazed site in Wichita County. In addition, the moderately grazed site was compared to the intensely grazed site and no significant differences in vegetative richness or percentages grass and forforb were found between sites. Height of vegetation, percentage bare ground and woody species coverage were significantly different in comparisons between the two grazed sites. Because the two sites contained populations of D. elator, it appears that they can use moderately to heavily grazed habitats as burrow locations and can tolerate significant differences in vegetation height and amount of bare ground and woody vegetations. They rarely use ungrazed sites as habitat and, in a previous investigation, an ungrazed site was significantly different from the grazed sites in vegetational height, percentage bare ground, and percentage grass coverage. Grazing regimes, amount of bare ground coverage, and introduction of tall, dense-growing grasses may be important considerations in managing habitat for Texas kangaroo rats. Moderately to heavily grazed sites may provide better habitat for these state-threatened mammals.

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Dipodomys elator (Merriam 1894) is a state threatened species. The International Union for the Conservation of Nature (1986) lists habitat loss and degradation resulting from agricultureagricultural and infrastructure development as major threats. Though degradation such as fragmentation and loss of habitat have played important roles, changes in vegetation patterns may also be important. Much historic range of the Texas kangaroo rat has been fragmented by extensive cultivation within the Rolling Plains Region of Texas and adjacent regions of Oklahoma (Correll & Johnston 1970).

Cultivation fragmented the grasslands and only those areas unsuitable for cultivation were left in their natural state. These fragments were fenced by ranchers and grazed by cattle and provided some habitat for the Texas kangaroo rat. However, because of a decline in ranching, some of these pasturelands are no longer grazed and have been invaded by introduced species such as Japanese brome. Additionally, lack of fire has allowed the increase of woody vegetation such as honey mesquite (Prosopis glandulosa) and lotebush (Ziziphus obtusifolia). In some cases, Texas kangaroo rats used these woody species as burrow sites because the plants collected wind-blown soil and the Texas kangaroo rats dig burrows at their bases. However, as mesquites mature, their shade changes the vegetation composition sometimes favoring introduced grasses like Japanese brome, which grows densely and changes the habitat so that it is more suitable for other types of small mammals.

Mesquite forest was not seen as a problem by the Texas Parks and Wildlife Department (TPWD) because of the assumption that this habitat was required for Texas kangaroo rats. However, perceptions regarding threats to the species and ideas about future management have changed. Research has suggested that (1) mesquite is not a critical component of D. elator habitat (Stangl et al. 1992; Goetze et al. 2007), (2) grazing may benefit D. elator (Stangl et al. 1992; Stasey 2005; Goetze et al. 2007), and (3) Texas kangaroo rats opportunistically use human structures that collect friable soils as burrow sites (Stangl et al. 1992; Stasey 2005; Goetze et al. 2007). Schmidly (2004), which is used by TPWD as their main source of small mammal data for the state, states that heavily grazed rangeland and the eroded sites of rangeland roadways may provide optimum habitat. NatureServe (2008) states that vegetation has become overgrown and that rangeland practices that result in dense growth of grasses or invasion of non-native grasses have degraded habitat because the Texas kangaroo rat thrives in heavily grazed or otherwise disturbed conditions. NatureServe (2008) further states that habitat for D. elator consists of sparsely vegetated areas that may or may not include honey mesquite, including heavily grazed land, disturbed areas, and areas along fencerows adjacent to cultivated fields and roads. These current statements are quite different from Davis & Schmidly (1994), who stated that Texas kangaroo rat burrows invariably entered the ground at the base of a mesquite and the primary threat contributing to the rarity of the species was the clearing of mesquite brush.

Currently, the only relatively large populations of Texas kangaroo rats known in Wichita County occur in pastures with small or scattered mesquite, and burrows are often not associated with mesquite at all, but rather with lotebush, prairie mounds (natural, elevated, and relatively bare areas possibly uplifted by clay soils swelling in cracks; Diggs et al. 1999), or in areas where man-made berms occur due to road, fence, and oilfield construction, or in association with old (>30 years), unburned brush piles where wood has decayed leaving a mound of loose friable soil (Stangl et al. 1992; Goetze et al. 2007). Stangl et al. (1992) hypothesized that grazing bison and prairie dogs, along with fire, historically maintained the type of disturbances needed by the Texas kangaroo rat. Also, prolonged drought likely played an important role in fire frequency and maintaining short vegetation with intermittent bare patches of soil. Natural prairie heterogeneity such as prairie mounds (Diggs et al. 1999; Goetze et al. 2007) appear to be important in providing the type of habitat needed by the Texas kangaroo rat before cattle grazing and human mediated disturbances were used opportunistically as burrow sites (Stangl et al. 1992). In areas where cattle no longer graze or at sites where native vegetation has been replaced by introduced species, it appears that populations of Texas kangaroo rats have declined.

