Habitat attributes and population size of Texas kangaroo rats on an intensely grazed pasture in Wichita County, Texas.
The Texas kangaroo rat (Dipodomys elator) is listed as a threatened species by the Texas Parks and Wildlife Department (Schmidly 2004). Reasons for D. elator being listed are largely based on its apparent scarcity and small geographic range. The International Union for Conservation of Nature (1986) lists habitat loss and degradation resulting from expanding agricultural and infrastructure development as the major threats to continued existence of D. elator.
An association between honey mesquite (Prosopis glandulosa) and the Texas kangaroo rat has been well documented (Bailey 1905; Blair 1954; Carter et al. 1985; Chapman 1972; Dalquest & Collier 1964; Martin & Matocha 1972, 1991; Roberts & Packard 1973; Schmidly 2004). More recently Stangl et al. (1992) noted that extensive stands of mesquite within the range of D. elator may not be as critical as once believed for survival and persistence of the Texas kangaroo rat. Also, while working at the Wichita County study site and another nearby site, Stangl et al. (1992) discussed opportunistic use of habitat affected by human disturbances. However, during that investigation, characteristics of burrows and size of populations was not quantified. Obtaining information on habitat characteristics critical to species survival and documenting demographic changes of D. elator populations must be accomplished to evaluate the conservation status of the Texas kangaroo rat (Jones et al. 1988). Therefore, the purposes of this study were to locate potential burrows of D. elator within this study site, relate associations of burrows with soils and human disturbance, quantify vegetation associated with burrows, and examine whether there is a correlation between number of burrows and size of population.
The study site, located in Wichita County, Texas, 3 km N Lake Buffalo Creek Reservoir (location in decimal degrees; 34.02653 N, 98.75991 W), was previously described by Stangl et al. (1992) as study site 2--Goetze property. The grazing regimen of this site has remained virtually unchanged since described by Stangl et al. (1992). The 15-ha area is fenced and bordered by wheat fields on its eastern and western sides and contains constructed earthen ponds and erosion features, such as rills and gullies. There is additional habitat occupied by Texas kangaroo rats to the north and south of this study site and it probably is open to migration from these areas.
Beginning in 1975, the study site was mechanically cleared of brush and subsequently sprayed with an herbicide four times over five year intervals. This brush was piled into mounds scattered throughout this pasture (Stangl et al. 1992), but was left unburned. Over a period of 30 years, the brush decayed and collected soil to form low earthen mounds about 0.5 m in height. A few dead stumps still persist in most of these elevated brush piles. As a result of mechanical and chemical treatments, brush at the site is short (1-2 m in height) but still relatively dense. A count of 54 honey mesquites, two lotebushes, and two net-leaf hackberry (Celtis laevigata var. reticulata) in a one-ha quadrat was considered typical of the site. About 30 large, sandstone rocks are associated with constructed earthen dams surrounding the three ponds within the study site.
The study site was surveyed for burrows of the Texas kangaroo rat by the investigators walking about 3 m apart over the entire area. The survey occurred on 18-26 May 2005. Burrows belonging to D. elator were identified based on diameter and orientation of entrance/exit hole (see Fig. 3 in Stangl et al. 1992). Stasey (2005) noted a significant difference between size of burrow entrance and angle of entry to the burrow between D. elator and other rodents of similar size. Distinct trails and dust-bathing areas led away from these burrows and these runways sometimes connected to other distant burrows. The specific location of each burrow was recorded in decimal degrees using a Garmin GPS-12 unit and were categorized as being associated with human-mediated disturbances such as old brush piles, fence rows, and rocks associated with constructed earthen dams, or other available habitats that contained elevated, open areas, lotebushes, or honey mesquites.
