28 December 2008

Killer Bamboo

The bamboo forests of the western highlands of China are known throughout the world as the vanishing habitat of the last remaining pandas. With farms and villages now encroaching on its forest territory, the panda, which depends on bamboo as its only food, is fighting a desperate battle for survival. Bamboo is usually seen as merely the passive backdrop for this evolutionary tussle. But Truman Young, an ecologist at the Gallmann Memorial Foundation in Kenya, has put an entirely new spin on the scenario. The squeeze for space, he suspects, has intensified a much older battle that began when pandas took up eating bamboo in the first place. Killing pandas, he suggests, may be the plant’s time-honored strategy for ensuring its own survival.

The clue to bamboo’s insidious nature is the plant’s bizarre sex life. For most of its life, this large woody grass reproduces asexually, by extending new shoots from underground runners. Over the years large areas may consequently be covered by genetically identical clones of single plant. Then, like clockwork, all the individuals in a species go into a sexual phase when they flower, seed, and die. Only the sprouting seedlings survive to recognize the area, but ten or more years may have to pass before they’re a viable food source again. Pandas can survive these die offs if they can switch to another type of bamboo, but as their range has shrunk, so has their selections of bamboo species. When several species go to seed simultaneously, which happened in the mid-1970s in the Min Mountains of northern Sichuan and Gansu, pandas simple starve.

Young asked himself why bamboo plants do this. Bamboo, of course, is by no means the only type of plant or animal to reproduce in sync. The same strategy is used by perennial flowers, by many antelope, and by salmon who clog their native streams when they return to spawn. The idea from an evolutionary standpoint is to glut your predators with more offspring than they can possibly eat, so that at least some of your offspring survive.
Bamboo, however, goes in for an eccentric variation of this evolutionary strategy. It indulges in sexual reproduction at only very long intervals – after 15 years or, in a few species, more than 120 years. Why bamboo waits so long to flower has been a puzzle. “But when you look at it from the bamboo’s point of view,” says Young, “it all makes sense.”

Synchronous mass seedings let bamboo stay one step ahead of predators like rats and insects, which eat the plant’s seeds when they are plentiful but revert to other types of food when the pickings get slim. For bamboo the biggest obstacle to staying alive long enough to flower and seed again is the panda, who insists on eating this plant only.

Young hypothesizes that the bamboo’s peculiar reproductive cycle may have evolved to rid itself of this nuisance. It’s a slow, sly two-part strategy. First, bamboo proliferates asexually, providing pandas with an abundant larder of food, allowing even these notoriously slow-breeding animals to gradually build up their numbers. Then, when the panda population has grown, the bamboo abruptly dies off, leaving the animals on the brink of famine. To make matters worse, the animals that are most likely that are pregnant or nursing, since they have the highest nutritional needs.

Once you have heard this theory, you will never look at bamboo the same way again. Green and slender, deceptively innocent-looking, it spreads out slowly, year by year, until it has its victims surrounded. Meanwhile the pandas, poor patsies, are eating out the bamboo’s hand. Only when the pandas are well and truly dependent on it does the bamboo deal its coup de grace. It flowers and seeds, thus ensuring its own survival as a species. And then, in an act of sweet self-sacrifice, it dies, taking its arch-enemy with it.

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