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Changes in trophic structure along a gradient of water availability in temperate montane grasslands Harrower, William Laughton


Predators and plants are inextricably linked by the flow of energy in ecosystems. However, we still lack good descriptions of how predators affect the diversity, function, and stability of ecosystems under different environmental conditions. If water availability to plants modifies the interactions between predators and plants, and changes in these species interactions lead to a modification of trophic structure, then the direction and strength of trophic cascades must depend on the availability of water to plants. I use the unifying concept of the trophic cascade and an underlying gradient of water availability to investigate how species interactions in a montane grassland influence the diversity and function of these ecosystems. Firstly, I examine the distribution, abundance, and community composition of grassland songbirds. I show that as water becomes more abundant, the number of species increases more slowly than the number of songbird individuals. Second, I manipulate the presence of vertebrate predators along the gradient. I found that changes in the abundance of both songbirds, small mammals and their prey alters feeding behavior and restructures communities. These changes in intraguild predation work in conjunction with the metabolic demands of consumers to determine trophic structure and alter the strength of the trophic cascade in response to water availability. Finally, I show that predators mediate apparent competition between herbivore and detritivore food chains. In the montane grassland I studied, an apparent trophic cascade becomes established as detritus food chains emerge with increasing water availability. I show that the reversal of trophic control typical of ecosystems with allochthonous external subsidies does not occur with autochthonous detritus subsidies in my system. The direction of trophic control in the grazing food chain remains bottom-up, but the detritus food chain is instead controlled by generalist predators. These interactions between predators and plants regulate the diversity of plants and arthropod functional groups, and affect ecosystem functions such as plant biomass production and decomposition. My results show that as water availability to plants increases in semi-arid and temperate grasslands, food webs become shorter, broader, more reticulate, and are more resistant to the effect of species losses and drought.

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