Thousands of strangely serene caterpillars began showing up in wilderness areas around the world recently, from the Arctic Circle to southern Australia. They confused a variety of predators who tried to eat them, and then mysteriously disappeared.
Those predators may never understand what happened, but we do. And thanks to all their earnest attempts to eat these weird caterpillars, we also now know more about the predators themselves — and about the key ecological roles they play.
Scientists who study predators sometimes have to use bogus prey as bait, like fake plasticine "caterpillars" (see photo above). Many researchers have done this before, but a newly published study is the first to do it on a global scale. By gluing nearly 3,000 fake caterpillars onto plants at 31 sites across six continents, the study's authors reveal big insights about predation patterns around the planet.
It's well-known that tropical habitats bustle with life, typically hosting far more species than areas at higher latitudes. This biodiversity is good for life in general (including people), but as the new study shows, living closer to the tropics also makes life demonstrably more dangerous for certain animals. Daily attack rates on the fake caterpillars were 2.7 percent lower for every degree of latitude — about 69 miles, or 111 kilometers — farther from the equator, going either north or south.
That's because lower latitudes are teeming with predators, and not just mammals, birds, reptiles or amphibians. In fact, the study suggests a less obvious reason why predation is more prolific closer to the equator: tiny arthropods, especially ants.
Trouble in paradise
The study's authors placed 2,879 green plasticine caterpillars at 31 locations around the world, hitting every continent but Antarctica. The caterpillars were all glued onto plants so they couldn't actually be eaten, but that didn't stop predators from trying. The researchers then removed all the decoys after four to 18 days, carefully preserving any bite marks so they could be analyzed.
"The great thing about this method is that you can track down who the predator was by inspecting the attack marks," says study co-author Eleanor Slade, a zoology researcher at the universities of Oxford and Lancaster, in a statement. "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."
Decoys in more northern and southern locations had significantly fewer bite marks than those closer to the equator. But aside from latitude, higher elevation also seemed to reduce the pressure from predators, points out fellow co-author and University of Helsinki ecologist Tomas Roslin.
"The pattern was not only mirrored on both sides of the equator, but also appeared across elevational gradients," Roslin says. "Moving up a mountain slope, you find the same decrease in predation risk as when moving toward the poles. This suggests a common driver could be controlling species interactions at a global scale."
A labor of larvae
The idea for this study came up when Slade and Roslin were discussing results from fake-caterpillar research at very different latitudes. "Tomas had used plasticine caterpillars in Greenland, and thought they didn't work when he found very low attack rates," Slade explains. "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."
Doing field research on a global scale is hard, though. All the experiments must be standardized, for example, to make sure results can be compared. That's why all the decoys were made at a single "hatchery" — they're designed to mimic looper caterpillars (see photo above) — and packed into kits for each site. The kits even included glue for attaching decoys to plants, ensuring a consistent look and smell.
Research of this scale also requires a lot of scientists. In this case, it took 40 researchers from 21 countries, whose combined efforts yielded an unusually huge perspective. "This is the beauty of what are called 'distributed experiments,'" says co-author and University of Helsinki lab manager Bess Hardwick.
"As ecologists, we typically ask questions about patterns and processes much larger than we as single researchers or teams can examine," she adds. "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."
Ants and plants
After examining all the bite marks, the study's authors identified what they call a "clear culprit" behind the higher attack rates at lower latitudes. This phenomenon isn't driven by big-bodied carnivores, they conclude, or even by vertebrates at all.
"People often think of vertebrates as the most important predators in the tropics," notes co-author Will Petry, a plant ecologist at ETH Zurich, "but birds and mammals weren't the groups responsible for the increase in predation risk toward the equator. Instead, tiny arthropod predators like ants drove the pattern."
Ants rarely get the respect they deserve from humanity, although that has been changing in recent decades. (That's largely due to advocates like renowned biologist E.O. Wilson, who released his landmark book "The Ants" in 1990). We've learned to see ant colonies as "superorganisms," with individual ants acting like cells, and we're increasingly aware of their amazing abilities and ecological influence. According to some experts, ants may even "control the planet" as much as we do.
Aside from offering more reasons to be in awe of ants, this study could also shed light on the evolution of plant-eating insects, its authors say. "Our results suggest that tropical caterpillars would do well to target their defenses and camouflage specifically against arthropod predators," Petry says. "Closer to the poles, lower predation may allow caterpillars to let their guard down."
It's still unclear if this applies to other kinds of herbivores, the researchers write, or if it translates from the forest understory up to the canopy. They say they hope to inspire more big, ambitious studies like this, and that future research will reveal if these patterns have cascading effects on forest ecosystems overall.
In the meantime, though, they suggest we don't take ants for granted.
"To understand why the world stays green and is not fully consumed by hordes of caterpillars," Roslin says, "we should appreciate the role of arthropod predators."