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Distribution and habitat attributes of an endemic subspecies of pocket gopher.

INTRODUCTION

Geomys bursarius, plains pocket gopher, occurs throughout the Midwest from southern Canada to Texas encompassing a variety of grassy habitats (Elrod et al., 2000). They feed primarily on grasses but consume forbs as well (Behrend and Tester, 1988). Although the grassy habitats preferred by the plains pocket gopher occur throughout its range, these habitat types occur in a matrix of others including woodlands and agricultural areas. Soil type is another important factor in pocket gopher habitat suitability due to differences in energy costs of burrowing (Vleck, 1981). Plains pocket gophers prefer soils with a high sand and low clay and silt contents (Downhower and Hall, 1966; Schmidly, 1983), thereby being restricted to habitat based on soil content consisting of sand and loam more so than vegetational structure. Soil particle size, topsoil depth and food availability are important in the suitability of potential habitat for pocket gophers.

Until the mid 1990s, Geomys breviceps, Baird's pocket gopher, was the only species of pocket gopher thought to exist in Arkansas (Sealander and Heidt, 1990). However, Elrod et al. (1996a) determined by using allozyme analysis and identification of chewing lice that a second species of pocket gopher, the plains pocket gopher, inhabited Arkansas. Furthermore, Elrod et al. (2000) described a new subspecies of G. bursarius from this disjunct population in Izard County, Arkansas (Fig. 1) as G. b. ozarkensis, Ozark pocket gopher, based on cytochrome-b gene of mtDNA and cranial morphology. This subspecies was possibly isolated by a glaciation event and subsequent retreat from an ancestral form of a plains pocket gopher occurring from the Ozark highlands to the western foothills of the Appalachian Mountains (Elrod et al., 2000). Kershen (2004) estimated the population of this subspecies at 3500 individuals, with a range entirely contained within Izard County. This subspecies has been identified as a "Species of Greatest Conservation Need" in the Arkansas Wildlife Action Plan (Anderson, 2006).

Limited published data exists on the Ozark pocket gopher, with the exception of the initial distribution, historical biogeography, trapping techniques and community ecology (Elrod et al., 2000; Connior et al., 2008; Kovarik et al., 2008; Connior and Risch, 2009, 2010). Ozark pocket gopher habitat harbors numerous species of herpetofauna and small mammals that either use or potentially use the burrows of the pocket gophers (Connior et al., 2008). Furthermore, Ozark pocket gophers are most active in tunneling and foraging in the winter and spring seasons (Connior and Risch, 2010), which provides refugia or prey for many of these vertebrate associates. Although these isolated pocket gophers have a large impact on the ecosystem, the habitat requirements and possible dispersal routes has not been assessed. Thus, the objective of this manuscript was to analyze multi-year data in an effort to provide a synthesis of current habitat requirements and distribution and determine possible dispersal corridors and future range potential for gophers in Izard County.

METHODS

DISTRIBUTION

Presence of pocket gophers was identified via automobile and helicopter surveys of Izard and surrounding counties. The original distribution for Geomys bursarius ozarkensis is published in Elrod et al. (2000) and was determined by examining fields along all roads within a 30 km radius of the assumed epicenter. We conducted additional automobile surveys in 2002, 2007 and 2008. In 2008, we focused additional searching efforts in the area just south of the White River near Guion in Stone County, Arkansas.

Helicopter flights were conducted in Izard, Fulton, Sharp and Randolph Counties in Arkansas for additional pocket gopher locations (Fig. 1). Helicopter flights were completed in a search of above ground sign for new pocket gopher locations during 2002 in Izard County. Helicopter flights were conducted on 19 Apr. 2002 lasting approximately 4 h and 24 Nov. 2002 lasting approximately 3 h. Flights followed a 1.2 km grid over Izard County with a ~3.2 km overlap into each surrounding county for a total coverage of ~1737 [km.sup.2]. During flights, coordinates of Ozark pocket gopher locations were recorded with a global positioning system (GPS) unit. Flights were also conducted along the floodplains of streams and rivers on 12-14 Mar., 27 Apr. and 21-22 Dec. 2005 with a total of 25 h being flown to the north and northeast of known range. These flights encompassed all major drainages in Fulton, Sharp and western Randolph Counties. We recorded locations using a GPS of probable gopher colonies identified during the flights. Later, we ground-truthed these probable gopher colonies to determine if gophers were present. If confirmed, these sites were recorded with GPS and combined with all known locations into one shapefile for a minimum convex polygon analysis of animal locations using ArcGIS (ESRI, Redlands, CA).

