As long as there have been humans, we have gazed at beautiful butterflies, but climate change may alter all that. Rising carbon dioxide levels associated with global warming may affect interactions between plants and the insects that eat them, altering the course of plant evolution. This is particularly true of milkweed, the food that the caterpillars that turn into beautiful Monarch butterflies eat. The plant tastes bitter to other insects, but Monarchs have no problem with it. But if higher CO2 levels make the plant less bitter, other insects would no longer hesitate to feed on milkweed, leaving less for the Monarchs, which might eventually disappear.

Plant defenses—and insect eating patterns—also respond to environmental factors such as rising carbon dioxide. This suggests that elevated carbon dioxide could affect plant evolution by altering the "selection pressure" that plant-eating insects exert on plants. Selection pressure, the driving force of evolution, induces changes in the genetic composition of a population. It works like this: if insects inflict too much damage on plants, the plants can’t reproduce as successfully. This sets up a situation in which any plants that, by chance, have inherited insect-deterring traits are at an advantage. Because of that advantage, such traits are likely to spread through the population, urged on by "pressure" from the insects. Researcher Rachel Vannette found that milkweed grown in a high CO2 environment had as much as 50% less of its insect-repelling toxins. Vannette says, "That’s a big difference if you’re a caterpillar." Will the plants’ changing defense strategies help or hinder monarchs? "We don’t know yet," Vannette says, "but that’s a question we’re investigating."

So far, thank goodness, butterflies seem to be doing all right. Researcher Craig Wilson says that despite recent cold snaps that brought record-setting low temperatures over much of Texas, the outlook for the annual Monarch butterfly invasion to the state looks promising “and better than expected.” He says the numbers of Monarchs entering the state over the next few weeks should be very strong: “The numbers are better than expected and the Monarch appears to have recovered to 2008-09 levels in their over-wintering sites in Mexico.”

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