Date of Award:

5-1-2005

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Life Sciences: Biology

Committee Chair(s)

James A. MacMahon

Committee

James A. MacMahon

Committee

Edward W. Evans

Committee

Edmund D. Brodie Jr.

Committee

Gary E. Belovsky

Committee

Joseph R. Mendelson III

Abstract

The primary objective of this study was to attempt to disentangle the relative importance of biotic vs. abiotic factors on the distribution and abundance of desert horned lizards (Phrynosoma platyrhinos). Research was conducted along a Great Basin shrub-steppe bajada that examined interactions among P. platyrhinos, ants, plants, and soil characteristics. Studies focused on: (1) pattern description and habitat associations of P. platyrhinos, and (2) evaluating specific biotic and abiotic pathways underlying observed patterns of lizard abundance and distribution. Examination of lizard habitat associations along the elevational gradient revealed that unvegetated spaces and cryptobiotic crust cover were the best predictors of P. platyrhinos occurrence. Further, lizards responded strongly to abiotic attributes of shrubs (shade and undershrub soil mounds); lizards preferred high-shade model shrubs (82% of day selections) and microsites with soil mounds (85% of total selections). These findings suggest that avoidance of temperature extremes through shade-seeking and burrowing may be critical to habitat selection by desert horned lizards. Analyses of ant availability and lizard diet selection under a range of circumstances revealed that lizards do not show disproportionate preference for harvester ants as originally suspected. Instead, P. platyrhinos selected ants based on their size and abundance, highlighting the opportunistic tendencies of P. platyrhinos feeding behavior. There was a significant negative association between cheatgrass cover and lizard scat abundance across the bajada. Sprint performance trials using field-established raceways revealed that adult and juvenile sprint performance were reduced on grass raceways by 50-70% of their bare-substrate speed. These results suggest that lizards may avoid areas with dense vegetation because of the negative impact of vegetation on lizard mobility and the associated fitness consequences of reduced locomotor performance. In the cattle exclusion study, shift in habitat use by lizards from protected to grazed areas may also reflect avoidance of dense vegetation; this shift in habitat use coincided with a decline in vegetation cover in grazed areas. In this semiarid shrub-steppe community, P. platyrhinos are more constrained by abiotic rather than biotic factors. Further, the coupling of small-scale experimental studies with a gradient analysis proved to be a particularly effective approach for disentangling complex, community-level interactions.

Comments

Ecology

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