Ecological characterization of burrows in situations that lacked grazing as a component are rare (Martin & Matocha 1992; Stasey 2005). Martin & Matocha (1992) trapped for 10 trap nights and characterized a burrow where a single Texas kangaroo rat was trapped in association with a fence row adjacent to a gravel county road in Motley County, Texas. No other mammals were captured. This capture site contained vegetation characteristic of a disturbed site and was adjacent to a field of sudan grass. At the capture site there was 30.2% bare ground, 65% grasses and 4.8% forb (Martin & Matocha 1992).

Stasey (2005) trapped for 972 trap nights in a mesquite forest with an understory dominated by Japanese brome. His site was within 4.0 km of a large and persistent Texas kangaroo rat population in Wichita County where the grazing regime is heavy (Goetze et al. 2007). During his investigation, he caught only two kangaroo rats on the periphery of the mesquite forest habitat. One was caught in a friable clay soil that had blown in and accumulated around the corner posts that supported the gate leading into the pasture and the other was captured along the fence separating this pasture from a wheat field where a berm had accumulated due to plowing next to the fence line. Stasey (2005) caught Sigmodon hispidus, Peromyscus leucopus, and P. maniculatus in the core of the ungrazed site. In the core of the ungrazed pastureland, where no Texas kangaroo rats were captured, seven quadrats were sampled to assess vegetation characteristics. Percent bare ground had a mean of 10.9%, grasses 63.7%, and forbs 16.1%, whereas the mean average herbaceous height was 49.0 cm (Stasey et al. 2005).

Because identifying habitat critical to the survival of the species is a research priority (Jones et al. 1988), the purpose of this investigation was to compare a moderately grazed site to that of a heavily grazed site, both of which have populations of kangaroo rats. These data were then compared to data from ungrazed sites dominated by Japanese brome (Stasey 2005) and vegetation associated with a disturbed roadside (Martin & Matocha 1992).

MATERIALS AND METHODS

The study area is in Wichita County on the east side of the intersection of highways 1739 and 2384 and is a privately owned ranch that is moderately grazed pasture (0.30 head per ha). Coordinates at the entrance of the ranch are 34.05423 N, 98.81721 W. The pastureland is fenced, has small mesquite (less than 2 m. in height), and has several old oil field storage sites. Mesquite density at the site is 168/ha. This site was compared to a nearby population of Texas kangaroo rat known in Wichita County and its locality and history have previously been described (Stangl et al. 1992; Goetze et al. 2007). Grazing at this site is intense (0.81 head/ha) and it has about 54 small (less than 2m in height with most under 1 m) mesquite/ha (Goetze et al. 2007).

All trapping was done using 7.5 by 8.8 by 30 cm Sherman traps with rolled oats as bait. Traps were set just before dark and checked early the next day. Based on parameters set by Stangl et al. (1992) and Stasey (2005) regarding burrow entrance diameter, angle of entry, and vegetation, suspected burrows were selected at the sites and three traps were placed around each burrow entrance.

All vegetation data was quantified in May so that direct comparisons could be made, thus eliminating seasonal vegetative changes. For burrows where at least one Texas kangaroo rat was caught, one square meter quadrats were centered around burrow entrances and percentage cover, grass, forb, bare ground, and woody vegetation (when present) was recorded (Goetze et al. 2007). Vegetative richness and height were measured and the dominant grass, forb, and woody species of each quadrat were identified (Goetze et al. 2007). Quantitative data was compared using SPSS 14.0 (SPSS, Inc. 2005). A Mann Whitney test was used to test for significant differences in richness and percentages of grasses, forbs, bare ground, and woody vegetation between the two grazed sites. In addition, data from a previous study (Stasey 2005) at a site that was ungrazed was included in a Kruskal-Wallis analysis to test for significant differences between the grazed and ungrazed sites.

Dominant vegetation at the site was identified using floras for the state and for north central Texas (Correll & Johnson 1970; Diggs et al. 1999). Voucher specimens are deposited in the Tarleton State University Herbarium (TAC).

The specific location of each burrow was recorded in decimal degrees using a Garmin GPS-12 unit and burrows were classified as being associated with human-mediated disturbances such as old brush piles and fence rows, or other available natural habitats that included prairie mounds, which are elevated, open areas formed by clay soil shrinkage and swelling (Diggs et al. 1999), or accumulation of soil at the base of lotebushes or honey mesquites (Goetze et al. 2007; Table 1).

