One Plant Species, Two Different Pollinators — Where’s This Headed?

Biologists at the University of California, San Diego have discovered that the future of red and yellow varieties of a San Diego wildflower may depend on the fates of two different animals. They report in the current issue of the Journal of Evolutionary Biology that monkeyflowers have two different animal pollinators. The red form, common along the coast, is strongly preferred by hummingbirds, while yellow monkeyflowers, found east of I-15, are favored by hawkmoths.

The researchers suspect the recent increase in San Diego’s hummingbird population, fueled by the growth of suburban developments and gardens, will eventually favor the red over the yellow variety. “Hummingbirds are now three times as dense inland as they were on the coast 50 years ago,” said Joshua Kohn, an associate professor of biology at UCSD, who conducted the study with Matthew Streisfeld, now a researcher at Duke University. “This increase in hummingbirds may be tipping the balance of selection from favoring yellow to favoring red flowers.”

“The shift between the red- and yellow-flowered forms can be seen along any road running from the coast inland in San Diego county and is one of the sharpest natural patterns residents can view while driving in the late spring,” he adds. “We have shown that this shift is very likely due to selection by different types of pollinators. The abundance of at least one of these pollinators, hummingbirds, has recently increased dramatically and may well favor an eastern expansion of the red-flowered form.”

“The striking geographic pattern of flower color—where we never see a red-flowered plant in the east or a yellow-flowered plant along the coast—speaks wonders of the power of natural selection to maintain these differences,” said Streisfeld.