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Nutria (myocastor coypus) in big bend national park; a non-native species in desert wetlands.

Abstract-Nutria are large, semi-aquatic rodents first introduced into the United States from South America as a fur resource during the 1890s. Nutria first were reported at Rio Grande Village, Big Bend National Park, in 1993. During 2004 and 2005, more than 30 locations of nutria activity were documented along a 16 km section of the Rio Grande River from 7.6 km upstream of Rio Grande Village to Boquillas Canyon including the Rio Grande Village beaver pond. Seventeen nutria were captured, marked, and released. Using the Schnabel and Chapman methods, 38-74 nutria were estimated to inhabit the RGV area. Stomach contents (n = 14) contained common reed (Phragmites australis), water pennywort (Hydroelyle umbellata), giant reed (Arundo donax), spi kerush (Eleocharis caribaea), bermudagrass (Cynodon dactylon), water hyssop (Bacopa monnieri), foxtail (Alopecurus sp). and flatsedge (Cyperus sp.). Seven adult nutria were radio-collared and released between May 2004 and June 2005. The mean home-range size was estimated to be 10.0 ha (14.8 ha for males, 2.9 for females), and the mean maximum daily distance moved was estimated to 637.4 m (738.3 m for males, 486 m for females).

Nutria (Myocastor coypus) are large hystricomorph South American rodents adapted to semi-aquatic environments (Gosling 1981). This monotypic species is native to Brazil, Bolivia, Paraguay, Uruguay, Argentina, and Chile (Carter & Leonard 2002) where it occupies wetland habitats such as ponds, streams, rivers, and marshes. Nutria occupy similar habitats throughout their range in North America, including marshes and swamps in both freshwater and brackish water communities (Borgnia et al. 2000).

Nutria were introduced or migrated into 30 states of the United States beginning in California in 1899 (Carter & Leonard 2002) and are considered an aggressively invasive species. High nutria densities are found in the south-central, southeastern, and Atlantic coastal areas of the United States with largest populations occurring in Louisiana and Maryland (Carter & Leonard 2002). The nutria continues to expand its range in some areas of the United States and elsewhere (Bounds et al. 2001).

Nutria were introduced into rivers of arid eastern New Mexico as early as 1938 and now are year-around residents along the Pecos and Rio Grande rivers in Texas and New Mexico (Findley et al. 1975; Schmidly 2004). In Texas, nutria have spread westward to the Trans-Pecos ecological region since their entry into east Texas in the early 1940s (Dozier 1952; Swank & Petrides 1954; Evans 1983; Schmidly 1983). They first were reported from Big Bend National Park (BBNP) along the eastern park boundary in 1993 (R. Skiles, pers. comm.). Restricted riparian habitat along waterways in arid regions limits nutria population size, although colonies of these rodents are capable of living in high densities in small areas (Brown 1975).

The primary foods of nutria are emergent plant species in wetland habitats (Borgnia et al. 2000). Intensive foraging by nutria in wild areas has severely damaged wetlands (Jenkins 2002). They are an increasing concern in areas with limited wetland habitat. Desert wetlands, such as springs, seeps, and riparian corridors are fragile and small in size (Hubbs 1977; Schmidly 1977; Wauer 1977). Nutria activity in these areas could lead to irreparable damage of complex ecological communities and associated indigenous taxa such as the endangered Big Bend gambusia (Gamhusia gaigei) (Williams et al. 1989; Reeder 2001; National Park Service 2006) and the endangered Mexican beaver (Castor canadensis mexicanus). Disruption of forag ing sites, shade, and sheltering vegetation may place the Big Bend gambusia population at critical risk (Rio Grande Fishes Recovery Team 1984). The Rio Grande Village (RGV) beaver population maintains a beaver dam which forms the largest pond containing Big Bend gambusia (Reeder 2001). Should the presence of nutria disrupt the maintenance of the beaver dam and pond, the gambusia population would be further jeopardized (Rio Grande Fishes Recovery Team 1984; National Park Service Water Resources Division 1992).

The objectives of this study were to: 1) determine distribution of nutria along the Rio Grande River from the gravel pit to the mouth of Boquillas Canyon, including Hot Springs, RGV, and Beaver Pond; 2) describe and quantify centers of nutria activity in the research area; 3) quantify the food habits of nutria; 4) estimate nutria population size in the study area; and 5) use radio telemetry to estimate home range size and movements of nutria living in the limited wetland habitat of the Chihuahuan Desert.


