RH: McDonald---Habitats of muskrats in Oklahoma
USE OF HABITAT DURING DROUGHT BY THE COMMON MUSKRAT (ONDATRA
ZIBETHICUS) IN SOUTHWESTERN OKLAHOMA
BRANDON MCDONALD
Department of Biology, Midwestern State University, Wichita Falls, TX 76308
Present Address: Department of Biological Sciences, Cameron University, Lawton, OK
73505
Correspondent: bmcdonal@cameron.edu
ABSTRACT---Status of the common muskrat (Ondatra zibethicus) in the southern
High Plains and western Rolling Plains has remained uncertain due to a scarcity of records.
I examined diversity of riparian habitats, frequency of occurrence of habitats, availability
of habitats, and use of habitats by muskrats within an irrigation drainage system in
southwestern Oklahoma during prolonged drought. Six major habitats were supported in
31 km of drainage channels. These included, in order of descending frequency, emergent
cattails, prairie sedge meadow, cattail-forested transition, forest, turf grass, and pioneer
mudflat. Of habitats in drainage systems, 60% were available for muskrats as defined by
presence of water. Availability differed among habitats; prairie sedge meadow had highest
availability and emergent cattails was the lowest in availability. Use of habitats differed
significantly among habitats and was driven by availability.
RESUMEN---El estatus de las almizcleras (Ondatra zibethicus) en las altiplanicies
meridionales y las planicies ondulantes occidentales se ha mantenido incierto debido a la
escasez de récordes. Yo examiné la diversidad de hábitats ribereños, la frecuencia de
ocurrencia de diferentes tipos de hábitats, la disponibilidad de dichos tipos de hábitats, y el
uso por las almizcleras de un sistema de drenaje de irrigación en el suroeste de Oklahoma
durante un periodo de sequía prolongado. Treinta y un kilómetros de canales de drenaje
sostenían una riqueza de seis tipos principales de hábitats; incluyendo en orden de
frecuencia descendiente: anea emergente, juncia de prado, anea en transición a forestación,
anea forestada, césped y marisma pionera. El 60% de los hábitats del sistema de drenaje
fueron clasificados como disponibles para las almizcleras de acuerdo a la presencia de
agua. La disponibilidad difería entre los tipos de hábitats; la juncia de prado tenía la
disponibilidad más alta mientras que la anea emergente tenía la más baja. El uso de los
hábitats por las almizcleras variaba significativamente entre los tipos de hábitats, y era
determinado por su disponibilidad.
The common muskrat (Ondatra zibethicus) is the most widespread and abundant
semi-aquatic fur-bearer in North America. Its extensive geographical range coincides with
an abundance of suitable wetland habitats (Wilner et al., 1980; Parker and Maxwell, 1984;
Dalquest and Schultz, 1992; Clark and Kroeker, 1993; Virgil and Messier, 1996; Engeman
and Whisson, 2003). Arid regions of North America where, rainfall is annually and
seasonally variable, watercourses often are ephemeral, and permanent bodies of water are
few and isolated, mark the southwestern boundaries of the geographic range of muskrats.
Southwestern populations persist in isolated habitats such as desert water holes, man-made
reservoirs, and agricultural irrigation systems (Dixon, 1922; Storer, 1937; Errington,
1951). Isolation of habitats is attributed to a lack of wetland corridors across southwestern
margins of the range (Bolen et al., 1989).
Verified records of muskrats in the High Plains-Rolling Plains interface are few
and isolated in Texas (Bailey, 1905; Jones et al., 1988; Stangl et al., 1989; Dalquest and
Schultz, 1992; Schmidly, 2004a) and Oklahoma (Caire et al., 1989; Braun and Relevez,
2005; McDonald, 2006a). During the 20th century, muskrats largely have disappeared
from tributaries of the Pecos and Canadian rivers in western and habitats in northern
Texas, respectively (Schmidly, 2004b; F. B. Stangl, pers. comm.)
