Ecological Methods and Research in Reptile Ecology-Grasshopper Distribution and Abundance in the Alvord Basin

Investigating Methods to Document Grasshopper Distribution and Abundance in the Alvord Basin

Jim Garner, Amy Hummer, Brandon Saimo, and Michelle Smith

Summer Session 2001 Field Biology Course, BIOL 417a Department of Biology
Western Washington University
Bellingham, WA 98225

Introduction

Methods

Results

Conclusions

Figures

Introduction
The long-nose leopard lizard, Gambelia wislizenii is a species that is particularly well suited for study as it is relatively easy to capture and observe, even among lizards. The species can be found in flat desert areas with sandy to gravelly soil and scattered shrubs, and ranges from southern Oregon to Baja California. G. wislizenii is an ambush predator as well as a cruising forager, and eats lizards and large arthropods. The species apparently eats more grasshoppers and fewer lizards in the northern part of its geographic range than in the regions to the south. Grasshoppers are not as high quality a food source as lizards. If grasshoppers are prevalent in the diet then which grasshoppers do they eat and why? This research utilizes several sampling techniques to describe the relative abundance and distribution of grasshoppers in the Alvord Basin. The Alvord Basin is part of the Great Basin Desert of western North America. This area was chosen for study because it contains a large population of G. wislizenii near the northern latitudinal and altitudinal limits of their geographic range. Knowing the relative abundance of grasshoppers among plants and across the habitat may help predict the foraging behavior of G. wislizenii.

Methods
Investigaing where grasshoppers choose to reside in the Alvord desert during the day required at least two separate procedures for measuring their distribution and abundance. We used one procedure on the open ground and low-lying grasses, and we used another for analyzing grasshoppers on the shrubs.

Distribution amongst plants

The Desert Shrub Inhabitant Isolator (DSII) was created to isolate particular plants from their surrounding environments while all of the grasshoppers residing on them were removed by the researcher. Two DSIIs were created, one large and one small (fig. 1)
The dominant shrub species in the Alvord Basin are Artemesia tridentata (the Basin Big Sagebrush), Sarcobatus vermiculatus (Greasewood), and Atriplex confertifolia (Shadscale). 130 plants were sampled in the area surrounding the preexisting field course south plot, including: 50 S. vermiculatus, 50 A. tridentata, and 30 A. confertifolia.
The DSIIs were then used to isolate plants. The researchers worked in two person teams to move the box over a plant and place it down on the ground. One researcher would then act as a flusher, standing inside the large box or reaching into the small one and prodding the plant with a small metal rod. Any grasshoppers that fell from the plant to the ground or hopped onto the screen were collected in vials. After the entire plant had been prodded, a sweep net was used to vigorously shake it and remove the remaining grasshoppers.
After the flusher was done looking for grasshoppers, the other researcher recorded the following data for each plant: length, width, and height of the plant, temperature at 2m, temperature at 20cm, distance to the nearest neighbor in each compass direction (up to 2m) and the species of that neighbor, and whether or not the plant was typical (defined by whether or not its volume could accurately be estimated by that of a half sphere).
The samplings were conducted over an 11-day period in early July (including a variety of weather), and across time of day from 0800 to 2000 hours.
Data were analyzed with a Poisson distribution and a Chi-square test.

Distribution in open areas

Two person teams walked through the open areas at a rate of 0.5m/s sweeping out a 2m wide path with long handled sweep nets. One person walked in front with the sweep net while the other followed and tried to spot grasshoppers that were disturbed by the motion.
A stopwatch was started at the beginning of the search, and was stopped every time a grasshopper was sighted.
The searchers then captured the grasshopper using the net and a vial and stored it for analysis. Notes were made of estimated distances for the first and second hops of the grasshopper, the substrate that it was found on, the substrate that it was found on, the substrate that it hopped to, and the species and dimensions of the nearest shrub neighbor.
A square meter was then marked around the area where the grasshopper was found. This was accomplished using two 1m pieces of PVC pipe tied together with two pieces of rope that were also 1m long. The general grass and shrub composition of the square was then recorded.
All of these data will be used to establish correlations between the different grasshopper species and specific microhabitats.

