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, research at the Univ. of Michigan
suggests.
The research focused on the effects of elevated carbon
dioxide on common milkweed, Asclepias
syriaca. Milkweed is one of many plants that produce toxic or bitter
chemical compounds to protect themselves from being eaten by insects. These
chemical defenses are the result of a long history of interactions between the
plants and insects such as monarch caterpillars that feed on them.
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.
Researchers Rachel Vannette and Mark Hunter investigated
whether different genetic “families” of the common milkweed from a
single population in Northern Michigan would
respond differently to increasing carbon dioxide levels in the atmosphere and
if so, how those responses might affect the plants’ chances of being eaten by
insects.
“Specifically, we examined the response of milkweed
plants to elevated carbon dioxide in terms of plant growth, asexual
reproduction, and the production of chemical and physical defenses,”
Vannette said. Although all plants grew larger in response to elevated carbon
dioxide, and all plant families showed similar growth and reproductive
responses, plant families responded differently to elevated carbon dioxide in
their production of chemical and physical defenses against plant-eating
insects.
In particular, their production of heart poisons called
cardenolides differed. While some plant families responded to elevated carbon
dioxide by increasing cardenolide production, most decreased production—by as
much as 50%.
“That’s a big difference if you’re a caterpillar,”
said Vannette, who is a graduate student in Hunter’s research group. Hunter is
the Henry A. Gleason Collegiate Professor of Ecology and Evolutionary Biology.
Because the insects that consume milkweed, including monarch caterpillars,
choose their host plants carefully and select specific plants based on the
plants’ concentration of toxic compounds, these specialist insects can act as
agents of selection on milkweed plants.
Countering the shift away from chemical defenses was a shift
toward physical defenses and resistance. “The plants had tougher leaves,
and they were better at tolerating herbivory by caterpillars—they grew back
faster,” Vannette said.
Taken together, the results provide evidence that in response
to elevated carbon dioxide, genetically-based differences in plant defense
mechanisms and the changing plant-insect interactions that result may influence
how plants adapt to changing climate.
Will the plants’ changing defense strategies help or hinder
monarchs?
“We don’t know yet,” Vannette said, “but
that’s a question we’re investigating.”
The findings appear Global
Change Biology.
Conducted at the U-M Biological Station near Pellston, Michigan,
the research was funded by the National Science Foundation.
