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14 pp. / A primary expression of stocking levels on rangeland vegetation is utilization defined as the proportion or degree of current years forage production that is consumed or destroyed by animals (including insects). Utilization may refer either to a single plant species, a group of species, or the vegetation as a whole. Utilization is an important factor in influencing changes in the soil, water, animal, and vegetation resources. The impact of a specific intensity of use on a plant species is highly variable depending on past and present use, period of use, duration of use, inter-specific competition, weather, availability of soil moisture for regrowth, and how these factors interact. Utilization data can be used as a guideline for moving livestock within an allotment with due consideration to season, weather conditions and the availability of forage and water in pastures scheduled for use during the same grazing season. In combination with actual use and climatic data, utilization measurements on key areas and utilization pattern mapping are useful for estimating proper stocking levels under current management. Utilization studies are helpful in identifying key and problem areas, and in identifying range improvements needed to improve livestock distribution.

In this work, the method of streaming rays have been expanded to two spatial dimensions (three phase space dimensions, x, y, and s) and was used as a basis for the development of the electron transport computer code SR2D. The streaming ray algorithm is an Eulerian-Lagrangian hybrid. Electrons are followed as they traverse the medium along specified streaming rays. Fluxes, however, are computed at the centers of the fixed cells. The development of the SR2D code required the specification of a Lagrangian streaming ray network overlaying a three dimensional Eulerian grid. In contrast to its one dimensional predecessor, the SR2D code accommodates non-uniform cell sizes and allows for arbitrary quadrature sets (S₂, S₄, S₆, S₈, S₁₂, or S₁₆). The critical aspect of the streaming ray method is the determination of the pathlengths of each and every streaming ray through all of the Eulerian cells. These values must be precalculated and stored because of the iterative nature of the solution scheme. Although the number of pathlengths may be exceedingly large, computer memory requirements are minimized, however, in the two dimensional algorithm by the symmetry of the geometry in each pathlength increment. The SR2D code was used to calculate the energy deposition profile for two kinds of sources, an isotropic point source and a monodirectional point at the periphery of a two dimensional medium. For each case, we chose aluminum with dimensions 0.01g/cm² thick by 0.02g/cm² wide as the medium and specified a grid of 5 by 10 uniform cells, respectively. The pathlength increment was 0.002g/cm² with 25 pathlength increments chosen. An S₈ quadrature set was selected for the monodirectional point source while an S₁₂ quadrature set was used for the isotropic point source. Both sources were normalized to one incident particle with an energy of 200 keV. SR2D results were compared with those from the electron/photon Monte Carlo code TIGER. The total energy deposited in the medium and peak cell was selected to facilitate the comparison. For the monodirectional point source SR2D results were within 1% for total energy deposited into the medium and peak cell energy. The total energy deposited for the isotropic point source was within 1%, but peak cell energy varied by 4%.

Tests of optimal foraging theory have shown that many predators are selective about which prey and which patches should be utilized. I hypothesize that prey species "exploit" this choosiness by evolving characteristics that cause predators to choose alternate prey. Specifically, prey should evolve traits that increase the probability of predator death, decrease the per prey or per patch nutritional intake, increase processing time, and advertise (or mimic advertisements of) undesirable traits. Predator choosiness allows prey to divert the predator instead of defeating it. The evolution of a long-term, prudent foraging strategy requires that three conditions be met: (1) The forager must use resources from a discrete subpopulation; (2) use of that subpopulation must be relatively exclusive; (3) the resource population must respond in such a way that a long-term strategy provides an economic advantage. For the horned lizard-ant system, conditions (1) and (2) were tested by tagging lizards with transmitters or radioactive tags. Horned lizards occupy home ranges much smaller than would be expected if they moved at random and home range overlap was less than expected by random placement of home ranges, thus conditions (1) and (2) were not rejected. Most techniques of home range study do not distinguish random from nonrandom movement. Condition (3) was tested by subjecting ant colonies to various levels of artificial predation. In none of five experiments was the result obtained that an increased harvest intensity led to a decrease in long-term yield; condition (3) is tentatively rejected. Ant colonies shut down in response to predation; this puts a ceiling on their losses. Short-term foraging models were tested for horned lizards foraging at ant colonies. A prediction of the marginal value theorem was not rejected: Horned lizards tended to leave colonies when their instantaneous rate of harvest at that colony had fallen to their average rate of harvest for the day. Another short-term prediction, however, was rejected: Lizards did not stay longer at the "better" of two colonies. A more liberal version of the same prediction was not rejected. Apparently, horned lizards forage adaptively but not optimally.

