email a friend iconprinter friendly iconCarnivorous Plants
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Evolution has repeatedly made this trade-off. By comparing the DNA of carnivorous plants with other species, scientists have found that they evolved independently on at least six separate occasions. Some carnivorous plants that look nearly identical turn out to be distantly related. Both kinds of pitcher plants—the tropical genus Nepenthes and the North American Sarracenia—grow deep pitcher-shaped leaves and employ the same strategy for capturing prey. Yet they evolved from different ancestors.

In several cases scientists can see how complex carnivorous plants evolved from simpler ones. Venus flytraps, for example, share an ancestor with Portuguese sundews, which only make passive flypaper glands on their stems. They share a more recent ancestor with Drosera sundews, which not only make flypaper glands but can also curl their leaves over their prey. Venus flytraps appear to have evolved an even more elaborate version of this kind of trap, complete with jawlike leaves.

Unfortunately, the adaptations that enable carnivorous plants to thrive in marginal habitats also make them exquisitely sensitive to environmental changes. Agricultural runoff and pollution from power plants are adding extra nitrogen to many bogs in North America. Carnivorous plants are so finely tuned to low levels of nitrogen that this extra fertilizer is overloading their systems. "They eventually burn themselves out," says Ellison.

Humans also threaten carnivorous plants in other ways. The black market trade in exotic carnivorous plants is so vigorous now that botanists are keeping the location of some rare species a secret. Venus flytraps are being poached from North Carolina by the thousands to be sold at roadside stands. The North Carolina Department of Agriculture has been dabbing wild Venus flytraps with harmless dye that's normally invisible but glows in UV light so that inspectors who come across Venus flytraps for sale can quickly determine if the plants were raised in a greenhouse or poached from the wild. But even if the poaching of carnivorous plants can be halted (a very big if), they will continue to suffer from other assaults. Their habitat is disappearing, to be replaced by shopping centers and houses. Fires are being suppressed, allowing other plants to grow quickly and outcompete the Venus flytraps. Good news, perhaps, for flies. But a loss for all who delight in the sheer inventiveness of evolution. 

Carl Zimmer's most recent book is The Tangled Bank: An Introduction to Evolution. Swedish photographer Helene Schmitz shot less assertive plants for a National Geographic story on Carl Linnaeus in June 2007.
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