Interactions between megaherbivores and microherbivores : elephant browsing reduces host plant quality for caterpillars

Abstract

Direct effects of herbivory, and indirect effects through induced responses to herbivory, can both influence the susceptibility of plants to subsequent attacks by herbivores. There has, however, been very little research (if any) to investigate how the large-scale effects of browsing by megaherbivores (.1000 kg body mass) on woody plants might influence the subsequent use of those plants by phytophagous insects. We conducted a field study in Kruger National Park, South Africa, to investigate whether browsing by elephants (Loxodonta africana) on mopane trees (Colophospermum mopane) had any influence on the subsequent selection of those trees by ovipositing mopane moths (Imbrasia belina). Our results showed that, after controlling for differences in canopy volume, the density of egg masses was almost halved in mopane woodlands recovering from severe elephant browsing in the previous season. This is despite the regrowth on heavily browsed trees having lower tannin:protein ratios and longer shoots. Our suggested explanation is that large monophagous caterpillars can only feed in the canopies of the trees in which they hatch and so the quantity of food in each canopy is more important than its quality. There are implications for the sustainable harvesting of mopane caterpillars, which represent an important food resource for rural communities in southern Africa. Future avenues for research include patch selection by large herbivores in response to local nutrient enrichment by frass deposited during caterpillar outbreaks.