A geo-referenced base map was produced with a Manifold 5.0 (Manifold System Ltd, 2003) GIS system using a 1-m resolution, digital orthophoto quadrangle obtained from IntraSearch, Denver, Colorado. This digital map was loaded into a Manifold 5.50 GIS system. Spatial and habitat data were entered into data tables and imported onto the base map as layers. A geo-referenced drawing of the major soil series of the study site was added to the layered map to ascertain whether a specific type of soil was most favored by the Texas kangaroo rat. Information about soils was obtained from the soil survey of Wichita County, Texas (Richardson et al. 1977). Area of the study site and the extent of soil coverage (in [m.sup.2]) were obtained using the Tracker function in the Manifold software.
Vegetation was sampled during May 2005. A 1-[m.sup.2] quadrat made from PVC pipe was placed directly over 10 burrows where D. elator was captured. Within each quadrat, vegetative richness was recorded as total number of species present (Brower et al. 1990). Percentage coverage of grass, forbs, bare ground, woody vegetation, and rocks or stumps within each quadrat were recorded, as was average herbaceous vegetation height (obtained by averaging the height of the herbaceous vegetation 15 cm interior to each corner of the quadrat). If woody vegetation was present, its height also was recorded. To quantify vegetation of habitat away from burrows, 20 m north-to-south and east-to-west transects, bisecting at the burrow entrance, were evaluated. Vegetation type (grass, forb, bare ground, woody, or other), height, and vertical intercept (distance from ground to woody species in cm directly over meter-point) was recorded at each meter-point for a total of 40 data points for each burrow (Brower et al. 1990). Specimens of the dominant herbaceous and woody plants were collected and placed in a plant press. These vegetation vouchers were deposited in the herbarium of Tarleton State University (TAC).
Ordinal vegetation percentage data between quadrats and transects were compared within the study site with the Wilcoxon Mann-Whitney test (SAS Institute 1999). A paired t-test (SAS Institute 1999) was used to compare vegetational height. Herbaceous height and percentages of grasses, forbs, bare ground, woody, and other categories were all evaluated for statistically significant (P<0.05) differences.
To test accuracy of counting burrows to estimate population size, trapping was conducted by placing three 7.5 by 8.8 by 30 cm Sherman Live Traps within 0.10 to 0.50 m of each burrow entrance, with the open end of each trap facing the entrance (Cross & Waser 2000). Traps were baited with dry oatmeal. Trapping was conducted during these dates in 2005 (parentheses indicate number of trap nights): May 18-24 (178), June 21-25 (210), July 6-7 (84), and July 19-22 (168).
Captured animals were tagged with passive integrated transponders (PIT tags) to determine rates of recapture for specific individuals. PIT tags were implanted subcutaneously immediately posterior to the cranium by means of a syringe. To minimize handling time and other stresses, anesthesia was not used (Schooley et al. 1993). Syringes were sterilized between implantations with 91% alcohol. Each animal's total length, gender, and reproductive condition were recorded at the time of capture. As a result of trap mortality, one individual was prepared as a skin and skull voucher specimen (TSU 1294) on 20 May 2005. Program MARK was used to estimate size of population from the trapping data (White & Burnham 1997) with the integrated POPAN program using the Jolly-Seber algorithm and assuming an open population with equal probabilities of capture and survival.
Of the two major soil associations within the study area, most D. elator burrows (56) were in Kamay silt loam soils (9.8 ha) and the remaining 10 burrows occurred in Asa-Portales soils (5.2 ha). Twenty-one of the 22 Texas kangaroo rats were captured in the Kamay soil association.
Of the vegetation sampled within quadrats, mean percentage cover of grasses was 25.1 (range 1-55) and little barley (Hordium pusillum) was always the dominant grass (Table 1). Mean percentage cover of forbs was 17 (range 1-35) and most quadrats contained Virginia pepperweed (Lepidium virginicum). Other herbaceous dominants included common broomweed (Gutierrezia dracunculoides), hog potato (Hoffmannseggia glauca), and western ragweed (Ambrosia psilostachya). Mean percentage cover of woody vegetation was six (range 0-50), evenly distributed between lotebush and honey mesquite. Mean percentage of bare ground was 49.9 (range 0-80) and stumps comprised a minor component of the habitat. Mean percentage richness was 5.8 (range 3-10). Mean herbaceous height was 7.1 cm (range 2-40 cm) and mean woody height was 15.9 cm (range 20-121 cm).
