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J.D. Cortez, S.E. Henke, D.W. Wiemers, T.E. Fulbright, D.B. Wester, and R. Riddle
22001155 SOUTHEASTERN NATURALIST Vo1l4.( 114):,4 N1–o5. 61
Distribution and Habitat Selection by the
Maritime Pocket Gopher
Jorge D. Cortez1, Scott E. Henke1,*, Dean W. Wiemers1, Timothy E. Fulbright1,
David B. Wester1, and Richard Riddle2
Abstract - Geomys personatus maritimus (Maritime Pocket Gopher) is a genetically distinct
subspecies that only occurs in deep sandy soils of Nueces and Kleberg counties of
southern Texas. The US Fish and Wildlife Service has considered recommending the Maritime
Pocket Gopher for federal listing. Because a large proportion of this gopher’s current
range occurs on US Navy (hereafter, Navy) property, active management by the Navy plays
a key role in the conservation of this subspecies. Therefore, our objective was to assess
the distribution and habitat preferences of Maritime Pocket Gophers on Navy properties in
southern Texas. We conducted a strip-transect survey to evaluate the number and distribution
of gopher mounds on the Navy base. We created GIS layers of the different disturbance
types and plant communities, and overlaid them onto the mapped gopher-mound density
layer. If the proportion of available area used by Maritime Pocket Gophers was below or
above the corresponding 90% confidence interval, we categorized disturbance types and
plant communities as used more often or less often, respectively. The distribution of Maritime
Pocket Gophers was influenced by soil-particle size, plant community, type of habitat
disturbance, and quantity of leaf litter. Maritime Pocket Gophers favored areas with sandy
soil in frequently mowed native prairie, restoration fields, and Cynodon dactylon (Bermuda
Grass) with little to no ground litter (i.e., dead vegetation, debris).
Introduction
Geomys personatus maritimus Davis (Maritime Pocket Gopher) is one of 7
subspecies of Geomys personatus True (Texas Pocket Gopher) and is endemic to
the sandy soils of the mainland in Kleberg and Nueces counties of coastal South
Texas, between Baffin Bay and Flour Bluff (Williams and Genoways 1981). This
subspecies was closely associated with native coastal prairie and deep sandy soils;
however, development of agricultural fields and urbanization have minimized and
fragmented sandy grasslands, resulting in a patchy distribution of this subspecies.
Published data on Maritime Pocket Gophers focused on general characteristics of
morphology, distribution, and habitat (Williams 1982). Accordingly, the assumed
current geographic distribution of the Maritime Pocket Gopher may be outdated because
it was determined more than 30 years ago by Williams and Genoways (1981).
Potential threats to persistence of the Maritime Pocket Gopher include habitat
loss, degradation, and fragmentation (by urbanization and agricultural conversion)
resulting in reduced genetic variability and gene flow. In addition, native
1Caesar Kleberg Wildlife Research Institute, MSC 218, 1150 Engineering Avenue, Texas
A&M University-Kingsville, Kingsville, TX 78363-8202. 2US Navy, 8851 Ocean Drive,
Corpus Christi, TX 78419-5226. *Corresponding author - scott.henke@tamuk.edu.
Manuscript Editor: Allan O’Connel
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vegetation has been replaced by the sub-tropical exotic grasses Urochloa maxima
(Jacq.) R. Webster (Guinea Grass), Pennisetum ciliare (L.) Link (Buffel Grass), and
Cynodon dactylon (L.) Pers. (Bermuda Grass) in much of the region (Larson et al.
2010). Exotic grass invasion has been associated with the decline of small-mammal
populations (Germano et al. 2001). The threat of predation is relatively small because
gophers are subterranean (Andersen 1987) and few endemic predators (e.g.,
feral cats, snakes) present on the study site have the means and persistence to dig a
gopher out of its elaborate tunnel system. The Maritime Pocket Gopher’s restricted
distribution makes it particularly vulnerable to adverse human impacts such as
urbanization (Hafner 2000). Accordingly, the US Fish and Wildlife Service has
labeled the Maritime Pocket Gopher as a species of concern and has considered
recommending it for federal listing. Because the potential threats are mainly habitat-
related, it is important to determine the habitat preferences of Maritime Pocket
Gophers to help managers develop habitat-management practices that are attuned
to the requirements for this subspecies.
