Spatiotemporal patterns of desert grasshoppers and their predator, the leopard lizard, Gambelia wislizenii

 

 

 

 

 

Janet Cray, Colleen Duck, and Colin Hume

Biol 417a,b,  Summer Session 2002

 

 

(poster edited by Dr. Anderson to fit website format)

 

 

 

 

 

 

Introduction

 

Across a south-to-north gradient in the deserts of the western U.S.A, body sizes decrease, population densities increase, and diets shift from mostly lizards to mostly grasshoppers in populations of the long-nosed leopard lizard Gambelia wislizenii.  Study of a population of G. wislizenii near the northern and upper elevation extremes of the species’ geographic range should provide a useful base of knowledge to compare with data on southern populations, and thus lead to an understanding of the causes for the geographic trends. 

 

A convenient study locale is in the northern Pueblo Valley in the Alvord Basin, a part of the Great Basin Desert Scrub in the rain shadow of the Steens Mountain uplift.  The vegetation in the Alvord Basin consists primarily of perennial shrubs.  The ground substratum is mostly sandy, varying from loose, fine sand on the dunes in the greasewood-dominated habitats lower in the basin, to compact sand and gravel higher up slope in the sage dominated habitat.

 

Arthropods appear to be abundant in the study area, and grasshoppers appear especially easy to encounter.  Spatiotemporal patterns of the distribution and abundance of grasshoppers in the study area may influence the spatiotemporal patterns of Gambelia.  We decided to investigate the spatiotemporal distribution of grasshoppers in the study area and compare sighting frequency of Gambelia with the spatiotemporal patterns of grasshoppers. We also decided to observe grasshopper escape behavior during our encounters with an anticipation that grasshopper evasiveness may vary with ontogeny, time of day, and plant structure, as well as among grasshopper species.  The spatiotemporal distributions, abundances, and evasiveness of grasshoppers are expected to influence the food acquisition behavior of Gambelia wislizenii.


Methods

 

  • After a few days of anecdotal surveys we decided to compare among four obviously different habitats for their spatiotemporal patterns of grasshoppers and Gambelia wislizenii: 1) sage-dominated habitats upslope on compact sand and gravel (upper transition), 2) sage and greasewood as co-dominants downslope on the looser sand of sandy flats, small dunes, albeit mixed with some hardpan depressions (lower transition), 3) larger dunes further downslope, and 4) grassy hardpan salt flats in the valley basin.  We used areas of approximately 40m x 400 m in the grassy salt flats and the upper and lower transitional.  Out of 11 dunes available, 3 dunes were randomly selected, and a 150 m long section of each of the 3 dunes was randomly chosen for the study.  Plant species designations: Table 1.

 

  • Distribution, abundance, areal cover and volume of each perennial plant species was estimated with three 50m line intercepts on each plot (and on dunes verified by measuring all perennials on six 5m2 quadrats on each of three dunes); lines and quadrats were randomly chosen on each habitat area.

 

  • Each grasshopper and Gambelia sampling episode covered a unique 10 x 50 area and lasted no longer than an hour.   Time taken to cover 10 x 50 m areas in all four habitats in a collection period depended on number of grasshoppers encountered, but generally required about 4 hours.  Collection time periods were 0730 to 1200 and 1530 to 2000.  We decided on 9 collection episodes per habitat per collection time period.  At beginning and end of each collection episode at each site, these microclimate data were collected: wind speed, ground surface temperature, air temperature at 2m above ground.  Chance encounters with Gambelia were noted.

 

  • Grasshopper collection methods:  3 persons with sweep nets spread out side by side to cover a “foraging width” of 10 m.  The general method was to look for a grasshopper, and if none were readily apparent, then to brush the bushes with the net handles or sweep the open ground with the nets to flush them out.  Grasshoppers spotted on the bushes were caught using the larger (11 cm) snap-lock vials, quickly trapping them into the vial with the lids.  Grasshoppers spotted on the ground were captured by quickly slamming the nets down over them, which often caused them to jump up into the nets for easier collection.  If the grasshoppers were flushed from a perennial, a flagging weighted with a large washer was tossed on the ground near the initial sighting location of the grasshopper.  Retractable metal measuring tapes were used to measure the horizontal and vertical distances of the first hop or flight. 

