Status of the parthenogenetic lizards of the Cnemidophorus laredoensis complex in Texas: re-survey after eleven years.
The Laredo Striped Whiptail, Cnemidophorus laredoensis (Sauria: Teiidae), is a complex of two all-female lizard species (one undescribed) that reproduce by obligate parthenogenesis. The complex is found in the Rio Grande valley of Texas and Mexico from Del Rio/Ciudad Acuna southeastward to the Brownsville/Matamoros area and in a few outlying areas in Dimmit, LaSalle and Starr counties (Walker 1987a; 1987b; Walker et al. 1990). The two species are provisionally designated C. laredoensis A, or LAR-A (the form of C. laredoensis originally described by McKinney et al. 1973) and C. laredoensis B, or LAR-B, following Walker (1986); see Paulissen & Walker (1998) for a discussion of nomenclatural issues. Like all parthenogenetic Cnemidophorus, LAR-A and LAR-B originated from hybrids between bisexual (gonochoristic) species, in this case between the Texas spotted whiptail, C. gularis, and the six-lined racerunner, C. sexlineatus (cf. Bickham et al. 1976; Wright et al. 1983; Parker et al. 1989; Abuhteba et al. 2000). Histocompatibility studies indicate LAR-A and LAR-B arose independently from separate hybridization events (Abuhteba et al. 2001).
The first specimens of C. laredoensis (=LAR-A) were collected from two sites approximately one mile apart within the city of Laredo in 1970 and 1971 (McKinney et al. 1973). No new information on the distribution of C. laredoensis was discovered until 1983. Between 1983 and 1989, a series of field trips were made to the Rio Grande valley of Texas and Mexico. This extensive field survey resulted in the discovery of a second parthenogenetic species, LAR-B, and elucidation of the geographic range of both LAR-A and LAR-B in Texas and Mexico (Walker 1987a; 1987b; Walker et al. 1990; Paulissen & Walker 1998). In Texas, LAR-A has been recorded from 29 sites in Dimmit, Webb, LaSalle, Zapata, Starr and Hidalgo counties (Walker 1987a; Walker et al. 1990), and LAR-B has been recorded from 17 sites in Val Verde, Maverick, Webb, Starr, Hidalgo and Cameron counties (Walker 1987b; Walker et al. 1990). The parthenogens have been found together at several sites in Starr, Hidalgo and Webb counties. Both LAR-A and LAR-B are found only in habitats with some form of disturbance, such as grazing or heavily used dirt roads, which produce large expanses of bare ground interspersed with bunchgrasses and weeds. The gonochoristic species C. gularis occurs at most sites occupied by one or both parthenogens.
There are several reasons to suspect that the collection sites and lizard populations that were studied in the 1980s have undergone dramatic changes in the eleven years since these sites were last studied. First, a lengthy field study conducted in Bentsen-Rio Grande Valley State Park in the 1990s showed that the whiptail lizard community changed dramatically since the Park was initially surveyed. In the 1980s, LAR-B was extremely abundant; LAR-A was rare, and C. gularis was completely absent, but by the middle 1990s, LAR-A was abundant, LAR-B was uncommon, and C. gularis had become established in low numbers (Walker et al. 1996). Second, many of the sites at which LAR-A, LAR-B or both had been found in the 1980s were within or near towns where catastrophic disturbance or destruction of a site by urban development was likely to occur. A few cases of this have already been documented in the cities of Eagle Pass (Maverick County) and Laredo (Webb County) (Walker 1987c; Walker et al. 1990). Third, southern Texas has suffered a drought in the 1990s, with lower than average rainfall most years during the decade. It is well known that drought can negatively impact lizard populations (Ballinger 1977; Pianka 1986; Anderson 1994), including LAR-A (Paulissen 1999).
Although a few field studies of lizard populations have been conducted in the Rio Grande valley in the 1990s, these studies have focused on only a few sites such as Bentsen-Rio Grande Valley State Park in Hidalgo County (Paulissen 1994; 1995; 1997; 1999; 2001), and Mines Road in Webb County (Walker et al. 2001). In the spring of 2000, a study of the genetic variation of LAR-A and LAR-B throughout their geographic ranges was begun, necessitating the collection of lizards from numerous sites to obtain tissue samples for analysis. This presented an opportunity to re-survey most of the sites in Texas at which LAR-A, LAR-B, or both had been recorded in the 1980s. This paper reports the results of this re-survey, in particular information on whether or not the site still exists, what major changes in vegetation and whiptail lizard communities have occurred.
