The likelihood that a caterpillar will fall prey to another animal increases substantially the closer the caterpillar is to the equator, as well as at lower elevations, according to a new study in the May 19 issue of Science.
In an experiment that deployed 2,879 artificial caterpillars across a span of latitudes, the researchers found that for every 1° of latitude away from the equator, the daily odds of a caterpillar being attacked decreased by 2.7%. Predation rates also decreased with increasing elevation, where every additional 100 meters of elevation decreased the odds of predation by 6.6%.
"I was most surprised about the fact that the pattern found across latitudes was so neatly mirrored across elevations," said Tomas Roslin of the Swedish University of Agricultural Sciences and co-author of the paper. He initially only analyzed patterns across latitudes, but when a colleague prompted him to analyze elevation as well, the patterns across altitude were striking. "Finding the same pattern in a horizontal and vertical direction just stunned me."
Wedge-shaped marks at the end of this plasticine worm were made a bird beak. | Tomas Roslin
Tropical regions near the equator are the most biodiverse regions in the world and typically the total number of different species in an area decreases incrementally as one moves closer to the North and South Poles. But does increased biodiversity mean more interactions among species and therefore an increased chance of being preyed upon?
Many scientists think that interactions among species, and subsequently rates of predation, do indeed become increasingly strong at lower latitudes, those close to the equator. However, assessing such a theory can be challenging because of the complexity of large-scale patterns of species interactions.
"In different parts of the world, you will have a lot of different species interacting with each other. If you then go out and just randomly pick some species per site, you will essentially end up comparing apples with oranges," explained Roslin.
Taking a more simplified approach, Roslin and his colleagues created caterpillars out of green plasticine, the same modeling material that kids use to make figurines, placing the fake critters across an 11,660-kilometer latitudinal gradient spanning six continents and monitoring their fates over four to 18 days.
Why caterpillars? Insects are the most abundant type of animal in the world. "The type of green 'inchworm' or 'looper' caterpillar that we chose is a very generic design for an insect. You will find such species in the natural fauna of every site targeted by our study," said Roslin. "[That's why] we selected them as a standard, common type of prey that predators would interact with all over the world."
Based on the type of bite marks left in the plasticine, the researchers also determined which animals attacked the fake caterpillars. Interestingly, predation by other insects was more common than predation by birds or mammals.
While this study cannot tell us exactly why there is such a difference in predation across latitude and elevation, Roslin has a suspicion. "The one thing that varies both toward the pole and the sky is temperature, essentially energy ... Insects are ectotherms and thus dependent on external energy for their activity. With warmer temperatures you have more active insects, which attack more prey; it is perhaps as simple as that."
Roslin is working on five new projects that rely on a similar modeling technique used in this study. One project includes working with scientists in the Arctic to understand how pollinator and predator communities are structured across the region and how that structure is related to pollination success.