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Abstract
Cheatgrass (Bromus tectorum), an invasive exotic annual plant, is
known to diminish many shrub-dependent organisms; however, relatively
little research has been done on how it affects snake communities. I
conducted a study on Antelope Island State Park, Davis County, Utah,
where cheatgrass is abundant and may be negatively impacting snake
populations. Four study sites with different cheatgrass cover (37.90 –
69.23%) were established to measure relative abundance of snakes. I
trapped snakes using funnel traps attached to drift fence arrays (one
array per site). A total of 35 individuals were captured between June
and September 2005 (221 trap-array days) representing two snake species:
the western yellow bellied racer (Coluber constrictor mormon) and
the Great Basin gopher snake (Pituophis catenifer deserticola).
Linear regression was used to determine if a relationship existed
between the percent of cheatgrass cover and species relative
abundances. A negative relationship was found between the relative
abundances of both snake species and increasing cheatgrass density.
These results suggest that cheatgrass may negatively affect the snake
community in shrub-steppe habitat.
Introduction
Cheatgrass (Bromus tectorum), an invasive
annual plant
introduced from Eurasia, was detected in the western U.S. as early as
the 1890’s (Mack, 1981; Novak and Mack, 2001). Since its introduction
in the Intermountain West, cheatgrass has come to dominate at least
200,000 km² of the shrub-steppe landscape (Mack, 1989). Characteristics
of cheatgrass that allow it to out-compete native perennials include
abundant seed production, rapid germination (Stewart and Hull, 1949) and
its superior competitiveness (Holmgren, 1956; Harris, 1967; Melgoza et
al., 1990). Cheatgrass alters plant community structure (Hulbert, 1955;
Brooks, 2000), soil nutrient cycling (Walker and Smith, 1997; Evans et
al., 2001; Wolfe and Klironomos, 2005), microclimate (D’Antonio and
Vitousek, 1992) and fire frequency (Stewart and Hull, 1949;
Young and Evans, 1978; Walker and
Smith, 1997).
Cheatgrass has diminished many shrub-dependent organisms (Pimentel et
al., 2000) such as shrubland birds (Wiens and
Rottenberry, 1985; Knick and Rotenberry, 1995, 2000), small mammals (Yensen
et al., 1992; Gitzen et al., 2001), and lizards (Newbold, 2005);
however, little is known about how it affects snake communities.
Cheatgrass cover does reduce the detectability of snakes (Hirth et al.,
1969; Mortensen, 2004), but there may be potentially negative
implications to the management and preservation of snakes inhabiting
cheatgrass areas that have yet to be studied (e.g., locomotive
performance and prey carrying capacity may be limited in dense
cheatgrass). In northern Utah, cheatgrass is widespread in shrub areas
with native grasses and may negatively impact snake communities. My
objective was to determine if cheatgrass adversely affects snake
abundance.
Materials and Methods
Study Area. – This study was performed on Antelope Island State Park
(UTM: Zone 12; 395,961 E;
4,545,924 N; elevation 1,310 m; 11, 311 ha) located in
Davis County, Utah.
Antelope Island is the
largest island in the Great Salt Lake (Figure 1) and is not a true
island due to its causeway and a naturally occurring land-bridge during
dry years. Cheatgrass is one of the most abundant plants on the island;
other common shrub vegetation includes grasses and plants such as
bluegrass (Poa spp.), buckwheat (Eriogonum spp.),
wheatgrass (Agropyron spp.), rabbitbrush (Chrysothamnus
spp.), and
sagebrush (Artemisia spp.) (Marshall, 1940).
Field Procedures. – From April to July 2005 four study sites (3
ha each) were selected. Sites were located ≥ 1 km of one another; sites
C and D were established in Bridger Bay and sites A and B in White Rock
Bay (Figure 1). These bays were chosen for their like habitat and
variety of cheatgrass densities. Cheatgrass is common throughout the
island, and each site included some; however, sites differed in percent
of cover. Cheatgrass coverage was determined by walking 20 transects
from the center of each plot, placing a 1-m² wooden frame on the ground
every 10 m and estimating cheatgrass cover within the square to the
nearest 5% (Daubenmire, 1959), for a total of 200 samples per site.
Samples were then calculated to find the mean cheatgrass percentage / m²
for each site.
In the center of each site, a hardware cloth drift fence trap array
(0.635 cm² mesh, 40 cm high × 30 m long) was established in an “X”
pattern. To eliminate gaps between the fence and soil irregularities,
fences were buried at least 5 cm. Attached to each fence were 10
double-ended, hardware cloth funnel traps (0.635 cm² mesh, 38 cm high ×
1 m long)(Cavitt, 2000) placed 5 m apart. Traps were covered with white
corrugated plastic to reduce heat stress of captured animals. Funnel
traps were used as the primary method for data collection (Fitch,
1987). Snakes captured by hand within sites were also recorded.
