Short-term Effects of Single Species Browsing Release by Different-sized Herbivores on Sand Forest Vegetation Community, South Africa.

ABSTRACT Manipulations of herbivores in protected areas may have profound effects on ecosystems. We examine short-term effects on tree species assemblages and resource utilization by a mesoherbivore and small-size herbivores (ungulates <20 kg) in Sand Forest, after browsing release from a megaherbivore (elephant), or both a mega- and mesoherbivore (nyala), respectively. Effects were experimentally separated using replicated exclosures where all trees were counted, identified to species and browsing events recorded. Tree species assemblages were impacted by both elephant and nyala, and by each herbivore species individually. Tree turnover rates were higher where both herbivore species were present than in their combined absence. Diet was segregated among elephant, nyala and small-size herbivores. Both resource specificity and browsing pressure by nyala increased in absence of elephant; small-size herbivores increased resource specificity in absence of elephant, and increased browsing pressure in absence of both elephant and nyala. This implies interference competition with competitive release. The indirect effect of the manipulation of herbivore populations, through the removal of one or two herbivore species, caused a shift in tree species composition and diet of smaller-size herbivores. These indirect effects, especially on tree species composition, can become critical as they affect vegetation dynamics, biodiversity and ecosystem processes. Therefore, in order to conserve habitats and biodiversity across all trophic levels, conservation managers should consider the effects of: (1) the full herbivore assemblage present; and (2) any effects of altering the relative and absolute abundance of different herbivore species on other herbivore species and vegetation. [ABSTRACT FROM AUTHOR]