MATERIALS AND METHODS
To facilitate reporting results of the 1980s survey, a system of site codes was devised which was comprised of the first letter of the county in which the site was located plus a site number. For example, BentsenRio Grande Valley State Park in Hidalgo County is site H-3; Mines Road in Webb County is site W-15. These codes were first published in Walker (1987a; 1987b) and Walker et al. (1990) and are used again in this paper. Sites at which either LAR-A, LAR-B or both had been found previously were visited at least once between March 13 and May 24, 2000. Data from several collecting trips made during the 1990s are also included where appropriate. All surveys were conducted when the weather was at least partly sunny and the air temperature was between 22 and 40[degrees]C.
Notes on site size, vegetation and general appearance with reference to lizard habitat requirements were taken following the terminology of Walker (1987a; 1987b). An attempt was made to ascertain which species of lizards were present at each site via visual observation of active lizards in combination with capture of live lizards via noosing. Voucher specimens were kept to document new or unusual records or when field identification of specimens was uncertain. These specimens are deposited with the holdings of the University of Arkansas Department of Zoology (UADZ). Unfortunately, poor weather, especially cloudy skies and cool temperatures, prevented lizards from being active at some sites at the time the sites were visited. If these sites could not be visited again, an assessment of the potential of the site to support lizards was made based on overall habitat characteristics plus tangible signs of lizard presence such as tracks, dried scat, and burrow entrances. In a few cases, it was not possible to find the exact location of a site listed in Walker (1987a; 1987b). When this occurred, one or two sites in the immediate area where the "missing" site was located were surveyed for lizards. These alternate sites were chosen on the basis of habitat and soil characteristics, specifically sites with sandy or sandy-loamy soil with patches of bunchgrass and weeds adjacent to or mixed with mesquite forest close (< 5 km) to the Rio Grande. To facilitate comparison to earlier work, this study followed Walker (1987b) in dividing the Rio Grande valley into three sections: (1) upper Rio Grande Valley (URGV): Val Verde, Kinney, Maverick and northwestern Webb counties; (2) middle Rio Grande Valley (MRGV): the remainder of Webb County (including the city of Laredo), Zapata, Dimmit, LaSalle and Starr counties west of Rio Grande City; and (3) lower Rio Grande Valley (LRGV): the remainder of Starr County (including Rio Grande City), Hidalgo and Cameron counties.
A summary of the habitat characteristics and lizard populations at each site visited in Spring 2000 is presented in Table 1. The table also includes a summary of site characteristics in the 1980s as reported by Walker (1987a; 1987b) and Walker et al. (1990).
Upper Rio Grande Valley (URGV). -- VAL VERDE COUNTY: Two sites at which LAR-B had been found in the 1980s were visited. The first was V-10 located 0.8 km N of the international bridge on the west side of U.S. Hwy. 277S/State Hwy. 239. This site no longer has suitable lizard habitat: the bunchgrass, weed, thorn forest association reported in Walker (1987b) has been replaced by small businesses, dirt parking lots and driveways. A new site discovered near the Rio Grande approximately 0.4 km W of the intersection of Garza and Las Vacas Roads appeared to have suitable habitat, consisting of bunchgrass and weeds along several small dirt roads near a moderately disturbed mesquite forest. However a search of this site on April 6 found only large numbers of the bisexual species C. gularis. The second site was V-13, located 10.8 km SW of Twin Lakes Duckpond on Cienegas Rd. paralleling the Rio Grande (this site was listed as being NW of the junction of Spur 239 and US Hwy 90/277 by Walker et al. 1990). This site consists of bunchgrasses and weeds along the side of two dirt roads running parallel to each other between the Rio Grande and a railroad track. The whole site is adjacent to Chihuahuan desert vegetation that slopes up on a hillside on the north side of the railroad tracks. This is essentially unchanged from the way the site was described in Walker et al. (1990). Three surveys of the site yielded sightings of about 90 C. gularis and three LAR-B, one of which was kept as a voucher (UADZ 6867).
MAVERICK COUNTY: Walker (1987b) listed two sites in the city of Eagle Pass from which LAR-B had been collected: M-2 just N of the international bridge, and M-3 located 1.6 km S of the traffic signal nearest the bridge between a city park and the Rio Grande. Walker et al. (1990) reported that M-2 had been bulldozed and that M-3 had been bulldozed and burned in 1988. Visits to these sites on April 8 revealed that M-2 was now a series of soccer fields with mown grass surrounded by dirt roads lined by a few small patches of bunchgrass and weeds. The second site, M-3 has been replaced by facilities surrounding a newly constructed international bridge to the south and part of a golf course to the north; only a few small patches of bunchgrass by some abandoned roads remains. Since neither Eagle Pass site has suitable habitat left, it is almost certain that LAR-B has been extirpated from these sites.