[FIGURE 1 OMITTED]

HABITAT ANALYSIS

We used five randomly spaced circular plots with a diameter of 88.9 cm to determine percent ground cover of major vegetation species in each of 10 sites within Izard County, Arkansas containing abundant gopher activity, which was defined as having a large number of pocket gopher mounds. We determined percent ground cover by combining the percent ground cover of each plant type from all plots surveyed and dividing by the total possible percentage. We identified each plant type to the lowest possible taxonomic level using dichotomous keys listed in Waterfall (1969), Hitchcock (1971), Great Plains Floral Association (1986), Tyrl et al. (1998), Diggs et al. (1999) and U.S. Department of Agriculture (2002).

We collected and analyzed three random samples of approximately 300 g per each of the same 10 vegetation sites for analyses of soil texture as defined by Gee and Bander (1986), particle size, percent organic matter and pH. Soil profiles for all samples were created to a minimum depth of 35.56 cm and samples were collected from the middle of each profile and sealed in a plastic bag (Soil Survey Staff, 1974). Detailed soil analyses methods are described in Kershen (2004). We used visible color change to identify topsoil depth by measuring distance to B horizon on soil profiles in the field. We conducted laboratory analyses by taking the total sample weight after equilibrating samples to room temperature and moisture levels, sieving out particles >2 mm then splitting the samples into the correct weights for the various tests. Moisture was removed from ~10 g of soil by placing the sample in a 105 C oven overnight. Percent organic matter and pH were documented using the protocol outlined in Buol et al. (1989). Soil particle size was determined on ~50-60 g of soil by employing the pipette method protocol outlined in Gee and Bander (1986). We calculated percent clay by taking the original sample weights along with the difference between empty 40 ml beakers and the beakers with dried samples. The cylinders were then rinsed completely using de-ionized water through a 63 [micro]m sieve, which trapped the sand. Original sample weights and sand weights were used to calculate percent sand of the samples. Silt was calculated as the remainder of the soil not accounted for by clay and sand weights.

We used Minitab 14 (Minitab, Inc., State College, Pennsylvania) to conduct multiple regression analyses to determine if density of pocket gophers (see below) is related to soil particle size, organic matter or pH in 10 of the sites where density was estimated. We estimated density of pocket gophers in 17 sites ranging from 0.28 to 4.18 ha by using a modified version of density estimation for pocket gophers detailed in Howard (1961). Pocket gopher mounds were counted and their arrangements recorded on a grid. This was accomplished by setting belt transects with colored flags at 30 m increments. Only fresh mounds within 9.5 m of either side of transect lines were counted and if it occurred >5 m from any previous mound within the same transect. Fresh mounds must not have any vegetation or tracks of any kind through the middle and soil must still appear granular and not compacted. In addition, a saturated trapping effort provided the actual density of pocket gophers. As traps were set their location was recorded on the documented grid as were location of trapped animals. Other studies (Blackburn and Gaston, 1996; Smallwood et al., 1996; Smallwood and Schonewald, 1996; Smallwood and Morrison, 1999) have found high correlations between mammalian density estimates and the size of study area sampled; therefore, we conducted a linear regression between log-transformed density estimates and the corresponding log-transformed study areas in order to control for the variation in density explained by the study area size (Fig. 2). Then, we used the residuals of this analysis as an index of density in subsequent tests of association with soil particle size, organic matter or hydrogen ion concentration to correct for variation in study size area.

[FIGURE 2 OMITTED]

In addition to soil samples, we used Geographic Information System (GIS) to identify all soils that Ozark pocket gophers occupy by relating pocketgopher occupancy to these soils. We used coordinates in the UTM NAD83 coordinate system of known pocket gopher locations from all automobile and helicopter surveys. We imported the GPS locations into ArcView 9.0. For reference, we overlaid the pocket gopher location data on a 2000-2002 county mosaic ftp file and a Natural Resources Conservation Service (NRCS) 2003 SSURGO soil map to investigate soil associations. We identified potential habitat in Izard County by creating a shapefile of all soil types that contained pocket gophers throughout Izard County. The 'Clip' feature in ArcView 9.0 was used to form the entire data set to the Izard County boundary line.

