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Vegetation of South Padre Island, Texas: freshwater and brackish wetlands.

Abstract.--Species composition and importance, species richness, species diversity, and evenness were compared among 14 freshwater and brackish wetlands in the secondary dunes and vegetated flats topographic zone of South Padre Island, Texas. Twenty-four different species were found among the sites, but the greatest species richness at a site was only 14. Mean species diversity was 0.855 and mean evenness was 0.797. Both parameters showed little variability among sites. South Padre Island wetlands had significantly lower species richness and species diversity than freshwater and brackish marshes of the adjacent Texas mainland. Conversely, evenness was similar among island and mainland wetlands. Most of the island wetlands shared 62% to 67% of their species and similarity in species composition was significantly greater for island wetlands than mainland wetlands. Bulrush (Schoenoplectus pungens) was the dominant species at nine wetlands. Umbrella grass (Fuirena simplex) and marshhay cordgrass (Spartina patens) each were dominant in two wetlands. Bushy bluestem (Andropogon glomeratus) was dominant in one wetland. The meager data currently available suggests that species richness of wetland communities is similar on northern and southern portions of Padre Island, but similarity in species composition is modest. Species importance in wetland communities varies markedly between northern and southern areas of Padre Island.

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The most conspicuous physiographic feature along the Gulf of Mexico coastline of Texas is a series of five barrier islands that enclose several shallow bays. The flora and vegetation of South Padre Island are relatively well known compared to other areas of the barrier island chain (Dahl et al. 1974; Judd et al. 1977; Lonard et al. 1978; Lonard & Judd 1980; Lonard & Judd 1981; Judd & Sides 1983; Judd & Lonard 1985; Judd & Lonard 1987; Judd et al. 1989; Lonard & Judd 1989; Judd et al. 1991; Lonard et al. 1991; Everitt et al. 1991; Everitt et al. 1992; Lonard & Judd 1993; Judd et al. 1994; Lonard & Judd 1997; Judd et al. 1998; Lonard & Judd 1999; Everitt et al. 1999; Lonard et al. 1999; Judd et al. 2007) but little is known of the composition of freshwater and brackish wetland communities on South Padre Island or any of the other Texas barrier islands.

Judd et al. (1977) reported that dense meadows of fresh water marsh communities occurred within the secondary dunes and vegetated flats topographic zone of South Padre Island and identified five common species. Similarly, Lonard et al. (1999) commented that depressions that often support development of marsh communities occur frequently in the secondary dunes and vegetated flats topographic zone. These wetland plant communities were said to be dominated by a combination of sedges and grasses including Scirpus pungens (= Schoenoplectus pungens), Spartina patens, Fimbristylis castanea, Fuirena simplex and Rhynchospora colorata by Lonard et al. (1999). Thus, the only information available is a partial list of species that occur in the wetlands of South Padre Island. A complete list of species for even one wetland is lacking and there is no information on the abundance of the species comprising a wetland community or the fidelity of species among wetland sites. Consequently, a quantitative comparison of wetland communities at 14 sites on South Padre Island is provided herein to rectify these insufficiencies in the knowledge of the vegetation of South Padre Island.

MATERIALS AND METHODS

The locations of wetland communities studied are given in Table 1. Four of the 14 sites contained standing water at the time of sampling. Salinity was recorded at each of the four sites using a temperature compensated hand-held refractometer (Table 1). Marshes were considered freshwater if salinity was 0.0 to 0.5 ppt and brackish if salinity was 0.5 to 17.0 ppt.
Table 1. Study site locations, dates of sampling, and mean salinity of
sites with standing water.

Site Location Sample Mean
 Date Salinity
 (ppt)