Walker (1987b) reported seeing LAR-B and C. gularis at a third site in Maverick county, M-4, located between the small town of El Indio and the Rio Grande about 30 km S of Eagle Pass. This site is on private land, but the senior author was permitted access for 40 minutes on April 9. The site appeared unchanged from the bunchgrass/weed/mesquite habitats lining dirt roads next to the Rio Grande described by Walker (1987b). Unfortunately, the weather was mostly cloudy and cool (20[degrees]C) during the visit so any lizards that existed there were not active. The presence of dried lizard scat, obvious burrow entrances, and suitable habitat strongly suggest whiptail lizards are common at M-4, though it is not known if they are LAR-B, C. gularis or both (Table 1).
WEBB COUNTY: The one URGV site in Webb county is W-15, Mines Road. The day this site was visited, the weather was mostly cloudy and cool with occasional light sprinkles so very few lizards were seen, although five C. gularis and one LAR-B were identified. However, this site has been visited many times in the 1980s and 1990s producing a wealth of data on the distributional ecology of LAR-A, LAR-B, C. gularis and C, sexlineatus (cf. Walker et al. 1990; 2001).
Middle Rio Grande Valley (MRGV). -- WEBB COUNTY: Walker (1987a) reported eight sites within the city of Laredo from which LAR-A had been collected; two of these were the sites from which the original series used to describe Cnemidophorus laredoensis was collected (McKinney et al. 1973). All eight sites were re-surveyed in this study. The first site, W-1, the type locality of LAR-A (=C. laredoensis), is located where U.S. Hwy. 83 crosses Chacon Creek on the south side of the city. A chain link fence now restricts access to most of the site, and the vegetation has changed from the bunchgrass/Russian thistle/mesquite habitat reported in Walker (1987a) to a degraded mesquite forest with scattered weeds and dense grass along the edge of the creek. Two visits to W-1 resulted in sighting of 12 individuals of C. gularis, one of which was kept as a voucher (UADZ 6864) but only one LAR-A. The finding that C. gularis is much more abundant at W-1 than LAR-A is surprising since no C. gularis were seen or collected there in the 1980s. Site W-2 is located on the south bank of Chacon Creek just N of the end of Arkansas Avenue (across U.S. Hwy. 83 from W-1). In the 1980s, the vegetation here consisted of dense bunchgrass along the edges of paths, dirt roads, and the bank of Chacon Creek with a few mesquite trees and a large amount of cactus near the creek (Walker 1987a). The only change is that there is now much less cactus and more weeds. A brief visit resulted in sighting of one LAR-A and one C. gularis; this is similar to what was reported for W-2 in the 1980s (Table 1).
Site W-3, located 2 km S of Chacon Creek on the E side of U.S. Hwy. 83 at Saltillo St., was studied extensively in the 1980s. In 1983, the site was an isolated patch of degraded thorn shrub habitat supporting only LAR-A. The site was bulldozed in May 1984 and only a few lizards were found there in June 1984. By July 1985, a thick growth of grasses and weeds had developed and LAR-A were abundant (Walker 1987c). However, by 1989, Saltillo St. was the center of a new subdivision with recently built homes (Walker et al. 1990). A visit to W-3 on March 21 showed subdivision development had been completed: all streets (including Saltillo St.) had been paved and lawns and gardens surrounded all the houses leaving no habitat for whiptail lizards. Site W-4, located across U.S. Hwy, 83 from W-3, is still present, though much changed from the 1980s (Table 1). Walker (1987a) reported this site shifted from a bunchgrass/mesquite habitat to a bunchgrass/sunflower/mesquite habitat between 1983 and 1984. Two visits to W-4 revealed the site now consists of low-growing (<20 cm) grass along the edges of dirt roads and that the whole site is surrounded by dense mesquite and thorn shrub forest (part of which extends to the Rio Grande approximately 1.5 km to the east). Only two LAR-A were seen, but over a dozen C. gularis were seen including one kept as a voucher (UADZ 6865). The presence and numerical superiority of C. gularis at this site is a significant change from what was observed in the 1980s.
Site W-5 is located 2.2 km S of Chacon Creek on the E side of U.S. Hwy. 83. When first surveyed in 1984, it was a vegetationally complex thorn shrub habitat with large patches of bunchgrass and cactus (Walker 1987a). By July 1985 however, the entire site had been bulldozed and supported only dense thickets of bunchgrass, weeds and Russian thistle (Walker 1987a; Walker et al. 1990). Even in this highly altered state, site W-5 still supported LAR-A, primarily in the narrow zone where thistles and weeds grew at the side of the road. A visit to the site on March 21 showed that even the thistle-weed habitat is gone; the entire site is now a city park with tennis courts and mowed fields, habitats not suitable for lizards. The situation is different at W-6, located about 500 m S of W-5 (approximately 2.6 km S of Chacon Creek on the E side of U.S. Hwy. 83 near Norton St.). This site consists of a series of dirt paths and gulleys supporting numerous patches of bunchgrass, some prickly pear cactus, and many mesquite trees. This is essentially the same habitat present at W-6 when it was first visited in 1984, but is an obvious change, due to ecological succession, from the bunchgrass/weed/Russian thistle habitat that existed at W-6 in 1985 (Walker 1987a). Both LAR-A and C. gularis were common at W-6; just as was the case when the site was surveyed in the 1980s.