Captured snakes were identified (Collins and Taggart,
2002) and sexed by hemipenal probing (Schaefer, 1934) with the
exception of some neonates, in which case hemipenal eversion was
performed (Rosen, 1991). Snout-vent lengths (SVL) were measured by
contouring a metric vinyl tape along side the individual’s snout to its
vent. Tail lengths (TL) were also recorded by measuring from the vent
to the tail tip. Snakes were weighed to the nearest ±1 g using a Pesola®
spring scale. Captured snakes were individually marked by clipping
unique combinations of ventral scales (Spellerberg, 1977). Once marked,
snakes were released at the point of capture.
As a result of an abnormally wet and cold spring, sites C and D were
first opened in early-June. Sites A and B however, were relocated to
White Rock Bay to avoid bison (Bos bison) grazing areas
and were reopened in mid-June and early-July, respectively. Traps
were closed from mid-July to early-August to prevent heat stress and
trap mortality due to air temperatures exceeding 38° C. Traps were
closed in late September after an early frost.
Data
Analyses. – Site-specific relative abundances (number of snakes
trapped / 10 trap-array days) were calculated for each species. Linear
regression was used to determine if a relationship existed between
species relative abundance and the percent cover of cheatgrass. SVL
measurements were calculated to find the species mean according to
site. Linear regression was performed for both species to determine if
dense cheatgrass cover affected snake SVL. Chi-square analyses
were used to verify if sex ratios (males:females) differed significantly
from parity. Statistical tests for regression and nonparametric
analyses were conducted using Statistical Package for the Social
Sciences (SPSS) software, version 13.0. Significance level was set at α
= 0.05 for all statistical tests.
Results
The cheatgrass cover percentage analysis determined individual
percentages for each site. All four study sites were distinct in
cheatgrass cover percentage (Table 1). Twenty-eight western
yellow-bellied racers (Coluber constrictor mormon) and seven
Great Basin gopher snakes (Pituophis catenifer deserticola) were
the only species of snakes observed and trapped during 221 trap-array
days. Racers were captured in all four sites, whereas gopher
snakes were found in only three sites (A, B and C; Figure 2). The
desert-striped whipsnake (Masticophis taeniatus taeniatus) was
not observed during the course of this study despite its recorded
presence on the island (Mortensen, 2004).
Relative
Abundance. – Only one marked snake (C. constrictor; site C)
was recaptured. Consequently, no estimates of population size could be
derived. Relative abundance of racers was negatively associated
with percentage of cheatgrass cover (F = 22.065; df model,
error = 1, 2; P = 0.042; R² = 0.917; Figure 2). Likewise, gopher
snake relative abundance showed a significant negative relationship to
increased cheatgrass cover (F = 18.582; df model, error = 1,
2; P = 0.049; R² = 0.903; Figure 2).
SVL Analyses. – We found no significant relationship between
racer and gopher snake SVL with the percent of cheatgrass (F = 0; df
model, error = 1, 2; P = 0.994; R² = 0; F = 4.895, df
model, error = 1, 2; P = 0.270; R² = 0.830).
Sex Ratios. – The sex ratio (males:females) for racers did not
significantly differ from parity (11:16; χ2 = 0.46, df = 1, P > 0.05).
There were too few data to statistically compare the sex ratio for
gopher snakes (3:4).
Discussion
These findings show that snake abundance is comparatively lower in cheatgrass on Antelope Island, suggesting that increasing cheatgrass
density negatively impacts snake communities. However, during a
replicate study (one performed 20 y after the original to monitor
changes in reptile status in Idaho) Cossel (2003) found that snake |
abundance was not cheatgrass.
Yet, the difference between our results may lie in the method through
which we ranked vegetation abundance. Cossel used transect walk-through
and point / line intercept surveys to classify all the major cover types
of vegetation on a four-point scale. Subsequently, he could not detect
a percentage difference among his highest (four, on his scale)
cheatgrass rankings. Furthermore, since Cossel’s study was a replicate,
he was required to utilize previous study sites which exemplified an
array of habitat variables; whereas I was able to limit my sites to a
similar habitat pattern with the only differences being cheatgrass
densities. Thus, he was unable to directly measure cheatgrass effects
on snake abundance using similar habitat with the sole variable being
cheatgrass percentages.
Gopher snakes exhibited lower relative abundances than racers in all
sites, particularly in site D, where gopher snakes were absent. Their
absence could be attributed to lower overall vertebrate prey abundance
upon which they are dependent (Fitch, 1949; Rodríguez-Robles, 1998).