Big Bend National Park consists of 324,219 ha located in the Trans-Pecos ecological region of Texas. This represents the largest area of protected Chihuahuan Desert habitat in the United States. The Rio Grande River is the 190 km southern border of the park and creates a productive riparian corridor that is refuge to numerous species of concern. Approximately 10,000 ha of wetlands and 315 water sources exist within BBNP, many found near or along the Rio Grande (Shaw & Finch 1996).

This research project was conducted along a 16 km stretch of the Rio Grande River from a gravel pit located 7.6 km upstream of Rio Grande Village (29[degrees]9.1'N, 103[degrees]0.2'W) to the mouth of Boquillas Canyon (29[degrees]12.2'N, 102[degrees]54.8'W), including the RGV campground (29[degrees]10.8'N, 102[degrees]57.7'W) and adjacent areas, and the Beaver Pond (29[degrees]10.7'N, 102[degrees]57.2'W)(Fig. 1).


The Beaver Pond is bisected by a boardwalk used for visitor access to hiking trails. Farms and villages are located across the Rio Grande River in Mexico. Boquillas del Carmen, Coahuila, Mexico, is the largest village in the area bordering approximately 1.5 km of the Rio Grande.

Dense stands of native common reed (Phragmites australis) and the introduced giant reed (Arundo donax) were found on the United States side of the river, whereas willow baccharis (Baccharis glutinosa), salt cedar (Tamarix sp.), and bermudagrass (Cynodon dactylon) comprised the majority of vegetation on the Mexican side. Common reed was the primary plant occupying the shoreline and interior of the Beaver Pond. Giant reed was more common closer to the river with only a few remaining stands of common cane found along the river bank. Other emergent and submerged vegetation in the Beaver pond included: cattail (Typha latifolia), water hyssop (Bacopa monnieri), spikerush (Eleocharis caribaea), and water pennywort (Hydrocotyle umbellata). Woody vegetation near the Beaver Pond included cottonwoods (Populus acuminata) and huisache (Acacia smallii). Within a short distance of the riparian zone, the vegetation was primarily a thorn-shrub desert characterized by ocotillo (Fouquieria splendens), a variety of yuccas (Yucca sp.), chollas and cactuses (Opuntia sp. and Echinocereus sp., respectively), cenizas (Leucophyllum sp.), Texas persimmon (Diospyros texana), agarito (Berberis trifobliolata), and many other xeric adapted plant species (Wauer 1992; Evans 1998).


Nutria were captured using Tomahawk live traps (90 by 33 by 30 cm, Tomahawk Live Trap Company), baited with sweet potatoes and Victor 2 1/2 cushioned leg-hold traps (Texas State University IACUC #HOASJQ 02, NPS Permit # BIBE-2003-SCI-0003, and BIBE-2004-SCI-0043). Captured nutria were weighed using a spring scale and then sedated with ketamine hydrochloride at 0.25 cc/kg (Jalanka & Roeken 1990; Bo et al. 1994). Gender and standard external measurements of nutria along with field data were recorded. Age was estimated using the hind foot measurement (Adams 1956; Towns et al. 2003). Tags were attached to the hind-foot webbing (right hind-foot, male; left hind-foot, female). Commercial hair bleach and developer (Clairol Company[R]) was applied to the top of the nutria's head and shoulders for improved visibility during visual recaptures (Johnson 1992). Global Positioning System (GPS) data were taken at the site of capture and release. Diurnal and nocturnal surveys were conducted to assess nutria numbers and activity patterns throughout the study.

The nutria population was estimated from capture data using the Schnabel formula (Schnabel 1938) and a modified Chapman formula (Schneider 1998). These estimates were based on number of trap nights, trap success, and number of recaptures. Though the Schnabel method typically is used for closed systems, it allows for multiple trapping efforts where accumulation of captured and marked animals is allowed (Krebs 1989). The Chapman variation of the Petersen estimate also was used because it allows for population estimates to be calculated months after recapture (Schneider 1998).

Fourteen nutria were euthanized to obtain stomach samples for dietary analysis. Stomach contents were placed in 10% formalin for later laboratory analysis. Voucher material (10 skulls, TTU-M 100646 to TTU-M 100655) were deposited in the mammal collection at the Natural Science Research Laboratory, The Museum, Texas Tech University.

