Other Environmental Themes

A Co-Evolving Partnership

Some pollinators like honeybees are generalists, visiting the flowers of many species on a given day or season. Other pollinators are highly specialized, sometimes attending to the needs of a single flowering plant. In such cases, the interactions between plant and animal can be amazingly well choreographed with two species evolving together over the course of millions of years, each adapting to the other in mutually beneficial ways. The size of a pollinator's tongue or shape of its bill may fit a target plant like a key sliding into a lock. Charles Darwin introduced the idea of "co-evolution"—a joint process of natural selection—in his 1871 work, "The Ascent of Man."

One puzzle in Darwin's era was posed by a Madagascar orchid called the Bethlehem star, which stored nectar in the bottom of a slender, foot-long tube. No creatures were known with a tongue long enough and narrow enough to reach the nectar, but Darwin was convinced a pollinator had to exist, most likely a moth. Sure enough, a hawkmoth with an 11-inch tongue was discovered in 1903, decades after his prediction.

Wild fig trees, to take another example, are pollinated by fig wasps, and the life cycles of these insects and plants are tightly coupled, though the details of pollination vary for different fig trees. Attracted by the scent of certain trees, female wasps push their way into an opening of an immature fig, where they lay their eggs and die. Newly born males mate with young females and die within the tree after boring holes that release carbon dioxide, thereby triggering pollen formation. The pollen, in turn, is carried to other fig trees by female wasps searching for a place to deposit their eggs.

Only one creature, the black-and-white ruffed lemur is nimble enough to pollinate the flowers of the travelers tree in the Madagascar rainforest. If the lemur goes extinct, no other animal can take its place, leaving the trees unable to reproduce. The tremendous biodiversity we see in nature is supported by countless partnerships like these. The ties that bind species are also points of vulnerability, so we must take pains to protect pollinators and their dependent plants.