Stalking the green meat eaters: why ecologists love these toothless predators.

Ecologist Nick Gotelli walks on water, and he Lappears to have every confidence that his two visitors soon will too. A man who speaks and moves with precision, he sounds plausible. It s easy to overlook slightly alarming details about the field trip that he's leading for his two visitors, an ecologist from Spain and a reporter.

He's taking his guests to see his study subjects: meat-eating pitcher plants. They absorb nutrients from ants or other little animals that slip into their pitcher-shaped leaves and drown in the liquid at the bottom. The plants that Gotelli studies spend their lives, some longer than 50 years, rooted in mats of sphagnum moss that float on wetlands like soggy, giant sponges. The moss mats can build up to such a cushy, buoyant thickness that, Gotelli says, a person can walk out onto them and admire the plants at will.

Gotelli's visiting colleague, Fernando Maestre of King Juan Carlos University in Madrid, is spending the summer working with bogmeister Gotelli at the University of Vermont in Burlington even though Maestre specializes in very dry places. Both researchers, Gotelli explains, are developing statistical methods for analyzing communities. He says that pitcher plants make good organisms for studying ideas that apply to many kinds of communities.

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Gotelli and other researchers are designing such studies on several levels. Bogs hosting pitcher plants offer nicely isolated systems for studying the impact of environmental change. And Gotelli's also working on the habitat within pitcher plant leaves.

Death traps for some visitors, these pools are also home to aquatic creatures that stay very much alive. Ecologists use the pools for experiments that they could otherwise do only in their dreams. Researchers add top predators or diminish the habitat with a few dips of a pipette, something that they could never do with lions on the Serengeti or acreage in Yellowstone.

A pitcher plant is "an ecosystem you can hold in the palm of your hand," says Gotelli.

PUDDLES Gotelli is leading the way to an unspoiled gem of a bog with plenty of northern pitcher plants (Sarracenia purpurea). The landscape looks green enough, but now we've turned down a farm road and eased to the edge of woods beside a cultivated field. Gotelli gets out. A little way into the woods lies the bog, he says. But shouldn't things be, well, wetter?

No, and that's part of the beauty of this kind of bog, he explains. As glaciers scoured this landscape 10,000 years ago, they gouged big holes in otherwise firm ground. No rivers ran into or out of them. Filled with rainwater and snowmelt, they became water worlds virtually isolated from the rest of the local hydrology. The bog we are visiting is "like a sealed bowl" Gotelli says.

Its isolation made the bog a good field site for a study that Gotelli and Aaron Ellison of Harvard University have done of humanity's nitrogen excesses. In recent decades especially, farming and the burning of fossil fuels have added nitrogen in biologically reactive form to the air and water.

Earlier studies of the impact of excess nitrogen on forests and streams were complicated by the fact that these ecosystems get nitrogen through many different routes. In the bog, almost all the extra nitrogen arrives via rain and snow. From 1998 through 2000, Gotelli and Ellison used the nitrogen-absorbing power of pitcher leaves as a handy way to tinker with nutrients.

When a new pitcher leaf formed on one of their test plants, the researchers pipetted out its natural pool of liquid and replaced it with one of nine watery solutions dosed, for example, with extra nitrogen or with nitrogen plus phosphorus. Every 2 weeks, each pitcher got a refill. Gagging the plants by stuffing a wad of glass wool in the mouths of the leaf traps, the researchers kept insects from complicating the experiments by adding more nitrogen to the mix.

Gotelli and Ellison measured how much plants grew and reproduced on various nitrogen diets and then created a computer model to predict pitcher plants' futures under various nitrogen regimens.

"Nitrogen is a fertilizer--in some respects, it's a good thing," Gotelli says. But he and Ellison found that more is not better for pitcher plants. Boosting nitrogen pushed up the death toll among young plants. The researchers predicted that even a 1 percent annual increase in extra nitrogen in the environment would raise a substantial risk of the population going extinct within the next century.

STOPPING BY WOODS Gotelli pushes into low, shrubby woods with black spruce and larch trees almost dense enough for their branches to interlock like Velcro. Dead tree trunks have wedged aslant among the living. We work our way about 10 feet forward, and Gotelli suggests that he push ahead alone to search for the path.

While Gotelli threads his way into the foliage, Maestre tells me that in his dry-land work, he comes across occasional carnivorous plants. They're unusual, according to a specialist I call later, Barry Rice of Davis, California, conservation chair of the International Carnivorous Plant Society. Meat-eating plants typically grow in wet places with poor nutrition but plenty of sunlight. Because their leaves have to assume shapes that work as traps, "they suck at capturing light," says Rice. So pitcher plants don't have an edge over others, except in fringe habitats.

An estimated 500 to 700 plant species around the world kill and eat meat to some degree. Asked how many pitcher plant species North America has, Rice says, "It depends on who you want to get into a fistfight with." Answers range from 8 to 12.

The one that Gotelli studies grows from Canada to New Jersey. Pitcher plants can't stand too much competition from other plants, Gotelli explains--a point that becomes clear during our trip through the woods. As we clamber toward his voice, we see life aplenty: lichen beards on twigs, ankle-high bunchberries, waist-high blueberry bushes, and, at last, a continuous, sort carpet of lettuce-green sphagnum moss. But in this dense jumble, we don't see pitcher plants.