Stomach contents were cleaned following methods used by Towns et al. (2003). Approximately 10% of each sample was removed for analysis. Samples were cleared with 6% sodium hypochlorite according to Holechek & Valdez (1985). A ten-point frame (Chamrad & Box 1964) was used to select samples for mounting on slides for identification. Fifty slides were prepared from each stomach sample using Mount-Quick[R] mounting medium and 22 by 22 mm cover slip. Slides were allowed to dry for at least five hours before inspection. A National microscope (MFG# 163-ASC) was used to view each slide. Two fields of view per slide were randomly selected for comparative analysis with reference slides. Plant fragments closest to the pointer within the microscope field of view were identified. One hundred fields of view were examined for each stomach.

For identification, reference slides were prepared according to Green et al. (1985) from roots, stems, and leaves of wetland plants of the RGV area for use in identifying foods eaten by nutria. Epidermal layers from both sides of leaves were used (Korschgen 1980; Towns et al. 2003). Reference material was cleared with bleach as discussed above, mounted to slides using Permount[R] and a 22 by 22 mm coverslip, and allowed to dry for at least two days (Baumgartner & Martin 1939; Green et al. 1985; Litvaitis et al. 1996; Towns et al. 2003). Photographs, using a Nikon Cool Pix 995 camera mounted on the National microscope, were taken and cataloged for comparison.

Percent composition and frequency of occurrence of each plant species per stomach were calculated following Fracker & Brischle (1944) formulae. Data were pooled and an overall percentile calculated based on 1400 fields of view.

Radio collars (Model # HLPM-3180 or Model # HLPM-3210, Wildlife Materials, Murphysboro, IL) were placed on seven nutria. Location data were gathered using a portable receiver (model TRX 100S, Wildlife Materials International, Murphysboro, IL) and Yagi antenna (Y-4FL 151-153 MHz, Televilt TVP Positioning AB, Lindesberg, Sweden). Locations of collared nutria were determined by triangulation. A Garmin GPS 12XL unit (Olathe, KS) was used to record positions. Location data were gathered as close as possible to 300 h, 900 h, 1500 h, and 2100 h each day of tracking.

A digital orthophoto using GIS software (ArcMap 8.3 ESRI[R], Redlands, CA) was used to plot tracking data. Minimum convex polygons (MCP) were constructed to represent the home range for each animal (Mohr 1947). Unsuitable habitat outside wetlands was removed from each MCP using GIS software. Home range area was determined using the GIS software's field calculations function (August et al. 1996; Ostro et al. 1999). Home ranges were calculated only for individuals with < 20 locations. Also, daily linear movement was calculated for each nutria.


A total of 17 nutria were captured, marked, and released during 234 trap nights from March through November 2003. There were five recaptures. The Schnabel population estimate was 38 nutria and the Chapman variation suggested a population of 74 nutria in the study area.

Percent composition in the diet for each food species was 59.9% common reed, 12.7% water pennywort, 6.3% giant reed, 6.1% spikerush, 4.8% bermudagrass, 2.0% water hyssop, 0.9% foxtail (Alopecurus sp.), 0.7% flatsedge (Cyperus sp.), and 6.6% unidentified fragments (Table 1). Frequency of occurrence of food plants among stomach samples was 100 % common reed, 92.9% bermudagrass, 85.7% giant reed, 50.0%) spikeush, 50.0% water hyssop, 50% water pennywort, 28.6% foxtail, 14.3% flatsedge, and 100%) unidentified fragments (Table 1).
Table 1. Percent composition in the diet and frequency of occurrence
of food plants in nutria stomachs from Big Bend National Park.

Species       Percent Composition   Frequency of
                  [+ or -] SE      Occurrence (%)

Common reed   59.9                          100.0
              [+ or -]7.6
Water         12.7                           50.0
pennywort     [+ or -]5.1
Giant reed    6.3                            85.7
              [+ or -] 1.5
Canada        6.1                            50.0
spikesedge    [+ or -]3.5
Bermudagrass  4.8                            92.9
              [+ or -] 0.8
Water         2.0                            50.0
hyssop        [+ or -]0.7
Flatsedge     0.7                            14.3
              [+ or -] 0.6
Foxtail       0.9                            28.6
              [+ or -] 0.5
Unidentified  6.6                           100.0
              [+ or -]0.6

Five adult nutria (three males and two females) were captured and fitted with radio transmitter collars between 13 May 2004 and 24 August 2004. Trapping occurred on 47 nights for a total of 530 trap nights (488 live trap nights and 42 leg-hold trap nights). Data on tracking time period for each animal, MCP home ranges, and mean and maximum daily linear movement are presented in Table 2.
Table 2. Home range sizes and distances moved daily for nulria
inhabiting the Rio Grande River near Rio Grande Village, Big Bend
National Park. MCP (Minimum convex polygons).

