Nicole Addington, Renee Dufek, and Jesse Johnson

BIOL 417a & 417b

Summer Session 2003

Western Washington University

Bellingham, WA

Introduction

Deserts provide a rich opportunity to study the basic ecological questions about distribution and abundance of organisms. Desert habitats are relatively simple systems wherein much of the biota can be observed and measured readily. The Great Basin Desert, located principally in Nevada and Utah, is especially tractable because habitats (of shrubs and bunchgrasses) are more stable and weather is more predictable (e.g., fewer sheet floods from summer rains) there than in southwestern U.S. deserts.

Reptiles and arthropods are the prevalent diurnal animal denizens of most desert habitats. Desert lizards mostly eat arthropods. Arthropods are the abundant primary and secondary consumers near the base of the desert food web. Spatial variation in the desert habitat, as represented by variation in soil type, ground topography, type and physiognomy of dominant perennial plants may cause significant variation in arthropod species abundance and diversity over small spatial scales, such as in mesohabitats and microhabitats. Gaining knowledge of the spatial patterns of distribution, abundance, and diversity of the arthropods is essential to understanding the spatiotemporal patterns of foraging desert lizards. Knowing the spatiotemporal patterns of predator and prey is essential to understanding the population consequences of those interactions, one of the primary goals in ecology.

The western whiptail lizard Cnemidophorus tigris, is a species whose populations inhabit most deserts in the western U.S. These lizards are a wide, intensive foraging, arthopodivores. They spend most of their daily activity periods moving over a large home range in search of arthropods under perennial plants, and must make frequent choices among microhabitats (e.g., the plants) in which to forage (Anderson 1993). The primary prey of C. tigris are eggs, larvae, pupae, and adults of relatively sedentary species; termites are especially common (Anderson 1993). Cnemidophorus tigris, however, are opportunistic predators (Scudday and Dixon, 1973), and with the exception of adult beetles and ants, will eat the more abundant arthropods (Anderson 1993, 1994). This study examined the distribution and abundance of arthropods in a desert scrub of Great Basin Desert, and compare the spatial patterns of arthropods with spatiotemporal patterns of foraging Cnemidophorus tigris.

Pit-trapping for arthropods and standardized searches for the lizard, along with some focal observations of the lizard documented mesohabitat and microhabitat use by prey and predator. These spatiotemporal patterns of lizards and the data from the pit trap study provide preliminary answers to basic questions and enable several hypotheses to be tested.

The following hypotheses can be tested propitiously:

· Hypothesis 1: Larger plants have more standing crop biomass and a greater variety of nanohabitats than do smaller plants, hence larger plants will have higher arthropod abundance and diversity.

· Hypothesis 2: The succulent evergreen perennial Sarcobatus vermiculatus has more edible biomass for a longer time during the activity season than the drought-deciduous perennial Artemisia tridentata. Thus S. vermiculatus will have higher arthropod abundance and diversity than will an A. tridentata of the same size.

· Hypothesis 3: The number of arthropods captured in a pit trap will vary inversely with the distance between perennial plants and the pit trap.

· Hypothesis 4: Mesohabitats with more and larger perennial plants will have a greater abundance and diversity of arthropods captured in pit traps than will mesohabitats with fewer and smaller perennials. Hence, in the Pueblo Valley, the dune mesohabitat will have the greatest abundance and diversity of arthropods, whereas sandy flats will have fewer, and hardpan will have the lowest abundance and diversity.

· Hypothesis 5: The lizard Cnemidophorus tigris will selectively forage in mesohabitats and under plants where their preferred arthropod prey are most abundant.

Study Area and Methods

The chosen study area, the annual study area for Biol 417a,b, is in the north end of the Pueblo Valley, situated in the Alvord Basin. The Alvord Basin is in the northern extreme of the Great Basin Desert. The study area is an ecotone from Great Basin Sage Association upslope to Greasewood-Salt Grass Association down slope. The mesohabitats studied in the study area are dunes, sandy flats, and hardpan.

The pit traps were placed out in the open in each of the mesohabitats and under dominant plant species to investigate spatial variation in arthropod diversity and abundance, and thus, yielding knowledge to predict the prevalent locations for Cnemidophorus tigris foraging. Pit trapping (methods as in 2002 pit trapping study for Biol 417a) was done in the three primary mesohabitats (dunes, sandy flats, and hardpan), and under the two dominant perennial plant species, Artemisia tridentata (ARTR) and Sarcobatus vermiculatus (SAVE).