 

  • Data taken for each grasshopper:  date, plot, time, species, adult/nymph, initial behavior, head direction, escape direction, escape angle (oblique or straight, towards or away), horizontal and vertical hop distances, capture vial number, initial substrate and final substrate.  An effort was made to note and identify any grasshoppers that escaped.   All grasshoppers collected were placed in vials and preserved in a mixture of ethylene glycol and isopropyl alcohol.

Results I. Distribution and abundance of perennials on the four habitats:

 

·        Dunes:  Quadrat technique on the dunes show the dominant species to be the greasewood Sarcobatus vermiculatus (SAVE) for both percent cover and percent volume, (Figs. 1, 2). The line intercept technique also revealed SAVE as the dominant species.

 

·        Upper Transition:  The line intercepts technique shows the dominant species to be the Basin Big Sagebrush Artemisia tridentata (ARTR, Fig. 1).

 

·        Lower Transition: The line intercept technique shows the dominant species to be Basin Big Sagebrush Artemisia tridentata (Fig. 1).

 

·        Grassy Salt Flats:  This area had such low perennial plant diversity and percent cover that a rough estimate of 5% Sarcobatus vermiculatus (Greasewood) and 5% Panicum occidentale (PAOC) was used.

 

Results II.  Microhabitats compared for grasshopper abundance

 

·        Out of 701 total grasshoppers observed, the grassy flats harbored  significantly fewer grasshoppers (Fig 3).

 

·        Out of all grasshoppers observed, 51% were first seen on plants, 43% were first seen on the ground in the open and for 4% the location of first sighting was uncertain (Fig. 4).

 

·        The nymphs were found more on the plants, but adults (presumably involved in courtship) were found more on the ground in the open (Fig. 5). 

 

·        Of 289 grasshoppers found on plants, most were on Artemisia tridentata and Sarcobatus vermiculatus (Fig. 6). 

 

·        Grasshopper species richness was highest on PAOC, SAVE, and ARTR (Fig 7).  Note that the grass Panicum occidentale (PAOC), was the prevalent plant on the salt flats, and that one portion of the salt flats habitat was very close to a dune; hence, the wind and proximity of dunes may have inflated the species list of “resident” grasshoppers with transients.  SAVE was dominant on dunes and ARTR was dominant on the other two habitats.

 

·        Although there was no correlation between the volume of a plant and the number of grasshoppers on the plant (perhaps a sample size or methods problem), there was a significant relationship between percent cover of a perennial plant species on a plot and the number of grasshoppers found on that plant species on the plot (R2 = 0.545 with a p value of 0.044).

 

·        Fifteen tentative taxa of grasshoppers were designated (Table 2, includes nymph, adult male, and adult female categories) and only about one-third could be considered “common” (Fig. 8)

 

·        Among initial evasion behaviors of all grasshoppers, the most common was hopping (Fig 9) although for species T5, T6, T7, T9 flying was the prevalent predator evasion behavior.   Note that sample sizes were very small for some grasshopper species.

 

·        Evasion directions and distances of grasshoppers revealed horizontal movements to be several times greater than vertical movements (Fig 10); sample sizes were similar with about 8 individuals sampled per movement type per species. 


Results III. Microhabitats compared for Gambelia wislizenii abundance.

 

·        Out of the limited number of G. wislizenii observed during grasshopper collection periods most were seen in the lower transitional habitat, but sample sizes were low.  Hence we used data on chance encounters with Gambelia on the field course study site, in the same habitat as LT. Most Gambelia were first seen in the open rather than directly under plants (Fig. 11).  Note also the microhabitats of adult grasshoppers (Fig 4).

 

·        Of the G. wislizenii that were observed under plants, most were under Sarcobatus vermiculatus and Artemisia tridentata (Fig. 12).  Note also the grasshopper abundance related to plant cover (Fig 5).

