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of fleretology, Vol. 35, No. 2, pp. 326-330, 2001 Jowrnal Copyright 2001t Society for the Study of Amphibians anid Reptiles
Microhabitat Use and Activity Patterns of Holbrookia niaculata and Sceloporus u-ndulatus at White Sands National Monument, New Mexico STPHIEN B. HAGER'
Department of Biology, Nezw Mexico State University, Lis Cruces, New Mexico 88003, USA White Sands National Montument (WVSNM), Ntlw Mexico, was established as a National P['ark to preserve the biotic and abiotic features of the "white" gypsum dunes environment. The lizards of WSNM are characterized as having special adaptive qualities that enable them to survive in gypsum sand. The lesser earless lizard (Hoihbrookiza mnt olata) and the eastern fence lizard (Sceioporus unduwatus) are white formas of their respective species that reside at the Monurment (Ruthven, 1907; Smith, 1943; 1owe and Norris, 1956). At WSNM, Hoibrookia onac1u/ota is short, stout, and dorsally pallid with fairly well-defined ventro-lateral blotcles, and Sccloportus wndulatus is medium-sized with a dorsal coloration of pale gray to nearly white with faint black shoulder patches (Smith, 1943). Dixon and Medica (1966) reported food of these lizards, and Dixotn (1967) studied their activity and reproduction. MacFarland (1969) found that H. m;aciilata and S. imdulattus overlap greatly in daily activity, behavior, and food bitt differ in substrate preferences in senminatuiral enclosures at WSNM. Holbrookia maocuiata at WSNM had lower bodv temperatures and thermoregulated differently than a population of H. rnaculata from outside the dunes proper (I-ager, 200()). Human-induced landscape alterationis and increased land management practices threaten taxa adapted to aeolian habitats. Preservation of these organisrns requlires basic knowledge of their biology. Consequently, my goal was to understand more about the activity patterns and habitat tLse of H. maculata and S. undulatus in the gypsum dmne field at WSNM. To rny knowledge, this is the first systematic field sttidy of these two lizards in their natural habitat across an entire activity season. For each species, I evaluated daily and seasonal surface activity and how this activity relates to the lizard's structural environment, use of microhabitats, proximity to vegetation, and population densitv. 't-hese data will allow monument personnel to make more iiiformed rnianagement decisions on behalf of these liza rds and the biotic resources, in general, and will ultinately contribute toward the preservation of the biodiversitv of the gypsum environment. In broad terms, this information sheds more light on community structure in sand dune habitats. White Sands National Monument, Ne.w Mexico, (32'45'N; 106''l5'W) encompasses approximately 450 kim2 of active, moving dunes. Its edges house various IPresent Address: Department of Biology, Augustana College, Rock Island, Illinois 61201-2296, USA; FE-mail:
[email protected].