There was significantly more bare ground within quadrats (49.9%) than along transects (22.3%; Table 2). Likewise, there was significantly less grass within quadrats (25.1%) than along transects (54.3%). There were no significant differences between other compared parameters (Table 1).
Sixty-six active burrows were found within the study site and the burrow associations were as follows: Four active burrows were found at or near the bases of honey mesquite trees, and five active burrows were underneath large, sandstone rocks. Six active burrows were underneath lotebushes. Seven active burrows had been constructed in the elevated soils associated with fence rows. Nineteen active burrows were found in elevated, open areas, and soils associated with 30 year old brush piles contained the greatest number (25) active burrows.
A total of 640 trap nights was conducted within the sampling area. Forty-five captures of D. elator, representing 18 individuals, were obtained. Ten Chaetodipus hispidus were captured over the course of the trapping period. Also, six Spermophilus tridecemlineatus, four Neotoma micropus, and two Peromyscus leucopus were captured. The 18 Texas kangaroo rats were captured at 22 different burrows. Habitat associations of the 22 burrows where D. elator was captured were as follows: mesquite (1), rock (1), lotebush (2), brush piles (8), and prairie mounds (10).
Utilizing the MARK program, a population estimate of 33 [+ or -] 6 individuals was obtained for the study area. Ninety-five percent confidence limits ranged from 25-49 individuals. Of the 18 D. elator captured, eight were caught at multiple burrows. Of those eight individuals, five were captured at two different burrows, and three were captured at three different burrows. Of the 22 burrows where D. elator was captured, multiple individuals were captured at five burrows. Multiple D. elator were caught at the same burrow on the same trap night only twice. On a single occasion, a male and female where captured together at the same burrow.
Soils. -- Both major soil associations within the study site are categorized as clay loams. The Kamay soils are more favored by D. elator and dominate throughout most of the study site. Kamay soils are gently sloping and were formed over ancient alluvium deposits from red-bed clay and shale. These soils are well-drained but slowly permeable. The shrink-swell potential in some areas may be severe because of high underlying clay content (Richardson et al. 1977). Asa-Portales soils formed in alluvial materials of recent age and occur in creeks and intermittent streams (Richardson et al. 1977). Because Asa-Portales soils are subject to frequent flooding, they provide only marginal habitat for Texas kangaroo rats. Although 15% of burrows occurred in this soil association, most of the burrows were found along the margins of the Asa-Portales soils.
Packard & Roberts (1973) noted that D. elator was not in sandy soils of the Red River terraces within Wichita County north of the study site. However, Martin & Matocha (1991) indicated that clay or clay loam soils may not always be used for burrow sites because a few D. elator have been found in soils with high sand content.
Kamay soils at the study site are classified as alfisols. Alfisols of the Blackland Prairie region of North Central Texas contain soil features known as prairie mounds (Diggs et al. 1999). These mounds were formed as a result of shrink-swell and soil overturn properties of clays underlying alfisols. Although raised, open areas are of small elevation (about 20 cm or less) at the study site, the mounds appear to provide favorable burrow sites for D. elator as they comprised 29% of the total habitat associations. Thus, prairie mounds and other naturally occurring habitat heterogeneity features likely played important roles in determining distribution patterns of D. elator within Wichita County. Stangl et al. (1992) proposed that past activity of Bison bison may have created suitable habitat for D. elator by creating disturbances such as wallows and by intense grazing. However, because of the migratory nature of bison and its large range, it seems unlikely that bison alone could have created and maintained suitable habitat for Texas kangaroo rats. It is proposed that natural habitat heterogeneity, due in part to soil properties that form prairie mounds, also may be an important factor in determining the past and present distribution of the Texas kangaroo rat.