RESULTS

DISTRIBUTION

As of 2008, the current range of the Ozark pocket gopher remains in the southern portion of Izard County with a total area of ~2300 [km.sup.2] calculated using a minimum convex polygon (Fig. 3). Yet, some individuals or relict populations may still occur undetected adjacent to the White River in Stone County. We live trapped an adult male in 2007 in northeastern Stone County ca. 50 m from the White River. This individual remained at the site throughout the spring and summer of 2007, but in Jan. of 2008 we did not find any evidence (i.e., new mounds) of the individual. In other areas, we detected nine possible pocket gopher sites in adjacent Fulton and Sharp Counties during the 2005 aerial surveys, however subsequent ground-truthing in 2007 found no pocket gopher activity.

[FIGURE 3 OMITTED]

HABITAT ANALYSIS

Within the total of 50 plots (5 plots in each of the 10 sites), we identified 21 families of vegetation in Ozark pocket gopher sites (Table 1). Poaceae, the grass family, had the highest representation at 30 plots, which composed 66% of identifiable vegetation. All other identifiable plant species were forbs and combined made up 26%. Members of family Poaceae, grass family, were identified to family in 5 plots, genus in 3 plots and species in 22 plots. Euphorbiaceae, the spurge family, was collected at 12 plots and Asteraceae, the sunflower family, was collected at 11 plots. Cynodon dactylon, Bermuda grass, was the most common species collected at 7 plots, the next most numerous member of this family was Sorghum halapense, Johnson grass, collected at 3 plots. All Other species were collected only once. The most common genus was Sporobolus collected at 3 plots. We identified two genera of the family Euphorbiaceae. Four species of Croton were identified: C. glandulosis, vente conmigo (4 plots), C. monanthogynus, prairie tea (2 plots), C. texensis, Texas croton (1 plot), C. capitatus, hogwort (1 plot) and Chamaesyce nutans, eyebane, (1 plot). Three other representatives of the Euphorbiaceae family were identified. Of the Asteraceae family Tetraneuris, bitterweed, was most frequently collected (3 plots). We identified three other representatives to family, five to genera and three to species, the latter two groups represented by a collection in a single plot (see Table 1).

We determined in-field topsoil depth, or distance to B horizon, in only one of 30 samples which was determined by a visible lightening of the soil at a depth of 25.4 cm. We could not visually identify any color change in all other samples within the exposed profile of 35.56 cm. Most sampled sites were located at the base of a moderate hill and/or near a waterway. Both of these geographical positions would allow for deposition of soil, thus creating a deeper than expected topsoil (C. R. Ferring, pers. comm.). The pH had a median value of 5.05, (so = 0.97; range 4.03 to 7.12). Percent organic matter averaged 6.72% (SD = 3.36; range 0.32 to 16), gravel content averaged 1.87% (SD = 3.64; range zero to 18.64%), sand content averaged of 74.5% (SD = 13.15; range 41.77 to 97.44%), silt content averaged 20.97% (SD = 11.55; range 1.31 to 48.17%) and clay content averaged 4.63% (SD = 2.36; range 1.25 to 11.8%). Six soil textures were recorded with laboratory analyses and comprised the following of the total: (1) loamy sand 40%; (2) sandy loam 23%; (3) sand 17%; (4) gravely sandy loam 10%; (5) loam 7%; and (6) silt loam 3%.

The number of pocket gophers estimated per site ranged from 4.0 to 60.4 per hectare with an average density of 20.4 animals per hectare. These density estimates were highly correlated to size of area sampled with a regression equation of: Log Density/Ha = 1.19 0.943 Log Size (P-value < 0.001; [r.sup.2] = 0.765; Fig. 2). No significant relationship existed between the corrected density of pocket gophers versus soil particle size, percent organic matter or pH.