Plants chosen for pit trapping were quasi-randomly, and a plant was used if it met one of the predetermined size classes. Microhabitats sampled were small, medium, and large Artemisia tridentata in the sandy flats (1 pit trap per small, 2 per medium, and 4 per large), medium Sarcobatus vermiculatus on sandy flats and dunes, medium A. tridentata on dunes, and open areas in sandy flats and hardpan. Traps were left out for six days to collect a representative sample of arthropods. Contents of the pit traps were analyzed in the laboratory. All arthropods were extracted from sand and litter, then each arthropod was identified to the taxonomic level of order, then placed in alcohol-filled storage vials.

During the field course, students encountered C. tigris frequently, thus permitting statistically analyzable data on the mesohabitats and microhabitats of the lizards when they were first seen. Moreover, some lizards were chosen for focal observations, thereby providing more detailed information on microhabitat and nanohabitat use of focal lizards.

Standard plot searches for Cnemidophorus tigris were conducted on the 150m x 150m study plot daily. When a lizard was first seen, its location parameters (plot coordinates, substratum, microhabitat) and behavior were recorded. If the lizard could not be identified by unique paint marks for each individual identification lizard, then it was captured, examined for sex and reproductive condition, weighed, measured, given a unique toe-clip number if it did not already have one, painted, then released where captured. Initial and repeat sightings of individual lizards provide information on patterns of habitat use.

Focal observations (20-30 minutes) were performed on 15 Cnemidophorus tigris as they foraged. Searchers described such behaviors as shrub entries and exits, changes in direction of travel, changes in substratum traveled on, details of foraging behavior (e.g., scratching, digging, climbing), when light and shade patterns striking the lizard changed, and more. The microclimate and nanoclimate conditions at the beginning and end of focal observations were recorded.

Results

Out of 278 sightings of C. tigris during standardized searches of the 150m x 150m plot (June 27-July 15, 2003), the relative abundance of sightings among mesohabitats are: 41% percent on dunes, 32.5% on sandy flats, and 26.5% on the hardpan. The mesohabitat distribution on the 150m x 150m plot is: 29% dune, 43% sandy flats, and 28% hardpan, as estimated from plot maps made by students in Biol 417 from 2001 and 2003. Thus, C. tigris prefers foraging on dune. In addition to the standard plot surveys, our focal observations of C. tigris observations revealed them to forage under shrubs in approximate proportion to their abundance on site. Whiptails spent approximately 60% of their time foraging under Artemisia tridentata (ARTR) the most common perennial on site (Table 1). The mean height of ARTR foraged under was similar to the mean height of the available array of ARTR on the plot.

We analyzed 306 pit-traps for distribution and relative abundance of each major arthropod order. More arthropods were found in the pit traps under taller, larger plants (Figure 1). Ants (taxonomic family Formicidae, in the order Hymenoptera) were excluded from the majority of analyses both because ants are so numerically preponderant in the pit traps and because ants are not typically a prey item of C. tigris. Ants accounted for 56% of the total captured insects (Figure 2 and Table 2). Following ants the most abundant arthropods are in the orders Coleoptera, Diptera, Hymenoptera (non-ants), Thysanura, Neuroptera, Aranae, and Lepidoptera (most to least, respectively) collectively comprising 35% of the total arthropods (Figure 2).

An ANCOVA statistical analysis (P = 0.05) on a per pit trap basis was used to determine if there were significant differences between the interactions of plant type, size, and mesohabitat. Comparing the total number of arthropods without Formicidae showed significant differences between the plant types (P = 0.002) and the mesohabitats (p = 0.0001). These differences between plant types were attributable to differences among plant categories for Coleoptera, Lepidoptera and non-ant Hymenoptera ( P = 0.001, 0.47 and 0.002, respectively). There were no significant differences comparing the number of Diptera.

Mesohabitat effects on numbers of arthropods in pit traps under plants were not obvious (Figure 3). There were no significant differences between the medium SAVE on the sandy flat versus the dune. Neither were there mesohabitat differences in arthropod abundance for the medium ARTR. The arthropod numbers were similar for open pit traps in sandy flat and hardpan.

Small, medium, and large ARTR had relatively similar numbers of arthropods per pit trap (no significant differences) and all plants had significantly more arthropods per trap than did lone pit traps in the open (about 1 m from nearest plant). When volants were included, medium Sarcobatus vermiculatus (SAVE) had nearly a significantly greater number of arthropods (Figure 3 and Figure 4).

Statistically significant differences in numbers of arthropods trapped per plant existed between all combinations of sandy flat microhabitats: Within ARTR, as plant size increased, the number of pit-trapped arthropods caught under the plant increased. Areal projection of plant cover was compared with total numbers of arthropods, without Formicidae, for all the ARTR size groups on the sandy flats. (Figure 5). Statistically (t-tests, P < 0.05), greater numbers of arthropods were found in a) pit traps under small plants than in open pit traps (P = 0.0001), b) under medium size plants than under small plants (P = 0.018), and c) under large plants than under medium plants (P = 0.00001).

