Date of Award:

5-1-1986

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Biology Ecology

Committee Chair(s)

Ivan G. Palmblad

Committee

Ivan G. Palmblad

Committee

Martyn Caldwell

Committee

Austin Haws

Committee

Ting Hsiao

Committee

Jim MacMahon

Abstract

In the Grand Canyon, grasses with 3 carbon photosynthetic pathway and C4 grasses have different distribution patterns: C4 grasses are more abundant at lower elevations, C3 species are more common at higher elevations. Differences in C3 and C4 grass abundances at different sites were used to test the following hypotheses using three species of grasshopper: 1. C4 plants are avoided by herbivores; and 2. sparse grasses are preferred as food over abundant grasses by herbivores. Food habits of the grasshoppers were analyzed using carbon isotope ratios and feeding trials. Carbon isotope analysis gave estimates of the proportion of individual grasshopper diets that were comprised of C4 grass tissue (%C4 diet values). Parapomala pallida ate both C3 and C4 grasses; %C4 diet values, and feeding trials indicated that P. pallida ate grasses in proportions similar to their relative abundances. Opeia obscura exhibited a strong preference for C3 grasses based on %C4 diet values, but feeding trial results showed that O. obscura individuals will eat C4 grasses. Ageneotettix deorum also avoided C4 grasses, but to a lesser extent than did O. obscura. Some C4 grass was eaten in the feeding trials, indicating that C4 grasses were not completely avoided. No grasshopper species showed a preference for uncommon grasses. Food habits and distribution patterns of the three species were compared. There was considerable variation among %C4 diet values from individual grasshoppers, even from a single site. The significance of these values is discussed in reference to recent work on the importance of variability among individuals in plant-herbivore interactions. Ageneotettix deorum is described as primarily a grass-feeder. At my study sites, it ate more nongrasses than has been reported for other parts of its range. Possible explanations for the observed results are suggested. The method I used to evaluate feeding trial results was more tedious, but logically appeared to be more accurate than the conventional technique. Comparisons between the two techniques showed almost perfect agreement in estimating proportional contribution of a plant species to a grasshopper's diet.

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