20. 06. 2017

Ecologists from the Biology Centre of the CAS and Faculty of Science of the University of South Bohemia in České Budějovice engaged in the unique, unprecedented complex worldwide experiment. Just like 40 scientists in 21 other countries, they glued dozens of artificial caterpillars on trees in selected forests and then determined how many and which kinds of predators attacked them. The study, published in the prestigious journal Science, presents new knowledge on the mutual ties between animal species from the Arctic Circle to South Australia and can provide a number of incentives for agriculture and environmental protection in the battle against insect pests.

It is well known that many more species of animals live in the tropics than in the areas closer to the poles, but it has remained a question if there are stronger and even mutual ties between species moving closer to the equator. In the extensive research, 40 researchers sought the answer in 31 localities spread all over the world within an 11,635-kilometre-long gradient of geographical width. Scientists used thousands of caterpillars produced from children’s plastiline, which they glued to the leaves of trees and then left them exposed to the attacks of predators, such as birds and ants. According to prints of the beaks or mandibles in the plasticine, the scientists then determined how frequent the attacks of predators were, how long a caterpillar would last, and how many predators would attack it.

“The jaws of the ants leave two small bites, whereas birds’ beaks leave a print in the shape of a V on both sides of the caterpillar. Mammal teeth imprints are also really easy to identify,” describes Kateřina Sam from the Institute of Entomology of the Biology Center of the CAS, which has already used the method as a component of its research for many years. In this research, she installed and observed fake caterpillars in three localities, in the Czech Republic in a mixed forest in the Šumava Mountains area, in the tropical rainforest of Papua, New Guinea and in a eucalyptus forest in Australia. It arose from her work that in Bohemia birds and ants equally wanted to eat the caterpillar but in the tropics predatory insects attack with a clear predominance.

“People often think that vertebrates are the most important predators of an insect in tropical areas, but neither birds nor mammals were the groups responsible for the increase of the risk of predation at the equator. It was precisely the predatory insect, particularly ants and wasps,” explains Kateřina Sam. It shows that in the tropics the role of the predatory insect is crucial in the control of herbivorous pests and helps us understand why the world, including farm crops in the fields, remains green and not entirely consumed by hordes of caterpillars.

From the results of the entire research project, the scientists then calculated that unlike the caterpillar at the equator, which has the greatest chance of being attacked by a predator, the caterpillar near the pole already has only an eighth of the risk. It was also fascinating that the result was not only mirrored on both sides of the equator but was also manifested with changes in height above sea level.

The uniqueness of this ecological study consisted of the tremendous complexity of the proposed experiment, which was possible only through massive joint efforts among researchers around the world. “As ecologists, we usually ask questions about processes that are far more extensive than would be manageable by a single researcher or team. By designing experiments that can be divided into smaller parts, we can involve scientists from all over the world and work together to understand the broader picture,” explains the Czech ecologist. Not only did scientists at all sites use the same methods and procedures, but they had exactly identical fake caterpillars. These were created at the University of Helsinki in Finland from the same green plasticine and of the same shape. Even the glue used to stick them to the plants was included in the kit that each researcher received to ensure the same look and smell of the caterpillar. After exposure to the plants, the caterpillars were carefully separated from the leaves and returned to small plastic bottles back to Helsinki, where a small team of scientists verified the identity of predator bites. Only in that way was the complete standardization of the experiment ensured.

Prepared by: Biology Center of the CAS
Photo: Biology Center of the CAS