We made the tentative assumption that the mean number of arthropods in the sandy flat open pit traps, 5, represented the general habitat effect on numbers of arthropods under the plant. Thus, we subtracted 5 arthropods from the mean for each of the three size classes of ARTR. The result is that although the larger size class covered four times the area that a small size class did, the larger size class had five times more arthropods than did the smaller size class. Hence, there may be an increase in the number of arthropods per unit area of cover with increasing plant size.

Discussion and Conclusion

Some of our predictions were not supported by our data from pitfall traps. For example, we predicted that pit traps under plants in the dune mesohabitat would capture more non-ant arthropods than would pit traps under plants of the same size and species in the sandy flats mesohabitat. Our results showed that pitfall traps under perennial plants of the same size and species had similar numbers of arthropods, despite the different habitat. We also predicted that among plants of similar size, pit traps under the succulent, heavily-leaved SAVEs would have more arthropods than would pit traps under the less palatable, sparsely-leaved ARTRs. Although there was a trend, the difference was not statistically significant; perhaps we did not sample enough SAVEs.

Comparisons of captures rates of arthropods in pit traps among plants varying in size produced results ambiguous results as well. The total number of arthropods captured in all pit traps under a plant varied directly with plant size. There were significant differences in number of non-volant, non-ant arthropods among the small, medium, and large ARTRs on the sandy flats. It is possible, however, that the greater number of pit traps under the larger plants was partially or entirely the reason for higher numbers of arthropods captured under larger plants. Thus, our analysis of the number of non-volant, non-ant arthropods caught per pit trap revealed no significant differences among plants varying in size.

The only significant differences in arthropod numbers were between pit traps in the open and pit traps under plants. Out in the open, there is no refuge on the ground surface from the extreme heat of midday, and for all times of day there is little food and virtually no refuge from predators in the open microhabitat. Thus, even if many of the arthropods were nocturnal, it would be expected that they would associate more with the plants than the relatively sterile open ground.

We infer from the data in our standard plot surveys, observations of western whiptails, and pit trapping that for the period of our study these lizards neither did nor should use plant size as a cue for greater food availability. The ARTRs were the most common species visited and foraged in by C. tigris. But frequency of foraging under ARTRS may be the simple result of ARTRs being the most common species of perennial on the plot.

Although pit traps are one of the best ways to estimate relative arthropod abundance and density, it is unlikely that pit traps capture numerically representative samples of all the arthropods that C. tigris tends to prefer. These lizards eat a lot of arthropods that are hidden in soil and leaf litter, such as termites, quasi-sedentary larvae, and profoundly sedentary pupae and eggs (Anderson 1993). Foraging whiptail lizards tend to rely heavily on chemoreception to locate hidden prey (Etheridge and Wit). Cnemidophorus tigris is an opportunistic forager, however. For example, when lepidopteran larvae seemed to be plentiful during the first week of the field course, we made frequent anecdotal observations of C. tigris actively seeking these caterpillars to the exclusion of other prey. These lizards often climbed for the caterpillars that were feeding on vegetation and dug for caterpillars beginning to pupate in the soil. In the second and third weeks of the field course, however, digging became relatively uncommon, and the searches for caterpillars by C. tigris appeared rare. Instead, the lizards seemed to do more climbing, jumping, and running after more mobile and volant insects.

An experiment varying the number of pit traps within and among perennial plants varying in sizes may definitively reveal whether larger perennial plants harbor more arthropods per unit area of plant cover than do smaller perennials. Moreover, larger sample sizes should be used to compare arthropod abundance under different species of perennials.

Our preliminary analysis of a limited portion of the available C. tigris foraging data is that there is too much variation in lizard behavior relative to the sample size analyzed. We anticipate that full analyses of field course foraging data from (1999 and 2003) will reveal some understanding of the spatiotemporal patterns of foraging in Cnemidophorus tigris.

Table 1. Summary data from focal observations on Western Whiptails (N=8).

avg ARTR height	shortest ARTR	tallest ARTR	# of plants	time observed (sec)	total dist traveled	avg dist traveled	est time under ARTR (sec)	% est time under ARTRs
0.8125	0.75	1	5	1410	9	0.9	606	0.429787
0.6071	0.25	0.75	7	1153	24.25	1.68	846	0.733738
0.5833	0.5	0.75	5	1240	11	0.5	709	0.571774
0.6944	0.5	1	9	1302	27.25	3.41	1260	0.967742
0.75	0.25	1.75	19	1179	47.75	2.98	522	0.442748
0.3214	0.25	0.5	7	1500	51.75	2.88	483	0.322
0.5794	0.25	1	17	2259	91.75	2.62	1180	0.522355
0.6964	0.25	1	14	1416	52.25	2.01	1122	0.792373
0.6046			10.375				841	0.597815