Mesohabitat Use of Phrynosoma platyrhinos in Relation to Ant Distribution and Abundance

BIOL 417a,b, Summer Session 2004

Biology Department

Western Washington University

Abstract

A three-week investigation was conducted within three different mesohabitats, sandy flats, dune, and hardpan, of the Alvord Basin assessing Formicidae genera abundance and activity. In addition, observations discerning mesohabitat uses and distinct homeranges of individually identified Phyrynosoma platyrhinos were also carried out in order to determine relationships between ecological factors and preferences among the P. platyrhinos. Individually identified P. platyrhinos were telemetrically tracked daily and biotic and abiotic observations as well as mesohabitat type were recorded at each relocation site. Abundances of specific Formicidae genera including Pogonomermex, Formica, Veromessor, and Myrmecosystus were recorded. Results of this research indicate individual P. platyrhinos exhibit selective choice for food preferences despite predominant mesohabitat uses by those individuals and greater ant genera abundances within that particular mesohabitat. Specific mesohabitat uses may result in distinct size and distribution of homeranges for each P. platyrhinos individual.

Introduction

The Alvord Basin of Southern Oregon is located within the northern boundary of the Great Basin Desert. The three dominant mesohabitats are sandy flats, dune, and alkaline hardpan. Vegetation is composed of low desert shrubs and grasses. This ecosystem is home to the lizard Phrynosoma platyrhinos (Phrynosoma = toad body; platyrhinos = flat nose). Phrynosoma platyrhinos are solitary dwellers and generally inhabit a specific homerange. The P. platyrhinos is a myrmecophage, feeding primarily upon local ant populations. Homeranges vary in size and shape for specific individuals, but locality is often based upon subsistence abundance. The ants that compose the majority of the P. platyrhinos' diet within the Alvord Basin are members of the family Formicidae and include genera Pogonomermex, Formica, Veromessor, and Myrmecosystus, the first two being terrestrial harvester ants and the latter arboreal honey ants.

A three-week investigation was conducted assessing ant abundance and activity within the three different mesohabitats of the Alvord Basin in conjunction with observations discerning distinct homeranges of individual P. platyrhinos. The questions that were to be addressed included: the relative distribution of the P. platyrhinos in relation to the biotic and abiotic factors affecting this distribution, as well as the ecological factors determining the size of particular homeranges. Further questions included the relationship between Formicidae distribution and abundance as compared to P. platyrhinos mesohabitat preference, and the preference of P. platyrhinos to habitats possessing particular genera of Formicidae. The intent of this research is to understand the relationship between distribution and abundance of Formicidae in comparison to P. platyrhinos subsistence preference as determined from collected fecal pellets and mesohabitat use determined from observed individual homeranges.

Methods and Materials

§ Phrynosoma platyrhinos were originally captured as a result of haphazard search on and around a 150-meter, 150-meter plot. Each P. platyrhinos was given an individual identification in the form of toe-clip number and paint coloring.

§ Small radio transmitters, each assigned a specific radio frequency, were relayed to a directional antenna and a telemetry receiver to repeatedly relocate individual P. platyrhinos. Beaded collars were attached to the transmitters using gossamer floss and epoxy in order to prevent the transmitter from being lost.

§ Each P. platyrhinos was tracked once per day on a rotating schedule of early morning, late morning, and evening.

When a P. platyrhinos was relocated, location coordinates, habitat characteristics, substrate temperature, lizard behavior at the time of sighting, and lizard body temperature were recorded. Upon sighting, a waypoint was created for each lizard using a SporTrek handheld GPS unit. The lizard location was marked with a flag attached to the nearest shrub. Collected fecal pellets were later dried, weighed, and analyzed for Formicidae genera in lab.

§ Within nine 20-meter, 20-meter subplots (three for each of the three mesohabitats) shrubs containing honey ants, Veromessor and Myrmecosystus were flagged. Total number of shrubs were also counted to determine the percentage of shrubs used compared to number of available shrubs.

§ Moderate plot searches for terrestrial ant colonies of Pogonomyrmex, and Formica provided locations for daily hour-long observations conducted during late morning (09:00-12:00) and/or late afternoon (16:00-19:00). The purpose was to determine the relative levels of ant activity differentiated by genera and time of day. Activity was determined by marking a 0.5-meter diameter ring around the colony entrance and recording incoming and outgoing individuals in each compass direction.

Results

¨ Plot mesohabitats were determined to be 32.63% sandy flat, 40.83% hardpan, and 26.54% dune (Figure 1). Phrynosoma platyrhinos were found on 55.95% sandy flat, 22.67% hardpan, and 21.37% dune. P. platyrhinos used a larger percentage of sandy flat than both hardpan and dune, despite a greater availability of hardpan. Data shows that individuals exhibited variability of habitat use (Figure 2).