Racers were perhaps more abundant because they can seemingly maneuver
more easily in cheatgrass (Hirth et al., 1969) than the slower, more
robust gopher snakes (Mosauer, 1935). However, studies measuring garter
snake (Thamnophis elegans) locomotion in differing push-point
densities (representing vegetation stalk thickness by inserting nails
into a board), speed was reduced in both experimental populations of
garter snakes as push-(cont'd from the print edition) point densities surpassed intermediate levels
(Jayne, 1986; Kelley et al., 1997). Coupling this could be the
impediment that dense vegetation would have on burst speed, a trait that
is a selected for among garter snakes (Jayne and Bennett, 1990) and a
primary method by which racers obtain prey (Fitch, 1963). Field
observations of racers and gopher snakes at sites with greater cheatgrass density suggest that speed is hindered in both snake species
(personal observation), which may affect the ability to forage and
escape predators.
Alternatively, cheatgrass may negatively affect prey abundance, as seen
in some lizard species. Though lizards do not represent the majority of
gopher snake and racer diets (Klimstra, 1959; Rodriguez-Robles, 1998),
their negative association with cheatgrass is noteworthy. Working in
northern Utah, Newbold (2005) showed that cheatgrass decreased desert
horned lizard (Phrynosoma platyrhinos) locomotive performance and
abundance. Mortensen (2004) found lizard abundance and diversity to be
considerably lower in areas dominated by cheatgrass on Antelope Island.
This may tentatively explain the absence of the lizard-consuming
whipsnake (Parker and Brown, 1980; Camper and Dixon, 2000) during my
survey and provide some explanation for the reduced abundance of gopher
snakes and racers in high cheatgrass sites.
Birds and their eggs make up a small
percentage of gopher snake and
racer diets (Fitch, 1963; Parker and Brown,
1980; Rodríguez-Robles, 1998). Decreasing populations have been
observed among Sage Sparrows (Amphispiza belli), Brewer’s
Sparrows (Spizella breweri), and Sage Thrashers (Oreoscoptes
montanus) because of habitat loss due to wildfire and consequently,
exotic grass invasions (Wiens and Rottenberry, 1985; Knick and
Rottenberry, 1995, 2000). This, in turn, would reduce nesting sites of
shrubland birds in areas dominated by cheatgrass,
discouraging snake foraging in them.
Small mammals are important in gopher snake and racer diets (Fitch,
1949, 1963; Klimstra, 1959; Rodríguez-Robles, 1998; Shewchuk and Austin,
2001). In a small mammal survey performed in the shrub-steppe of
Washington, Gitzen et al. (2001) found that small mammal capture rates
were reduced in cheatgrass sites. Furthermore, in southern Idaho,
Townsend’s ground squirrels (Spermophilus townsendii) and their
burrows were also less abundant in areas with cheatgrass (Yensen et al.,
1992; Van Horne et al., 1997). During spring and summer censuses on
Antelope Island in 2006, I found deer mouse (Peromyscus maniculatus)
relative abundances steadily decreasing as cheatgrass density
increased (L. Hall, unpublished data). Reduced small mammal abundance
does not only limit prey abundance for snakes, but also diminishes
burrow system availability. Both snake species use rodent burrows for
oviposition (Fitch, 1949; Parker and Brown, 1972) and thermoregulation
(Brown, 1973; Huey et al., 1989).
Insects, principally orthopterans, constitute the bulk of racer diet (Klimstra,
1959, Fitch, 1963; Brown, 1973), whereas gopher snakes rely much less on
insects (Parker and Brown, 1980; Rodríguez-Robles, 1998). Among
orthopterans, acridid grasshoppers are favored by racers (Brown, 1973;
Shewchuk and Austin, 2001. In Colorado, Craig et al. (1999) discovered
acridids in a variety of habitats, some of which included cheatgrass.
In a related study from south-central Idaho, Fielding and Brusven
(1993) found relatively high density, but reduced diversity of acridids
in exotic annual sites. Unfortunately, partially digested grasshoppers
recovered from racer stomachs are only identified to family level,
probably because of the difficulty of recognizing any distinguishing
species features beyond that level. Therefore, it is problematic when
concluding if racers are selecting the few acridid species abundant in
high cheatgrass areas; nevertheless, it could be one reason for racer
occurrence (although lowered) in sites C and D.
Invasive plant species are important contributors to biodiversity
declines (Parker et al., 1999; Davis, 2003). Racers and gopher snakes
are essential predators and prey to a variety of organisms in
shrub-steppe food webs (Fitch, 1949, 1963; Parker and Brown, 1980).
Consequently, negatively impacted population dynamics of these snakes
could lead to biological disruptions in shrub ecosystems. The results
from this study support my initial hypothesis that cheatgrass cover
negatively affects snake abundance. It may do so by encumbering
locomotion and by not supporting an adequate density / diversity of
potential prey species.
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