Finally, we step into an almost perfectly circular clearing. The dimpled carpet of sphagnum rolls out of the woods and onward some 30 feet ahead of us to the bog's central circle of open water. The moss turns sunburn red, and hardly anything grows taller than our knees. Only out here--in the full sun and amid the squelching moss where most other plants would drown--do we finally sec pitchers.

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On each plant, pitchers grow in rosettes, presenting irregular circles of potbellied vases. Each is leathery green, red veined, and lightly furred with pale, downward-pointing hairs.

Exotic looking they may be, but they're not particularly deadly, says Gotelli. In the 1990s, Sandra Newell of Indiana University of Pennsylvania and a colleague videotaped pitchers and found that they actually trapped fewer than 1 of 100 of their insect visitors.

As we pick our way among the pitchers and a scattering of young, ankle-high cranberry plants, our rubber boots sink into the moss as if we're walking in mud. I ask Gotelli to point out where the solid ground will give way to the floating mats. "Back there," he says. "See. You can jump up and down here, and the sphagnum will bounce." He demonstrates. The moss around him heaves and wobbles in slow motion. I start to ask how deep the water is below us but decide the question can wait.

LEAVES STAY I admire the pitchers Gotelli is pointing out. One leaf swells into a plump, full-throated tube. The other's just a skinny vase with a flattened frill, or keel, down its side.

When Gotelli and Ellison were tinkering with the nutrient supplies in pitcher pools, they found that extra nitrogen decreased the proportion of plump pitchers to thinner-leaf tubes that catch light better, starting even in the first season of the test. "We were surprised to sec the response so rapidly," he says.

With more nitrogen available from the environment, plants need less from insects. As a result, the plants make smaller pitchers or even fiat leaves with no pitcher at all, Gotelli and Ellison reported in 2002. Ifs not a conscious decision, of course, but the plants start investing in flatter leaves that capture sunlight better. "It's almost like thinking about the plant in terms of an animal," says Gotelli.

In a sense, the northern pitcher plant is part animal. To show us, Gotelli whisks out a pipette and draws the liquid out of a pitcher, emptying it into a petri dish that he's conjured from another pocket. In this liquid, he ex-plains, live small creatures such as insect larvae, rotifers, protozoa, and bacteria. When an unlucky ant expires, the pool's residents break it down, providing nutrition for the pitcher plant.

The water has just a tinge of brown, a hint of tea. He waves the dish under our noses. "It's had dead animals in there, but it doesn't smell, and it's clear," he says. Like all photosynthesizing plants, the pitcher releases oxygen, which helps keep the water fresh.

The denizens of this liquid constitute a miniature ecosystem. Midge and fly larvae and mites rip the carcass of an expired insect into small pieces. The small chunks nourish a community of bacteria, which in turn feeds micropredators such as proto zoa and rotifers. These provide food for the next level of predators, particularly the aquatic larvae of the mosquito Wyeomyia smithii.

"Top predator" may be an odd term for something you can hold in a petri dish, but at least the mosquito larvae are visible to the naked eye. Barely. Gotelli points out squirming beige threads perhaps a quarter of an inch long.

They live what seems to be a precariously specialized life. Adult female W. smithii lay their eggs only in the pools within pitchers. The species achieved notoriety in 2001 when William Bradshaw and Christina Holzapfel of the University of Oregon in Eugene reported finding genetic changes in the mosquito resulting from climate change. As winters become milder, mosquitoes can wait until later in the year, when the days are shorter, to begin hibernation. The length of day that triggers hibernation is a genetic trait, and Bradshaw and Holzapfel found that over the past 30 years, mosquitoes with shorter-day genes have gotten more common.

LITTLE BIG GUYS Gotelli and Ellison have devised a string of experiments that rely on manipulating the contents of pitcher plant pools. In this way they plan to explore the way in which a variety of organisms comes together to form an ecosystem, with implications far beyond this particular case.

For example, the researchers can shrink a habitat simply by reducing the amount of liquid in a pitcher. That holds lessons for what happens to birds and bears forced to live in the narrowing confines of forests falling to loggers. To explore the importance of losing big predators, on the other hand, the researchers plucked the W. smithii mosquito larvae and other top predators from the pitcher liquid.

After altering pitcher ecosystems and letting them develop over a growing season, the researchers counted the remaining residents and compared the numbers with predictions from ecological models. Even though habitat shrinkage gets a lot of attention in the larger world, models that took it as the most important factor didn't make successful predictions. Models made better predictions when they emphasized interactions of the food web, for example, showing that a loss of top predators lets midlevel ones boom, Which in turn overeat and cut the numbers of their prey.

Gotelli starts to look for one of the animals that specializes in eating pitcher plants, a moth whose caterpillars feed on the leaves. I start to move but my leg won't budge. While I was listening, I've sunk almost up to my knees in the sphagnum. "Oh, it's a good idea to keep moving around," says Gotelli. Walking on water isn't so hard. But standing still ...