Disentangling the effects of two co-occurring biotic modifiers on vegetation properties in a semi-arid savannah ecosystem

Abstract

Biotic modifiers are species that strongly alter environmental conditions, and that often shape local communities. When more than one biotic modifier co-occurs, the potential for interactive effects exists, where the effects of one species may negate or enhance the impacts of a second biotic modifier. Here we test for an interactive effect of aardvark (Orycteropus afer) burrows and tree presence on soil (soil temperature, moisture and compaction) and understorey vegetation properties (plant cover, species richness and composition) in a semi-arid savannah. Aardvark burrowing altered abiotic conditions, with soil temperature and compaction being lower, and soil moisture higher, in burrow entrances than at the burrow mounds and control microsites. In addition, the impact of aardvark burrowing on soil moisture and soil compaction was mediated by the occurrence of trees. In contrast, vegetation cover and species richness were independently affected by burrowing and trees, while vegetation composition was only affected by the occurrence of trees, but not by burrowing. This research suggests that co-occurring biotic modifiers can have both independent and interactive effects, and disentangling their impacts will help understand the mechanisms underlying dryland plant communities.