 

 

Discussion and Conclusions

 

During our short-term study, adult grasshoppers were more common on the ground in the open, in easy view for the lizard, but as a combined total of nymphs and adults there were more grasshoppers on the perennial plants.  Both nymphs and adults on plants, however, should be less visually obvious than adult grasshoppers on the ground in the open.  Moreover, grasshopper nymphs were found most often in and near the top of perennial shrubs. The large size of adult grasshoppers and female adult grasshoppers laden with eggs should be more preferred than the smaller nymphs by Gambelia wislizenii.  Adult grasshoppers, however, can fly and can evade a pursuing Gambelia more easily than can nymphs, hence nymphs and the adults on the perennials should not be ignored by Gambelia.  Moreover, earlier in the activity season most grasshoppers would be nymphs, and .  Thus, early in the activity season either Gambelia should station themselves near the plant perimeters to catch grasshopper nymphs or Gambelia should be stationed under plants to facilitate ambushing of lizards.  But during our study adult grasshoppers were common in the open (Figure 5), hence we expected Gambelia to be stationed more in the open where the adult grasshoppers were.  Indeed, microhabitat use data from sightings of Gambelia wislizenii on the field course study site, the same habitat as the LT habitat supported this prediction (Figure 11).

 

We caught more grasshoppers on the two dominant perennial plant species  than on other perennial plant species (Figure 6).  Hence, we expected Gambelia wislizenii to be positioned more closely to these two dominant perennials, Basin Big Sage (Artemisia tridentata) and Greasewood (Sarcobatus vermiculatus), than near other plants on the field course study site, in the LT habitat.  Indeed, our expectations were met (Figure 12). 

 

Too few Gambelia were seen among all habitats during our grasshopper collection episodes to discern confidently a differential habitat use by Gambelia.  But 1) our anecdotal lizard sightings data, 2) fewer grasshoppers seen per unit area in the salt flats, and 3) very little shade in the salt flats, all support a prediction of fewer Gambelia in salt flats than in the other three habitats.  The prediction is supported by data in J. Steffen’s master’s thesis; moreover, the apparent higher abundance of Gambelia on the LT also are corroborated Steffen’s data.  More detailed observations and analyses, however, of spatiotemporal patterns of grasshoppers, prey lizard species, and Gambelia among mesohabitats (e.g., dune, hardpan, sandy flats), microhabitats (e.g., at perennials) and nanohabitats (e.g., precisely where on or under perennials) on the field course study site should help elucidate the causes and consequences of Gambelia spatiotemporal patterns, feeding rates, body sizes, and diets across the geographic range of G. wislizenii.

 

Tables and Figures

 

 

Table 1.  Plant and Substratum Names and Codes

 

 

Plant Code

Species Identification

Common Name

ACHY

Achnatherum hymenoides

Indian rice grass

ARSP

Artemisia spinescens

Bud Sage

ARTR

Artemisia tridentata

Basin big sagebrush

ATCA

Atriplex canescens

Four wing saltbrush

ATCO

Atriplex confertifolia

Shadscale

BRTE

Bromus tectorum

Cheat Grass

CWD

 

Coarse woody debris

DISP

Distichlis spicata

Salt grass

ERNA

Ericameria nauseosa

Gray rabbitbrush

ERVI

Ericameria viscidifloria

Green rabbitbrush

GRSP

Grayia spinosa

Spiny hopsage

PAOC

Panicum occidentale

Witchgrass

SAVE

Sarcobatus vermiculatus

Greasewood

TEGL

Tetradymia glabrata

Littleleaf horsebrush

TESP

Tetradynia spinosa

Cat claw horsebrush

HP

 

Hardpan substratum

 

 

 

 

Table 2:   Identification of grasshoppers caught in Alvord Basin, OR, July 6-12, 2002

              

 

SPECIES CODE

 

 

SM1 A1

PROBABLE GENUS

 

 

 

Melanoplus

SUBFAMILY OF ACRIDIDAE

 

 

Melanoplinae

SM1 A2

Melanoplus

Melanoplinae

S? N1

Melanoplus

Melanoplinae

SM2 A1

Melanoplus

Melanoplinae

SM2 A2

Melanoplus

Melanoplinae

SM2 N1

Melanoplus

Melanoplinae

S? A1

 

Melanoplinae

S? A2

 

