Caterpillars living near the equator are eight times more likely to be eaten by predators than those living closer to the poles, according to new research.
An international team made the discovery by gluing thousands of dummy caterpillars made of Plasticine on plants at 31 sites along an 11,635 km gradient from the Arctic Circle to southern Australia.
They then left them exposed to predator attack, revisiting them several times to check for attack marks. The predators of caterpillars, such as birds and ants, were tricked into thinking the model caterpillars were the real thing, only realising they had been deceived when they had taken a bite. These bite and strike marks were then counted to uncover predator hotspots across the globe.
“What was most fascinating was that the pattern was not only mirrored on both sides of the Equator, but also appeared across elevational gradients,” said Tomas Roslin of the Swedish University of Agricultural Sciences 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.”
This study, published in the journal Science, is one of the first to attempt to look at global patterns in how species interact. The results help shed light on the way that ecosystems are structured and function at a large scale, teaching us more about the growth and survival of plants, vital eco-services such as carbon storage and crop yields, and even how long-term changes in the environment may affect biodiversity.
Importantly, it also revealed the potential importance of arthropod predators, such as ants, in regulating plant-eating insects, which might otherwise devour the leaves of trees and saplings.
Eleanor Slade, a researcher at the Universities of Oxford and Lancaster, UK, who designed the globally-consistent approach said: “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.”
To do this, a total of 40 researchers from 21 countries worked together. Consistency and standardisation 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 moulded 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,” said Will Petry, who contributed data from California, and also helped analyse the data.
“The findings may also affect herbivore evolution,” said Petry. “Our results suggest 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.”
“To understand why the world stays green and is not fully consumed by hordes of caterpillars, we should appreciate the role of arthropod predators (such as ants),” said 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.“
It was a discussion between Roslin and Slade that triggered the whole project. She said: “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.”
T. Roslin et al. Higher Predation Risk for Insect Prey at Low Latitudes and Elevations. Science, in press, online 17 May 2017