A Geographic Information System analysis of all known Ozark pocket gopher occupancy sites identified 11 soils in which pocket gophers resided with Izard County containing 10 of those soil types and the last was found in Stone County where the individual gopher captured in 2007 was located in the Wideman loamy fine sand type (Table 2). Potential soil habitat occurs to the East, North and West of its current distribution (Fig. 3). Aerial photography reveals much of this potential habitat is presently Geomys bursarius ozarkensis' preferred vegetational structure of grassy vegetation.

DISCUSSION

Ozark pocket gophers are restricted to the southern one-third of Izard County. All sampling locations that contained pocket gophers were grazing land, hay fields or frequently mowed areas. Although aerial surveys detected possible sites in surrounding counties, no active populations were found during subsequent ground surveys of these sites. It is possible that pocket gophers may have been present at the time of the aerial surveys but later perished or dispersed. In the late 1980s, two specimens were collected in Stone County (Elrod et al., 2000). However, these specimens were in central Stone County and were not near the capture location of the individual in 2007. Subsequent surveys of Stone County did not detect any pocket gophers occupying the area (current study; Elrod et al., 1996b). The White River is a significant southern barrier to the Ozark pocket gopher, separating existing gopher colonies in Izard County from potential habitats in Stone County. Geomys bursarius ozarkensis has been documented travelling above-ground (Connior and Risch, 2010), making the Hwy 58 White River bridge a possible conduit for dispersal. This bridge spans the White River near the southernmost active colonies in Izard County. Another possibility is that the pocket gopher swam the White River, which is a dammed river and thus has variable stream flow. Pocket gophers are able to cross rivers with variable stream flow and narrower widths of waterways during droughts; however, they are unable to cross large widths with steady stream flow (Kennerly, 1959; Wilkins, 1985; Sudman et al., 1987; Wilkins, 1987; Wilkins and Swearingen, 1990). Additional surveys and monitoring in both Izard and Stone counties may determine the status of the White River as a barrier to the dispersal and distribution of the Ozark pocket gopher.

Previous studies have reported that Geomys bursarius occur in nonnative grasslands (Downhower and Hall, 1966; Reichman and Smith, 1985; Behrend and Tester, 1988; Broussard, 1996; Pitts and Choate, 1997). Concurrent with these other studies, sample sites in this study were dominated by invasive, non-native grass species. Although Bermuda grass, Cynodon dactylon, is by far the most prevalent species, it is quite likely that Bermuda grass was planted by landowners to establish a lawn, hay fields or grazing lands. In addition to Bermuda grass, our study area vegetation was dominated by grasses of different species, such as Sporobolus and Digitaria. We collected 26% forbs in our vegetation samples, far less than the amount of grass varieties. With such a wide array of vegetation present in pocket gopher habitat it appears that the Ozark pocket gopher inhabits areas not based on specific plant species but rather on the availability of abundant forage. Annual grasses and forbs and perennial forbs decline as vegetation reaches climax communities. Thus, pocket gophers maintain fields at an earlier stage of plant succession due to soil mounding (Inouye et al., 1987). Therefore, Ozark pocket gophers maintain habitat for organisms that prefer non-climax vegetation type prairies.

Thickened topsoil, which has looser, sandier soil than lower horizons, allows the pocket gophers to have variability in depth of their tunnel system to aid in water drainage and temperature fluctuations (Kennerly, 1964; Downhower and Hall, 1966). In this study, the soil had a thickened A horizon indicated by the only visible color change at 25.4 cm most likely due to slope position. Yet, pocket gopher activity in the area may have increased the topsoil depth as well (Schwartz and Schwartz, 1981; Jones et al., 1983).

Our findings concur with previous studies of Geomys bursanus' preference of high sand and low clay contents (Downhower and Hall, 1966; Schmidly, 1983) with the exception of those relating to silt. High clay and silt content as well as soil moisture may be important variables to pocket gopher occupancy because these factors reduce the aeration of the soil and gas exchange with the external environment (Moulton et al., 1983). Our findings reveal that Ozark pocket gophers tend to be more prevalent in loamy sand soils. This soil type contains more silt content possibly supporting more plant life due to availability of nutrients while still being conducive to pocket gopher tunneling. Loamy soils also have a higher water holding capacity than do sandy soils, which would facilitate plant growth and ease of digging. We found 21% of soil samples were sandy loam. Sandy loam is a soil with high sand, low clay and moderate silt contents. This result suggests that some silt is necessary for the establishment and survival of vegetation consumed by pocket gophers.

