Dartmouth College Professor Richard Holmes has been counting caterpillars at the Hubbard Brook Experimental Forest for more than 20 years. He was mostly interested in understanding the abundance of caterpillars available as food for birds, but the tremendous fluctuations he found from year to year in the overall abundance of caterpillars – sometimes as much as a 30-fold difference – raised questions of great interest to his colleagues.
Dartmouth Professor Matthew Ayres and student Lindsay Reynolds were curious about what caused the populations to fluctuate so wildly. There are at least 400 species of butterfly and moth caterpillars in the Hubbard Brook forest, and it wasn’t just a few species that were rising and falling. “We found that the fluctuations happened in synchrony across large geographic areas and many species,” said Ayres, “so there must be a large scale driver causing it.”
So Ayres and Reynolds tested what seemed to be the most likely driver – weather. Perhaps extremely cold winters caused a significant drop in caterpillar abundance the following summer, they hypothesized, or hot summers led to an increase in caterpillar numbers. By comparing historic climate data with the caterpillar abundance data that Holmes collected, the Dartmouth scientists found that cold winters have little effect on caterpillar populations but long warm springs do. In years when temperatures were high for an extended period in the summer, caterpillar populations increased.
The key to this phenomenon is what Ayres calls the phenological race.
“Caterpillar abundance fluctuations are influenced most by those caterpillars that feed early in the season when tree buds are just beginning to break and leaves are expanding,” he explained. “During that time, the leaves have a higher nutritional value to the caterpillars. Temperature affects the development rate of the leaves and the development rate of the caterpillars, and that’s where the phenological race comes in. In warmer years, caterpillars can achieve more of their feeding and development before the leaves expand and tighten up their defenses.”
As the climate continues to warm, Ayres believes that caterpillar populations will increase, but that’s not necessarily bad. From the perspective of the birds, this is a good thing, he said, because they’ll have more food available. “The caterpillars we’re talking about aren’t forest pests; they’re native fauna and a critical link in the structure of the ecosystem. They won’t harm the forest. Some caterpillars are pests and could have ecological and economic impacts, but that’s not the case here.”
Dartmouth postdoctoral fellow Erik Stange is following up on Ayres’ work, trying to determine if caterpillars from particular families or those with common ecologies might be most responsible for the fluctuations. After collecting and sorting 50,000 individual moths, he found that abundance fluctuations occurred across all caterpillar groupings, but those that feed in the spring show the greatest variation. “It’s a communitywide trend in abundance,” Stange said. “Years that are good for some caterpillars are good for many kinds of caterpillars.”