A large proportion of this gopher’s current distribution occurs on Navy property,
thus, active management by the Navy plays a key role in the conservation
of the subspecies. Therefore, our objectives were to assess the distribution and
habitat preferences of Maritime Pocket Gophers on Navy properties in southern
Texas. We sought to determine Maritime Pocket Gopher distribution patterns and
the preference or avoidance of plant communities and types of habitat disturbance
on Naval Air Station (NAS) Corpus Christi, and Navy Landing Airfield (NALF)
Waldron Station. We also assessed how the presence of exotic grasses versus native
grasses and forbs, the presence of ground litter and roads, and soil chemical
and physical properties influenced site selection by Maritime Pocket Gophers.
Our ultimate objective was to use our findings to propose management options for
this subspecies.
Field-site Description
The study was conducted on two Navy properties, NAS Corpus Christi
(27°41'33.47''N, 97°17'28.36"W) and NALF Waldron (27°38'05.68''N,
97°18'43.90"W) in the Flour Bluff region (16.1 km southeast of Corpus Christi) in
Nueces County, TX. Flour Bluff lies in the Gulf Prairies and Marshes ecoregion and
is surrounded by water on 3 sides (i.e., Corpus Christi Bay to the north, Oso Bay
to the west, and the Laguna Madre to the east). NAS Corpus Christi is 1049 ha in
extent and is home to the US Coast Guard, Army Depot, and Navy. The landscape of
NAS Corpus Christi mirrors that of the entire Flour Bluff region in that it is highly
developed with new construction encroaching on the fragmented areas of grassland
and scrubland habitat. NLAF Waldron is 352 ha in extent, 7 km south of NAS Corpus
Christi, and consists of an airfield, control tower, and recreational sport fields.
This station is less developed and has a more contiguous area of mid-grass prairie
and scrubland communities than NAS Corpus Christi.
Only 2 soil series occur at both study sites, Galveston (mixed, hyperthermic
Typic Udipsamments) and Mustang (siliceous, hyperthermic Typic Psammaquents)
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fine sand and dredge spoils (fine-loamy, mixed, superactive, frigid Typic Haploxerolls),
which consist of clay loam (NRCS 1960). The percentage of Galveston and
Mustang fine sand to clay loam on NAS Corpus Christi are 80% and 20%, respectively.
The soil in the NALF Waldron study area is entirely Galveston and Mustang
fine sand (NRCS 1960).
Both study areas consist of predominantly coastal, mid-grass-prairie grasslands
and scrub-dominated, mixed grassland communities. Both communities occur on
Galveston and Mustang fine sand and clay loam. Grass species include Aristida
purpurea var. wrightii (Nash) Allred (Wright’s Threeawn), Dichanthelium oligosanthes
(Schult.) Gould (Scribner’s Panicum), Spartina spartinae (Trin.) Merr. Ex
Hitchc. (Gulf Cordgrass), Paspalum plicatulum Michx. (Brownseed Paspalum),
Andropogon virginicus L. (Broomsedge Bluestem), Paspalum monostachyum Vasey
(Gulfdune Paspalum), Schizachyrium scoparium var. litoralis (Seacoast Bluestem),
and Dicanthium annulatum (Forssk.) Stapf (Kleberg Bluestem) (Garcia 2001). The
forb species include Baptisia alba (L.) Vent. (Whitestem Wild Indigo), Phlox drummondii
Hook. (Annual Phlox), Ambrosia psilostachya (Cuman Ragweed), Physalis
spp. (ground cherry), Croton spp. (croton), Rayjacksonia phyllocephala (DC.) R.L.
Hartm. & M.A. Lane (Camphor Daisy), Rhynchosia americana (Houst. Ex Mill.)
M.C. Metz (American Snoutbean), Cassia fasciculata (Michx.) Greene (Partridge
Pea), and Amorpha fruticosa L. (False Indigo) (Garcia 2001). The predominant
woody species are Quercus virginiana Mill. (Live Oak), Salix nigra Marshall (Black
Willow), and Prosopis glandulosa Torr. (Honey Mesquite). Opuntia engelmannii
Salm-Dyck ex Engelm. (Prickly Pear Cactus) is also present. Additionally, 4 nonnative
grasses occur on the sites: Bermuda grass, Stenotaphrum secundatum (Walter)
Kuntze (St. Augustine Grass ), Buffel Grass, and Guinea Grass.