RESULTS

At the moderately grazed site, three traps were placed around each of 12 burrows which resulted in the capture of 10 D. elator. No other rodents were captured. Three of the animals were caught in each of the traps placed at one burrow and two were caught in the three traps surrounding another burrow site. The dominant grass at five of the seven burrows was little barley (Hordeum pusillum) with buffalograss (Buchloe dactyloides) and rescue grass (Bromus catharticus) being dominant at the other two burrows (Table 1). The dominant forb at six of the seven burrows was Texas broom weed (Gutierrezia texana) with Virginia pepperweed (Lepidium virginicum) being dominat at the other burrow (Table 1). Five of the seven quadrats had honey mesquite (Prosopis glandulosa) as the dominant woody vegetation and two contained no woody vegetation (Table 1). Two burrows were associated with fence rows and five with honey mesquite (Table 1).

At the heavily grazed site, three traps were placed around each of 22 burrows which resulted in the capture of 18 D. elator. Of these, 10 burrow sites were analyzed for vegetation and burrow associations. At the heavily grazed site, little barley was always the dominant grass and most quadrats contained Virginia pepperweed as the dominant forb (Table 1). Other herbaceous dominants included Texas broomweed, hog potato (Hoffmannseggia glauca), and western ragweed (Ambrosia psilostachya) (Table 1). Woody vegetation was evenly distributed between lotebush and honey mesquite. At the heavily grazed site, five burrows were associated with old brush piles, two with prairie mounds, one with a fence row, and one each with lotebush and honey mesquite (Table 1).

Average herbaceous height, percentage bare ground, and percentage woody vegetation were significantly different between heavily and moderately grazed sites (Table 2). Percentage forbs, grasses, and other categories such as rocks and stumps, as well as richness were not significantly different between sites (Table 2).

Average herbaceous height (P = 0.001), percentage bare ground (P = 0.009) and grasses (P = 0.009) were significantly different when comparing the moderately and heavily grazed sites as well as the ungrazed site examined by Stasey (2005).

DISCUSSION

Little barley, Texas broomweed, hog potato, Virginia pepperweed, and western ragweed occur in disturbed habitats (Diggs et al. 1999). These plants were dominant species associated with burrows of Texas kangaroo rats and their occurrence is likely caused by disturbances such as grazing of cattle and rodent activity around the burrows. Habitat of D. elator was dominated by short, herbaceous vegetation (2.0 - 40.0 cm in height). There is general agreement that D. elator requires a sparse, short-grassland habitat (Dalquest & Collier 1964; Roberts & Packard 1973; Carter et al. 1985; Stangl et al. 1992), and findings from this current study support this conclusion. These findings also indicate that grazing may be important in maintaining sparse, short grassland habitat. When comparing two grazed sites and an ungrazed site (Stasey 2005), the only significant differences were in average herbaceous vegetation height and percentage bare ground and grass. Grazing can change these three parameters, which appear to be important in maintaining Texas kangaroo rat habitat. As previously discussed, this is complicated by the dominance of the introduced grass, Bromus japonicas at the ungrazed site and additional studies need to be conducted at ungrazed sites containing native vegetation. However, based on the lack of D. elator at this site and the relative abundance at the grazed sites, it appears that grazing plays a role in maintaining suitable habitat for Texas kangaroo rats. Lack of grazing significantly increased vegetation height at the ungrazed site, which on average was double the moderately grazed site and seven times greater than the heavily grazed site. Percentage of grasses was about three times greater on average at the ungrazed site when compared to grazed sites. Percentage bare ground, on average was about one-half that of the moderately grazed site and about one-fourth that of the heavily grazed site. This tall, dense coverage by grasses may impede Texas kangaroo rat movement, inhibit their ability to see potential predators, and may make burrow construction difficult. Lack of bare ground likely inhibits their dust bathing activities.

Burrows at the grazed site compare favorably to the burrow ecological characteristics reported from Motley County (Martin & Matocha 1992). In the classification system used for burrows and described in the Materials and Methods, this burrow would have been a fence line association and the animals were likely using soil that accumulated at the base of the fence. Although no grazing was reported at this site, the value reported for bare ground percentage (30.2%) compares favorably with the grazed sites in this investigation but forb percentage (4.8%) was low and grass percentage was high (65%). The location of this burrow in a fence row berm may have provided the friable soil preferred for burrow development. Its location at the edge of a sudan field adjacent to a gravel road (Martin & Matocha 1992) may have provided enough disturbance to maintain bare patches for dust bathing and for foraging trails that still allow the rats to spot predators.