Nutria    Days    MCP  Mean daily movement (m)  Maximum daily
        Tracked  (ha)        [+ or -]SD         movement (m)

Nl           53  30.6     362.3 [+ or -] 240.4           830
N2           42   1.0      116.0 [+ or -] 72.9           244
N3           18   2.2       72.9 [+ or -] 69.9           223
N4           14   3.6     285.8 [+ or -] 246.8           749
N5           48  12.8     302.8 [+ or -] 354.0          1141

Home range was calculated for five animals using an average of 27 locations per animal. Duration of tracking ranged from 14 to 53 days. The mean size of nutria home ranges in BBNP was 10.1 ha (SE = 5.55). The mean home range of male nutria was 14.8 ha (SE - 8.60) and the mean home range of females was 2.9 ha (SE = 0.67).

The mean maximum distance traveled per day by nutria near RGV was 637.4 m (SE = 177.44 m) (Table 2). The mean maximum distance traveled was 738 m (SE = 262.97) for males, and 486 m (SE = 263.04) for females. The mean distance traveled by males was 260.3 m (SE = 74.18) per day, while females traveled 179.4 m (SE = 106.46) per day. Two nutria, male N2 and female N3, rarely left the beaver pond. Their movement distances were less than nutria inhabiting riparian areas. The maximum distance traveled by nutria in the beaver pond was < 250 m.

Approximately 30 locations of nutria activity as indicated by tracks and scat were found along the 16 km study area of the Rio Grande. At nutria activity sites, the riparian zone width averaged approximately 25 m. At shoreline areas with nutria sign, vegetation was composed of bermudagrass, salt cedar, willow baccharis, and giant reed. Very little nutria sign was noted in fast moving portions of the river, shallow areas, or within canyons. Nutria activity typically occurred in deep, slow-moving pools with emergent shoreline vegetation and a low or moderate shoreline slope. The majority of nutria activity was recorded in or near the Beaver Pond at Rio Grande Village.


Sample sizes in this study were low because nutria populations are limited to small and dispersed wetlands in this arid environment. After an initial capture, adult nutria avoided traps, thus affecting population estimates. Although nutria appeared abundant in the study area, population estimates may be inflated due to "trap-shy" behavior after handling and marking (Simpson & Swank 1979).

Although nutria sign was noted throughout the study area, nutria activity was greatest in riparian areas with abundant food sources and deeper waters, such as near Hot Springs, the boat ramp at RGV, Boquillas crossing, and in the Beaver Pond. Results from radio telemetry data suggest nutria move freely between the river and adjacent wetland areas.

Dietary components of nutria in BBNP generally conformed to items reported in nutria diets elsewhere. Towns et al. (2003) reported nutria eating water hyssop in the Hill Country of Texas, while Borgnia et al. (2000) found bermudagrass, spikerush, and water pennywort in diets of nutria from the Argentinean pampas. Shirley et al. (1981) and Willner et al. (1979) reported nutria feeding on spikerush, water pennywort, and common reed in Louisiana and Maryland. These plant taxa comprised 78.7% of the nutria's diet from collected individuals in RGV, with common reed contributing 59.9%. Non-native, invasive plants such as bermudagrass and giant reed also were consumed, but in small amounts.

Activity of the Mexican beaver was observed alongside that of nutria in the same regions of the Rio Grande River and the Beaver Pond. These semi-aquatic mammals have similar habitat requirements (Retzer et al. 1956; Novak 1987). Whereas nutria do not typically consume woody vegetation, beaver depend on herbaceous vegetation in habitats with limited woody vegetation (Schmidly 2004) as found in the Beaver Pond and along the Rio Grande River. If nutria foraging activities disrupt the maintenance of the beaver dam and pond by beaver, this could jeopardize the endangered Big Bend gambusia population (Rio Grande Fishes Recovery Team 1984, National Park Service Water Resources Division 1992).