Vegetation. -- Little barley, common broomweed, hog potato, Virginia pepperweed, and western ragweed occur in disturbed habitats, and common broomweed is often an indicator of heavy grazing (Diggs et al. 1999). These plants were dominant species associated with burrows of Texas kangaroo rats (Table 1) and their occurrence is likely caused by intense grazing by cattle and rodent activity around the burrows. Habitat of D. elator was dominated by short, herbaceous vegetation (2.0-40.0 cm in height) with little overhead woody cover, and there was a significantly greater amount of bare ground and less grass within quadrats as compared to transects leading away from the burrow. There is general agreement that D. elator requires a sparse, short-grassland habitat (Carter et al. 1985; Dalquest & Collier 1964; Roberts & Packard 1973; Stangl et al. 1992), and findings from this current study support this conclusion, although as discussed earlier, there has been disagreement concerning the importance of mesquite within habitats.
At present, habitat as described above usually is not typical for Wichita County, Texas. Nearly all tillable land is under cultivation within the range of D. elator. Routine tilling and resulting monocultures of these fields render these areas uninhabitable for Texas kangaroo rats (Stangl et al. 1992). Areas not in crop production are developed for gas and oil exploration or used as rangeland. Associated disturbances, such as road construction, and discarded equipment that accumulates soil are opportunistically used by Texas kangaroo rats (Roberts & Packard 1973; Stangl et al. 1992).
The use of fire to control woody species is precluded by presence of oil field equipment, and costs of mechanical brush control often are prohibitive. These circumstances may allow areas to develop dense stands of mature honey mesquite, wherein the herbaceous vegetation becomes tall and dense. In 1985, D. elator was recorded for two separate locations in Hardeman County, Texas. When the sites were visited in 1990, the vegetation had become much denser and D. elator had been extirpated from the locations (Stangl et al. 1992). The Goetze property is a location of intensive grazing by cattle and the population of D. elator is known to have persisted at this site since at least 1930 (Oscar and Ernest Goetze, pers. comm.). Intensive grazing for maintenance of D. elator habitat has been documented (Chapman 1972; Stangl et al. 1992) and the grazing regime at the study site may be a factor accounting for the long-term persistence of the population.
Burrow associations. -- The greatest number of burrows of the Texas kangaroo rat were associated with loose, elevated soils where 30-year-old, unburned brush piles had decayed (n = 25). Burrows also were found in fence rows (7) adjacent to wheat fields located on the east and west sides of the study site and under large rocks occurring in earthen dams (5). This is in agreement with Stangl et al. (1992), who concluded that D. elator opportunistically used disturbed habitats including brush piles and fence rows.
Second in abundance were associations with elevated, open areas (n = 19), some of which could have formed due to disturbance during the clearing of brush whereas others may be prairie mounds formed by alternating cycles of soil shrinkage and swell to form prairie mounds. Honey mesquite and lotebush associations were less abundant (4 and 6 burrows, respectively). Many researchers have emphasized an affinity between D. elator and mesquite (Bailey 1905; Blair 1954; Carter et al. 1985; Chapman 1972; Dalquest & Collier 1964; Martin & Matocha 1972, 1991; Roberts & Packard 1973; Schmidly 2004). Although a high number of small honey mesquite (54/ha) was present on the study area, it was found that loose, elevated soils formed from decay of old brush piles and elevated, open areas were the most important habitats at this site. Additionally, lotebushes were of greater importance (based on this shrub's overall lower density and greater number of burrow associations) as a habitat feature at the site than honey mesquites.
Burrow use and population estimates. -- It is suggested that D. elator may occupy more than one burrow. Forty-four percent of the captured animals possibly used more than one burrow. Upon release, these individuals always returned to the nearest burrow. On two occasions, two individuals were captured at the same burrow entrance. Runways were observed connecting burrows and, by using night-vision scopes, animals were observed leaving one burrow and traveling along the runway to enter an adjacent burrow. Therefore, the current results differ from those of Packard & Roberts (1973), who reported rare use of multiple burrows.