Of the 17 burrows examined in this investigation, 47% were associated with human disturbances including old brush piles and fence rows. The other burrows were associated with more natural sites such as shrubs and prairie mounds. In the heavily grazed site most burrows were associated with 30 year old brush piles, while at the moderately grazed site most were associated with honey mesquite. Second in number of burrow associations at the heavily grazed site was prairie mounds and none of this association was observed at the moderately grazed site. Heavy grazing might make prairie mounds more suitable as burrow sites, since that was the only place this habitat association occurred. At the heavily grazed site, lotebush and honey mesquite associations were equal in number while no lotebush associations were observed at the moderately grazed site. This is likely because the moderately grazed site had woody vegetation dominated by honey mesquite whereas the heavily grazed site had some lotebush available. The type of shrub is probably not as important as is the accumulation of loose, friable soil at the base of the shrubs.

Extrinsic disturbance caused by grazing, fire, or drought, and natural landscape heterogeneity such as prairie mounds probably is important for burrow site selection in Texas kangaroo rats. The slight elevation of prairie mounds may provide more bare ground because of a drier microclimate and better drainage. Also, the animals can dig their more characteristically horizontal openings into the sides of these mounds. Opportunistic use of any natural or manmade disturbance where friable soil accumulates such as around shrubs, the bases of rocks, fence lines, cattle pens, pasture and oil field roads, abandoned equipment, old, unburned brush piles with most of the wood decayed have been observed (Stangl et al. 1992; Goetze et al. 2007). Almost one-half of the burrows in this investigation may be characterized as such, supporting hypotheses of opportunistic use made by others (Stangl et al. 1992; Stasey 2005; Goetze et al. 2007). These manmade disturbances likely mimic natural prairie heterogeneity.

Investigations of Texas kangaroo rats that examine symbiosis with prairie dogs, effects of fire in maintaining habitat, the role of drought on habitat, and additional research into the influence of natural prairie heterogeneity and grazing regimes are critical for understanding the animal's niche. Surveys need to be conducted at Buffalo Creek Reservoir and Lake Arrowhead State Park to ascertain if D. elator occurs in any protected natural areas (pnas) in Wichita County or if not, if suitable habitat is available in protected natural areas. Also, based on the results of this investigation, managers of pnas may need to consider grazing as a management practice to promote the development of habitat for the Texas kangaroo rat and other organisms that require grazing as a disturbance.

ACKNOWLEDGMENTS

We would like to thank Oscar and Edith Goetze for allowing access to their properties in Wichita County and for room and board while conducting fieldwork. Tarleton State University Organized Faculty Research provided funding for parts of this project. This study was conducted under Texas Parks and Wildlife permit SPR-0496-775.

LITERATURE CITED

Carter, D. C., W. D. Webster, J. K. Jones, Jr., C. Jones & R. D. Suttkus. 1985. Dipodomys elator. Mammalian Species, 232:1-3.

Correll, D. S. & M. C. Johnston. 1970. Manual of the vascular plants of Texas. Texas Research Foundation. Renner, Texas. 1083 pp.

Dalquest, W. W. & G. Collier. 1964. Notes on Dipodomys elator, a rare kangaroo rat. Southwestern Nat., 9:146-150.

Davis, W. B. & D. J. Schmidly. 1994. The mammals of Texas. Texas Parks and Wildlife Press. Austin, TX, 338 pp.

Diggs, G. M., B. L. Lipscomb & R. J. O'Kennon. 1999. Shinners & Mahler's Illustrated Flora of North Central Texas. Botanical Research Institute of Texas. Fort Worth, Texas, 1626 pp.

Goetze J. R, W. C. Stasey, A. D. Nelson, & P. D. Sudman, 2007. Habitat attributes and population size of Texas kangaroo rats on an intensely grazed pasture in Wichita County, Texas. The Texas J. of Sci., 59(1): 11-22.

International Union for Conservation of Nature and Natural Resources. 1986. 1986 IUCN red List of threatened animals. IUCN. Cambridge, U. K., 105 pp.

Jones, C., M. A. Bogan & L. M. Mount. 1988. Status of the Texas kangaroo rat (Dipodomys elator). The Texas J. of Sci., 40(3):249-258.

Martin, R. E. & K. G. Matocha. 1991. The Texas kangaroo rat, Dipodomys elator, from Motley County, Texas, with notes on habitat attributes. Southwestern Nat., 36:354-356.

Merriam, C. H. 1894. Preliminary descriptions of eleven new kangaroo rats of the genera Dipodomys and Perodipus. Proc. Biol. Soc. Washington, 9:109-116.

NatureServe. 2008. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.0. NatureServe, Arlington, Virginia. Available http://www.natureserve.org/explorer. (Accessed: September 4, 2008).