Methods
Gopher population assessment
We surveyed the occurrence of Maritime Pocket Gopher mounds on both study
sites employing strip-line transects (Witmer et al. 1999). We used a 2004 US Department
of Agriculture (USDA) aerial photograph of the study sites to create a
strip-line transect map. With Using ArcGIS 9.1, we laid a 3-ha-grid matrix over the
aerial photograph and subdivided each grid into 1-ha strips. We chose one of the
three 1-ha strips from each grid selected by a random number generator in Microsoft
Excel. Due to the configuration of the study area, transect lengths ranged from
250 m to 536 m, and all transects were >15 m apart. The surveyor walked each 1-ha
strip using a Trimble GPS unit (model GeoExplorer III DGPS with beacon receiver,
accurate to within less than 0.5 m; Trimble, Sunnyvale, CA) to stay within the strip area
and recorded the location of every mound within the strip. We counted all mounds,
regardless of condition—either fresh or old—because old-appearing burrow systems
still may have been active (Proulx 2002). We assumed that we detected 100%
of mounds present because our transects were relatively narrow, making it unlikely
that we missed any Maritime Pocket Gopher mounds. We verified suspected burrow
systems by visual estimation of mound proximity, and mapped them using
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a Trimble GPS unit. We considered mounds > 15 m apart to be associated with
separate burrow systems (Witmer et al. 1999). We determined the area of each burrow
system using ArcGIS 9.1 XTools analysis and recorded the number of mounds
within each suspected burrow system.
Macrohabitat assessment
We imported gopher-burrow system locations into ArcGIS 9.1 and created a
burrow-system map. We delineated plant communities and habitat-disturbance
areas with the Trimble GPS unit and then categorized them (Appendices 1, 2).
We classified plant communities as salt marsh, Bermuda Grass, Bermuda Grass/
Guinea Grass, debris-native vegetation/Guinea Grass, golf course, Guinea Grass,
Guinea Grass/Buffel Grass, Gulf Cordgrass, Live Oak, Honey Mesquite/Guinea
Grass, native prairie, restoration field, softball field or park, St. Augustine Grass,
and vegetated runway (Appendix 1). We categorized disturbance types as construction,
developed, manicured, mowed, previously disturbed, shrub removal, or
untouched (Appendix 2). Differentiation of plant communities and habitat-disturbance
types are explained in Appendix 1 and Appendix 2, respectively. We used
the geoprocessing tool, Dissolve, on the gopher-burrow -system layer to combine
overlapping gopher systems (ERSI) and clip analysis in ArcGIS 9.1 on both the
plant community layer and habitat-disturbance layer overlaid upon the gopherburrow
layer. This analysis enabled us to determine the area (m2) for each burrow
system in each plant community and habitat-disturbance type. We converted
burrow-system area into proportion of area observed in each plant community and
habitat-disturbance type.
We determined gopher use of plant community and habitat-disturbance types as
described by Neu et al. (1974) using chi-square analysis and Bonferroni Z-statistics
to control experiment-wise error at P = 0.10. Plant communities and habitat-disturbance
types were considered used more often or less often, respectively, if the
proportion of available area used by Maritime Pocket Gophers was below or above
the corresponding 90% confidence interval.
Microhabitat assessment
We used a restricted random sampling design to examine microhabitat selection
by Maritime Pocket Gophers (Keating and Cherry 2004). We randomly placed
10-m-long transects within each vegetation community at NAS Corpus Christi and
NALF Waldron using Hawth’s Analysis Tools 3.8 in ArcGIS 9.1 (Beyer 2004).
Transects were proportionally allocated to communities based on their spatial extent;
we placed a minimum of 5 transects in the communities that comprised the
smallest proportion of the study sites. We established a total of 196 transects resulting
in a sampling density of 1 transect/3.5 ha.
We recorded the presence or absence of Maritime Pocket Gopher mounds within
a 4 m x 10 m belt along each transect. We chose a 4-m-wide belt because we could
easily observe pocket gopher mounds on either side of the center of the belt. We
calculated the distance of each transect from roads using ArcGIS 9.1 software.
We estimated percent cover of exotic grass species, native grasses and grass-like
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plants, forbs, bare ground, and ground litter within three 20 cm x 50 cm quadrats
placed at 0, 5, and 10 m along each of the 196 transects using methods of Daubenmire
(1959). We collected soil from the upper 12 cm at 0 m, 5 m, and 10 m along
each transect using a 2-cm-diameter soil probe. Samples from within a transect
were combined, placed in pans, spread out uniformly, and dried at room temperature
to a constant mass. We sieved the samples using a 5-mm sieve to remove roots
and other debris and analyzed the composite soil samples from each transect to determine
soil texture, pH, nitrate nitrogen (NO3-N), phosphorous (P), and potassium
(K). We used the hydrometer procecure to estimate soil texture (Day 1965, Murphy
and Riley 1962), a 1 N KCl solution to evaluate NO3-N, a cadmium column
and spectrophotometer measurement to estimate the reduction of nitrate to nitrite
(Keeney and Nelson 1982), a hydrogen-selective electrode to determine soil pH, a
conductivity probe to estimate conductivity (Schofield and Taylor 1955), and the
inductively coupled plasma (ICP) method and Mehlich III extractant d to determine
P and K (Mehlich 1978, 1984).