 1 26[degrees]09'43.26" N, 97[degrees]10'27.49" W 5-21-07 dry
 2 26[degrees]10'13.16" N, 97[degrees]10'32.18" W 5-21-07 dry
 3 26[degrees]13'00.52" N, 97[degrees]10'56.16" W 6-14-07 dry
 4 26[degrees]13'04.89" N, 97[degrees]10'57.26" W 6-14-07 dry
 5 26[degrees]13'25.55" N, 97[degrees]10'57.67" W 5-21-07 2.0
 6 26[degrees]14'22.16" N, 97[degrees]11'10.28" W 8-4-07 0.0
 7 26[degrees]14'28.30" N, 97[degrees]11'09.16" W 9-24-07 5.5
 8 26[degrees]14'47.10" N, 97[degrees]11'14.44" W 9-24-07 dry
 9 26[degrees]17'03.69" N, 97[degrees]11'45.26" W 11-5-07 dry
10 26[degrees]17'51.20" N, 97[degrees]11'55.86" W 11-5-07 dry
11 26[degrees]19'12.36" N, 97[degrees]12'11.11" W 3-26-08 dry
12 26[degrees]19'29.37" N, 97[degrees]12'21.68" W 3-26-08 12.0
13 26[degrees]20'44.42" N, 97[degrees]12'36.77" W 3-27-08 dry
14 26[degrees]21'33.38" N, 97[degrees]12'48.32" W 3-27-08 dry


The line intercept method (Canfield 1941) was used to quantify species abundance. Seventy meters of transects were sampled at each site. Each transect was divided into 10 m intervals and readings were taken along the length of each interval. First, the total cover of dead vegetation intercepted was recorded without regard to species. For live vegetation, each species intercepted by the line was rated individually without separation into strata. Species and foliage cover were recorded and from these data the frequency of occurrence, relative frequency, relative cover and an importance value, which is the sum of relative frequency and relative cover, were calculated. The importance value was used to determine dominant species.

Similarity of species composition among wetland sites was calculated using Sorensen's Coefficient of Community (Krebs 1999). Species importance value was used as the measure of abundance for calculating species diversity indices. Species diversity was assessed using the Shannon diversity index (Brower et al. 1998; Krebs 1999). Evenness was determined as the ratio of heterogeneity (H') to maximum heterogeneity (H' max) (Brower et al. 1998; Krebs 1999). Nomenclature and common names follow Jones & Wipff (2003).

RESULTS

Study sites were located in the southern two-thirds of South Padre Island along a south to north axis in the secondary dune and vegetated flats topographic zone (Judd et al. 1977). Distance between successive study locations ranged from 120 m to 5,010 m (mean = 1,846.6 m, SD = 1,764.9 m). Twenty-four different species were found in the 14 sites sampled (Table 2). Twenty of the species were perennials and four were annuals. Eight species were grasses, five were sedges, 10 were broad-leaved herbaceous species and one species was a cattail (Table 2). No woody species were found. Thirteen of the species occurred in more than half of the sites and Iva texensis (sumpweed) was present at each site. Conoclinium betoncifolium (mist flower) and Schoenoplectus pungens (bulrush) each occurred in all but one of the wetlands (92.9%).
Table 2. Species present, their periodicity, growth form and percent
occurrence among sites.

Species Periodicity Growth Form % Occurrence

Andropogon Perennial Grass 78.6
glomeratus

Eragrostis Perennial Grass 7.1
secundiflora

Panicum amarum Perennial Grass 57.1

Paspalum Perennial Grass 64.3
monostachyum

Schizachyrium Perennial Grass 50.0

littorale

Spartina Perennial Grass 71.4
patens

Sporobolus Perennial Grass 14.3
airoides

Sporobolus Perennial Grass 71.4
virginicus

Eleocharis Annual Sedge 42.9
geniculata

Fimbristylis Perennial Sedge 21.4
castanea

Fuirena Perennial Sedge 85.7
simplex

Rhynchospora Perennial Sedge 57.1
colorata

Schoenoplectus Perennial Sedge 92.9
pungens

Typha Perennial Cattail 14.3
domingensis

Agalinus sp. Annual Broad-leaved herbaceous 28.6

Bacopa Perennial Broad-leaved herbaceous 21.4
monnieri

Blutaparon Perennial Broad-leaved herbaceous 7.1
vermiculare

Borrichia Perennial Broad-leaved suffrutescent 35.7
frutescens

Conoclinium Perennial Broad-leaved herbaceous 92.9
betonicifolium

Flaveria Annual Broad-leaved herbaceous 7.1
brownii

Iva texensis Annual Broad-leaved herbaceous 100.0

Lythrum alatum Perennial Broad-leaved herbaceous 21.4

Samolus Perennial Broad-leaved herbaceous 71.4
ebracteatus

Solidago Perennial Broad-leaved herbaceous 78.6
sempervirens


Sampling at Site 1 produced 14 species (58.3% of the total). Site 2 added four new species and resulted in 75% of the total for the 14 sites. After Site 2, no more than a single species was added at a site. Sampling through Site 9 produced 95.8% of the total species and only a single new species was added thereafter (at Site 12).