Site W-7, located between U.S. Hwy 83 and Lake Casa Blanca Dam at Chacon Creek, was an abandoned drive-in theater parking lot with scattered small patches of soil and weeds surrounded on three sides by bunchgrass/mesquite/thorn shrub habitat in 1984 (Walker 1987a; Walker & Cordes, 1990). This site supported both LAR-A and C. gularis at that time. In 1988, the site was bulldozed, completely eliminating the habitat and all the lizards in it (Walker et al. 1990). However, visits to W-7 in 2000 revealed that LAR-A was present, though not abundant, along small trails through clumps of bunchgrass and weeds on the S side of Chacon Creek. No C. gularis were seen at the site, though it may be present in a large tract of thorn shrub habitat on the south side of U.S. Hwy. 59 about 200 m south of Chacon Creek.
The final site in Laredo, W-8, is located at the west end of Masterson St. at the Rio Grande approximately 4.5 km S of Chacon Creek. In 1984, the vegetation at W-8 consisted of bunchgrass, weeds and clematis. When visited on April 11, the bunchgrass was still abundant along dirt roads in the area, but large mesquite trees had grown along the river. Only a perfunctory search was made at this site because of warnings from passing U.S. Border Patrol agents that the site was a dangerous smuggling zone. The presence of lizard burrows and "tail drag" impressions in the soil indicate whiptail lizards are present at this site, though it is not known what species exist there. Both LAR-A and C. gularis were present at the site in the 1980s (Table 1).
ZAPATA COUNTY: There are only three sites in Zapata county from which LAR-A has been recorded, one of which, Z-2 within the city of Zapata, was not visited. Site Z-1 is located between a Texas roadside park and the Rio Grande on U.S. Hwy. 83 4.8 km N of San Ygnacio. In 1984, the site was a bunchgrass-Russian thistle habitat within a degraded thorn shrub forest; both LAR-A and C. gularis were collected there (Walker 1987a). By March 2000, the habitat had changed dramatically: there was very little grass left, and what was present was dead, leaving only woody shrubs. This habitat is suitable for C. gularis, but not for LAR-A, so it is unlikely that the parthenogen population still exists at Z-1. Unfortunately, no lizards were observed during the brief stop at Z-1 so this speculation is unconfirmed.
Site Z-3 is located on the shore of Falcon Reservoir at the end of FM 496 about 5 km SW of U.S. Hwy. 83. In 1987, the habitat consisted of grass and weeds surrounded by thorn shrub habitat; both LAR-A and C. gularis were found there (Walker et al. 1990). By May 2000 the thorn shrub habitat had expanded to near the shore of the lake but there was still some grass along the dirt road to the lake and in between the trees. Three LAR-A but no C. gularis were seen. It is probable that C. gularis is still in the area, perhaps further back into the thorn shrub, but none were located because the grassy areas by the side of the dirt roads received the most attention.
STARR COUNTY: The part of Starr County that lies in the MRGV extends from the Zapata County line to just west of Rio Grande City (Walker 1987b). There are three MRGV sites in Starr County, one of which, S-2, was visited twice during this study (Table 1). This site is located along an unsurfaced road running south from the corner of River and Ebano streets in the small town of Fronton. The vegetation is essentially the same moderately degraded bunchgrass/cactus/mesquite habitat reported to be present at S-2 in the 1980s (Table 1). A total of 32 LAR-A and 36 C. gularis were seen in the two visits combined suggesting these two species are very common at S-2 and are about equally abundant.
LASALLE COUNTY: There are two sites in LaSalle County from which LAR-A had been reported. One of these, L-2, located at the junction of I-35 and FM 133 on the west side of the interstate highway at Artesia Wells, was visited briefly on March 20. This site is a degraded bunchgrass/weed/mesquite/thorn shrub habitat along sandy roads and is essentially unchanged from the habitat found there in the 1980s (Table 1). No lizards were seen during the visit, perhaps due to the time of day. Several C. gularis and one LAR-A were collected at L-2 on September 7, 1996 and four LAR-A were collected at L-2 on August 31, 1997. This suggests the population of LAR-A and C. gularis remained intact at L-2 through the 1990s.