Melanoplinae

P1 A1

Paropomala

Gomphocerinae

P1 A2

Paropomala

Gomphocerinae

P1 N1

Paropomala

Gomphocerinae

P1 N2*

Paropomala

Gomphocerinae

P2 A1

Paropomala

Gomphocerinae

P2 A2

Paropomala

Gomphocerinae

P2 N

Paropomala

Gomphocerinae

C1 A1

Cordillacris

Gomphocerinae

C1 A2

Cordillacris

Gomphocerinae

C1 N1

Cordillacris

Gomphocerinae

T3 A1

Trimerotropis

Oedipodinae

T3 A2

Trimerotropis

Oedipodinae

T4 A1

Trimerotropis

Oedipodinae

T5 A1

Trimerotropis

Oedipodinae

T5 A2

Trimerotropis

Oedipodinae

T6 A1

Trimerotropis

Oedipodinae

T6 A2

Trimerotropis

Oedipodinae

T7 A1

Trimerotropis

Oedipodinae

T7 A2

Trimerotropis

Oedipodinae

T8 A1

Trimerotropis

Oedipodinae

T9 A1

Trimerotropis

Oedipodinae

T9 A2

Trimerotropis

Oedipodinae

T10 A1

Trimerotropis

Oedipodinae

CO1 A1

Conozoa

Oedipodinae

CO1 A2

Conozoa

Oedipodinae

ST? A

 

Melanoplinae

ST? N

 

Melanoplinae

G? A

 

Gomphocerinae

G? N

 

Gomphocerinae

P? A

 

Gomphocerinae,paropomal

P? N

 

Gomphocerinae,paropomal

T? A

 

Oedipodinae

T? N

 

Oedipodinae

TET A

 

Tettigionidae

TET N

 

Tettigionidae

 

Life stage, sex, and species designations assigned by Colin Hume, with the aid of the following references:

1) Book: North American Grasshoppers by Daniel Otte,

2) Website: Grasshoppers of Wyoming and the West,

 3) Research Report: OSU survey species list for the Alvord Basin, 1981.

 

   Figure 1.  Comparisons among perennial plant species for percent

                     cover, on three of four habitats studied; dunes are

 shown individually; see Table 1 for plant species codes.

 

 

 

 

Figure 2. The combined averages for measures of relative % volume

       of each perennial plant species among all perennials on three

       dunes, as measured by six 5m2 quadrats on each dune.


 

 


 

Figure 3.  The mean number of grasshoppers observed per similar area

       traversed during collection episodes on each of four habitats sampled

       in the Alvord Basin, OR, 7/6/02-7/11/02.  SF=salt flats, D= dunes,  

       LT=lower transition, UT = upper transition (LT is the main field

       course study area, it is an ecotone between sage-dominated vegetation

       upslope to greasewood-dominated vegetation downslope).  The mean

       for SF is significantly less than the other sites, which do not differ

       from each other (ANOVA, F ratio = 8, error df = 20.  Trt ms = 5,

       N = 701 grasshopper sightings).

 

 

 

 

 

Figure 4.  Grasshopper microhabitat use; combined observations

                 of all grasshoppers from all four habitats.

 

 

Figure 5.   Microhabitat use by nymphal and adult grasshoppers;

        combined observations from all four habitats.

 

 

 

 

 

 

 

 

Figure 6.   Relative abundance of grasshoppers among perennial plant species; 

                  these are combined observations combined from all four habitats.

 


 

 

Figure 7.  Grasshopper species richness on perennial plants and substrata;

       see Table 1 for designations.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 8.   The total number of each grasshopper “species” caught among

        four habitats in the Alvord Basin, OR, 7/6/02 – 7/11/02.  N= 322. 

                   See Table 2 for grasshopper species codes.

 

 

 

 

 

 

 

Figure 9.   Distribution of evasion behaviors of grasshoppers, 

                  N = 11.4 + 12.2 evasions per species, range 1-41;

                   see Table 2 for grasshopper species codes.

 

 

 

 

 

 

 

Figure 10. Horizontal and vertical evasion distances by grasshoppers. 

         Sample sizes per species are 8 + 8.8 (range 0-32) horizontal

         evasions and 7.6 + 10.3 (range 1-31) for vertical evasions.

 

 

 

 

 

 

Figure 11.   Gambelia microhabitat use: location of each lizard

                    when it was first seen during chance encounters on

                    the field  course study site, in the LT habitat;  

                    N = 65 lizards encountered. 

 

 

 

 

 

Figure 12.  Use of plant microhabitats (= on ground under plant)

                    by Gambelia wislizenii.  N = 18 observations of

                    G. wislizenii under a perennial plant when the lizard

                    was first seen during a chance encounter. 

                    See Table 1 for plant codes.