Tiny bite marks reveal a global pattern in predation

A new study deploys "dummy caterpillars" across the world to reveal predation hotspots - and who is behind them.

It is well known that the tropics have many more species than the poles. But do the interactions among species also increase or become stronger nearer the Equator? A new study published in Science says yes, revealing a global pattern of predation on insect herbivores.

An international team made the discovery by examining the fraction of caterpillars eaten along an 11,635 km gradient from the Arctic Circle to southern Australia. What they found was that a caterpillar close to the poles has only one-eighth of the chance of being eaten compared to a caterpillar at the Equator.

"What was most fascinating was that the pattern was not only mirrored on both sides of the Equator, but also appeared across elevational gradients," says Tomas Roslin, who led the analyses. "Moving up a mountain slope, you find the same decrease in predation risk as when moving towards the poles. This suggests a common driver could be controlling species interactions at a global scale."

The findings were achieved with remarkably simple materials. To measure local predation rates, researchers glued thousands of caterpillars made from children's plasticine, a type of modeling clay, to plants across 31 sites around the globe. They then left these "dummy caterpillars" exposed to predator attack, and revisited them several times to check for attack marks. The predators of caterpillars, such as birds and ants, are tricked into thinking the model caterpillars the real thing, only realizing they have been deceived when they have taken a bite.

"The great thing about this method is that you can track down who the predator was by inspecting the attack marks. The jaws of an insect, like an ant, will leave two small piercings, whereas a bird beak will cause wedge-shaped marks. Mammals will leave teeth marks - well, you get the idea" explains Eleanor Slade, a researcher at the Universities of Oxford and Lancaster, UK, who designed the globally-consistent approach.

To do this, a total of 40 researchers from 21 countries worked together. Consistency and standardization were key to make the data comparable across these far-flung locations. From a dummy caterpillar "hatchery" at the University of Helsinki, Finland, each researcher was mailed caterpillars all molded from the same green plasticine and all shaped as "loopers" (or "inchworms"). Even the glue used to attach them to plants was included in the kit to ensure the same look and smell of caterpillars across sites.

After exposure, the caterpillars were carefully detached from the leaves and returned to Helsinki. Back in the lab, a small team led by Bess Hardwick pored over each caterpillar to score them for damage. By attributing each attack mark to a specific predator group, the team was then able to identify a clear culprit behind the latitudinal gradient in attack rates.

"People often think of vertebrates as the most important predators in the tropics, but birds and mammals weren't the groups responsible for the increase in predation risk towards the Equator. Instead tiny arthropod predators like ants drove the pattern", explains Will Petry, who contributed data from California, and also helped analyze the data.

"The findings may also affect herbivore evolution", says Petry. "Our results suggest that tropical caterpillars would do well to target their defenses and camouflage specifically against arthropod predators. Closer to the poles, lower predation may allow caterpillars to let their guard down."

What the patterns also suggest is that arthropods are some of the strongest agents keeping plant-feeding arthropods in check. "To understand why the world stays green and is not fully consumed by hordes of caterpillars, we should appreciate the role of arthropod predators", says Roslin. "What our findings suggest is that their role may be even further accentuated towards the Equator."

Hardwick stresses that the current findings were only made possible by the massive collaborative effort between researchers across the world: "This is the beauty of what are called 'distributed experiments'. As ecologists, we typically ask questions about patterns and processes much larger than we as single researchers or teams can examine. But by designing experiments that can be split into smaller work packages, we can involve collaborators all over the world, and work together to understand the bigger picture."

Incidentally, it was a discussion between Roslin and Slade that triggered the whole project. "Tomas had used plasticine caterpillars in Greenland and thought they didn't work when he found very low attack rates. I had used them in the rainforests in Borneo, and had detected very high attack rates. Just imagine if these are the two end points of a global pattern, we thought. And that is exactly what they turned out to be", concludes Slade.

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Citation

T. Roslin et al. Higher Predation Risk for Insect Prey at Low Latitudes and Elevations. Science, in press, online XX May 2017 LINK HERE

For more information, please contact the first or last author of the article:

Prof Tomas Roslin, Swedish University of Agricultural Sciences, Sweden (tomas.roslin@slu.se), ?ph +358 40 595 8098

Dr Eleanor Slade, Universities of Oxford and Lancaster, UK (eleanor.slade@zoo.ox.ac.uk), ?ph. +44 7765044742.

and/ or co-authors from the following continents and countries:

North America

Canada: Dr. David Hik, University of Alberta, david.hik@ualberta.ca; ph. +1 780 4929878

USA: Dr. William Petry, ETH Zürich (Swiss Federal Institute of Technology), william.petry@usys.ethz.ch; ?ph. +1 805 686 1941.

USA: Dr. Elizabeth Nichols, Swarthmore College enichol1@swarthmore.edu; ph. +1 610-328-8296

USA: Dr. Saskya van Nouhuys, Cornell University, saskya@cornell.edu

Mexico: Dr. Wesley Dáttilo, Red de Ecoetología, Instituto de Ecología AC., wesley.dattilo@inecol.mx; Phone: +52 (228) 842 18 00. Ext. 4123

Central America

Panama: Beth King, Smithsonian Tropical Research Institute, kingb@si.edu

South America

Ecuador: Dr. David A. Donoso, Escuela Politécnica Nacional, Quito david.donosov@epn.edu.ec; ?ph +593 984 110 610

Brazil: Dr. Andrea Larissa Boesing, Departamento de Ecologia, Universidade de São Paulo, lari.boesing@gmail.com

Europe

Iceland: Dr. Isabel C Barrio, Institute of Life and Environmental Sciences (University of Iceland), icbarrio@hi.is; ph. +354 869 3876.

Finland: Tapani Hopkins, Biodiversity Unit, University of Turku, taesho@utu.fi

Norway: Professor Vigdis Vandvik, University of Bergen, Vigdis.vandvik@uib.no; ph. +4747301794

Denmark and Greenland: Dr. Niels Martin Schmidt, Aarhus University, Roskilde, nms@bios.au.dk; ?ph. +45 8715 8683.

Czech Republic: Dr. Katerina Sam, Biology Centre CAS, Institute of Entomology, Ceske Budejovice, Katerina.sam.cz@gmail.com, ph. +420 387775031

Switzerland: Dr. William Petry, ETH Zürich (Swiss Federal Institute of Technology), william.petry@usys.ethz.ch; ph. +1 805 686 1941.

Asia, People's Republic of China

Mainland China: Dr Akihiro Nakamura, Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, a.nakamura@xtbg.ac.cn, ph. +86 184 8793 0911.

Hong Kong: Dr. Timothy Bonebrake, The University of Hong Kong, email tbone@hku.hk

Australia

Associate Professor Nigel Andrew, University of New England, Armidale, NSW; nigel.andrew@une.edu.au; ph +61 2 6773 2937

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