Predatory behavior in tropics driven by ants and others, study says

By the A.M Costa Rica staff

A new Oxford University collaboration revealing the world’s prime insect predation hotspots, achieved its landmark findings using an unusual aid: plastic dummy caterpillars.

The new study published in “Science” has revealed a global pattern of predation on insect herbivores. The trends observed were surprising, revealing that predatory behavior in the tropics is not driven by birds or mammals but by ants and other small arthropods.

The international team made the discovery by examining a fraction of caterpillars eaten along an 11,635 km gradient from the Arctic Circle to southern Australia. The work revealed that a caterpillar living near the Equator is eight times more likely to be eaten, than a caterpillar at the poles.

The findings were achieved with surprisingly simple materials. To measure local predation rates, researchers glued thousands of artificial caterpillars made from plasticine, to plants across 31 sites around the world.

Predators of caterpillars, such as birds and ants, are then tricked into thinking the decoy insects are the real thing, and only realize that they have been deceived when they have taken a bite. The team then revisited sites several times to check for bite marks, and evidence of any predatory behavior.

Eleanor Slade, one of the study’s lead authors and a researcher in Zoology at the Universities of Oxford and Lancaster, 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 – well, you get the idea.”

Tomas Roslin, the other lead author and professor of Agricultural Sciences at the University of Helsinki, said:

“The pattern was not only mirrored on both sides of the Equator, but also appeared across elevational gradients. 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.”

Decoy insects have been used in previous ecology studies, but never before on such a broad scale. It was comparing results from these smaller initiatives that inspired the team to take their research to the next level.

The replica insects were placed at far-flung locations across the globe, which meant that gathering the corresponding data required a lot of team work, with a total of 40 researchers from 21 countries working on the project.

For the insights gained to be comparable, consistency and standardization were key. The dummy caterpillars were designed at and sent to researchers from the projects central hatchery, at the University of Helsinki, Finland. Moulded from green plasticine, the materials were shaped to closely resemble 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 all sites.

A total of 2,879 were used in the study and left at the sites over the course of four to 18 days. The caterpillars were then carefully detached from the leaves and returned to Helsinki for analysis. Led by Bess Hardwick, the team inspected the caterpillars for signs of predator-induced damage, such as bite marks.

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.

Will Petry, Postdoctoral Research Associate at Zurich, who contributed data and supported analysis from California, said: “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.”

The findings may also have implications for herbivore evolution, Petry said: “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.”

Roslin said: ‘To understand why the world stays green and is not fully consumed by hordes of caterpillars, we should appreciate the role of arthropod predators. Our findings suggest that their role may be even further accentuated towards the Equator.”

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