The Processing Plant

Bugs that fall into a purple pitcher plant get drowned in acid. Their carcasses are then ground up by a microscopic disassembly line: a chain of insect larvae that thrive in the pitcher pool, cooperating to feed themselves--and the plant.

By Carl Zimmer|Friday, September 01, 1995
Not all ecologists spend their days in the presence of nature’s majesty, contemplating redwood groves or mountain lakes or rain forest canopies. Since 1988, Stephen Heard has been peering into a gruesome little roadside attraction called the purple pitcher plant. This bog dweller is actually quite charming to look at--it has a nodding brick-red bell of a flower and purple-veined leaves that curve in on themselves to form scalloped pitchers--but what Heard is interested in are the plant’s feeding habits. The Venus flytrap screams, ‘I’m eating insects!’ says Heard, a Canadian ecologist now at the University of Iowa. The pitcher plant does not. But if you look closely, you can see that inside the leaf are hairs that all point down. Once you, as an insect, start wandering down, it’s hard to turn around. Below those hairs is a slick, slippery band, and when you step onto that--whish, you slide down.

The sweet smell of nectar had attracted you as an insect, but instead of drinking that nectar you end up plummeting into the pool of rainwater at the bottom of the pitcher. The rainwater is not pure: the plant secretes chemicals into it that cause the water to coat your body like oil, and it pumps out protons until the water is as acidic as vinegar. Before long you drown. Then you become food for the pitcher plant.

And not just for the pitcher plant. Unbelievably, the larvae of three different insect species--a fly, a midge, and a mosquito--somehow manage to thrive in the lethal acid bath inside the pitcher. In fact, they exist only in the pitcher plant, where they form their own tiny ecosystem. This is what Heard studies. He has discovered that the larval insects feed in a peculiar way on the carcasses of adult insects that tumble into the pitcher. Rather than fighting over the food all at once, they go about eating in assembly-line fashion, with one species waiting until the food has been properly prepared by another. Last in line, apparently, is the pitcher plant itself.

Life in a bog forces pitcher plants to eat flesh. The ground is so acidic and waterlogged that bacteria can’t finish the job of breaking down dead plant matter into a nitrogen-rich soil; a far less nutritious peat forms instead. As a result, the pitcher plant must draw its nitrogen from animal prey--just as other bog plants do, such as the Venus flytrap, the sundew, and the bladderwort. Those other carnivores, though, don’t share their catch with a microcosm of larvae; they do the digesting themselves, with enzymes. When Heard first started wandering into bogs, as a University of Pennsylvania graduate student in search of a research topic, he realized that the pitcher plant’s complex life might have several dissertations to offer. He began by taking measurements of hundreds of plants in Gros Morne National Park in Newfoundland.

The larvae inside a Newfoundland pitcher plant, he soon found, are finely tuned to the plant’s life cycle. Each leaf lives only about a year, and that’s how long the larval stage lasts, too. In early July adult flies, midges, and mosquitoes seek out new pitchers in which to deposit their young. The larvae feast on the carcasses of other insects until the days become short and it’s time to prepare for winter. Then they fast, so that when the plant is buried in snow and the pitcher pool freezes solid, no particle will remain in their digestive systems to sprout ice crystals and burst their guts. In spring the insects thaw out and start eating again. By early July they reach adulthood and fly off in search of a mate and a new pitcher to hold the next generation.

All three pitcher residents follow this cycle, but their behavior as larvae is quite distinct. For the half-inch-long flesh fly larva, childhood is often a brutal experience. If two maggots find themselves floating in the same pitcher, they wrap their legless bodies around each other and wrestle until one manages to drown the other. (The flesh fly doesn’t have the luxury of laying defenseless eggs, as most flies do; it must give birth to live maggots that are ready for battle.) The lone surviving maggot floats at the top of the pitcher pool, where it chews on floating insect carcasses. Meanwhile dozens of midge and mosquito larvae, around a quarter-inch long, lurk underwater, noshing on carcasses or carcass parts that escape the maggot.

Early in his research Heard became fascinated with those midges and mosquitoes. When you see two animals that get the same resource, if you’re not careful, you might think, ‘They ought to be competing with each other,’ he says. But while a midge and a mosquito might both relish eating an ant, they attack it in completely different ways. The midge has sharp mandibles that allow it to tear right into a carcass. The mosquito’s utensils, on the other hand, are broomlike hairs; it has to wait until the ant has turned into ant particles that it can sweep into its mouth. An ant in pitcher water will gradually disintegrate on its own, but the midges, Heard noticed, seem to greatly accelerate the process. They do not have perfect table manners (It’s like kids eating Oreos, says Heard), and the crumbs they let fall, as well as the droppings they excrete, fall to the mosquitoes. The mosquitoes feed on those particles, and above all on the bacteria that grow on the particles’ surface.

Thus although the midges and mosquitoes eat the same food, it looked to Heard as if they don’t compete, because the mosquitoes depend on the midges to process the food first. After his initial summer at Gros Morne, Heard decided to do the experiments necessary to see whether this odd arrangement was really in effect. Some of his experiments involved real pitcher plants--for instance, he sprinkled extra mosquito larvae into pitchers to see what effect they had on the midges. But he also made artificial pitchers out of plastic tubes, planting them in a bog and filling them with real pitcher water; that allowed him to run the experiment with a fixed number of midges or mosquitoes. In both types of experiment, after letting the larvae feed for anywhere from a week to an entire season, Heard removed the pitchers from the bog and brought them back to his lab. There he measured how many bacteria were in each pool--an indication of how many food particles there were--and counted how many midges and mosquitoes had survived. Finally he weighed the bugs to see if his manipulations had left them thin or plump.

