Abstract
Herbivore size matters for productivity‐richness relationships in African savannas: Commentary on Burkepile <i>et al</i>. (2017)
Highlights
Since the Late Pleistocene, human impact has increasingly resulted in defaunation, or human-caused animal loss, leading to largely impoverished vertebrate communities (Dirzo et al 2014; McCauley et al 2015)
Burkepile et al (2017) performed such a test and address the question how the loss of vertebrate herbivores affects plant species richness in African savanna. They incorporated the size-selectivity of herbivore extinctions in their design by using size-selective exclosures to separate the effect of losing only the larger herbivores as elephant, zebra and wildebeest from that of losing all vertebrate herbivores >1 kg together. They subsequently replicated this design across a 10-fold gradient in plant production within Kruger National Park, South Africa
That is only part of the answer as it is the feeding strategy of the impala causing the impact on plant species richness: their diet consists mostly of grasses, forbs profit from impala grazing on grasses and subsequent suppression of grass competition
Summary
The study design incorporates the notion that herbivores affect plant species richness systematically across a productivity gradient: herbivores reduce plant species richness at low production and increase it at high productivity (Bakker et al 2006; Borer et al 2014). Nothing new, as Burkepile et al (2017) confirm this pattern These previous tests worked mostly with presence or absence of herbivores only, whereas the size-selective exclosures give more information on the role of the different herbivores. Feeding on the dominant competitor warrants their impact on plant species richness This effect is usually attributed to large bulk grazers, this study shows that medium-sized herbivores may have similar effects. Burkepile et al (2017) find a strong relation between the dominance of the plant species and the impact of the herbivores on plant species richness: when reducing the dominant, the impacts on species richness are increasingly positive This sheds new light on the current theory: it is not just a matter of large herbivores being bulk grazers, removing dominant plants at high production, thereby preventing light limitation among the plants and promoting species richness (Borer et al 2014). That herbivores regulate plant diversity through their impact on plant dominance is a principle already outlined by Olff & Ritchie (1998), but is not often taken into consideration
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