McDonald (2006b) suggested that gradients of abundance of muskrats from east to
west and northeast to southeast, as previously described by Caire et al. (1989), are
associated with large-scale spatial variations in environmental factors (e.g., average annual
precipitation) across Oklahoma. Seasonal variation in rainfall in western portions of
Oklahoma contributes to temporally narrow corridors of dispersal and seasonal
fluctuations in otherwise suitable habitats. Combinations of natural effects and
anthropogenic alterations of the landscape likely have left western populations disjunct
(McDonald, 2006b), although viability of populations and connectivity of suitable habitats
remain speculative (McDonald, 2006a). Glass (1952) noted that muskrats in Oklahoma
are of interest to researchers examining the ecology of a species in marginal environmental
conditions.
Drought in southwestern Oklahoma that peaked in 2006, afforded an opportunity to
investigate ecological characteristics of a peripheral population of muskrats in marginal
conditions. I examined an irrigation drainage system and described the diversity of
riparian wetland habitats, relative frequencies of habitats, availability of habitats, and use
of habitats by muskrats.
MATERIALS AND METHODS---Study Area---The Blair irrigation drainages are
human-made channels lying parallel and perpendicular to the Altus-Lugert Main Irrigation
Canal in and around the town of Blair, Jackson County, Oklahoma (Fig. 1). Nine
drainages total ca. 31 km of drainage channels. Channels are steep banked (50-90◦) and
reach heights at banks of 6 m. Drainages function in collecting excess surface water from
occasional heavy rainfall and overflows of the main irrigation canal; thereby, preventing
damage from floods to property, soil, and crops near Blair. The drainages have existed as
permanent sources of water since the early 1940s, when the United States Bureau of
Reclamation constructed the Altus-Lugert Irrigation District. Drainage channels now
support riparian habitats and associated wetland flora and fauna.
Classification of Habitat---I randomly selected 75-m sampling transects to
represent every 200-m length of each drainage channel. I randomly assigned two sampling
points per transect, each intersecting the channel by 90º allowing for a cross-section
measurement of 34 environmental variables within the drainage channel and extending into
the bank-side herbaceous belt. Most variables estimated were regarded previously as
significant to suitability of habitat for muskrats (Wilner et al., 1980; Nadeau et al., 1995).
Environmental variables included height of bank, slope of bank, cover of bank-side by
grasses, cover of bank-side by forbs, cover of bank-side by woody plants, cover of bankside by litter, cover of bank-side by bare soil, width of bank-side herbaceous belt, cover of
bank-side over-story canopy, width of channel, width of water, depth of water at bank,
depth of water between bank and center of channel, depth of water at center of channel,
greatest depth of water, percentage water capacity of channel, percentage of open water
(i.e., no cover by aquatic vegetation), cover of submerged vegetation at bank, cover of
submerged vegetation at ¼ of channel, cover of submerged vegetation at center of channel,
cover of floating vegetation at bank, cover of floating vegetation at ¼ of channel, cover of
floating vegetation at center of channel, cover of emergent sedge-rush-grass vegetation at
bank, cover of emergent sedge-rush-grass vegetation at ¼ of channel, cover of sedge-rushgrass vegetation at center of channel, cover of cattails at bank, cover of cattails at ¼ of
channel, cover of cattails at center of channel, cover of over-story canopy at center of
channel, distance to nearest tree, diameter at breast height (dbh) of nearest tree, distance to
nearest neighboring tree, and dbh of nearest neighboring tree. I used a 1-m² PVC frame to
estimate values of surface cover. I estimated cover of overstory canopy with the aid of a
handheld densiometer (Model-C, R. Lemmon Forest Densiometers, Bartlesville,
Oklahoma). I used a 2-m PVC rod marked with 0.1-m increments to measure depth of
water, height of bank, and dbh.
Principal-components analysis was used to determine which variables significantly
contributed to overall variation among transects. Loadings on components >0.30, with
>100 samples, were accepted as significant, equivocally at the 99% confidence level (i.e.,
P < 0.01; following Hair et al., 1987; Tabachnick and Fidell, 1989; McCarigal et al., 2000;
Perez-Neto et al., 2003). I used results of principal-components analysis and qualitative
notes to determine major habitats present within the drainage system. I assigned titles to
habitat categories following a plant-functional-group method similar to Harris and
Marshall (1962), Day et al. (1988), and Galatowitsch and van der Valk (1996).