A major challenge in plant ecology is in understanding how species strategies mediate interactions between the environment and fitness. Variation in niche strategies that affect phenological, physiological, and reproductive traits will allow species to partition resources differently in space and time, allowing for coexistence of many species and strategies within a community. How species differentially respond to variable environments will ultimately influence their population dynamics and geographic distribution. This dissertation approaches this topic from two perspectives: (a) examining the interaction between biogeography and variable demographic strategies in desert annual plants, and (b) examining the costs and benefits of contrasting reproductive strategies in co-occurring selfing and outcrossing desert annuals. Firstly, I tested the abundant center model to determine the role of range position on plant population dynamics. I examined how the geographic and climatic position of 13 desert annuals found at a common location, the Desert Laboratory at Tumamoc Hill in Tucson, Arizona, related to their demography over a 25-year time span. I found that species for which the Desert Laboratory was close to the center of their geographic range have less variable long-term survival and fecundity compared to species for which the Desert Laboratory was further from the center of their range. Secondly, I studied how related species with contrasting mating systems respond to variable environments to affect plant performance. In a three-year field study I investigated how inter-annual variation in plant reproductive phenology affects synchrony with pollinators and herbivores. Since selfing species are guaranteed to reproduce in the absence of pollinators, seasonal and annual variation in phenology resulted in less variable plant reproductive success compared to outcrossing species. Greater variation in reproduction in outcrossing species resulted from asynchrony in some years between plants and pollinators. In a greenhouse study examining the interaction between mating system and drought, I found that the physiological functioning and survival of outcrossing species was more strongly negatively affected by drought conditions, suggesting that selfing species have an advantage in more arid environments. These studies demonstrate how plant reproductive and physiological strategies can play a critical role in influencing fitness, population dynamics and geographic distribution.

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Two trials were conducted to demonstrate diet selected by grazing cattle. Selection criteria were investigated on the basis of cattle selection for plant species, green leaf in preference to dry leaf or dry stem and the chemical contents of esophageal samples versus those in the standing crop. The influence of residual old vegetation on selection ability was studied by mowing all or part of the pasture. Cattle consumed Lehmann lovegrass in preference to shrubby species (Table 1). The esophageal samples contained higher levels of protein, phosphorus and IVDMD than that in the standing crop in both Trial 1 and Trial 2. Neutral detergent fiber was higher in the standing crop than in the esophageal samples, whereas in Trial 2, there appeared to be no difference in NDF. Removal of old vegetation by mowing had no effect on selection for plant parts. Cattle selected green leaf, in preference to dry leaf or dry stem regardless of the presence of old material. Free ranging cattle selected a diet that was higher in nutrient density and more digestible than the available forage.

The objective of this work is to estimate the effect of floods on the riparian vegetation communities of the Gila Box, Arizona, based on historical maps, satellite images, and GPS-referenced airborne video. The condition and extent of selected riparian vegetation communities in 1973, 1982, and 1991 are documented. Changes in riparian vegetation communities for the time period from 1973 to 1982 and from 1982 to 1991 are analyzed to assess the potential effect of floods that occurred in 1978 and 1983, respectively. The results indicate that the floods in 1978 and 1983 may have had an impact on the mesquite riparian community. The effect of floods on riparian vegetation is most apparent in the downstream portion of the Gila River. The information collected and presented in this work can be used to formulate effective management plans to the Gila Box area.