Large stands of giant reed currently exist along the Rio Grande River and adjacent wetland areas. This non-native, invasive plant may dramatically alter the riparian habitat (Bell 1993). Rivers and ponds with a high density of giant reed typically show decreased water oxygen concentrations and increased pH resulting in lower diversity in the fish community of these systems (Dunne & Leopold 1978; Chadwick & Associates 1992). The giant reed also requires large amounts of water to support its rapid growth rate (Perdue 1958; Iverson 1994).

Nutria diets in the RGV area are composed of greater amounts of common reed than the more abundant giant reed. This might be due to toxic and unpalatable chemicals in the giant reed leaves which protect the plant (Bell 1993). If this disproportionate use of common reed by nutria continues, it might hasten the replacement of common reed stands by stands of giant reed, leaving little food resources for beaver in times of stress (Strong 1982; Bell 1993). Further research is needed on Mexican beaver populations and their response to nutria.

Home range sizes varied greatly between individual nutria. However, males had larger home ranges than females. Nutria living on or near the river had larger home ranges than those living in or near the pond. Doncaster & Micol (1989) documented the home ranges of nutria as 5.68 ha for males and 2.47 ha for females in France, and the difference between genders was significant. Denena et al. (2003) found home range size also varied by gender in Central Texas. In Mississippi, Lohmeier (1981) found male nutria to have smaller home ranges than females. This is similar to results observed for nutria that inhabited the beaver pond (male = 1.01 ha and female = 2.23 ha).

Variations in home range size might be directly associated with habitat. In limited habitats such as small ponds, home ranges are smaller, and in large marshes home ranges are larger. In BBNP, most of the habitat is a narrow riparian area bounding the Rio Grande River. This makes home ranges linear, following the river corridor. Food and adequate feeding and nesting platforms may be spread out over a greater distance causing increased movements. The cane marsh surrounding the Beaver Pond contains large amounts of food in a small area, thus producing smaller home ranges.

Movement data suggest that nutria are capable of traveling long distances along the riparian corridor. Distance traveled in one day suggests that nutria can travel significant distances up and down river. However, less suitable habitat (e.g., canyons) of several kilometers may be a significant barrier to movement.

An IPM (Integrated Pest Management) for nutria management within BBNP is essential to preserving the limited riparian wetland habitats found along the Rio Grande. The small size of the habitat may help to limit nutria densities in BBNP, however areas such as the Beaver Pond could serve as refugia for larger populations. Careful monitoring and management of nutria populations should to be implemented before irreparable damage is made to this sensitive desert wetland area. More data are needed for other locations along the Rio Grande River within the park.

Results reported in this study indicate that controlling nutria within the RGV area might be timely and imperative before their population size becomes too large to control effectively. Bertolino et al. (2005) suggest that nutria removal accelerates native species restoration. Nutria removal campaigns began in Britain (Baker & Clarke 1988) in April of 1981 because of nutria's destructive influence on native habitats. In the United States, Congress has approved, under the Coastal Wetlands Planning Protection and Restoration Act, spending $12.5 million to pay a $4 bounty in Louisiana and Maryland (Schmidly 2004). Management of this invasive species is necessary due to its potential impact to native species; specifically, the Mexican beaver, Big Bend gambusia, and the limited remaining stands of Phragmites within the RGV area.

A nutria management program under the Invasive Species Management Program (Clinton 1999) may be essential to preserving the Big Bend gambusia and Mexican beaver populations. Under NPS policies (National Park Service United States Department of the Interior 2006), an appropriate program includes Integrated Pest Management strategies for the nutria population, impact monitoring, establishment of thresholds for control, and a science-based control plan that accommodates local ecological conditions, best available methods, and social constraints as influenced by human use patterns.


We thank the National Park Service for their generous funding and National Park Service personnel, V. Davila, M. Paredes, and D. van Inwagen, for their assistance. Special thanks to R. Skiles, National Park Service, for invaluable assistance and sharing his extensive knowledge of Big Bend National Park. We thank F. Weckerly for his assistance in data analysis. We also thank two peer reviewers, whose suggestions improved the manuscript.


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TRS at: r_simpson@txstate,edu

Matthew T. Milholland, Jason P. Shumate, Thomas R. Simpson and Richard W. Manning *

Department of Biology Texas State University-San Marcos San Marcos, Texas 78666, and * 107 LBJ Cove, San Marcos, Texas 78666
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Author:Milholland, Matthew T.; Shumate, Jason P.; Simpson, Thomas R.; Manning, Richard W.
Publication:The Texas Journal of Science
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2010
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