Master of Science / Department of Biology / David C. Hartnett / A key feature of many grass-dominated ecosystems is the formation of grazing lawns, distinct patches characterized by intense grazing by mammalian herbivores and a dense short-statured grass canopy. A central concept of grazing lawns is the positive feedbacks between grazing animals and the grass resource. Intraspecific morphological plant trait changes and differences in plant species composition could both or individually play a role in the differences in characteristics of grazing lawns and neighboring tallgrass swards. I studied grazing lawns in North American tallgrass prairie to: a) test the ‘architectural shift hypothesis’ where continued grazing leads to changes in plant architecture resulting in more efficient foraging for grazers, creating a positive feedback that increases grazing and b) examine soil resource (nutrient and water) availability and grass nutritive quality on and off lawns to test the nutrient- and water-based pathways for grazing lawn maintenance. In a separate study (not reported here), we a) examined plant community structure on and off lawns to determine whether species composition differences account for the distinct grazing lawn characteristics and b) assessed effects of grazing lawn formation on tallgrass prairie plant species diversity. Several differences in morphological traits between dominant grasses on grazing lawns and tallgrass swards support the architectural shift hypothesis. For Sorghastrum nutans, Dichanthelium oligosanthes, and Pascopyrum smithii, leaf-to-stem ratio was twice as high on grazing lawns compared to surrounding matrix tallgrass vegetation and tiller branching was higher and culm internode lengths were shorter on grazing lawns for these species. However, Andropogon gerardii traits did not differ between grazing lawns and tallgrass vegetation. For all four species, above-ground tiller biomass and number of below-ground buds were both higher on grazing lawns. Overall, these morphological responses resulted in a higher grass canopy density (forage biomass per unit canopy volume) on grazing lawns and this increased grass canopy density in turn results in higher grazer foraging efficiency by increasing the amount of forage intake per bite and per unit time. D. oligosanthes, P. smithii, and S. nutans plants on grazing lawns had a significantly lower carbon-to-nitrogen ratio and higher nitrogen content than plants in the matrix tallgrass vegetation, while A. gerardii showed no significant difference in nitrogen content or in carbon-to-nitrogen ratio between grazing lawns and surrounding matrix tallgrass vegetation. With regards to the total grass canopy (all grass species combined), nitrogen content was significantly higher on grazing lawns compared to tallgrass vegetation for all three field seasons, 2016, 2017, and 2018. All measured soil nutrients, ammonium, nitrate, phosphorus, and sodium, were significantly higher on grazing lawns compared to soils of surrounding tallgrass swards, while water content showed no significant difference between grazing lawns and surrounding tallgrass vegetation. The results of this study strongly indicate that developmental and morphological shifts result in increased forage density and increased grazing efficiency on grazing lawns and that the frequent and intense activities of large grazers result in increased plant nitrogen content and lower C:N ratios in grasses on tallgrass prairie grazing lawns. Thus, at least two different mechanisms, plant architectural shifts and the nutrient-based pathway could both contribute to the positive feedbacks that encourage further grazing on lawns and grazing lawn maintenance on tallgrass prairie.

How so many species coexist while utilizing the same resources is both of ecological interest and important for the management of wildlife communities and parks. This thesis endeavours to understand how grazing herbivores co-exist, with special emphasis on understanding the mechanisms of competition and facilitation over temporal and spatial scales. I compared the dry season use of grasslands, grass species, grass height and grass greenness by white rhino and three other ungulate species, zebra, wildebeest, and impala. I was specifically interested in the extent to which white rhinos, with their capacity to graze both short and tall grass, either competed with or facilitated other grazers. In Chapter Two, I clarify the mechanisms of facilitation and competition in terms of temporal and spatial scales, and discuss why there is little evidence in the literature to support these mechanisms. I conclude that evidence for facilitation through stimulation of grass re-growth during the growing season appears stronger than that for increased resource access through removal of obstructing grass structures during the dormant season. Facilitation may benefit the nutritional gains obtained by certain species in the short term, but these benefits do not appear to be translated into the expected population consequences. In collaboration with co-author Norman Owen-Smith, we suggested this could be due to seasonal tradeoffs between facilitation and competition, as well as to restrictions on the spatial extent of trophic overlap. In Chapter 3 Norman Owen-Smith and I compared the grass height use in relation to body size. We expected that the grass height favoured would increase with the body size of the herbivore species, as suggested from past studies of resource partitioning among large mammalian herbivores. Instead we found that the largest of these species, white rhino, concentrated on the shortest grass, while the smallest species, impala, favoured grass heights intermediate between those grazed by wildebeest and zebra. Results suggest that the scaling of mouth width relative to body size, and hence metabolic demands, is the primary factor governing grass height selection, rather than body size alone. Hence grazing successions governed by body size differences may not be a typical feature of their ecology, contrary to past suggestions. Furthermore, there was considerable overlap in grass height grazed among these four species, indicating that niche separation by grass height is inadequate alone to explain their coexistence. More attention needs to be paid to other aspects such as the grass species selected and habitat structure favoured. Chapter Four compared the overlap in grassland use and grass species use, as well as grass height and grass greenness of swards utilized by the herbivores as the dry season advances. I show that all species prefer grazing lawns during times of abundance, and that zebra leave grazing lawns before other species, and wildebeest leave grazing lawns before white rhino and impala. This suggests zebra and wildebeest may be competitively excluded from grazing lawns through a reduction in grass height, by white rhino and impala, during the dry season. However, white rhino are also potentially the ‘supreme’ facilitator increasing the availability of nutritious grazing lawns, as well as increasing the quality of those lawns through grazing in the wet season. A better understanding of the trade-off between “habitat facilitation” and competitive exclusion by white rhino, allows us to better understand how grazing herbivores co-exist.