We employed t-tests to compare percent cover of ground litter, Bermuda Grass,
Buffel Grass, Guinea Grass, Kleberg Bluestem, native forbs, and native grasses;
% clay; NO3-N(μg/ml); K (μg/ml); pH; P (μg/ml); % sand; % silt; % bare ground;
and distance from roads (km) between transects with ≥1 Maritime Pocket Gopher
mound and transects that did not contain Maritime Pocket Gopher mounds . We
tested distributions of residual errors for normality via the Shapiro-Wilk test (Steel
and Torrie 1980). Variables that differed significantly (P < 0.05) between used and
non-used sites were included as predictor variables in further analyses using logistic
regression models to estimate 3rd-order resource-selection functions (Johnson
1980, Manly et al. 2002). The response variable in logistic regression was the log
odds of presence of Maritime Pocket Gopher mounds (Hosmer and Lemeshow
1989, Moruzzi et al. 2002). Predictor variables in logistic regression models included
litter percent cover, percent clay, Kleberg Bluestem percent canopy cover,
and native grass cover. We used combinations of the 4 variables to produce 15
candidate models. We used Akaike weights to compare models (Burnham and Anderson
2002) using program MuMIn (Barton 2013) in Program R.
Results
Gopher-population assessment
Of the 557 transects surveyed on NAS Corpus Christi and NALF Waldron, we
observed Maritime Pocket Gopher mounds on 308 (55.3%) transects. We documented
14,627 Maritime Pocket Gopher mounds constituting 1155 suspected burrow
systems on Navy property. The greatest mound density observed on a transect for
NAS Corpus Christi and NALF Waldron was 402 mounds/ha and 208 mounds/ha,
respectively. Most transects that did not contain observable Maritime Pocket Gopher
mounds were composed of large swaths of concrete or live oak. Maritime Pocket
Gopher mounds comprised 10 ha (1.5%) and 0.4 ha (0.2%) of useable space on NAS
Corpus Christi (669.5 ha) and NALF Waldron (190 ha), respectively.
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Macrohabitat assessment
We identified 15 plant communities and 7 habitat disturbance types on NAS
Corpus Christi and NALF Waldron (Tables 1, 2). The composition of plant communities
on NAS Corpus Christi and NALF Waldron was 3.7% salt marsh, 19.0%
Bermuda Grass, 3.3% Bermuda Grass and Guinea Grass mixture, 1.0% native
prairie and Guinea Grass intermixed with debris, 8.5% golf course grasses, 0.8%
Guinea Grass, 0.3% Guinea Grass and Buffel Grass mixture, 0.9% Gulf Cordgrass,
8.9% Live Oak, 4.6% Honey Mesquite and Guinea Grass, 43.2% native
prairie, 2.9% restoration field, 1.0% softball field or park, 1.4% St. Augustine
Grass, and 0.4% abandoned vegetated runway (Table 1). The composition of
habitat disturbances on NAS Corpus Christi and NALF Waldron was 1.4% construction,
32.0% developed, 7.3% manicured, 36.4% mowed, 5.3% previously
disturbed, 0.1% shrub removal, and 17.5% untouched (Table 2).
Macrohabitat selection
Gopher mounds were not distributed proportionately to habitat type (χ2 = 92,454;
P < 0.0001) or disturbance type (χ2 = 121,585; P < 0.0001). Nearly 92.5% of Maritime
Pocket Gopher mounds were observed in native prairie (48.9%), Bermuda
Grass (20.7%), restoration fields (16.6%), and Bermuda and Guinea grass associations
(6.2%) (Table 1). Maritime Pocket Gophers used all other habitat types less
often. The restoration fields accounted for nearly 74% of the observed χ2 value; the
Table 1. Occurrence of Maritime Pocket Gopher mounds within various habitat types on Naval Air
Station-Corpus Christi and Waldron Airfield, Corpus Christi, TX. χ 2 = 92,453.9; P < 0.0001. Outcomes:
P = preferred, A = avoided.