I used a handheld global positioning system (Garmin eTrex - Legend, Garmin
International, Olathe, Kansas) to assign latitude and longitude coordinates of sampling
points along each of the drainages. Coordinates in conjunction with United States
Geological Survey topographical maps were used to precisely record all pertinent spatial
information.
Relative Frequency of Habitats---Based on results of principal-components
analysis and qualitative data, I subdivided the drainage system into the following habitats:
emergent cattails (thick cattails), prairie sedge meadows (thick cover of grasses, sedges,
rushes), forest (thick cover by over-story canopy), cattail-forest transition (cattaildominated channel with minimal but significant cover by over-story canopy), pioneer
mudflat (high erosion potential and bank-side vegetation sparse and dominated by early
succession taxa), and turf-grass (bank-side cover of Bermuda grass Cynodon dactylon).
These six habitats parallel the marsh community, drought-induced, model of gradient
succession of Harris and Marshall (1962), where sequence of community seres is 1) annual
weeds, 2) perennial emergents, 3) composite of annuals, perennials, and woody plants, and
4) woody plants (a list of plants in each habitat is available from the author).
I assigned transects to appropriate habitats and calculated total counts for transects
in each habitat. Because number of transects in each habitat were unequal, I used relative
frequencies. I calculated relative frequencies by dividing number of transects of each
habitat by total number of transects in the drainage system. Values of relative frequency
were square-root-arcsine transformed as outlined by R. High
(http://www.uoregon.edu/~robinh/statistics.html) and compared with chi-squared analysis.
All statistical analyses were performed with PAST version 1.39 (Hammer et al., 2001).
Availability of Habitats---Availability was defined as transects with a minimum
depth of water of 0.10 m. Optimal depths of water for inhabitance by muskrats are 0.151.2 m (Allen and Hoffman, 1984; Schmidly, 2004b); however, based on prior experience,
0.10 m was judged minimally suitable for a combination of dispersal activities and
residency.
I divided number of available transects within each habitat by total number of
transects (available and unavailable) within each habitat to obtain percentage availability to
be compared among all habitats. I square-root-arcsine transformed data on availability of
habitat and compared relative availability among habitats with chi-squared analysis.
Use of Habitats---I examined each transect for evidence of muskrats to
diagnostically indicate use of habitats. Verifiable evidence included burrows, feeding
stations, and tracks. This census method has been employed routinely and is regarded as
non-intrusive, cost-effective, and labor efficient (Errington, 1940; Sprugel, 1951; Brooks
and Dodge, 1986; Nadeau et al., 1995). I recorded burrows and evidence of feeding as old
or recent. Active burrows were characterized by runways leading to the entrance, as well
as proximal recent evidence of feeding (middens). I visually estimated age of feeding
evidence as recent or old based on degree of desiccation and color of plant remains. I
recorded counts and GPS coordinates of burrows and evidence of feeding for each transect.
I plotted recent and old evidence on a hand-drawn map of the Blair drainage system to
depict an overall pattern of activity within the drainage system.
I pooled counts of burrows and evidence of feeding, both recent and old, for each
habitat. I calculated relative frequency of evidence by dividing number of stations with
evidence for each habitat by total count of stations with evidence in the drainage system.
Values for percentage frequency were square-root-arcsine transformed and compared
across habitats with chi-squared analysis. I used linear-regression analyses to examine
relationships between frequencies of evidence of muskrats among habitats (response
variable), where predictor variables included frequency and availability of habitat .
RESULTS---Classification of Habitats---Principal-components analysis yielded eight
variables that contributed to significant variation among 120 transects. The first two
components (Fig. 2) explained 62% of variation among transects. The first three
components collectively explained 77% of variation. Loadings on component 1 were
dominated by open water (0.55), cover of cattails at bank (-0.53), cover of cattails at center
of channel (-0.50), and distance to nearest tree (-0.31). Loadings on component 2 were
dominated by distance to nearest tree (-0.83) and bank-side over-story canopy (0.31).