DIMMIT COUNTY: Of the three sites surveyed in this county in the 1980s, one, D-3 at Catarina, was found to support a healthy population of LAR-A in 1986 (Walker 1987a). The Catarina site has been visited several times during the 1990s; the data collected during these visits will be reported elsewhere. A brief visit to D-3 in the late afternoon of April 9 resulted in the sighting of two LAR-A. Two C. gularis were seen there during a visit on March 20 (Table 1). These results confirm the continued existence of both species at this site for the past 14 years.
Lower Rio Grande Valley (LRGV). -- STARR COUNTY: The portion of Starr County within the LRGV extends from Rio Grande City east to the Hidalgo County line. Eight sites within this area supported populations of one or both parthenogens in the 1980s; five sites were resurveyed during this study. The first of these sites is S-4, located approximately 1.5 km SE of the jct. of FM 2644 and U.S. Hwy. 83 at Garceno. The habitat consists of a bunchgrass/cactus/mesquite forest bordering dirt roads that run to the Rio Grande. This vegetation is similar to what was present at this site in 1986 except that previously there were fewer large trees and little cactus (Table 1). Two visits to the site resulted in sightings of a total of 30 C. gularis and 15 LAR-A indicating the site supports healthy populations of both species as it did in the 1980s.
The situation is different at S-5, located within Rio Grande City between the international bridge and the old high school. In 1984 and 1985, the vegetation at this site was bunchgrass and sunflower along dirt roads with a few scattered mesquite trees (Walker 1987a). When visited March 23, much of the site had been replaced by a grain elevator operation and gravel roads leading to it; only a small area about 50 by 150 m of bunchgrass-mesquite habitat remained. The brief stop yielded no sightings of LAR-A but three sightings of C. gularis (one of which was kept as a voucher, UADZ 6866). This is a complete reversal from the lizard community in the 1980s which included only LAR-A (Table 1).
The last two sites in Starr County along the Rio Grande are S-7 and S-8, located between the small town of La Grulla and the Rio Grande. It was not possible to positively locate the two different sites; instead a single site, located 0.8 km S of Pvt. Solis St. was investigated. The site was a bunchgrass-weed-cactus habitat along dirt roads lined bordered by tracts of moderately degraded mesquite forest or cultivated fields; this is very similar to the habitat present at S-7 and S-8 in 1986 (Walker 1987b). A total of 13 C. gularis, 5 LAR-A, and a single LAR-B were seen at this site in two visits, and a single LAR-A X C. gularis hybrid was collected (UADZ 6869). Although all three species were collected from S-7 and S-8 in the 1980s, the 2000 survey shows a change in lizard community composition in that the bisexual species C. gularis is more common than the two parthenogens combined and that LAR-B in particular is rare.
There are three LRGV sites in Starr County located over 10 km north of the Rio Grande, two of which were visited briefly in the late afternoon of March 24. Site S-9 is located at the jct. of FM 649 and FM 2687 and is a bunchgrass/weed habitat with a few scattered mesquite trees along a dirt ranch road. Site S-10 is located at El Sauz and consists of a bunchgrass/weed habitat mixed with a mesquite/cactus/thorn shrub habitat along unsurfaced roads. One C. gularis was sighted at S-10, no lizards were seen at S-9, but this may due to the time of the visit. Both habitats appeared similar to the way they were in 1986 and probably support at least a few lizards, though which species is unknown (Table 1).
HIDALGO COUNTY: The most extensive studies of the ecology and behavior of LAR-A and LAR-B have been conducted at two sites in Hidalgo County: H-3, Bentsen-Rio Grande Valley State Park, and H-2, Los Ebanos. Site H-3 was the location of a long-term field study (Paulissen 1994; 1995; 1997; 1999; 2001; Walker et al. 1996). Site H-2 was the focus of an initial study by Paulissen et al. (1988) and of a number of subsequent collecting trips since, the data of which will be reported elsewhere. Two other sites in Hidalgo County, H-1 near the Los Ebanos Ferry, and H-4, Rio Rico Rd., were not surveyed. The one Hidalgo County site visited during this study was H-5, located 2.2 km W of the north-south levee just W of spur 359 at the international bridge at Hidalgo. The habitat was essentially unchanged from the bunchgrass/weed/mesquite habitat lining the dirt roads running along the Rio Grande that was present in 1988 (Table 1). However, when the site was visited in May 2000, all the grass had died and turned brown, presumably because of the drought. Only one lizard, a C. gularis, was seen during a brief stop, perhaps due to the extremely hot temperatures that day (air temperature 39[degrees]C). Other than the dried, dead grass, the site appeared suitable for lizards, though what species is/are present is unknown.
