Do positive interactions expand the upper distributional limits of vascular plant species on Marion Island?

Abstract

Biotic interactions may strongly shape species’ distributions as they contribute to determining species’ realized niches, potentially either constraining or expanding the range of conditions under which species occur. This study examined whether fine-scale plant-plant interactions scale up to shape plant species distributions using Azorella selago Hook, a widespread cushion plant (compact, hemispherical plants that create favourable microhabitats by ameliorating stresses. and the rest of the vascular flora from the sub-Antarctic Marion Island as a model system. We assessed the elevational distribution of vascular plant species when growing in association or away from A. selago to test how the interaction with this cushion plant species affect species’ ranges along: 1. twenty island-scale altitudinal transects, and 2. twenty-one landform-scale altitudinal transects. The upper distributional limits of most species did not differ significantly in the presence and absence of A. selago at either spatial scale. However, at the island-scale, A. selago had a positive effect on the upper range limit of one species, Aceana magellanica (mean difference = + 26 m., and had a strong positive effect on another species, Colobanthus kerguelensis (+ 104 m. in transects located on the south slope aspect of the island. Therefore, although A. selago strongly impacts some fine-scale species patterns, these local impacts could only scale up to shape the distribution of certain vascular plant species on Marion Island. Thus, plant-plant interactions have the potential to mediate range shifts by expanding species’ upper distributional limits, although the influence of these interactions may be species-specific. - Abstract as displayed in the - Abstract booklet. The presentation on the day may differ from the - Abstract.