Loadings on component 3 were highest for cover by bank-side grasses (0.63), cover of
emergent sedge-rush-grass vegetation (0.48), cover of bare soil (0.36), and bank-side overstory canopy -0.35).
Relative Frequency of Habitats---Habitats significantly differed (X ² = 24.07, df = 5,
P < 0.01) in relative frequency of occurrence throughout the drainage system (Fig. 3).
Emergent cattails (40% of total) was the most abundant habitat, while pioneer mudflat
(5%) and turf grass (6%) habitats were the rarest habitats in the drainage system.
Availability of Habitats---Within the Blair drainage system (31 km), 60% (18.5 km)
was available for habitation by muskrats (Fig. 1). Values for relative availability differed
significantly (X ² = 20.5, df = 5, P < 0.01) among the six habitats, prairie sedge meadows
(96% availability) were more available than all other habitats (Fig. 3).
Use of Habitats---A total of 21 old and 17 recent stations with evidence of
muskrats was identified within the drainage system (Fig. 1). Drainages a and b had
evidence of feeding, but no recent or abandoned burrows. Evidence of feeding was
accompanied by burrows throughout the remainder of drainages in the system (i.e.,
drainages c- i).
Relative frequencies of evidence differed significantly among the six habitats (X ² =
13.2, df = 5, P < 0.03), with prairie sedge meadows accounting for the most use by
muskrats (Fig. 3). Results of linear regression indicated a non-influential relationship
between evidence of muskrats and frequency of habitat (r² = 0.09, P = 0.55). A significant
model was revealed between evidence of muskrats and availability of habitat (r² = 0.87, P
< 0.01).
DISCUSSION---Habitat heterogeneity in the Blair drainage system is due to a
disturbance regime including anthropogenic and natural disturbances. However, drought
(a natural disturbance) is the most influential factor on prairie-wetland ecosystems (Shay
and Shay, 1986) and central to the theme of my study. Due to drought, only 60% of
drainage channels provided habitat for muskrats (Fig. 1). Such drawdowns (Kroll and
Meeks, 1985; Shay and Shay, 1986; Thurber et al., 1991) initially are driven by belowaverage rainfall, but subsequently compounded by prevalence of cattail-dominated
habitats. Cattails gain a competitive advantage over other wetland plants during drought
and rapidly form monotypic stands (Harris and Marshall, 1962; Urban et al., 1993; Shay
and Shay, 1986; Gunderson, 2001). Once a monotypic community of cattails is
established, rates of transpiration increase, thereby, further reducing available water
(Brenzy et al., 1973; Crundwell, 1986; Sanchez-Carrillo, 2004).
The high frequency of occurrence of habitats with emergent cattails (40% of total)
combined with decreased availability (only 40% available) illustrates fragmentation of
habitats within the drainage system. Other habitats with water were separated by extensive
stretches of dry habitats with emergent cattails. Muskrats seeking suitable habitat are
forced to travel without cover of water, thereby, exposing them to increased risks of
predation (Errington, 1940; Sprugel, 1951; Erickson, 1963; Thurber et al., 1991; Clark and
Kroeker, 1993). Peripheral populations suffer increased mortality from predation under
fluctuating water levels and dependence on marginal habitat for cover (Boyce, 1978;
Messier et. al., 1990; Virgil and Messier, 1996, 2000).
Stability of water level is the primary limiting factor for populations of muskrats
(Sather, 1958; Errington et al., 1963; Clark and Kroeker, 1993; Virgil and Messier, 1996,
2000). Distributional patterns of evidence of muskrats combined with results of linear
regression indicate that availability of habitat (i.e., presence of water) is a more influential
limiting factor than structure of plant community in drought conditions. Density of cattails
often is emphasized as a vital component of quality of habitat (Allen and Hoffman, 1984),
but results regarding how cattail marshes influence peripheral populations of muskrats are
absent. Populations in cattail marshes proliferate rapidly and can sustain high mortality
(Bailey, 1937), and are thus, ecologically significant source habitats. However, most
research has taken place in central regions where environmental conditions are most
favorable and large-scale, source-sink, population dynamics and general models of habitat
selection apply. Such results might not apply to peripheral populations in marginal
conditions, such as those throughout the High Plains-Rolling Plains interface.