The natural and artificial experiments told the same story. The more midges there were in the pitcher pool, Heard found, the faster dead insects turned into particles and bacteria, and the more fat and prosperous the mosquitoes became. If the mosquitoes were somehow beneficial to the midges in return, this would have been a case of symbiosis. Or the mosquitoes might have harmed the midges--they might have been parasites. But the mosquitoes in Heard’s experiments had no effect at all on the number or weight of the midges. This fit his hypothesis: since mosquitoes must wait to eat until midges have mashed up the carcasses, there is no way they can interfere with the midges’ meal.

Heard dubbed what he was seeing--one animal species preparing food for another--a processing chain. If you don’t have a word for a concept, you can’t really talk about it, he says. That’s my excuse for inventing a new term in a field that already has all kinds of jargon and doesn’t really need much more. It didn’t take him long, though, to realize that he had given a name to something that had been floating anonymously around ecology for a long time. Some wolf hauls down a moose and eats half of it, leaves the carcass, and then scavengers pick over the remains, says Heard, offering the most obvious example.

Another example he likes involves Swiss cheese. When you make Swiss cheese, you basically use two different bacteria to break down the milk sugars, Heard says. The first bacterium takes lactose and converts it into lactic acid. Then a second bacterium converts that into propionic acid and carbon dioxide. Propionic acid gives the cheese some of its flavor, and the carbon dioxide, of course, gives it those big bubbles. The basic idea is the same as in the pitcher plant: an upstream consumer, as Heard calls it, is processing a food, taking some of the energy out of it and leaving it in a form that is accessible to a downstream consumer.

Heard now has dozens of examples of processing chains. The hard numbers he got from his work on pitcher plants have allowed him to come up with a set of equations, in the form of a computer model, that can be applied to any such chain--and in particular to the question of whether the chain is doing the downstream animal much good. It is by no means a guaranteed boon. After all, if the upstream consumer is thorough, not much food will make it downstream. If you’re a scavenger, are you better off having lots of predators that take down prey, or are you better off waiting for the prey to die naturally? asks Heard. That’s the trade-off.

In the case of the pitcher plant mosquito, the trade turns out to be a good deal. If you’re a mosquito, you get the benefit of the extra processing the midge is doing right now, explains Heard. In principle you pay the cost later; the cost is stuff that the midge is consuming that otherwise might get processed by other means later. But the mosquito can’t wait for later--that is, for insect carcasses to disintegrate on their own. By its first birthday it must metamorphose into an adult and flee the dying pitcher leaf in search of a mate. The midge provides food when the mosquito needs it, in exchange for a payback time that never comes. It’s like borrowing from the bank, and before it’s time to pay back, the bank closes down or you fly off to Venezuela, says Heard.

There’s no reason, however, to assume that all processing chains offer such favorable terms to the downstream consumer. Heard thinks ecologists may have reported benefits in some cases simply because they haven’t watched the chain long enough to see payback time. In the case of the processing chain he’s now beginning to study, the issue is not just academic. Jobs may depend on it.

In forested streams, you have a lot of production coming from the breakdown of dead leaves that fall into the stream, says Heard. You have this class of insects called shredders, like stone fly larvae, that feed on leaves by chewing them and making little particles--just like my midges. Then there is another class called collectors, like mayfly and blackfly larvae, that feed on the particles. And finally there are larger animals that eat both classes of insect larvae--economically important animals like salmon and trout.

There’s a fair bit of interest in trying to fertilize streams to improve fish production, says Heard. The question is, how do you do that? The fish are eating insect larvae, so how do we get more insect larvae? If you improve the habitat for shredders, maybe by throwing more leaf litter into the stream, then what will be the net effect on fish? The current idea would be that if you did something good for shredders, it would also be good for collectors, and ultimately for fish too. But that may or may not be true; it could also be a negative interaction. The theoretical work I’ve done gives us the tools to do the experiment correctly to find out.

Until now Heard has kept his experiments simple by looking at the shortest possible processing chain, with only two links. But many chains have three or more. The trout stream is one example, and the pitcher plant itself is another: that chain doesn’t start or end with midges and mosquitoes. There’s also the flesh fly, which feeds on carcasses while they’re still floating on the top of the pitcher, says Heard. It eats some of that stuff, and the rest of it sinks faster down to the bottom, where the midge can get to it. And the midges and mosquitoes produce dissolved nutrients that are taken up by the plant itself. That’s the reason for being carnivorous: it wants things like nitrogen and calcium that are hard to get in a bog. But it can’t do much with just a beetle; I mean, there’s no way to chew it. The plant doesn’t produce enzymes of its own, so it uses the insects to do the job for it. So in fact, it’s a four- level chain.

It’s possible the plant doesn’t benefit from the chain; it might be better off not sharing its catch with the larvae and instead just waiting for bacteria in the pitcher to break down the beetle. Although Heard thinks the processing chain probably does benefit the pitcher plant, proving that would require the same sort of painstaking experiment he did to show that the mosquitoes benefit from the presence of the midges. Heard has moved on to other things. He plans to leave that experiment to the next student who goes Ph.D. hunting in a bog.
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