Species richness at the wetland sites ranged from 7 to 14 and 11 of the wetlands (78.6%) had 12 or more species (Table 3). Mean species richness was 11.9 (SD = 2.17) and the 95% confidence interval ranged from 10.6 to 13.2. Species diversity values ranged from 0.533 to 1.005 (Table 3). Mean species diversity was 0.855 (SD = 0.134) and the 95% confidence interval extended from 0.778 to 0.932. Evenness showed even less variation (Table 3). The mean was 0.797 (SD = 0.084) and the 95% confidence interval ranged from 0.750 to 0.844.
Table 3. Comparison of species richness (N), species diversity (H'),
and evenness (J') among wetlands on South Padre Island, Texas.

Wetland N H' J'

 1 14 1.005 0.877
 2 13 0.787 0.708
 3 13 0.848 0.761
 4 12 0.947 0.878
 5 14 1.001 0.873
 6 12 0.876 0.812
 7 14 0.918 0.801
 8 13 0.809 0.726
 9 13 0.994 0.892
 10 10 0.899 0.899
 11 12 0.775 0.719
 12 8 0.667 0.738
 13 7 0.533 0.631
 14 12 0.904 0.838


Judd & Lonard (2004) provide data on species richness, species diversity, and evenness in freshwater, brackish, and saltwater marshes in the Rio Grande Delta of the Texas mainland adjacent to South Padre Island. The means for freshwater and brackish marshes were calculated from their data and compared with the means for these parameters in the South Padre Island wetlands. Six mainland freshwater marshes had a mean species richness of 25.3 (SD = 6.25), mean species diversity of 1.394 (SD = 0.120), and mean evenness of 0.788 (SD = 0.05). Nine mainland brackish marshes had a mean species richness of 17.9 (SD = 5.578), mean species diversity of 1.262 (SD = 0.184) and mean evenness of 0.783 (SD = 0.056). South Padre Island wetlands had significantly lower species richness than both freshwater (t = 7.279, 18 df, P < 0.001) and brackish (t = 3.656, 21 df, P < 0.01) mainland marshes. Likewise, South Padre Island wetlands had significantly lower species diversity than mainland freshwater marshes (t = 8.502, 18 df, P < 0.001) or mainland brackish marshes (t = 5.700, 21 df, P < 0.001). However, there was no significant difference in evenness between South Padre Island wetlands and either mainland freshwater marshes (t = 0.243, 18 df, P > 0.5) or mainland brackish marshes (t = 0.44, 21 df, P > 0.5).

Similarity in species composition among marshes was high (Table 4) with coefficients for 91 pair-wise comparisons ranging from 0.300 to 0.857. The mean coefficient of similarity was 0.644 (SD = 0.138) and the 95% confidence interval of the mean ranged from 0.615 to 0.673. Most of the wetlands shared 62% to 67% of their species. The similarity is significantly greater (t = 9.499,104 df, P < 0.001) than the mean of 0.322 (SD = 0.116) for 15 coefficients of freshwater marshes and the mean of 0.258 (SD = 0.123) for 36 coefficients of brackish water mainland marshes (t = 16.176, 125 df, P < 0.001) reported for the adjacent Texas mainland by Judd & Lonard (2004).
Table 4. Comparison of Sorensen's community similarity coefficients
among freshwater and brackish wetlands of South Padre Island, Texas.

 Site

Site 2 3 4 5 6 7 8 9 10 11 12

1 .667 .593 .692 .714 .615 .786 .593 .593 .667 .615 .636
2 .846 .800 .741 .720 .815 .615 .692 .696 .720 .381
3 .720 .741 .720 .741 .615 .538 .696 .640 .381
4 .769 .750 .769 .720 .720 .818 .750 .500
5 .846 .857 .815 .741 .750 .692 .455
6 .769 .800 .640 .727 .667 .500
7 .741 .815 .750 .615 .455
8 .769 .696 .640 .476
9 .696 .640 .381
10 .727 .444
11 .500
12
13

