New Zealand’s South Island was once covered in dense forest, the trees breaking like a dark green tide around the grassy peaks. After the arrival of Māori settlers about 750 years ago, some hills were cleared from their trees by fire, and the foliage has not returned. For the organisms living in these forests, their habitat changed almost overnight from sheltered forests to exposed, windswept grasslands.
Since the forests burned down, small winged insects called stoneflies have also changed, researchers have found. In some sort of evolutionary pivot over just a handful of centuries, the stoneflies that live above the treeline have lost the ability to fly, suggesting that human-induced changes to an ecosystem, such as deforestation, are affecting the bodies of its inhabitants. The discovery was published Wednesday in the journal Biology Letters.
Charles Darwin noted that insects on islands have a curious tendency not to fly, perhaps because flying is dangerous when you are small and the wind is strong. In New Zealand, scientists had found flightless stoneflies on many different mountains, said Jon Waters, a professor at the University of Otago in New Zealand who is an author on the new paper. It wasn’t clear at first why – whether there was something about the altitude that favored a non-flying form, or if something else was going on.
To answer the question, he and his colleagues collected stoneflies from five locations, walking through the forests to the barren peaks of the mountains. They caught insects as they went up the slopes and recorded their locations. When they looked at all the data, they were surprised to find a very clear trend.
“We found that there was an amazing transition from winged populations to flightless populations as you went up,” said Dr. water. “Wherever we looked, that correlation was linked to where the trees stopped, not a particular height.”
Because the switchover occurred at the treeline, rather than at a specific elevation, this suggests that the exposed situation above the trees has promoted the flightlessness of stoneflies. Perhaps, as in the case of Darwin’s island insects, wind at altitude makes flying a liability.
It is possible that in some places, even before the forests burned down, there were already flying stoneflies that simply expanded their territory after the fires. A genetic analysis of the stoneflies showed that three of the five populations the researchers looked at were very different from their lowland winged brethren, implying that they may have evolved on their own for a while.
However, the other two had smaller differences, suggesting the change could be recent — recent enough to have been since people arrived on the island.
The apparent speed of change recalls the case of the peppered moth, whose color shifted from light to dark as air pollution from England’s industrial revolution obscured the trees it lived on — light moths, of course, were more visible to predators in their altered environment. It doesn’t take millennia for animal populations to change through natural selection, these cases show.
“You go into the trees and you suddenly end up in a different population. It’s almost magical that evolution seems to work so clearly and so effectively over a short distance in some of these cases and a short time frame,” said Dr. water.
Now the researchers are looking deeper into the stoneflies’ genetics to understand what it is that changes when the insects lose the ability to fly. The details can reveal whether stoneflies’ apparent flexibility stems from new mutations, or whether their flightlessness stems from variations that already existed in their ancestral populations and simply waited for the right moment.
