The Coevolutionary Matrix

Coevolution is far more powerful and widespread than Darwin ever imagined, even among the plants that inspired him. Scientists now recognize that the vast majority of flowering plants–290,000 species–depend on animals to spread their pollen (only 20,000 species can spread their pollen by wind or water). Instead of nectar, some of them get insects to spread their pollen by offering resins and oils the insects use to help build the walls of their hives. Tomatoes and certain other plants even offer up some of their own pollen. They typically keep their pollen in containers shaped like salt shakers, and when the insects land on a flower, they buzz their wings at a frequency that jostles the pollen free. The insects feast on the pollen, and in the process they get dusted with it as well.

While most of the animals that pollinate flowers are insects, about 1,200 species of vertebrates–mostly birds and bats–also do the job. Like insect pollinators, they have shaped the evolution of the plants they pollinate. The flowers that are pollinated by birds lure them with bright red petals (insects are blind to the color). Unlike fragrant orchids, bird‑pollinated flowers are scentless, since birds have a poor sense of smell. They keep their nectar in long, wide tubes to suit the long, stiff beaks of birds. On the other hand, the plants that depend on bats to spread their pollen open their flowers at night, when bats leave their roosts in search of food. To make themselves easy to find, some bat flowers have evolved into cuplike shapes that can reflect and focus sound waves that bats use to echolocate. These acoustic mirrors catch the bats’ attention and guide them to their meal.

Domesticated plants depend on pollinators just as their wild relatives do. Without them, an apple orchard would be fruitless, a cornfield cobless. But plants–both wild and domesticated–also depend on other coevolutionary partners to keep from starving to death. Plants use photosynthesis to turn carbon dioxide and water into organic carbon, but they have a harder time extracting nitrogen, phosphorus, and other nutrients from the soil. Fortunately, the roots of many species of plants are enmeshed in a vast filigree of fungus that can supply the nutrients they need.

The fungus produces enzymes that break down soil, allowing it to suck up phosphorus and other chemicals. It injects these nutrients into the plants, and in exchange, it draws out some of the organic carbon the plants create with photosynthesis. The fungus demands a steep cost for its services: about 15 percent of the organic carbon a tree creates in a year. But it’s a price worth paying; without fungus, many plants become stunted and gaunt. Some species of fungus can kill soil nematodes and other plant enemies and may even make plants more resistant to drought and other disasters. It can store carbon it extracts from trees and then shuttle it through its web. If one tree it is connected to is suffering a carbon shortfall, the fungus can pump carbon into its roots. Forests, prairies, and soybean fields are not a collection of lonely individuals: they are just the visible tips of a huge coevolutionary matrix.