Back at the lab

Voucher specimens of each grasshopper species were brought home for data analysis.
A G. wislizenii fecal analysis was performed to discover the species of grasshopper that the lizards chose most frequently as a prey item.
Data were analyzed with a Poisson distribution and a Chi-square test.

Results

We were not able to reject the hypothesis that the grasshopper population was randomly distributed amid the three shrub species.

Using both the small and large DSII we sampled 132 woody perennials bordering the southern field course.

Among the S. vermiculatus we came into contact with a total of eight grasshoppers including three of taxonomic unit one, taxonomic unit nine, seventeen (nymph), and three additional unidentified units.

The sweep net surveys turned up 152 individuals of 15 different "operational taxonomic units," out of these 132 were captured during times abundance walks.

We found 45 grasshoppers of OTU 1, 20 of OTU 2, 13 OTU 3, 6 OTU 5, 4 OTU 12, 2 each of OTUs 7,10, and 11, and 1 each of OTUs 4, 6, 9, 13, 14, and 16.

98 Grasshoppers were found on sand, 9 on hardpan, 7 on gravel, and 6 on vegetation.

The mean distance form the original location the grasshopper was spotted to the nearest vegetation was 0.52 meters, had a variance of 0.183 m, and ranged from 0 to 2 meters away

The types of vegetation nearest to the grasshoppers included 65 A. tridentata, 19 dead individuals, 9 S. vermiculatus, 14 A. confertifolia, 11 Ericameria vicidiflora, 5 Artemesia spinescens, 8 various grasses, 3 Tetradmia glabrata, and 2 each Ericameria nauseosa and Grayia spinosa

They were found at five different air temperatures ranging from 29.2 °C to 32.2 °C

Mean jump distance was 1.78 meters, with a variance of 3.38 meters, and a range from 0.05 meters, up to more than 10 meters. Their second jump was recorded where applicable had a mean of 1.81 meters, a variance 3.60 meters, and a range of 0.08 meters to 8 meters

In our timed sweep net search we covered approximately 12,800 square meters in 213 minutes and 14 seconds, in that time we flushed 132 grasshoppers resulting in an estimate of one grasshopper per every 93 m².

Conclusions

A goodness of fit test concluded that a Poisson distribution fit the Desert Shrub Inhabitant Isolator data and with a chi-square analysis we inferred that grasshoppers were randomly distributed among the three shrub species. We could not show that the grasshoppers preferred one shrub type or size to any other size or type.
The microhabitat correlation matrix resulted in a positive relationship only between mean first hop distance and mean second hop distance. Any further statistical analyses could not be performed due to incomplete data sets.
We concluded that adult grasshoppers were easiest to find on sandy substrates. Jump-evading grasshoppers landed on sand more often than on hardpan or vegetation. It is important to note that the majority of our net sweep sessions took place over regions that contained primarily sandy substrates. Surface sand temperatures in the Alvord reach highs near 60 °C during midday, but sunlit sand surface may be a good location for grasshoppers to thermoregulate, court, and mate early and late in the day.
Basin Big Sage was the most common type of vegetation found nearest to grasshoppers when they were first observed. The correlation may be a simple result of Basin Big Sage being he most abundant desert shrub in this part of the Alvord Basin. BBS does provide the densest cover in sandy regions.
Field observations of feeding Gambelia wislizenii and laboratory analyses of G. wislizenii fecal pellets revealed no evidence for preference by G. wislizenii for one grasshopper species relative to any other. We did notice, however, that the ability among grasshopper species to be cryptic did vary with the kind of substratum or nearby vegetation. Hence, the furtive, feeding nymphal grasshoppers on vegetation may be much more difficult to detect than adults actively courting out on the open sand. Future studies should compare distribution and abundance of nymphal and adult grasshoppers, and should use more observational sampling techniques and avoid the labor intensive sampling effort with the DSIIs.

Figure 1 - Number of Arthropods Found on Total Sample of Plants in Alvord Desert

Figure 4 - Taxonomic Units Found

Figure 5 - Grasshopper Substrate Preference

Figure 7 - Nearest Neighbor Identity

Figure 9 - Hop-Flight Substrate Preference

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