Many carnivorous plants have special glands that secrete enzymes powerful enough to penetrate the hard exoskeleton of insects so they can absorb nutrients from inside their prey. But the purple pitcher plant, which lives in bogs and infertile sandy soils in much of North America, enlists other organisms to digest its food. It is home to an intricate food web of mosquito larvae, midges, protozoans, and bacteria, many of which can survive only in this unique habitat. The animals shred the prey that fall into the pitcher, and the smaller organisms feed on the debris. Finally, the pitcher plant absorbs the nutrients released by the feeding frenzy. "Having the animals creates a processing chain that speeds up all the reactions," says Nicholas Gotelli of the University of Vermont. "And then the plant dumps oxygen back into the pitcher for the insects. It's a tight feedback loop."
Pitcher plants grow by the thousands in the bogs at the Harvard Forest in central Massachusetts. One late spring day Aaron Ellison took me on a tour, stopping from time to time to watch patiently as I pulled a sinking leg out of the muck. "You haven't had a real bog experience till you're up to your crotch in it," said Ellison, a senior ecologist at the forest. Little orange flags fluttered across the bogs. Each one marked a pitcher plant impressed into the service of science. In the distance a student was feeding flies to the flagged plants. The researchers raise these insects on food spiked with unusual forms of carbon and nitrogen so they can later harvest the pitcher plants and measure how much of each element from the flies has been absorbed into the plants. Because pitcher plants grow slowly (they can live for decades), the experiments can take years to yield results.
Ellison and Gotelli are trying to figure out what evolutionary forces pushed these plants toward a taste for meat. Carnivorous plants clearly benefit from eating animals; when the scientists feed pitcher plants extra bugs, the plants get bigger. But the benefits of eating flesh are not the ones you might expect. Carnivorous animals like ourselves use the carbon in protein and the fat in meat to build muscles and store energy. Carnivorous plants instead draw nitrogen, phosphorus, and other critical nutrients from their prey in order to build light-harvesting enzymes. Eating animals, in other words, lets carnivorous plants do what all plants do: grow by grabbing energy directly from the sun.
Alas, they do a lousy job of it. Carnivorous plants turn out to be very inefficient at converting sunlight into tissue. That's because they have to use a lot of energy to make the equipment they need to catch animals—the enzymes, the pumps, the sticky tentacles, and so on. A pitcher or a flytrap cannot carry out much photosynthesis because, unlike plants with ordinary leaves, they do not have flat solar panels that can grab lots of sunlight. Ellison and Gotelli suspect that only under special conditions do the benefits of carnivory outweigh the costs. The poor soil of bogs, for example, offers little nitrogen and phosphorus, so carnivorous plants enjoy an advantage there over plants that obtain these nutrients by more conventional means. Bogs are also flooded with sunshine, so even an inefficient carnivorous plant can carry out enough photosynthesis to survive. "They're stuck, and they're making the best of it," says Ellison.