Piospheres in semi-arid rangeland: Consequences of spatially constrained plant-herbivore interactions
Derry, Julian F
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This thesis explains two aspects of animal spatial foraging behaviour arising as a direct consequence of animals' need to drink water: the concentration of animal impacts, and the response of animals to those impacts. In semi-arid rangelands, the foraging range of free-ranging large mammalian herbivores is constrained by the distribution of drinking water during the dry season. Animal impacts become concentrated around these watering sites according to the geometrical relationship between the available foraging area and the distance from water, and the spatial distribution of animal impacts becomes organised along a utilisation gradient termed a "piosphere". During the dry season the temporal distribution of the impacts is determined by the day-to-day foraging behaviour of the animals. The specific conditions under which these spatial foraging processes determine the piosphere pattern have been identified in this thesis. At the core of this investigation are questions about the response of animals to the heterogeneity of their resources. Aspects of spatial foraging are widely commented on whilst explaining the consequences of piosphere phenomena for individual animal intake, population dynamics, feeding strategies and management. Implicated are our notions of optimal foraging, scale in animal response, and resource matching. This thesis addressed each of these. In the specific context of piospheres, the role of energy balance in optimal foraging was also tested. Field experiments for this thesis showed a relationship between goat browsing activity and measures of spatial impact. As a preliminary step to investigating animal response to resource heterogeneity, the spatial pattern of foraging behaviour/impacts was described using spatial statistics. Browsing activity varied daily revealing animal assessment of the spatial heterogeneity of their resources and an energetic basis for foraging decisions. This foraging behaviour was shown to be determined by individual plants rather than at larger scales of plant aggregation. A further experiment investigated the claim that defoliation has limited impact on browser intake rate, suggesting that piospheres may have few consequences for browser intake. This experiment identified a constraining influence of browse characteristics at the small scale on goat foraging by relating animal intake rate to plant bite size and distribution. Computer simulation experiments for this thesis supported these empirical findings by showing that the distribution of spatial impacts was sensitive to the marginal value of forage resources, and identified plant bite size and distribution as the causal factors in limiting animal intake rate in the presence of a piosphere. As a further description of spatial pattern, piospheres were characterised by applying a contemporary ecological theory that ranks resource patches into a spatial hierarchy. Ecosystem dynamics emerge from the interactions between these patches, with piospheres being an emergent property of a natural plant-herbivore system under specific conditions of constrained foraging. The generation of a piosphere was shown to be a function of intake constraints and available foraging area, whilst piosphere extent was shown to be independent of daily energy balance including expenditure on travel costs. A threshold distance for animal foraging range arising from a hypothesised conflict between daily energy intake and expenditure was shown not to exist, whereas evidence for an intermediate distance from water as a focus for accumulated foraging activity was identified. Individual animal foraging efficiency in the computer model was shown to be sensitive to the piosphere, while animal population dynamics were found to be determined in the longer term by dry season key resources near watering points. Time lags were found to operate in the maintenance of the gradient, and the density dependent moderation of the animal population. The latter was a direct result of the inability of animal populations to match the distribution of their resources with the distribution of their foraging behaviour, because of their daily drinking requirements. The result is that animal forage intake was compromised by the low density of dry season forage in the vicinity of a water point. This thesis also proposes that piospheres exert selection pressures on traits to maximise energy gain from the spatial heterogeneity of dry season resources, and that these have played a role in the evolution of large mammalian herbivores.