% of Area of % observed
Total total gopher mounds in each Confidence
Disturbance area (m2) area Observed Expected area interval Outcome
Salt Marsh 231,923.7 3.70 1885.3 3861.4 1.81 0.0170–0.0192 A
Bermuda Grass 1,191,263.5 19.00 21,561.5 19,828.9 20.66 0.2031–0.2101 P
Bermuda Grass/ 209,457.5 3.33 6455.3 3475.3 6.19 0.0597–0.0639 P
Guinea Grass
Debris-Native/ 64,655.1 1.03 347.2 1074.9 0.33 0.0028–0.0038 A
Guinea Grass
Golf course 535,332.6 8.51 2992.7 8881.3 2.87 0.0273–0.0301 A
Guinea Grass 50,604.3 0.81 133.4 845.3 0.13 0.0010–0.0016 A
Guinea Grass/ 18,129.9 0.29 0.0 302.7 0.00 0.0000–0.0000 A
Buffel Grass
Gulf Cordgrass 58,679.8 0.93 0.0 970.6 0.00 0.0000–0.0000 A
Live Oak 562,467.9 8.94 2081.8 9330.0 1.99 0.0187–0.0211 A
Honey Mesquite/ 286,113.1 4.55 155.6 4748.5 0.15 0.0012–0.0018 A
Guinea Grass
Native prairie 2,715,941.2 43.20 51,058.4 45,032.5 48.92 0.4847–0.4933 P
Restoration field 180,390.1 2.87 17,296.0 2995.2 16.57 0.1625–0.1689 P
Softball park 65,506.0 1.04 278.4 1085.4 0.27 0.0023–0.0031 A
St. Augustine Grass 91,182.4 1.45 117.1 1513.3 0.11 0.0008–0.0014 A
Vegetated runway 27,406.6 0.40 0.0 417.4 0.00 0.0000–0.0000 A
Total 6,289,053.7 100.00 104,362.7 104,362.7 100.00
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other frequently used habitat types cumulatively accounted for only 4.6% of the observed
χ2 value. Of the remaining 21% of the total χ2 value, Live Oak and the Honey
Mesquite and Guinea Grass association accounted for half (5.4% and 4.8%, respectively)
of the χ2 value.
Maritime Pocket Gophers frequently used areas that were mowed or disturbed
(i.e., construction, shrub removal, or previously mowed) and apparently did not
use developed, manicured, and untouched areas (Table 2). We observed the greatest
number of gopher mounds in mowed and previously disturbed areas (72.2%
and 21.2%, respectively). These same 2 disturbance types accounted for 62.5% of
the observed χ2 value. The remaining 2 disturbance types that were used relatively
more often (i.e., construction and shrub removal) accounted for only 0.5% of the
observed χ2 value. Of the disturbance types that were not apparently used by Maritime
Pocket Gophers, developed and untouched habitat disturbances accounted for
the majority (23.1% and 11.2%, respectively) of the remaining observed χ 2 value.
Microhabitat assessment
Maritime Pocket Gopher mounds were present on 28% (n = 196) of the transects
on NAS Corpus Christi and NALF Waldron. Bare ground; percent cover of Guinea
Grass, Buffel Grass, native grasses, and native forbs; soil pH, nitrate nitrogen, and
phosphorus did not differ significantly (P > 0.05) between transects with pocket
gopher mounds and those with no pocket gopher mounds (Appendix 3). Percent soil
potassium, sand, and silt were strongly related to percent clay (r ≥ 0.86) and were
not included in further analyses. Percent clay, percent litter cover, percent Kleberg
Bluestem cover, and percent native grass cover differed (P < 0.031) between transects
with and without Maritime Pocket Gopher mounds.
The logistic regression model with litter and percent clay as independent variables
and presence of pocket gopher mounds as the dependent variable was the
best model to explain habitat selection by Maritime Pocket Gopher based on AICc
weights (Tables 3, 4). Pocket gophers were most likely to be found on sandy soils
with ≤15% litter. Relative probability of use of a site by pocket gophers declined
dramatically with increasing soil-clay content (Fig. 1).
Table 2. Occurrence of Maritime Pocket Gopher mounds on various habitat disturbances on Naval Air
Station Corpus Christi and Waldron Airfield, Corpus Christi, TX. χ 2 = 121,585; P < 0001. Outcomes:
P = preferred, A = avoided.
