Inter- and intra-specific interactions affect the performance and distribution of plants in the sub-Antarctic

Abstract

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 and/or short-term experiments.

Second, I tested whether (inter-specific) facilitation by benefactors has consequences for (intra-specific) interactions among beneficiaries. Specifically, I hypothesized that where A. selago favours the establishment of high densities of A. magellanica, intra-specific competition may subsequently impede A. magellanica performance. However, observational data from six altitudinal transects showed that A. magellanica performance was significantly positively related to conspecific density (potentially reflecting facilitation among A. magellanica conspecifics), and this effect was significantly greater on A. selago cushion plants than on the adjacent substrate. In contrast, experimentally thinning of A. magellanica had no impact on A. magellanica performance, suggesting that A. magellanica may respond slowly to changes in biotic interactions. Therefore, in this study system, facilitation, both within and between plant species, may be more important than competition.

Finally, I tested the potential for fine-scale positive plant-plant interactions to expand species’ upper distributional limits, examining the influence of A. selago on the upper distributional limit of an entire vascular plant assemblage. Azorella selago had a positive impact on the upper altitudinal limits of three out of nineteen vascular plant species, suggesting that A. selago can allow species to occur in areas that would otherwise be abiotically unfavourable. Therefore, fine-scale positive plant-plant interactions do have the potential to expand species’ upper distributional limits, although their impact may be strongly species-specific.