Site H-7, located 10 km N of U.S. Hwy. 83 on FM 2221, then 200-800m west of the stop sign along an unsurfaced road, was not visited as part of this study but was visited several times during the 1990s. When first discovered in 1988, the vegetation was a bunchgrass/weed habitat along the side of the road, surrounded by a mesquite-cactus forest further away from the road. Some of the mesquite forest had been cleared away for cultivation, but even here the bunchgrasses and weeds remained by the side of the road. Both LAR-A and C. gularis were abundant at H-7 in the 1980s (Walker et al. 1990). However, in 1995 the 20m section of the mesquite forest nearest the road was cleared leaving the bunchgrass/weed habitat along side the road isolated from the mesquite forest. This habitat destruction took place during the severe drought in southern Texas; presumably one or both of these factors precipitated the marked decrease in lizard populations that occurred by 1997. Nonetheless, both LAR-A and C. gularis were still present at H-7 when the site was last visited on September 5, 1999.
CAMERON COUNTY: Only one site surveyed in the 1980s was surveyed for this study, C-1 located 3.0 km S of U.S. Hwy. 281 along the Rio Grande at the small town of Los Indios. The vegetation is essentially unchanged from the bunchgrass/mesquite habitat lining the dirt roads paralleling the Rio Grande, though the willow trees reported to be present in 1985 were not present when the site was visited May 2000 (Table 1). No lizards were seen, probably due to the lateness of the hour and intense heat. However, the presence of lizard scat, tail drag impressions in the soil, and reports of a Border Patrol agent familiar with the area indicate whiptail lizards are present at C-1, though which species is not known.
Site C-2, located between U.S. Hwy. 281 and the Rio Grande about 0.3 km W of the small town of El Ranchito, has been visited many times during the 1990s. The habitat varies from a dense thicket of mesquite and cactus to a bunchgrass-weed habitat along dirt roads and cultivated fields adjacent to the thicket. In March 1994, the grass and weeds were destroyed by burning, but since then they have regrown restoring the habitat to its pre-burn condition. In the 1980s, Walker (1987b) reported that C, gularis was extremely abundant, and LAR-B was present in low numbers at C-2. This situation has remained the same throughout the 1990s; for example, a total of 40 C. gularis and 6 LAR-B were seen at C-2 during four visits in Spring 2000.
The last site surveyed in the 1980s is C-3, located 13.2 km E of Brownsville, then 3.2 km S of State Hwy. 4 within 600 m of the Rio Grande. From 1984-1986, the site's vegetation was a bunchgrass/weed habitat along an unsurfaced road, giving way to dense cactus/mesquite/thorn shrub habitat away from the road (Walker 1987b). Cnemidophorus gularis was abundant, and LAR-B was present in moderate numbers at C-3. This site was last visited on September 3, 1993: the habitat was essentially unchanged; however, only C. gularis were seen (Table 1).
Forstner et al. (1998) reported collecting Cnemidophorus laredoensis, (species LAR-B, Dixon, pers. comm.) at a site on the northern edge of Harlingen. The site is located at the Harlingen municipal water tower about 1 km E of U.S. Hwy. 77 along loop 499. The habitat consists of dirt roads along and behind a canal lined with bunchgrass and weeds; mesquite and huisache trees grew in sparse clumps throughout the area. The site also has piles of wood, paper and roofing materials; the soil is loamy with little sand. Visits to the site on August 12, 1998, 31 March 1999, 20 May 1999 and 17 May 2000 found no LAR-A or LAR-B, though C. gularis was very abundant. This suggests that LAR-B must be rare or absent from this site today.
The Spring 2000 re-survey of sites at which one or both species of Cnemidophorus laredoensis complex was collected in the 1980s showed that some sites changed dramatically through the activities of humans, some sites changed only modestly through ecological succession, and some sites changed little at all. Though details vary from site to site, three broad trends stand out. First, sites in the larger cities are often substantially or completely destroyed by development. Walker et al. (1990) noted that LAR-B sites M-2 and M-3 in Eagle Pass and LAR-A sites W-3 and W-7 in Laredo were destroyed by development, extirpating the lizard populations there. The present study adds LAR-B site V-10 in Del Rio and LAR-A site W-5 in Laredo to this list. In addition, the type locality of LAR-A, W-1 in Laredo, and LAR-A site S-5 in Rio Grande City have been substantially reduced in size by development, perhaps contributing to the large decline in LAR-A population sizes at both sites (LAR-A was rare at W-1, not seen at S-5). Although it is true that LAR-A still live in the vicinity of W-7, sites W-2, W-4, and W-6 in Laredo are all adjacent to developed lots and may themselves be bulldozed and developed eliminating LAR-A populations at those places. Ironically, both parthenogens require habitats with at least some disturbance to promote the growth of bunchgrasses and weeds and to open up bare patches of ground (Walker 1987a, 1987b) and can even live in habitats which have no vegetation other than a few scattered clumps of weeds (Walker 1987c; Walker & Cordes 1990). But it appears that both LAR-A and LAR-B require at least this minimal habitat; the pavement, mowed lawns and gravel roads of development in large towns are habitat alterations too severe to permit existence of parthenogens. The situation is different at small towns such as Catarina (D-3), Fronton (S-2), La Grulla (S-7, S-8) and E1 Ranchito (C-2); the sites in these towns are moderately disturbed but have not been extensively developed so still support healthy populations of lizards. Clearly the development practices of small towns tend to help rather than hurt populations of parthenogens.