The Ozark pocket gopher does not appear to be limited by pH. Davis et al. (1938) also found no correlation between Geomys breviceps distribution and pH in Texas, with it occupying soil ranging from 4.5 to 8.0. Pocket gophers and their activities may elevate soil organic matter possibly explaining the expansive range values observed for organic matter. We did not find an association between pocket gopher density and any measured soil parameter. Based upon our observations, pocket gophers appear to be less numerous in areas of high gravel content and are more numerous in areas of high sand content possibly due to increased burrowing ease.

Flood plains tend to be more level than lands outside of flood plains (United States Department of Agriculture, 1984). This levelness reduces geological erosion which increases soil depth to bedrock as compared to non-flood plain areas. The majority of open, grassy fields in Izard County are located within flood plains due to historical and current farming practices. Open fields along small waterways such as creeks and streams may be more suitable to pocket gophers over fields along rivers as they have minimal flood inundation. Close proximity to a waterway provides a continuous deposition of sand from the waterway, creating excellent burrowing conditions. Moreover, nearly 59% of Izard County is described as moderate to steeply sloping with rocky outcroppings, and/or has high levels of surface stoniness and gravel, which highlights the importance to Ozark pocket gophers of additional sand deposition on relatively flat land. Pocket gophers may also use this riparian habitat as dispersal corridors from one suitable field of inhabitance to another. Maintaining the current environment of, and around, streams near known fields of inhabitance may promote dispersal of the Ozark pocket gopher as they seek more open fields of suitable habitat. Increasing suitable riparian habitat would decrease overcrowding in fields of known inhabitance creating a stronger healthier population. In general, Ozark pocket gophers are restricted in their ability to colonize new areas because of their fossorial lifestyle and that the majority of Izard County is not ideal habitat for pocket gophers. Therefore, maintaining riparian dispersal corridors is especially important to the conservation of the Ozark pocket gopher.

Acknowledgments.--We thank I. Guenther for assistance with fieldwork and the private landowners that allowed us access to their property. Portions of this study were funded by State Wildlife Grants from the Arkansas Game and Fish Commission and the U.S. Fish and Wildlife Service. This study was assisted by reassignment time granted to TSR from the Environmental Sciences Program at Arkansas State University.

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SUBMITTED 6 OCTOBER 2009

ACCEPTED 12 JANUARY 2010

MATTHEW B. CONNIOR (1)

Arkansas State University, Department of Biological Sciences, P.O. Box 599, State University 72467

AUDREY A. KERSHEN

University of North Texas, Department of Environmental Sciences, Denton 76203

REX E. MEDLIN, JR.

Arkansas State University, Department of Biological Sciences, P.O. Box 599, State University 72467

DOUGLAS A. ELROD

North Central Texas College, Science Department, 1500 North Corinth Street, Corinth 76208

D. BLAKE SASSE

Arkansas Game and Fish Commission, 213A Highway 89 South, Mayflower 72106

AND

THOMAS S. RISCH

Arkansas State University, Department of Biological Sciences, P.O. Box 599, State University 72467

(1) Corresponding author present address: South Arkansas Community College, P.O. Box 7010, 300 South West Avenue, El Dorado 71731; e-mail: mconnior@southark.edu
TABLE 1.--Identification and total percent ground cover of
collected vegetation from 10 sites of known Geomys bursarius
ozarkensis habitat in Izard County, Arkansas

Identification                      Common name             Percent
                                                            ground
                                                             cover

