| Summary: | Thesis (PhD (Plant Science))--University of Pretoria, 2019. Although the unidirectional impacts of benefactor species on associated beneficiary species have been well-established, beneficiary feedback effects (i.e. the impact of beneficiaries on their benefactors) and benefactor-mediated interactions among beneficiaries (i.e. interactions among beneficiaries that are the result of facilitation by benefactors) are relatively poorly understood. Additionally, fine-scale biotic interactions could influence broad-scale species distributions by either constraining (through negative interactions) or expanding (through positive interactions) the range of conditions under which a species can occur; however, while several studies have found evidence for this from species distribution modelling, none have explicitly tested this idea using field-quantified data. The aim of this project was, therefore, to examine the consequences of inter- and intra-specific interactions on species’ distributions and performance. I use a dominant species pair, Azorella selago (a widespread cushion plant) and Agrostis magellanica (a dominant perennial grass species that most frequently occurs with A. selago), on sub-Antarctic Marion Island as a model system. First, I examined the benefactor-beneficiary feedback effect of A. magellanica on A. selago, using a long-term dataset of repeated measures. I expected A. magellanica cover to have a negative effect on A. selago due to shading. However, A. magellanica had no long-term effect on the growth and vitality of A. selago. Therefore, for the first time using a long-term dataset, I show that the cost of facilitation to the benefactor may be negligible, in contrast to the majority of short-term studies. The effect of A. magellanica cover on A. selago performance did, however, vary between A. selago performance measures, when analysing data from a single time period. This, therefore, highlights that studies examining beneficiary feedback effects need to move beyond just using snapshot approaches ...
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