August 4, 2009
Animal and Plant Communication At The ESA Annual Meeting
Chemicals camouflage bugs, pitcher plant colors don't help attract prey, specialist caterpillars survive better than generalists
Animals and plants communicate with one another in a variety of ways: behavior, body patterns, and even chemistry. In a series of talks at the Ecological Society of America's annual meeting, to be held August 3-7 in Albuquerque, New Mexico, ecologists explore the myriad adaptations for exchanging information among living things.
Bugs pretending to be ants are protected against attack
A classic example of a mutualism, or a mutually beneficial relationship between two species, is that of warm-climate Acacia plants and their ant tenants. The plants provide the ants with shelter within their hollowed-out thorns and food in the form of nectar and protein. The ants, in return, defend the tree viciously, attacking anything that comes near "“ from other insects to birds and small mammals.
One species of bug, however, has exploited this system. These insects, in the family Coreidae, roam freely on one species of Acacia and feed on the plants' leaf tissue. Susan Whitehead of the University of Colorado wanted to know just what makes these bugs seemingly invisible to the watchdog ants.
Whitehead hypothesized that the bugs might be acting in some way that the ants found acceptable. Since ants use pheromones to communicate with one another, she also wondered if the bugs were mimicking the scent of the ants. When she and her colleagues immobilized the bugs, the ants still did not attack them. But when the researchers washed the bugs in a chemical solvent and returned them to the plants, the ants immediately swarmed the bugs.
The key, says Whitehead, was the removal of chemicals on the bugs' exoskeleton. Using chromatography and spectrometry, the researchers compared the bugs' exoskeletal chemicals with that of the ants.
"The chemicals in the bugs' cuticle matched that of the ants," says Whitehead. "The bugs mimic the hydrocarbons that the ants produce, so the ants don't recognize them as something foreign."
Even if this chemical camouflage is energetically costly for the bugs to produce, the benefits outweigh the costs, says Whitehead.
"Here, the bugs have tapped into a resource in a competition-free space," she says. "It emphasizes that even the most classic examples of mutualism in nature are not infallible."
Pitcher plants' red colors don't attract prey
Pitcher plants have distinctive adaptations for living in nutrient-poor soils: These carnivorous plants produce a pitcher-shaped structure with a pool of water in it. When insects investigate, they slide into the pitcher and meet a watery demise. The plant then dissolves the insect and uses it for food. Biologists have long assumed that in addition to their nectar-producing glands that attract prey to a potential food source, the plants' bright colors "“ mostly shades of red and green "“ help to attract insects. Like bees that are attracted to flowers there's a general idea that insects like bright colors, especially reds and yellows. But thus far there is little evidence that these colorful patterns attract prey to pitcher plants.
Aaron Ellison of Harvard University and his coauthor Katherine Bennett, a fifth grade teacher working under a National Science Foundation Research Experience for Teachers grant, placed painted tubes mimicking pitcher plants' natural color spectrum of redness into the plants' habitat. They found that red coloration didn't affect the number of ants "“ the pitchers' primary prey "“ that were captured.
These results are not surprising, says Ellison. Seventy percent of pitcher plants' prey are ants, which see the color red as grey. Since grey probably only weakly contrasts with the green base color of the pitcher, it's probable that the ants can't differentiate between mostly red and mostly green plants.
Ellison thinks this work is a good example of why biological concepts need to be tested and not simply assumed.
"There's no evidence that the coloration in pitcher plants is of any adaptive value for capturing prey," he says. "We don't need to assume that it has an adaptive value, or that it has anything to do with prey."
Specialists are better at avoiding predators
Insect herbivore species often specialize on the host plants that they eat, evolving adaptations to use a plant's unique set of resources. However, specialization doesn't come without costs.
"A lot of evolutionary ecologists have pondered the advantages of being a specialist, and there are presumably tradeoffs," says Michael Singer of Wesleyan University. "Specialists have a smaller resource base, but they might be better adapted to their niche."
Singer and his colleagues wondered if there could be other advantages to specialization than better utilization of host plants as food. Specialists might also be more adept than generalists, he postulated, at using their host plants for defense or refuge from predation, specifically by birds.
The team tested this idea by excluding birds from experimental plots in a temperate forest in Connecticut and surveying the density of generalist and specialist caterpillar species inside and outside the exclosures. In the exclosures, his team observed a surge in generalist density compared to natural areas. The number of specialists, however, only increased slightly. These results suggest that the bird predators were preferentially targeting generalists.
The difference is likely due to the specialists' ability to take better advantage of their host plants, says Singer. Many specialists use chemicals from their host plant's tissue to make themselves toxic. Others, like Singer's specialist caterpillars, might be more adept at camouflaging themselves by finding the best places to hide or to blend in.
Singer's experiment is the first to quantify bird predation on specialist and generalist herbivores, and he hopes it will spark further research. He says that the interactions among the three trophic levels "“ plants, herbivores and predators "“ are the key to understanding the species' ecology and evolution.
"Food webs are complex, and that complexity is fundamental to understanding ecological specialization and diversity in natural ecosystems," he says.
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