grass species, shrubs, and cottonwood trees. Vegetation in the interior of the dune field is niore sparse than at its edges and is concentrated in the hardpan flats between dunes. Climatic conditions at White Sands are generally mild and winidy in spring, hot and dry in the summer, mild in the fall, and cold and dry in the winter (National Oceanic and Atmospheric Administration, 1997). Temperatures peak in the summer with an average ambient high of 39'C in JuLne accompanied by very low relative humidity. Lowv temperatures in February average 7"C. Generally, gypsum soil temperatures are relatively cooler than adjacent, nongypsum substrate temperatures because of high rates of water evaporation from shallow subterranean water, high albedos (the ratio of the total reflective irradiance to the total incident irradiance on a surface with the solar spectrum), and the physical limitations of gypsum as a heat conductor (Chapman 1984; Weast 1986; Wheeler et al., 1994; V Gutschick, pers. comm.). I conducted field surveys in 1994 on a 2-ha study plot at White Sands National Monument. This site was characterized by large shrubs, such as Poliomnintha i1caira (Lamiaceae) and Yucca elata (Liliaceae), which had stabilized the dunes, and by hardpan tlats (interdune depressions) that were composed of hardened, encrusted gypsum with various bunchgrasses, such as Andropogon sp. and Sporobilis sp. (Poaceae) and Ephedra torree,una (Ephedraceae). The study site was representative of the interior of WSNM. Forty-three U. oawciiata and 47 S. itndolatuts were captured via noose, nmarked, and multiply recaptured at this site. Marking involved sewing a color-coded sequence of beads onto the base of the tail (Fisher and Muth, 1989); young of the year were toe clipped. I located lizards along 20 belt transects spaced every 10 in. Fieldwork was conducted approximately two days per week from May to August and approximately one day per week from September through Decembet. I alternated daily fieldwork between two time frames: 0700-1400 h and 1400-2000 h. Transects were walked continuously within these time frames and were temporarily halted when lizards were spotted. I resumed surveys following data collection, which lasted up to 15 min. Activity is defined as lizards observed above the soil surface during field surveys. I measured air temperature (Tl') 2.5 cm above the soil surface (shaded bulb) and surface tem0 peratures (TI) just below the soil surface (z 0.2 C) in various areas of the study plot approximately once per hour with a quick-reading, cloacal thermometerc At the initial capture of a lizard, I recorded the (1) tinme; (2) microhabitat in which lizard was first sighted-dune (rounded ridge of loose gypsum heaped up by wind and dotted with vegetation), hardpan flats (flat region boLnded by dwues housing stable vegetation on hardened, crusty soil), or transition (flat "ecotonal" area of the dune and hardpan- flat that lies between the windward edge of the flats and the lee side of an active, moving dune, and containing relatively stable vegetation and loose gypsumn); and (3) proximity to vegetation when first sighted-open (> 10 cm from edge of plant cover on ground), under (on ground and enclosed by plant from above), edge (< 10 crn away from the outer edge of vegetation on ground), and arboreal (within vegetation above ground and < 5 cm from outer edge). I characterized microhabitats by estimating soil cohesion, percent
SHORTER COMMUNICATIONS TABtL 1.
Microhabitat characteristics and differences of the study site at WSNM.
Mean soil comrpaction (mm ± 1 SE) Estimated % area % plant cover
Dume
Elardpan
Transition
Test
59.0 t 5.95 44 13.9
10.7 ± 0.89 35 25.9
30.6 ± 1.46 21 20.5
F X2
area, and percent plant cover. To quantify soil cohesion, I measured the depth (nearest mm) of a steel, flat-ended rod (mass = 200 g; diameter -1 cm; length = 50 cm) that penetrated the gypsum surface wihen dropped at a height of I m at 25 randomly chosen points for each microhabitat. T'he soil was considered compact if the steel rod penetratedi the soil surface to relatively shallow depths. I mapped the microhabitats of the study site using a Cartesian grid system with markers placed at every 10 m. Edges of microhabitats were delineated according to their proximity to these markers. Percent area was estimated for these microhabitats using SigmaScan software for the PC. I used line intercept transects to estimate percent cover for each microhabitat (Brower and Zar, 1980). Two replicate transects ranging from 70-90 m in length were completed in each microhabitat. I visually "recaptured" individuals and identified them by their bead colors. 'I'he time, microhabitat, and proximity to vegetation were recorded for all recaptures. I estimated population density by the number of residents on the 2-ha study site. Residents are defined as those individuals captured at least twice during the study and that remained on the site at least 30 days (Jones and Ballinger, 1987). All statistical tests were performed at at = 0.05. Means ± 1 SE are reported. Chi-squared tests of heterogeneity were performed on the following: (1) activity by hour and month between the species, to evaluate whether H. ndu12iata and S. uidtlatus were active at different temporal scales; (2) the percentage of lizards found within microhabitats relative to the estimated percent area of those microhabitats, to assess any differences in microhabitat use by species; (3) mi-
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