¨ Dunes contained the highest total number of Fomicidae and greatest abundance of Pogonomyrmex, Veromessor, and Myrmecosystus, and hardpan contained the lowest quantity of all genera (Figure 3). Formica are the most abundant on all mesohabitats, with the greatest abundance on sandy flats.

¨ Myrmecocystus sightings were greatest on sandy flat and lowest on dune (Figure 4). Shrub availability varied but was noticeably greater on dune and sandy flat and lowest on hardpan.

¨ Pogonomyrmex colonies were most abundant on dunes in both 2003 and 2004 (Figure 5). Formica colonies were most abundant on hardpan in 2003 and dune in 2004.

¨ Pogonomyrmex colony activity was greatest in the late afternoon (Figure 6). In contrast, Formica colony activity was greatest in the late morning.

¨ The largest average number of ants per P. platyrhinos fecal pellet were from the Pogonomyrmex genera, followed by Formica, Veromessor, and Myrmecosystus, respectively (Figure 7).

¨ Data shows variability in P. platyrhinos Formicidae consumption by mesohabitat use (Figure 8).

Discussion

Results of this research indicate individual Phrynosoma platyrhinos exhibit selective choice for food preferences despite predominant mesohabitat uses and greater ant genera abundances within that particular mesohabitat (Figure 2, 3, 8). Some individuals however, do consume the majority of the genera with the greatest abundance within that individual's preferred mesohabitat. Pogonomrymex colonies are more abundant on sandy flats mesohabitats and in relation more individual P. platyrhinos were relocated on sandy flats (Figure 5). Pogonomyrmex are highly active foragers in the late afternoon which therefore results in increased contact with P. platyrhinos (Figure 6). Both the predominance of Pogonomyrmex on sandy flats and the greater level of foraging activity make the Pogonomyrmex easy prey for the P. platyrhinos. It is evident that these individuals consume the more abundant prey as data indicates a higher consumption of Pogonomyrmex than any other genera in a single P. playtrhinos fecal pellet (Figure 7).

As exhibited by the presence of clustered relocation waypoints, P. platyrhinos have specific preferences that result in distinct, individual homeranges (see map). Despite the fact that the largest mesohabitat available in terms of overall area is hardpan, more than half of the telemetered P. platyrhinos were relocated on sandy flats (Figure 1, 8). Phrynosoma platyrhinos mesohabitat uses, and therefore preferences, may influence size and distribution of individual homeranges. Further research should be conducting relating P. platyrhinos mesohabitat preferences and size of individual homeranges in order to determine if the two correlate.

Acknowledgements

We would like to thank Dr. Anderson for his continual guidance and advice throughout this course. Dr. Lance MacBrayer and Steven Williams of Stephen F. Austin State University for providing the necessary telemeter equipment and direction to conduct the research. We are grateful to the grasshopper group and all SPS-ers for their continual search for ant colonies as well as fecal pellet analysis. Graduate T.A. Chris Fabry for unending support and for just being Chris. A thanks also to GIS Specialist, Stefan Freelan, Western Washington University, for technical assistance in mapping homeranges.

Spatial availability of sandy flats (SF), dune (D), and hardpan (Hp)

compared to the relative frequency of sightings of the horned lizard

Phrynosoma platyrhinos at those mesohabitats.

Figure 2: Percentage of relative sandy flats (SF), dune (D), and hardpan (Hp) mesohabitats used by individually identified Phrynosoma platyrhinos.

The Number of Individuals of each Major Ant Species

Caught per Pit Trap in 3 Mesohabitats

Figure 4: The number of observed shrubs containing Myrmecocystus compared to the total number of shrubs available for habitation in the three local mesohabitats: sandy flats(SF), dune (D), and hard pan (Hp).

Figure 5: Number of Pogonomyrmex and Formica (Aphenogaster?) colonies found on sandy flats (SF), dunes (D), and hard pan (Hp) mesohabitats in intensive searches performed in 2003 and moderate searches in 2004.

Figure 6: Average number of foraging ants in relatively active colonies of Pogonomyrmex and Formica (Aphenogaster?)during the late morning (09:00-12:00) and late afternoon (16:00-19:00).

Average number of ant heads of each presumptive ant OTU

counted in single fecal pellets from 30 P. platyrhinos.

Figure 8: Average number of ants from genera Pogonomyrmex, Formica (Aphenogaster?), Veromessor, and Myrmecocystus, observed as residual cranium in fecal pellets of individual telemetered P. platyrhinos separated by preferred mesohabitat relocations.