Site 13 14

1 .476 .692
2 .400 .640
3 .300 .640
4 .421 .750
5 .381 .769
6 .526 .750
7 .476 .692
8 .400 .640
9 .400 .640
10 .353 .818
11 .421 .833
12 .667 .600
13 .526


Cover of dead plant material ranged from 17.1% to 97.3% among the sites. The mean was 67.4% (SD = 26.1). Cover of live plants ranged from 27.4% to 80.5% and the mean was 56.9% (SD = 22.7). Much of the dead and live plant material overlapped, but relatively little of the surface was bare. The top three species in importance were abundant in both frequency (range 57.1% to 100%) and cover. The sum of the relative cover for the top three species in importance at a site ranged from 63.5% to 97.7%. The other species present at a site contributed little to the abundance of vegetation at the site. Consequently, data on species importance are presented for only the first three species in importance at each site (Table 5). Schoenoplectus pungens (bulrush) was among the top three species in importance in 13 of the wetlands (92.9%) and it was the dominant species in nine wetlands (64.3%) (Table 5). Fuirena simplex (umbrella grass) and Spartina patens (marshhay cordgrass) each were dominant in two wetlands and Andropogon glomeratus (bushy bluestem) was dominant in one (Table 5). Two of the dominants were sedges (bulrush and umbrella grass) and two were grasses (bushy bluestem and marshhay cordgrass). None of the broad-leaved species were dominants and only four were one of the top three species at a particular site.
Table 5. Comparison of species importance among freshwater and brackish
wetlands on South Padre Island, Texas. Freq. = frequency, Rel. Freq. =
relative frequency, Rel. Cover = Relative Cover, Imp. Val. = importance
value (sum of relative frequency and relative cover).

Site Species Freq. Rel. % Rel. Imp.Val.
 Freq. Cover Cover

1 Schoenoplectus pungens 100.0 14.3 14.23 35.5 49.8
 Spartina patens 85.7 12.2 11.21 28.0 40.2
 Iva texensis 85.7 12.2 5.61 14.0 26.2
 11 additional species Total Cover 40.03

2 Schoenoplectus pungens 100.0 15.7 65.64 81.6 97.3
 Iva texensis 87.5 13.7 4.31 5.4 19.1
 Conoclinium betonicifolium 87.5 13.7 2.16 2.7 16.4
 10 additional species Total Cover 80.5

3 Schoenoplectus pungens 100.0 17.9 13.13 47.9 65.8
 Iva texensis 100.0 17.9 10.31 37.6 55.8
 Spartina patens 85.7 15.4 1.09 4.0 19.4
 10 additional species Total Cover 27.44

4 Schoenoplectus pungens 100.0 13.7 15.34 36.2 49.9
 Solidago sempervirens 100.0 13.7 5.96 14.0 27.7
 Spartina patens 100.0 13.7 5.66 13.3 27.0
 9 additional species Total Cover 42.41

5 Schoenoplectus pungens 100.0 10.1 14.60 31.2 41.3
 Iva texensis 100.0 10.1 11.00 23.5 33.6
 Andropogon glomeratus 100.0 10.1 7.60 16.3 26.4
 11 additional species Total Cover 46.70

6 Schoenoplectus pungens 100.0 14.0 30.70 59.8 73.8
 Conoclinium betonicifolium 100.0 14.0 5.54 10.8 24.8
 Spartina patens 100.0 14.0 2.59 5.0 19.0
 9 additional species Total Cover 51.38

7 Fuirena simplex 100.0 12.3 34.86 55.6 67.9
 Andropogon glomeratus 100.0 12.3 16.84 26.9 39.2
 Schoenoplectus pungens 100.0 12.3 2.80 4.5 16.8
 11 additional species Total Cover 62.68

8 Fuirena simplex 100.0 14.3 62.61 79.9 94.2
 Schoenoplectus pungens 100.0 14.3 6.71 8.6 22.9
 Panicum amarnm 100.0 14.3 2.09 2.7 17.0
 10 additional species Total Cover 73.34

9 Schoenoplectus pungens 100.0 11.5 16.03 25.1 36.6
 Eleocharis geniculata 71.4 8.2 16.30 25.5 33.7
 Panicum amarum 100.0 11.5 10.36 16.2 27.7
 10 additional species Total Cover 63.89