% of Area of % observed
Total total gopher mounds in each Confidence
Disturbance area (m2) area Observed Expected area interval Outcome
Construction 183,131.0 1.4 1970.0 1280.9 2.2 0.0211–0.0232 P
Developed 4,047,333.6 32.0 577.1 29,278.0 0.6 0.0054–0.0066 A
Manicured 919,021.1 7.3 1,979.4 6679.1 2.2 0.0208–0.0232 A
Mowed 4,599,146.9 36.4 66,035.5 33,303.8 72.2 0.7184–0.7256 P
Previously disturbed 665,795.5 5.3 19,425.4 4849.2 21.2 0.2087–0.2153 P
Shrub removal 18,799.7 0.1 233.7 91.5 0.2 0.0016–0.0024 P
Untouched 2,211,130.4 17.5 1272.8 16,011.4 1.4 0.0130–0.0150 A
Total 12,644,358.0 100.0 91,493.9 91,493.9 100.0
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Discussion
The distribution of Maritime Pocket Gophers was influenced by soil-particle
size, plant community, type of habitat disturbance, and quantity of ground litter.
Distribution of pocket gophers typically is limited by the quality, type, and nutrient
content of soil (Huntly and Inouye 1988, Miller 1964). Because of their fossorial
nature, Geomys spp. prefer sandy to loamy soils (Baker et al. 2003) presumably to
facilitate the digging of burrows. The main habitat type where dredge spoils (i.e.,
clay soils) occurred was within the salt-marsh plant community, which was apparently
avoided by pocket gophers in our study. Although some gopher mounds were
located within this habitat type, most of the mounds occurred on the perimeter of
the area. Our finding that Maritime Pocket Gophers apparently avoid areas with
clay soils agrees with other researchers’ results, suggesting that clay soil represents
a geographic barrier for immigration by this species (Connior et al. 2010; Davis
1940, 1974; Hoffman et al. 2007). Rezsutek and Cameron (1998) found a direct
Table 3. Multiple logistic regression models with AICc values for predicting the relative probability
of use by pocket gophers (n = 196 observations), Nueces County, TX, June 2006.
AICc -2 Log
Independent variables k AICc ΔAICc weight likelihood
Clay, litter 3 205.82 0.00 0.430 199.70
Clay, Kleberg Bluestem, litter 4 206.83 1.01 0.260 198.62
Clay, litter, native grass 4 207.37 1.55 0.199 199.16
Clay, Kleberg Bluestem, litter, native grass 5 208.56 2.74 0.110 198.24
Clay, native grass 3 218.61 12.79 0.001 212.48
Clay 2 220.06 14.24 0.000 216.00
Clay, Kleberg Bluestem, native grass 4 220.29 14.46 0.000 212.08
Clay, Kleberg Bluestem 3 221.42 15.60 0.000 215.30
Kleberg Bluestem, litter 3 225.62 19.80 0.000 219.50
Kleberg Bluestem, litter native grass 4 225.71 19.89 0.000 217.50
Litter, native grass 3 226.87 21.05 0.000 220.75
Litter 2 228.21 22.38 0.000 224.14
Native grass 2 230.72 24.90 0.000 226.66
Kleberg Bluestem, native grass 3 230.76 24.94 0.000 224.64
Kleberg Bluestem 2 233.33 27.51 0.000 229.27
Null model 1 234.68 28.86 0.000 232.66
Table 4. Model-averaged parameter estimates, standard errors, an d confidence intervals.
Adjusted
Parameter Estimate Standard error standard error Lower 95% CI Upper 95% CI
Intercept 0.420 0.359 0.361 -0.288 1.127
Clay -0.088 0.023 0.023 -0.134 -0.042
Litter -0.038 0.012 0.012 -0.062 -0.015
Kleberg Bluestem -0.019 0.021 0.021 -0.060 0.022
Native grass 0.003 0.004 0.004 -0.006 0.012
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relationship between abundance of pocket gophers and dicots. Similarly, most
Maritime Pocket Gopher mounds in our study were located in habitat types with
the greatest abundance and diversity of forbs (i.e., native prairie and restoration
fields). Although pocket gophers in our study more often used the Bermuda Grass
monocultures, this habitat type was maintained in an early successional condition
by frequent mowing. Habitat disturbance is potentially disruptive to mammalian
populations, yet periodic mowing may be necessary to maintain the early successional
vegetation that some mammals prefer or require (Slade and Crain 2006).
