Herbivore damage increases avian and ant predation of caterpillars on trees along a complete elevational forest gradient in Papua New Guinea

Abstract

Signals given off by plants to alert predators to herbivore attack may provide exciting examples of coevolution among organisms from multiple trophic levels. We examined whether signals from mechanically damaged trees (simulating damage by herbivores) attract predators of insects along a complete elevational rainforest gradient in tropical region, where various predators are expected to occur at particular elevational belts. We studied predation of artificial caterpillars on trees with and without ‘herbivorous’ damage; as well as diversity and abundances of potential predators at eight study sites along the elevational gradient (200–3700 m a.s.l.). We focused on attacks by ants and birds, as the main predators of herbivorous insect. The predation rate decreased with elevation from 10% d−1 at 200 m a.s.l. to 1.8% d−1 at 3700 m a.s.l. Ants were relatively more important predators in the lowlands, while birds became dominant predators above 1700 m a.s.l. Caterpillars exposed on trees with herbivorous damage were attacked significantly more than caterpillars exposed on trees without damage. Results suggest that relative importance of predators varies along elevational gradient, and that observed predation rates correspond with abundances of predators. Results further show that herbivorous damage attracts both ants and birds, but its effect is stronger for ants.

Elevational gradients in plant defences and insect herbivory: recent advances in the field and prospects for future research

“…For example, studies in both tropical and temperate mountains have found increasing elevation to be associated with lower insect herbivore attack by parasitoids (Preszler andBoecklen 1996, Maunsell et al 2015), predatory arthropods (Koptur 1985, Rodríguez-Castañeda et al 2011, Rasmann et al 2014c, Sam et al 2015, Roslin et al 2017, and insectivorous birds (Sam et al 2015) (Fig. 1A).…”

“…Recent work has also pointed at the importance of considering changes in predator species or guild composition (e.g. increasing bird predation but decreasing ant predation with elevation; Sam et al 2015), which may lead to contrasting elevational gradients in plant traits mediating indirect defence assuming that different predators select upon different traits.…”

“…For instance, Sam et al (2015) recently found that artificially damaging leaves of tropical trees increased ant and bird predation on insect herbivores and that the effect of leaf damage on predation was stronger at lower elevations. Although predators were presumed to be attracted by volatile emissions or visual cues, the authors did not test these hypotheses or measure other plant traits.…”

Identifying indicator species of elevation: Comparing the utility of woody plants, ants and moths for long-term monitoring

“…The use of ants as indicator species for monitoring, however, requires a caveat from an ecological and conservation point of view as they were restricted largely to low and mid-elevational zones. Predation pressure exerted from ants was observed to decline with increasing elevation within our study location ) and elsewhere (Sam et al 2015) due to decline not only in species richness but total abundance of ants at high elevations. In Lamington National Park, there are many regional and local endemic species that are restricted to the 1100-m elevational zone, including plant (e.g.…”

“…Brehm & Fiedler 2003;Kitching et al 2011;Sam et al 2015). Brehm & Fiedler 2003;Kitching et al 2011;Sam et al 2015).…”