“In the polar regions, the likelihood that a caterpillar gets eaten is just a fraction of what it is on 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 Professor Tomas Roslin, who led the analyses.
Ecologists have known for two centuries that the tropics have more species than the polar regions. Are the many species of the tropics involved in more intense interaction than the few ones at the poles? Professor Roslin’s international study proves that this is indeed the case, by revealing a global pattern in the predation of herbivorous insects.
The international group observed the rate at which caterpillars were attacked by predators over an area spanning 11,635 kilometres from the north to the south – from Greenland to southern Australia. They discovered that the risk for a caterpillar of being eaten in the polar areas is only one eighth of what it is at the equator.
“Moving up a mountain slope, you find the same decrease in predation risk as when moving towards the poles. This suggests that a common driver, likely related to temperature, could be controlling species interactions at a global scale,” explains Roslin.
The research required the participation of 40 researchers from 21 countries. Consistency and standardisation were key to making the data comparable across these far-flung locations.
These findings were achieved through very simple methods. To measure local predation rates, that is the rate at which prey animals are eaten, researchers glued thousands of caterpillars made from children’s plasticine to plants across 31 sites around the globe. Two of these sites were in Finland: Turku in the south and Värriö in the north. The researchers then left these “dummy caterpillars” exposed to predators, and revisited them several times to check for attack marks.
“The great thing about this method is that you can track down who the predator was by inspecting the attack marks in the plasticine. 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,” describes Eleanor Slade, a researcher at the Universities of Oxford and Lancaster, UK, who designed the globally consistent approach.
The research required the participation of 40 researchers from 21 countries. Consistency and standardisation were key to making the data comparable across these far-flung locations. From a dummy caterpillar “hatchery” at the University of Helsinki, each researcher was mailed caterpillars moulded from the same plasticine and shaped as looped inchworms. Even the glue used to attach them to flora was included in the kit to ensure the same look and smell of all caterpillars at the sites. After their time in the field, the caterpillars were carefully detached and sent back to the University of Helsinki. There, a small team led by Bess Hardwick pored over each caterpillar to score them for damage. Hardwick emphasises that these findings were only made possible by an immense international collaborative effort.
“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,” says Hardwick.
Arthropod predators keep herbivores in check
By determining a specific predator group as responsible for each mark, 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 in the data analysis.
The group’s findings may also affect herbivore evolution, as they indicate that tropical caterpillars would do well to target their defences 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 arthropod predators, such as ants, 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. What our findings suggest is that their role may be even further accentuated towards the Equator,” states Roslin.
The whole project was triggered by a chance conversation between Roslin and Slade.
“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,” says Slade.
T. Roslin et al., Higher Predation Risk for Insect Prey at Low Latitudes and Elevations. Science DOI: 10.1126/science.aaj1631
Professor Tomas Roslin, University of Helsinki, Finland (firstname.lastname@example.org), tel. +358 40 595 8098)