Pocket gophers tend to be found in areas with high nutritional quality due to the
high energy requirements of fossorial animals (Inouye et al. 1997). Mowing maintained
habitat types at an early successional stage, and thus, kept vegetation at a
young growing state with higher nutritive value. Therefore, Maritime Pocket Gophers
in our study apparently preferred habitats that had a frequent mowing regime.
In addition, more pocket gopher mounds occurred in areas where there was less
litter cover, which coincided with mowed areas as opposed to undisturbed habitat
types. Buechner (1942), Connior et al. (2010), and Philips (1936) obtained similar
results for other species of pocket gophers and reported many more of them found
on lawns than on areas free from anthropomorphic influences.
Previous studies have noted the increase of pocket gopher density in grazed areas
(Buechner 1942, Reichman and Baker 1972). However, the apparent preference
of Maritime Pocket Gophers for grass areas due to their having been mowed has
not been previously reported. The effects of mowing and grazing on grassland communities
can be similar. Consequently, if the effects are similar and pocket gophers
preferred grazed areas in previous studies (Buechner 1942, Reichman and Baker
Figure 1. Relative
probability of occurrence
of a pocket
gopher mounds (n =
196 transects) in relation
to percent litter
cover and percent
clay, Nueces County,
TX, June 2006.
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50
1972), then it is reasonable to assume that pocket gophers also should prefer mowed
over unmowed areas. Buechner (1942) assumed that grazing provided more food
for gophers by keeping the vegetation in a productive stage of development, which
in turn, increased the number of gophers. Pocket gophers may also be attracted to
mowed or grazed habitats because of the greater root biomass of plants in those
areas. However, this effect is dependent on the intensity of grazing or mowing.
Below-ground biomass increases with increased mowing intensity (Dickinson and
Polwart 1982) and grazing (Milchunas and Lauenroth 1993, Pearson 1965, Sims
and Singh 1978). The proportion of roots in Maritime Pocket Gophers’ diet is unknown,
but roots do dominate the diet of a related gopher species, Geomys attwateri
Merriam (Attwater’s Pocket Gopher), for which up to 71% of its total diet consisted
of roots (Williams and Cameron 1986).
The apparent avoidance of shrubland and forested habitats (i.e., Honey Mesquite
and Live Oak, respectively) by Maritime Pocket Gophers is probably due to the loss
of on-ground primary productivity because of tree-canopy cover. Pocket gophers
tend to prefer areas of high primary productivity (Inouye et al. 1997). Additionally,
root mass of browse species may increase the difficulty of burrow construction.
Because our study area had a history of shredding such habitat within the recent
past (i.e., >15 years earlier), root-mass diameters of Live Oak and Mesquite often
exceeded 2 m, as witnessed from the excavation of similar root masses from the
nearby restoration habitat type.
Maritime Pocket Gophers apparently avoided the developed, manicured, and
untouched habitat disturbance types. Developed areas (i.e., asphalt, concrete, buildings,
etc.) were used less often for the obvious reasons of lack of available food
and reduced oxygen turnover rate. Small swaths of open ground occurred within
developed areas and we occasionally found gopher mounds in such areas. However,
it appeared that Maritime Pocket Gophers used developed areas mainly as travel
corridors to get to an adjoining hospitable habitat. When we found mounds in developed
areas, they usually occurred in a straight line along a concrete curb, sidewalk,
or led directly underneath a road and continued on the other side within available
open ground. Reasons for the avoidance of manicured areas are unclear.
Habitat fragmentation and urbanization also may have played a role in the lack
of Maritime Pocket Gophers in some areas. Some regions of potentially good habitat
for Maritime Pocket Gophers (i.e. deep sandy soils, mowed native prairie) were
devoid of gophers, but were surrounded on all sides by development (i.e., concrete,
buildings, and asphalt). Such areas may have been too far removed for pocket gophers
to gain access.
Management implications
Several management recommendations can be inferred from our research. Although
the Navy must maintain the Intercoastal Waterway, placement of dredged
soils—typically >70% clay, on top of sandy soils—should be avoided because the
presence of clay will limits Maritime Pocket Gopher use. Second, abandoned airplane
runways and dense oak secondary growth should be restored to native prairie to
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J.D. Cortez, S.E. Henke, D.W. Wiemers, T.E. Fulbright, D.B. Wester, and R. Riddle
2015 Vol. 14, No. 1
accommodate the overall habitat needs of this species. Corridors of suitable habitat
should be established and maintained to encourage use by Maritime Pocket Gophers;
however, minimum width and lengths of such corridors that will encourage use by
Maritime Pocket Gophers wil need to be determined. Although this gopher species
is under consideration for federal threatened or endangered status, the location of
gophers on DOD lands places them in conflict with naval operations. As a result,
additional research is needed to determine if Maritime Pocket Gophers can tolerate
relocation in the event a “pest-management” relocation program becomes necessary.