Roberts, J. D. & R. L. Packard. 1973. Comments on movements, home range and ecology of the Texas kangaroo rat, Dipodomys elator Merriam. J. Mamm., 54:957-962.

Schmidly, D. J. 2004. The mammals of Texas, revised edition. University of Texas Press, Austin, Texas, 501 pp.

Stangl, F. B., Jr., T. S. Schafer, J. R. Goetze & W. Pinchak. 1992. Opportunistic use of modified and disturbed habitat by the Texas kangaroo rat (Dipodomys elator). The Texas J. of Sci., 44(l):25-35.

Stasey, W. C. 2005. An evaluation of Texas kangaroo at (Dipodomys elator): Biological habits and population estimation. Unpublished Masters Thesis. Tarleton State University, 45 pp.

SPSS, Inc. SPSS 14.0 brief guide. 2005. Prentice Hall. Upper Saddle River, New Jersey, 245 pp.

ADN at: nelson@tarleton.edu

Allan D. Nelson, Jim R. Goetze *, Elizabeth Watson and Mark Nelson

Department of Biological Sciences, Box T-0100, Tarleton State University, Stephenville, Texas 76401 and * Science Department, Laredo Community College Laredo, Texas 78040
Table 1. Dominant vegetation and burrow classifications at the two
study sites. Dominant forbs are broomweed (Gutierrezia texana), hog
potato (Hoffmannsegia glauca). pepperweed (Lepidium virginicum), and
ragweed (Ambrosia psilostachya). Dominant grasses are barley (Hordeum
pusillum), buffalo grass (Buchloe dactyloides), and rescue grass
(Bromus catharticus). Woody vegetation includes lotebush (Zizyphus
obtusifolia) and mesquite (Prosopis glandulosa). Burrow classifications
included in this table are defined in the methods section of this
paper.

Moderately grazed burrows:

  Burrow #          1    2    3    4    5    6    7

  FORBS
  Broomweed         X    X         X    X    X    X
  Pepperweed                  X
  GRASS
  Barley            X    X    X         X         X
  Buffalo                          X
  Rescue                                     X
  WOODY VEG.
  Mesquite          X    X    X    X              X
  None                                  X    X
  Burrow assoc.
  Fence                                 X    X
  Mesquite          X    X    X    X              X

Heavily grazed burrows:

  Burrow #          1    2    3    4    5    6    7    8    9    10

  FORBS
  Broomweed                                  X
  Hog potato                                      X
  Pepperweed        X    X    X         X              X         X
  Ragweed                                                   X
  Unknown                          X
  GRASS
  Barley            X    X    X    X    X    X    X    X    X    X
  WOODY VEG.
  Lotebush          X
  Mesquite                                        X
  None                   X    X    X    X    X         X    X    X
  Burrow assoc.
  Fence                            X
  Lotebush          X
  Mesquite                                        X
  Old brush pile                        X    X         X    X    X
  Prairie mound     X    X


Table 2. Comparison of heavily and moderately grazed sites for average
herbaceous height, percentage coverage of bare ground, forbs, grasses,
woody, other (rocks, stumps, posts, etc.), and richness. Comparisons
are made using means, standard deviations (in parentheses), and ranges
[in brackets] and evaluated using Wilcoxon Mann-Whitney test.
Significant differences at P < 0.05 are denoted by an asterick.

               Heavily Grazed         Moderately Grazed      P-value

Avg. Herb.      7.1 ([+ or -] 7.9)    24.5 ([+ or -] 12.7)  0.0001 *
Height         [2.0-40.0]             [9.0-29.4]

% Bare Ground  49.9 ([+ or -] 24.0)   20.7 ([+ or -] 18.1)  0.024 *
               [0.0-80.0]             [5.0-60.0]

% Forbs        16.5 ([+ or -] 13.1)   33.7 ([+ or -] 22.6)  0.133
               [1.0-35.0]             [15.0-67.0]

% Grasses      24.60 ([+ or -] 18.9)  20.1 ([+ or -] 15.6)  0.623
               [1.0-55.0]             [1.0-45.0]

% Woody         6.0 ([+ or -] 15.8)   26.1 ([+ or -] 26.5)  0.037 *
               [0.0-50.0]             [0.0-60.0]

% Other         2.0 ([+ or -] 4.2)     0.0 ([+ or -] 0.0)   0.222
               [0.0-2.0]              [0.0-0.0]

Richness       6.2 ([+ or -] 2.4)      7.6 ([+ or -] 1.8)   0.137
               [3.0-10.0]             [6.0-10.0]
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