The population estimate of D. elator for this site was 33 [+ or -] 6 individuals. Sixty-six burrows were mapped and preliminary observations suggest multiple burrow use. Based upon the data and observations, D. elator appears to use two or more burrows per individual at this particular location. Intensive trapping (18 periods over 9.5 weeks) was conducted to obtain a reliable estimate of population size. However, intensive trapping of localities where D. elator occurs to estimate population sizes over a large geographic range seems impractical due to private ownership of most lands and fragmented habitats within this species' range. If a suitable conversion factor for number of burrows per animal can be obtained by additional studies, the burrow counting method could prove to be much quicker, less labor intensive, and less disruptive for D. elator populations than trapping.
By dividing the area of the study site (15 ha) by the population estimate (33), one obtains estimates of population densities of one D. elator /0.45 ha (or approximately 2/ha). This estimated density is much lower than the previously reported 10/ha for the same general area in Wichita County, Texas (Roberts & Packard 1973). This discrepancy is difficult to explain. Two possible reasons are (1) either the sites where Roberts & Packard (1973) conducted their research were more conducive to high populations or (2) overall population densities have declined since 1973. To determine if populations are indeed declining, many more population estimates need to be obtained throughout the range of the Texas kangaroo rat. Burrow mapping surveys might provide a cost-effective means of accomplishing this objective. Burrow counts and trapping to estimate population sizes have been employed of studies of the banner-tailed kangaroo rat, D. spectabilis, (Cross & Waser 2000) and Stephens' kangaroo rat, D. stephensi, (Brock & Kelt 2004). Results varied according to locality with D. stephensi, whereas burrow trapping was found to be a reliable method in censusing D. spectabilis. However, additional studies are needed to determine how accurately burrow counting methods may estimate D. elator population numbers.
We thank Mr. and Mrs. Oscar Goetze and Mr. Ernest Goetze for allowing us access to their properties within Wichita County. This study could not have been completed without their kind permission. We acknowledge R. Wittie and B. Lambert for assistance with statistical analyses and a number of reviewers for improving the paper. We thank Mike Miller and Texas Parks and Wildlife for providing PIT tags and associated equipment. This study was conducted under Texas Parks and Wildlife permit SPR-0496-775.
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JRG at: email@example.com
Jim R. Goetze*, William C. Stasey, Allan D. Nelson and Philip D. Sudman
*Science Department, Laredo Community College, Laredo, Texas 78040 and Department of Biology, Tarleton State University, Box T-0100
Stephenville, Texas 76402
Table 1. Mean percentage cover of bare ground, forbs, grasses, woody vegetation, and stumps as well as average herbaceous and woody vegetation heights compared between 10 quadrats and 40 transects in May 2005. Percentage data were evaluated by a Wilcoxon Mann-Whitney test. Height data were compared by a paired t-test. Standard deviations for data are indicated by parentheses and significant differences are denoted by an asterisk. Parameter Quadrats Transects P % Bare Ground 49.9 ([+ or -] 24.0) 22.3 ([+ or -] 8.7) 0.010* % Forbs 17.0 ([+ or -] 12.9) 22.9 ([+ or -] 9.8) 0.177 % Grasses 25.1 ([+ or -] 18.9) 54.3 ([+ or -] 9.5) 0.002* % Woody 6.0 ([+ or -] 15.8) 0.5 ([+ or -] 1.6) 0.256 % Stump 2.0 ([+ or -] 4.2) 0.0 ([+ or -] 0.0) 0.092 Herbaceous Height 7.1 ([+ or -] 6.7) 7.2 ([+ or -] 3.7) 0.934 (cm) Woody Height (cm) 15.9 ([+ or -] 38.8) 10.2 ([+ or -] 32.3) 0.700
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|Author:||Goetze, Jim R.; Stasey, William C.; Nelson, Allan D.; Sudman, Philip D.|
|Publication:||The Texas Journal of Science|
|Date:||Feb 1, 2007|
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