• Introduction
  
• Methods
  
• Results
  
• Discussion
  
• Figures
  

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.

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Methods

• A survey was conducted over a 100 by 80-meter area to locate and mark Formicidae colonies with numbered flags.

  Identifying ant species back at the lab.

• We designated the numbers 1-5 as operational taxonomic units to classify the five common species of ants found.
• We identified and mapped seventy-two colonies and recorded substrate type, species and coordinates.
• We randomly selected 36 colonies on which to focus our observations, including nests of all five species. Observations took place at set time intervals for three days, and consisted of taking ground, air and nest-entry temperatures and counting ant activity. Activity of each colony during an observation bout was measured with three 30-second counts of all ants entering and leaving the nest with a 30-second interval between each counting period.
• For the final two days, we focused on 17 colonies of the two species of Formicidae which are believed to be the primary prey items of Phrynosoma platyrhinos.
• Standard plot surveys were conducted to capture unmarked and record positions of marked Phrynosoma platyrhinos found in the field. Unmarked P. platyrhinos were marked with paint and permanent toe clips.
• P. platyrhinos fecal matter was collected in the field and expressed from captured lizards. Fecal matter was placed in vials for later dissection in the lab.

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Results

• Ant nests were located on the three predetermined substrates in the proportions. These frequencies were used to determine the distribution of each individual species (Figure 4).
• A total of 72 nests were found: 26 of Pogonomyrex californicus; 9 Myrmecocystus kennedyi; 20 species #3; 4 of species #4; 9 of species #5.
• There was a significant effect of observation time on the activity of ants across species for observational periods 2 and 3 (Figure 2).

  The head of ant species 2 recovered from a fecal pellet.

• A significant effect of temperature on all ants was found (F = 5.53, p = 0.000). The data were then split by species to determine which species were temperature dependent. Using ANOVA, we found significant effects as well for P.californicus, species 3, species 4, and species 5.
• 66 hours were spent dissecting P. platyrhinos feces. A total of 2,962 ant heads were counted; 1,130 P. californicus, 213 M. kennedyi; 15 of sp. #3; 0 sp. #4; 1,085 sp. #5; 239 sp. #6; 261 sp. #7; 3 sp. #8; 16 sp. #9. The minimum number of heads in a fecal pellet was 15 and the maximum was 312.
• To test if the P. platyrhinos eats ants according to availability we ran a Chi² analysis. This analysis showed that we were able to reject this null hypothesis that P. platyrhinos eat according to availability (?²_calc = 3,663, ?²_crit = 9.448). An ANOVA was run to test for sex differences of lizards who produced the fecal pellet examined, but found no significant effect (P.c.; F = 0.581, p = 0.455: sp. 2; F = 0.522, p = 0.455: sp. 5; F = 0.213, p = 0.649).

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Discussion
Our results show that of the three substrate types, sand was the most highly populated by ants (Figure 5). Substrate preferences vary between species and affect nest distribution (Figure 4). Different substrates may accommodate different species because of the types of plants they contain or the types of nests the ants build.

As was expected time of day and temperature have an effect on ant activity. Most of the observed ants showed low above ground activity when the soil temperature reached above 40 °C (Figure 3), and generally ant activity was higher at cooler times of day. Significant differences in activity between species occurred at these temperatures. The significant differences in ant activity came during 0900 - 1100 and 1100 - 1300 hours. P. californicus and species 5 were to some degree active at all observations times. This seemed true no matter the time of day or temperature.

We predicted that the P. platyrhinos would feed on the ants according to their availability. However, this was rejected by the Chi² test. This may be due to the distribution of the nests of certain species or that P. platyrhinos may be eating whatever is closest to them at the time. Further research is necessary to determine why P. platyrhinos do not eat in accordance to the ant availability.

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Figure 2 - Mean Ant Activity per Mound v. Observation Time

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Figure 3 - Mean Ant Activity per Mound v. Temperature

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Figure 4 - Ant Nest Distribution on FCS

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Figure 5 - Substrate Distribution v. Pp Spotting Distribution v. Ant Nest Distribution

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Chart 1 - Map of Ant Activity and Mound Locations

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