A second broad trend is that the bisexual species C. gularis has become established in many places from which it was absent in the 1980s and has increased in numbers relative to the parthenogens at some sites. The establishment of C. gularis at site H-3 (Bentsen-Rio Grande Valley State Park) in the 1990s has already been documented (Walker et al. 1996). Similarly, C. gularis is now present and more abundant than LAR-A at sites W-1 and W-4 in Laredo and S-5 in Rio Grande City; it was not found at any of these sites in the 1980s. Furthermore, C. gularis has substantially increased in abundance relative to LAR-A at sites S-2, S-4 and H-7 (Table 1). The influx of C. gularis into C. laredoensis habitat can potentially negatively affect the parthenogens in several ways, though earlier work suggests at least LAR-A is able to coexist indefinitely with C. gularis (cf. Paulissen et al. 1992).
The final trend is the stability of LAR-A populations relative to the decline of LAR-B. The Spring 2000 survey was conducted as part of a study of the genetic diversity of the two parthenogens, a study that required noosing live lizards to obtain tissue samples. Despite concerted efforts to locate and capture LAR-B at five sites (V-13, W-15, S-7/S-8, H-2 and C-2), only 10 lizards were obtained. This contrasts with attempts to capture LAR-A at 6 sites (S-2, S-4, S-7/S-8, H-2, D-3 and W-6) that yielded 19 captures. Furthermore, the pattern of LAR-A switching from being the less abundant parthenogen to the more abundant one documented at H-3 (Walker et al. 1996) is repeated at La Grulla (sites S-7/S-8). Since this site is close to H-3, it is possible that the LAR-B decline in the LRGV is part of a widespread phenomenon. The most obvious hypothesis for the decline is the drought in southern Texas, but habitat changes or competition with C. gularis may also play a role.
Table 1. Summary of habitat characteristics and whiptail lizard community composition at sites in southern Texas. Site codes follow Walker (1987a; 1987b) and Walker et al. (1990), see text for explanation. The left side of the table gives data from the 1980s when the sites were last surveyed. Sites which were surveyed more than once and which changed significantly during the 1980s are listed with each year during which samples were taken (Walker 1987a; Walker 1987b; Walker et al. 1990). The right side of the table lists results from the Spring 2000 re-survey, plus data from a few sites that were surveyed in the 1990s. Symbols used: Habitat Characteristics: BG = bunchgrass, WE = weeds, ME = mesquite trees, CA = cactus, TS = thorn shrub, RT = Russian thistle, SF = sunflower, CL = clematis, WI = willow; Catastrophic Alteration (Cat. Alt.): + = present, - = not present, d = site completely destroyed, r = site reduced in size by at least half; Species Present: A = LAR-A, B = LAR-B, G = C. gularis, P = whiptail lizards present based on the presence of scat, burrows or tracks. The relative abundance of species is indicated by <, >, and = signs; Ecological Succession (Ecol. Succ.): y = yes, n = little to none, n/a = not applicable. 