10 Schoenoplectus pungens 100.0 14.0 24.10 32.1 46.1
 Andropogon glomeratus 100.0 14.0 23.11 30.8 44.8
 Fuirena simplex 100.0 14.0 10.83 14.4 28.4
 7 additional species Total Cover 75.07

11 Schoenoplectus pungens 100.0 19.4 47.94 62.7 82.1
 Borrichia frutescens 100.0 19.4 21.40 28.0 47.4
 Bacopa monnieri 85.7 16.7 2.90 3.8 20.5
 9 additional species Total Cover 76.48

12 Spartina patens 100.0 25.0 30.36 54.0 79.0
 Schoenoplectus pungens 100.0 25.0 19.61 34.9 59.9
 Borrichia frutescens 85.7 21.4 4.97 8.8 30.2
 5 additional species Total Cover 56.18

13 Spartina patens 100.0 36.8 60.27 91.7 128.5
 Borrichia frutescens 57.1 21.1 0.63 1.0 22.2
 Iva texensis 42.9 15.8 1.17 1.8 17.6
 4 additional species Total Cover 65.74

14 Andropogon glomeratus 85.7 15.0 11.99 34.9 49.9
 Schoenoplectus pungens 100.0 17.5 7.73 22.5 40.0
 Spartina patens 100.0 17.5 5.81 16.9 34.4
 9 additional species Total Cover 34.33


DISCUSSION

Water in the wetlands of the secondary dunes and vegetated flats zone of South Padre Island occurs as a result of rainfall. Thus, it is fresh when it falls and it becomes progressively more brackish as water evaporates and the remaining standing water accumulates salt from wind transport. Consequently, a given wetland might contain freshwater if sampled soon after a rain and brackish water if sampled a week or more later. Thus, South Padre Island may not have wetlands that are permanently fresh or brackish. Salinity varies with the proximity of rain events. The wetlands are dry much of the time; especially so in drought years. Only 28.6% of the wetlands contained water at the time we sampled them. This was likely due to the fact that rainfall was below normal in seven of the 11 months of this study.

Each of the South Padre Island wetlands was sampled only once. Thus, it is not known if there is significant seasonal variation in any of the parameters studied here. However, it seems unlikely because only four species (16.7 % of the total, Table 2) are annuals and only two of these four species ranked in the top three species in importance at a given site (Table 5). None of the four species was dominant at a site.

South Padre Island freshwater and brackish wetlands have fewer species and lower species diversity than mainland freshwater and brackish marshes of the adjacent Rio Grande Delta (Judd & Lonard 2004), but evenness is similar in the island and mainland wetlands. Only six (24%) of the species occurring in South Padre Island wetlands in this study were also found in mainland wetlands by Judd & Lonard (2004). All six species occurred in brackish marshes on the mainland. Schoenoplectus pungens and Conoclinium betonicifolium occurred only in mainland brackish marshes, Typha domingensis and Bacopa monnieri were found in freshwater, brackish, and salt marshes of the adjacent mainland. Sporobolus virginicus and Borrichicia frutescens occurred in brackish and salt marshes on the mainland. Three of the species were dominant in one or more marshes on the mainland. Typha domingensis was dominant in one of six freshwater marshes and in three of nine brackish marshes on the mainland. Borrichia frutescens was dominant in one of nine brackish marshes and two of 11 salt marshes on the mainland. Sporobolus virginicus was dominant in two salt marshes on the mainland. None of the dominant species in island wetlands were also dominant in mainland wetland communities. Clearly the island wetland communities are markedly different than wetland communities of the adjacent mainland. They have fewer species, mostly different species, and different dominant species. It is unlikely that additional sampling on South Padre Island would have added many more species because sampling through the first nine sites resulted in 23 of the 24 species found. Sampling five more sites produced only one species. Similarity of species composition among freshwater and brackish wetlands of South Padre Island is high. The higher similarity among island wetlands is likely related to the much lower total species richness (24 species) compared to the mainland freshwater and brackish marshes (81 species each).

