Indirect effects of mammalian herbivores on invertebrates in a river gradient of the Kruger National Park, South Africa

Abstract: Savanna biomes have been heavily affected by fire suppression, and changed grazing and browsing regimes, caused by human interventions. As a result many biotic interactions, and ecosystem functions, have been altered. Earlier studies have described extensive indirect effects of herbivores on invertebrates in arid systems that depend on arthropods for their nutrient recycling. Animals that feed on invertebrates might also be affected by changes in invertebrate abundance and composition. The aim of this study was to assess indirect effects of mammalian herbivores on invertebrates in a vegetational gradient towards a river. The initial questions were: - Are there any differences in invertebrate richness and abundance along the catena, i.e. the vegetation gradient towards the river? - Will the invertebrate richness and abundance change with different grazing and browsing pressures? Invertebrate specimens were collected in three experimental sites, in three vegetation zones perpendicular to the Sabie River. The field work took place in March 2008, and resulted in information on the invertebrate richness and abundance. The results showed that large and medium-sized herbivores had effects on the invertebrate community, but also that the effects were site specific and different across invertebrate taxa. The river gradient had significant effects on the abundance of Araneae and Coleoptera, and the richness of Araneae and Formicidae. In general, the spider abundance and richness peaked at midrange from the river (in the foot slope). Coleopterans, on the other hand, were more abundant and taxon rich in the riparian zone. In addition to this, the riparian zone housed many ant taxa. The experimental treatments had significant effects on the total abundance of invertebrates in the herbaceous layer, and on the abundance of Araneae, Coleoptera, Orthoptera and Formicidae. The total abundance was depressed by large mammalian herbivores, most likely by elephants, and medium-sized mammals affected the abundance and richness of Coleoptera and Orthoptera negatively. The results on spider abundance were inconsistent in the sense that large mammals only occasionally affected them negatively. Nevertheless, the results showed that invertebrates can be severely hit by changes in game abundance. Human induced changes of ungulate densities might, therefore, not only affect invertebrates at Nkuhlu, but also taxa and ecosystems elsewhere.