With its brilliant red flowers, scarlet gilia (Ipomopsis aggregeta) stands out among the grass, shrubs, ferns, and pines found on the San Francisco Peaks in northern Arizona. The plant, which is also called skyrocket, is considered one of the most eyecatching wildflowers in the American West.
When Ken Paige of Northern Arizona University became interested in scarlet gilia in the early 1980s, two questions loomed large: Why do the petals change their shade from red to pink to white over the course of a flowering season which lasts from July to September? And how can the plant, a preferred food of elk and mule deer, withstand the extensive foraging from these animals? Some answers emerged after years of observations.
Paige and his colleague Thomas Whitham found that flowers produced in mid-July, early in the season, tended to be red, while those blooming in August and September were lighter. Similar changes were seen in individual plants, which shifted one to two color shades—say from red to dark pink or pink —every two weeks. These shifts correlated with changes in the plant's pollinators: The population of rufous hummingbirds, which are attracted to red flowers, drops sharply after July as the birds resume their migration toward Mexico. The population of hawkmoths, which are partial to lighter shades, remains stable through September. The color shift, an evolutionary adaptation, enables scarlet gilia to take advantage of the visual systems of its main pollinators. The plant has prospered by exploiting the pollination services of hawkmoths after the hummingbirds have left for warmer climes.
In separate experiments, Paige learned that browsing by elk and mule deer actually increases flower and fruit production. That's surprising, he says, because "most plants suffer when chewed or browsed—the resources put into restoring stems and leaves usually diminishing the size of roots, the number of fruits, and the number and quality of seeds." Paige concludes that scarlet gilia is "no passive beauty, but a chameleon-like strategist" that even takes advantage of being eaten.
More than 80 percent of the world's 250,000 flowering plants depend on animal pollinators, and roughly 15 to 30 percent of the food eaten throughout the world comes from plants pollinated by these creatures.