Baccus & Horton (1979) identify 13 species that occur in the low marshy areas of the heavily vegetated barrier flat at Padre Island National Seashore. Thus, species richness was similar to what we found at South Padre Island (mean = 11.9, 95% confidence interval = 10.6 to 13.2). However, similarity in species composition was modest. Only five of the 13 species Baccus & Horton (1979) list for Padre Island National Seashore were found among the 14 sampling sites in this study. This might be six species in common if the Eleocharis sp. they list was E. geniculata. Species in common increases to eight if species known to occur on South Padre Island are included (Lonard et al. 1978) but not found in this study, and it might be increased to nine if the Eleocharis sp. was E. geniculata, E. montevidensis, or E. obtusa, which are known to be present on South Padre Island (Lonard et al. 1978; this study). Nelson et al. (2001) compare the floras of Matagorda Island, Mustang Island, North Padre Island and South Padre Island. Each species in the wetlands of South Padre Island is known to occur on North Padre Island.

Baccus & Horton (1979) found that the dominant species among the wetland species was Eleocharis interstincta. Eleocharis interstincta was not found in this study and Lonard et al. (1978) did not find it on South Padre Island. A different species of Eleocharis (E. geniculata) was present at six of the 14 wetlands sampled, but it was abundant at only one site (2nd in importance at Site 9).

Nelson et al. (2000) reported on the vegetation and floristics of four communities in the Big Ball Hill region of Padre Island National Seashore. They identified Andropogon glomeratus and Spartina patens as dominants in the lowland subcommunity of the barrier flat community. Borrichia frutescens, Hydrocotyle bonariensis, Samolus ebracteatus, and Schoenoplectus pungens were listed as species typically present in the lowland sub-community. They do not suggest that this list is complete for the lowland sites. All of the species, except H. bonariensis were present among the sites studied at South Padre Island, but only Spartina patens occurred in Baccus & Horton's (1979) list of species occurring in low marshy areas of the vegetated flats of Padre Island National Seashore. Thus, there is less similarity in species present in lowland barrier flat communities on North Padre Island than there is between either of the studies and the species present in the wetlands of South Padre Island.

Mean species richness in the barrier flat community (both low and higher elevations) of Big Ball Hill region of Padre Island National Seashore was reported to be 20.3 (Nelson et al. 2000). This is almost double the mean species richness of 11.9 found among the wetlands at South Padre Island. It is not surprising since only the lowland component of the barrier flats was included. Nelson et al. (2000) reported a mean species diversity for the barrier flat community of 0.58 and a mean evenness of 0.61. They do not provide a measure of variance for either mean, but both means appear to be significantly lower than those reported here because the values fall below the 95% confidence intervals for species diversity and evenness in wetlands at South Padre Island.

CONCLUSIONS

Based on the meager data available, it appears that species richness at wetland sites is similar on the northern and southern portions of Padre Island, but there is only modest similarity in species composition of wetland communities. There is considerable variation in species importance among South Padre Island sites and marked variation between southern and northern portions of Padre Island. Additional study of Padre Island and other Texas barrier islands is needed to ascertain if the differences in composition of barrier island freshwater and brackish wetland communities reported here are stochastic or related to variation in environmental variables.

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Lonard, R.I. & F. W. Judd. 1999. The biological flora of coastal dunes and wetlands. Ipomoea imperati (Vahl) Griseb. Journal of Coastal Research, 15(3):645-652.

Lonard, R. I., F. W. Judd, J. H. Everitt, D. E. Escobar, M. A. Alaniz, I. Cavazos III & M. R. Davis. 1999. Vegetative change on South Padre Island, Texas, over twenty years and evaluation of multispectral videography in determining vegetative cover and species identity. Southwestern Naturalist, 44(3):261-271.

Nelson, A. D., J. R. Goetze, I. G. Negrete, V. E. French, M. P. Johnson & L. M. Macke. 2000. Vegetational analysis and floristics of four communities in the Big Ball Hill, region of Padre Island National Seashore. Southwestern Naturalist, 45(4):431-442.

Nelson, A., J. Goetze & A. Lucksinger. 2001. A comparison of the flora of northern Padre Island to that of Matagorda Island, Mustang Island and southern Padre Island, Texas. Occas. Papers, Museum of Texas Tech University, Number 209:1-23.

FWJ at: fjudd@utpa.edu

Frank W. Judd and Robert I. Lonard

Department of Biology, University of Texas Pan American Edinburg, Texas 78541
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Author:Judd, Frank W.; Lonard, Robert I.
Publication:The Texas Journal of Science
Article Type:Report
Geographic Code:1USA
Date:May 1, 2009
Words:5222
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