Muskrats in my study were restricted to isolated habitats, of which 70% were
prairie sedge meadows. Muskrats throughout these habitats were established residents as
indicated by burrows associated with evidence of feeding. Prairie sedge meadows offered
a rich plant community for foraging, although year-round cover provided by dense cattails
was lacking. Activities of muskrats in other isolated habitats without significant cover of
cattails (e.g., turf grass) support the conclusion that availability drives use of habitats in
marginal conditions. Previous authors have suggested that muskrats searching for habitat
in average conditions (Errington, 1951; Shanks and Arthur, 1952) and during drought
(Kroll and Meeks, 1985) are influenced primarily by continuation of water, leaving
composition of plant communities as a secondary and possibly indiscriminate factor in the
search.
Evidence of feeding by muskrats within drainages a and b were remnants of
dispersing individuals based on absence of established burrows, burrowing activity, or
temporary shelters indicative of residency or attempted short-term occupancy (Errington,
1939, 1940; Sprugel, 1951). Muskrats in western Oklahoma disperse in autumn
(McDonald, 2006a) and those in drainages a and b likely were moving in response to
seasonal dispersal cues (Beer and Meyer, 1951), drought (Takos, 1944), or both. Because
drainages a and b were 1,400-2,300 m from the nearest water (over land), it is unlikely that
resident muskrats from an independent water source would traverse such a distance on
regular feeding bouts. However, dispersing muskrats would be attracted to drainages a and
b, due to presence of water. Muskrats travel significant distances over land only when
driven by dispersal cues (Errington, 1940; Sprugel, 1951; Shanks and Arthur, 1952;
Errington et al., 1963; Thurber et al., 1991).
Adversities that muskrats face in drought conditions have been illustrated
throughout the range of this species (e.g., Bellrose and Low, 1943; Errington, 1939, 1963;
Boyce, 1978; Virgil and Messier, 1996). High fecundity, adequate habitats, and
dependable dispersal corridors, minimize potential local extirpation for populations in
central regions. However, peripheral populations exist in continually marginal
environmental conditions (Glass, 1952), which include isolation at the landscape level,
lessened viability of local populations through extirpations, temporally narrow corridors
for dispersal, fluctuations in otherwise suitable habitats, and increased predation facilitated
by exposure during frequent drawdown of water (Sojida and Solberg, 1993; McDonald,
2006b).
I thank the Altus-Lugert Irrigation District and landowners in Jackson County for
access to properties. Gratitude for critical reviews and assistance with this manuscript is
conveyed to F. Stangl, R. Vogtsberger, M. Shipley, M. Rincon-Zachary, M. Husak, and M
Santiago. Financial support was provided by the Texas Academy of Science.
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Submitted 14 September 2007. Accepted ????????????.
Associate Editor was Philip D. Sudman.
FIG. 1---Drainage system near Blair, Jackson County, Oklahoma: Altus-Lugert
Main Irrigation Canal (black and white line oriented north-south), drainages (solid lines ai), available habitats in drainage (wide corrugated line), recent evidence of muskrats (solid
circles), and old evidence of muskrats (open circles).
FIG. 2---Scatterplot of principal components 1 and 2 calculated from a study of
habitats used by common muskrats (Ondatra zibethicus) near Blair, Jackson County,
Oklahoma. Vectors (dashed lines) represent significant loadings of variables on
component axes: A, cover of cattails at bank; B, cover of cattails at center of channel; C,
cover by over-story at bank-side; D, cover of overstory in center of channel; E, open water;
F, distance to nearest tree. Triangles indicate transects in relation to respective component
axes and vectors of variables.
FIG. 3---Histograms displaying a) frequency of occurrence, b) availability, and c)
use of habitats by common muskrats (Ondatra zibethicus) near Blair, Jackson County,
Oklahoma: EC, emergent cattails; F, forested; CF, cattail-forested transition; PSM, prairie
sedge meadow; PM, pioneer mudflat; TG, turf grass.
Figure 1
Figure 2
Figure 3