Acknowledgments
Funding was provided by the US Navy. This is publication number 11-115 of the Caesar
Kleberg Wildlife Research Institute.
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Appendix 1. Plant community categories for NAS Corpus Christi and NALF Waldron Station
Plant community Description
Salt marsh Soil with high salinity, >95% salt-tolerant plants
Bermuda Grass >95% Bermuda Grass
Bermuda Grass/Guinea Grass less than 95% Bermuda + less than 95% Guinea grass, but >95%
Bermuda Grass and Guinea Grass combination
Debris-native/Guinea Grass less than 95% native prairie + less than 95% Guinea Grass, but >95%
native prairie and Guinea Grass combination with
scattered concrete and metal debris
Golf course >95% golf course grass species
Guinea Ggrass >95% Guinea Grass
Guinea Grass/Buffel Grass <95% Guinea Grass + <95% Buffel Grass, but > 95%
Guinea Grass and Buffel Grass combination
Gulf Cordgrass >95% Gulf Cordgrass
Live Oak >95% Live Oak forest
Honey Mesquite/Guinea Grass less than 95% Honey Mesquite forest + less than 95% Guinea Grass, but
>95% Guinea Grass and Honey Mesquite forest
combination
Native prairie >95% mixture of native grasses and forbs
Restoration field >95% mixture of native grasses with Guinea Grass and
Bermuda Grass
Softball field/park >95% softball field grass species
St. Augustine Grass >95% St. Augustine Grass
Vegetated runway >95% concrete slab with vegetative growth on top and
within cracks of concrete
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Appendix 2. Habitat-disturbance categories for NAS Corpus Christi and NALF Waldron
Station in southern Texas.
Habitat disturbance Description
Construction Active construction site
Developed Asphalt, concrete, or structure
Manicured Planted lawns, fertilized, watered, mowed weekly
Mowed Mowed (>2 weeks less than 6 months old)
Previously disturbed Previously mowed (6–12 months old)
Shrub removal Shredded Live Oak forest with tree bases uprooted
Untouched Undeveloped and not mowed (>12 months)
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Appendix 3. Means and lower (LCI) and upper (UCI) 95% confidence intervals and results of t-tests for vegetation and soil variables, bare
ground, and distance from roads at transects with ≥1 pocket gopher mound present and transects with no pocket gopher mounds present,
Nueces County, TX, June 2006.
Used sites (n = 55) Unused sites (n = 141) Difference (n = 194)
Variable Mean LCI UCI Mean LCI UCI Mean LCI UCI t P
Vegetation
Bermuda Grass (%) 17 9 25 12 8 16 5 -4 14 1.04 0.302
Buffel Grass (%) 3 less than 1 7 3 1 4 1 -3 5 0.50 0.618
Guinea Grass (%) 4 less than 1 7 8 4 12 -4 -10 1 -1.63 0.106
Kleberg Bluestem (%) 1 -1 3 5 2 8 -4 -7 1 -2.18 0.031
Native forbs (%) 21 16 26 17 14 21 4 -3 10 1.12 0.263
Native grasses (%) 54 43 65 38 32 45 16 3 29 2.48 0.014
Litter (%) 7 5 10 17 12 21 -9 -14 -5 -3.84 less than 0.001
Soil
Clay (%) 8 7 10 15 13 17 -7 -10 -4 -5.19 less than 0.001
NO3-N (μg/ml) 6 4 8 6 4 7 less than 1 -2 3 0.25 0.804
K (μg/ml) 121 98 143 232 199 264 -111 -151 -72 -5.60 less than 0.001
pH 7 7 8 7 7 8 less than 1 less than 1 less than 1 0.48 0.630
P (μg/ml) 24 9 39 15 13 18 8 -7 23 1.10 0.276
Sand (%) 86 84 88 75 72 78 11 7 15 5.92 less than 0.001
Silt (%) 6 5 6 10 9 11 -4 -6 -3 -5.61 less than 0.001
Other
Bare ground (%) 4 2 6 6 3 9 -2 -6 1 -1.32 0.188
Distance from roads (km) 65 51 80 57 48 65 8 -8 25 1.03 0.303