Every year, flocks of tiny white birds embark on an arduous, zigzagging journey from Greenland to Antarctica and then back again, flying more than 44,000 miles. In its lifetime, each of these arctic terns covers a distance equivalent to three or four round trips to the moon. Meanwhile, the dusky grouse, which lives at the edges of forests in mountainous regions of North America, travels but a fraction of a mile between its breeding grounds and its regular habitat. The great majority of bird species don’t migrate at all.
According to a study published in today’s issue of Nature Ecology & Evolution, those wild differences in migration patterns — and the resulting distribution of bird species around the world — reflect the efforts of those species to maintain an optimal energy balance within their competitive environments. The research makes a forceful argument that energy supply and demand is a driving ecological principle shaping the global structure of biodiversity. It also presents a powerful tool that environmental scientists and ecologists can use to predict how human activity will rapidly change that structure.
A Natural Experiment
In 1807, the naturalist and explorer Alexander von Humboldt wrote, “The nearer we approach the tropics, the greater the increase in the variety of structure, grace of form and mixture of colors, as also in perpetual youth and vigor of organic life.” He was among the first in a long line of biologists to note that diversity in plant and animal species (including birds) tends to increase from the poles toward the equator. But despite more than two centuries of observation since then, the mechanism responsible for this well-recognized pattern has continued to elude scientists.