Family Poaceae               Grass Family                    65.91
  Cynodon dactylon           Bermuda Grass                   41.96
  Sorghum halapense          Johnson Grass                    6.20
  Sporobolus species         Dropseed                         5.30
  Panicum species            Panicum                          4.90
  Digitaria species          Crabgrass                        3.20
  Setaria glauca             Yellow Foxtail                   1.10
  Tiidens flavus             Purpletop Tridens                1.00
  Sporobolus cryptandrus     Sand Dropseed                    0.91
  Sporobolus compositus      Composite Dropseed               0.50
  Polypogon monspeliensis    Annual Rabbitsfoot Grass         0.40
  Eragrostis pectinacea      Tufted Lovegrass                 0.20
  Cenehrus spinifex          Coastal Sandbur                  0.10
  Paspalum setaceum          Thin Paspalum                    0.10
Family Euphorbiacea          Spurge Family                    7.84
  Croton monanthogynus       Prairie Tea                      6.00
  Croton texensis            Texas Croton                     1.10
  Croton glandulosus         Vente Conmigo                    0.50
  Croton capitatus           Hogwort                          0.20
  Chamaesyre nulans          Eyebane                          0.04
Family Fabaceae              Pea Family                       3.51
  Strophostyles leiosperma   Slickseed Fuzzybean              1.35
  Trifolium species          Clover                           1.16
Family Asteraceae            Sunflower Family                 3.40
  Tetraneuris species        Four Nerve Daisy                 2.90
  Taraxacum species          Dandelion                        0.27
  Claysopsis pilosa          Soft Goldenaster                 0.09
  Silphium species           Rosinweed                        0.06
  Gnapthalium species        Cudweed                          0.04
  Rudbeckia hirta            Blackeyed Susan                  0.02
  Conyza canadensis          Canadian Horseweed               0.02
Family Cyperaceae            Sedge Family                     3.31
  Cyperus esculentus         Yellow Nutsedge                  2.20
  Cyperus species            Sedge                            1.11
Family Oxalidaceae           Wood Sorrel Family               1.46
  Oxalis stricta             Common Yellow Oxalis             1.04
  Oxalis corniculata         Creeping Woodsorrel              0.42
Family Solanaceae            Nightshade Family                1.29
  Solanum carolinense        Carolina Horsenettlc             1.16
  Solanum species            Horsenettle                      0.13
Family Rosaceae              Rose Family                      1.23
Family Apiaceae              Carrot Family                    1.11
Family Caryophyllaceae       Pink Family                      1.09
  Silene species             Catchfly                         1.00
  Stellaria calycantha       Northern Starwort                0.09
Family Amaranthaceae         Amaranth Family                  0.56
  Froelichia gracilus        Slender Snakecotton              0.56
Family Cactaceae             Cactus Family                    0.47
  Opuntia species            Prickly Pear                     0.47
Family Rubiaceae             Madder Family                    0.18
  Galium obtusum             Bluntleaf Bedstraw               0.18
Family Brassicaceae          Mustard Family                   0.15
  Lepidium virginicum        Virginia Pepperweed              0.15
Family Plantaginaceae        Plantain Family                  0.15
Family Lythraceae            Loosestrife Family               0.13
  Rotala ramosior            Lowland Rotala                   0.13
Family Hypericaceae          St. John's-wort Family           0.09
  Hypericum species          St. John's-wort                  0.09
Family Laminaceae            Mint Family                      0.05
  Clinopodium arkansanum     Limestone Calamint               0.05
Family Gentianceae           Gentian Family                   0.05
Family Commelinaceae         Spiderwort Family                0.02
  Commelina erecta           Whitemouth Dayflower             0.02
Unidentified or bare                                         11.47

TABLE 2.--Soil types of known Geomys bursarius
ozarkensis locations in Izard County, Arkansas

Soil type

Boden gravelly sandy loam, 3 to 8% slopes
Boden gravelly sandy loam, 8 to 20% slopes
Estate-Portian-Moko association, rolling
Estate-Portian-Moko association, steep
Peridge silt loam, 3 to 8% slopes
Portia sandy loam, 3 to 8% slopes, eroded
Portia sandy loam, 8 to 12% slopes, eroded
Sturkie silt loam, occassionally flooded
Wideman fine sand, frequently flooded
Wideman fine sandy loam, 0 to 3% slopes
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Article Details
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Author:Connior, Matthew B.; Kershen, Audrey A.; Medlin, Rex E., Jr.; Elrod, Douglas A.; Sasse, D. Blake; Ri
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1U7AR
Date:Oct 1, 2010
Words:5159
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