Mechanisms Determining Large-Herbivore Distribution

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Mechanisms Determining Large-Herbivore Distribution


H.T.T. Prins

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Grazing distribution is an important component of the foraging ecology of large herbivores. Recognising the differences in foraging behaviours that occur along spatial and temporal scales is critical for understanding the mechanisms that result in grazing distribution patterns. Abiotic factors such as topography, water availability and weather and biotic factors such as forage quantity and quality affect the distribution of large herbivores. Numerous empirical studies have shown that large herbivores typically match the time spent in an area with the quantity and quality of forage found there. Although the observed grazing patterns have been documented, the underlying behavioural processes are still being elucidated. Cognitive foraging mechanisms assume that animals can use spatial memory to remember the levels of forage resources in various locations, while non-cognitive mechanisms require that behaviours such as intake rate, movement rate and turning frequency vary in response to forage resource levels. The ability of animals to use spatial memory during foraging has been demonstrated in several species including livestock, which suggests cognitive mechanisms are possible. Optimal-foraging theory can also be used to help explain behavioural processes. Giving-up rules based on marginal-value theorem appear to work well for large herbivores when a patch or feeding site can be noticeably depleted within an appropriate temporal scale such as a grazing bout or when forage availability is limited. However, giving- up rules do not always explain movements among feeding sites when forage is plentiful. The satiety hypothesis has been used to explain the avoidance of toxins and the acquisition of nutrients in diet selection. We suggest the satiety hypothesis can be expanded to account better for the variability in feeding-site selection. Large herbivocres should move among feeding sites when forage availability becomes limiting or when animals become satiated. Satiation with feeding sites may occur because of the presence of toxins or nutrient imbalances or because of aversive external stimuli. Large herbivores may return to sites that were previously considered aversive due to a combination of individual animal variation and social factors. Large herbivores can now be readily tracked using global positioning system (GPS) technology, which will allow us to test predictions of th

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