Herbivore damage increases avian and ant predation of caterpillars on trees along a complete elevational forest gradient in Papua New GuineaSubmitted by editor on 13 May 2015. Get the paper!
Figure 1. Mt Wilhelm (4509 m a.s.l.; the highest peak of Papua New Guinea) hosts one the most complete and largely undisturbed forest elevational gradient in tropics. Here I took photograph of the upper part of the gradient, from 2000 m to the summit of Mt Wilhelm with its tree line at 3700 m a.s.l.
by Katerina Sam (katerina [dot] sam [dot] cz [at] gmail [dot] com)
We’re excited that our paper on the tritrophic relationships between plants, herbivorous insect and predators along an elevational gradient will come out in March edition of Ecography - a journal with a long history of publishing work about the patterns and causes of elevational diversity gradients.
It has been a very exciting period to plan, collect, analyse and consider these results – in particular since the work occurs along a beautiful elevational gradient of Mt Wilhelm in Papua New Guinea. Our work along Mt Wilhelm was supported by parataxonomist team and staff of the Binatang Reseach Center (BRC) for which we are very grateful.
I have been working along Mt Wilhelm gradient on bird communities from 2010. It was thus natural development that I continued with current research on the same study sites after I read about plants “crying for help” when attacked by herbivorous insect. This is when I decided to test “cry for help hypothesis” across various habitats of elevational gradient where predator communities are expected to be different.
The phenomenon known as ''crying for help“ was originally described as interactions between herbivore damaged plants, insect herbivores and parasitic wasps attacking larvae of the herbivore. Parasitoids and parasites were proved to be attracted to plant tissues damaged by herbivores significantly more than to undamaged plant tissue. It starts to be clear that also predators (e.g. ants, spiders) of herbivorous insect can be preferentially attracted to infested plants. Very importantly to me, behavioural experiments showed that birds could also use visible feeding marks on plant leaves and changes in reflectance as cues to find insect herbivores. As I was always interested in birds and ants, and I already knew that their species richness along Mt. Wilhelm differs considerably, it was only short step to my main research question: Can trees at all elevational sites call for help efficiently? What is the relative importance of individual predators along our gradient?
Results of our experiments showed that models of caterpillars (made under strict scientific protocols from plasticine) exposed on leaves with simulated herbivorous damage were always attacked more than caterpillar models exposed on undamaged leaves.
Figure 2. Model and real caterpillars of common genus Choreutis [A], on leaves with [B] and without [C] herbivorous damage.
We observed this pattern in lowland forests, where wasps and ants are the most important predators, as well as in forests at higher elevations, where birds are the most important predators. We further completed our survey of predation rate on plasticine caterpillars by extensive survey of predator communities – birds and ants, and found that abundances of predators correlate well with observed predation rate.
Figure 3. Examples of two beautiful birds often recorded along Mt Wilhelm gradient: common paradise kingfisher (Tanysiptera galatea) [A] and king bird of paradise (Cicinnurus regius) [B] hold by Stafford Hais.
Besides having great fun to survey ants and birds along the gradient, the main experiment focused on predation rate is not boring either. Let’s imagine that you have to make 300 artificial plasticine caterpillars (pressed by syringes) every day in the afternoon, pin them to styrofoam boards, take them to forest next day morning and pin them carefully to leaves to 30 trees located along more than 2 km long forest track. Next day, you have to recover all of these caterpillars, inspect them for possible bite marks, collect those that were “predated” and replaced them by new ones. The fun is not finished as the identification of the predators responsible for “death” of your caterpillars continues in the lab. For that reason, all birds mist-netted during bird survey were offered one artificial caterpillar so we were able to make a collection of bites which we completed by collection of bite marks by randomly caught predatory arthropods. The guide to bite marks is freely available or published as a part of another paper of ours.
Figure 4. Artificial plasticine caterpillar on a leaf together with first and third instar of a Choreutis spp. larvae. Predators responsible for bite marks in caterpillars – ant [A] and bee [B].
Our experiment with artificial caterpillars exposed on leaves with and without herbivorous damage showed that both chemically (e.g. ants) and visually (e.g. birds) prefer to search for food at trees crying for help, and they may use chemical signals from damaged plant tissues as well as visual appearance of damaged leaves as a clue. We further conclude that the change in dominant predator group with elevation could lead to dramatic changes in anti-predation strategies of herbivores, and the structure of local food webs, along elevational gradient.
We invite you to flip through our field photos (A + B + C), some more photos from the same study sites but various projects, or inspect our collections and some data which we made freely available, while we can share bird data upon request.
Figure 5. “Caterpillar cemetery”