1980S SURVEYS Sample Habitat Cat. Species Site Year Characteristics Alt. Present UPPER RIO GRANDE VALLEY V-10 1985 BG-WE-TS + to - G > B V-13 1988 BG-WE-TS + G > > B M-2 1985 BG-WE - B 1988 none d none M-3 1984 BG-WE-CA-ME - G > B 1988 none d none M-4 1987 BG-WE-CA-ME +/- G > B W-15 1989 BG-WE-ME +/- G > B MIDDLE RIO GRANDE VALLEY W-1 1983-4 BG-RT-ME +/- A W-2 1984 BG-CA-ME - G = A W-3 1983 BG-RT-CA-ME - A 1984-6 BG-WE + A 1989 WE d none W-4 1983 BG-ME + A 1984-5 BG-SF-ME - A W-5 1983-4 BG-RT-SF-CA-TS - A > G 1985 WE-BG-RT + A W-6 1984 BG-CA-ME-TS + A > G 1985 WE-BG-RT + A > G W-7 1984 BG-RT-WE-ME + G > A 1988 none d none W-8 1984 BG-CL-ME - A > G Z-1 1984 BG-RT-TS - A > G Z-3 1987-9 WE-TS - A > G S-2 1984-5 BG-CA-ME - A > G L-2 1986 BG-WE-ME-TS - G > A D-3 1986 WE-ME; BG-ME - A > G LOWER RIO GRANDE VALLEY S-4 1986 BG-ME +/- A > G S-5 1984-5 BG-SF-ME - A S-7/S-8 1986 BG-WE; CA-ME +/- G > B >A S-9 1986 BG-ME - G > A S-10 1986 BG-ME-CA-TS - G > A H-5 1987-8 BG-WE; ME +/- B > G H-7 1988-9 BG-WE-ME +/- A > G C-1 1985 BG-WE; WI +/- B C-2 1984-6 BG-WE; CA-TS +/- G > > B C-3 1984-6 BG-WE; CA-TS +/- G > B SPRING 2000 RE-SURVEY Samples Date(s) and Habitat Cat. Ecol. Species Site Time(s) Characteristics Alt. Succ. Present UPPER RIO GRANDE VALLEY V-10 4/6: 1330-1340 none d n/a none V-13 4/7: 1000-1500; 4/8: BG-WE-TS + n G > > B 1200-1600; 5/23: 0915- 1300 M-2 4/8: 1900 lawn d n/a none M-3 4/8: 1910 lawn d n/a none M-4 4/9: 1030-1110 BG-WE-ME + y P W-15 4/10: 1100-1630 BG-WE-ME-RT + n G > B MIDDLE RIO GRANDE VALLEY W-1 3/21: 1345-1500; 3/22: BG-ME r n G > A 0945-1200 W-2 4/11: 1100-1200 BG-WE-ME - n G = A W-3 3/21: 1510 none d n/a none W-4 3/21: 1530-1640; 3/22: BG-ME - n G > A 1230-1410 W-5 3/21: 1650 lawn d n/a none W-6 4/11: 1220-1320 BG-ME-TS - y A > G W-7 3/21: 0930-1115; 4/11: BG-WE-ME - y A 0850-1050 W-8 4/11: 1330-1350 BG-WE-ME - y P Z-1 3/22: 1510-1520 TS - y ? Z-3 5/22: 1035-1155 WE-ME - y A S-2 5/23: 0950-1310; 4/13: BG-CA-ME - n G = A 0920-1400 L-2 9/7/1996; 8/31/1997; BG-WE-ME-TS - n G > A 3/20: 1655 D-3 3/20: 1010-1500; 4/9: WE-ME; BG-ME - n A = G 1630-1700 LOWER RIO GRANDE VALLEY S-4 3/23: 1330-1540; 3/24: BG-ME-CA - y G > A 1220-1500 S-5 3/23: 1615-1655 BG-ME r y G S-7/S-8 3/19: 1410-1640: 3/21: BG-WE-ME-CA - y G > A > B 0850-1130 S-9 3/24: 1625-1645 BG-ME - y P S-10 3/24: 1530-1615 BG-ME-CA-TS - y G H-5 5/19: 1440-1530 BG-WE; ME + y G H-7 9/5/1999 BG-WE-ME + n A = G C-1 5/18: 1640-1720 BG-WE-ME + y P C-2 3/1: 0930-1500; 4/16: BG-WE; CA-TS + y G > > B 1220-1450; 5/18: 0900- 1445; 5/19: 0900-1330 C-3 9/3/1993 BG-WE; CA-TS +/- n G
Partial funding for travel was through a Shearman Research Grant issued to the senior author by McNeese State University. Mr. B. Maurer kindly permitted access to site M-4 at E1 Indio. Lodging during part of this study was provided by Mrs. B. Boothe and by the staff at Falcon State Park. T. Hibbs assisted in collecting lizards at C-2 and critiqued a draft of this manuscript. The courtesy of the agents of the U. S. Border Patrol encountered during this (and many other) field studies is also gratefully acknowledged. Voucher specimens were deposited in the University of Arkansas Department of Zoology (UADZ) collection. This study was conducted under authority of Texas Scientific Collecting Permit SPR-0691-408 issued to the senior author.
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Mark A. Paulissen, James M. Walker and James E. Cordes
Department of Biological and Environmental Sciences
McNeese State University, Lake Charles, Louisiana 70609
Department of Biological Sciences, University of Arkansas
Fayetteville, Arkansas 72701 and
Division of Sciences, Louisiana State University at Eunice
P.O. Box 1129, Eunice, Louisiana 70535
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|Author:||Paulissen, Mark A.; Walker, James M.; Cordes, James E.|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 2001|
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