For insight into fabulously complex ecological dynamics, Harvard University biologist Aaron Ellison peers into the cupped leaves of carnivorous pitcher plants.
At the bottom of each slippery-sided leaf is a pool of water into which unlucky insects fall and drown. The bugs sustain not only the plant, but an intricate food web of bacteria, plankton and invertebrates. Each pool is small enough to fit in a shot glass, and big enough to model the world.
"Each leaf is its own individual lake, its own individual ecosystem. Suddenly, in a bog I can walk to from my office, I've got 50,000 lakes to do experiments on. This is an opportunity to understand how a complete, functioning natural ecosystem works," said Ellison.
Understanding how ecosystems work is an important but challenging task for scientists. Though patterns can be described — nutrient levels shift, an animal population grows, another shrinks — it can be hard to know what's coincidental and what's linked.
If researchers can run experiments on an ecosystem, measuring exactly what goes in and out, tweaking different aspects and seeing what happens, then they can better decipher its underlying rules. That's the idea behind artificial ecosystems, all the way up to the infamous Biosphere II.
However, it's not easy to replicate nature, and it's even tougher to run these experiments in the wild. Many experiments are unethical: you can't take a lion from its home simply to study the effects of top-predator removal. Other experiments are impractical. It's hard to account for every variable in a rain forest.
Ecologists have had some success studying islands and lakes, which are fairly self-contained, and extrapolating those findings to the rest of the natural world. But not everyone is fortunate enough to have an island or lake to study.
"Islands are beloved by ecologists, because they're simplified fractions of the whole complex world. And one way to think about pitcher plants is as a modest-scale island," said Robert Holt, an eminent University of Florida ecologist who's tracked the pitcher plant work.
For the last fifteen years, Ellison and University of Vermont biologist Nicholas Gotelli have slogged through the bogs of New England, studying the life that exists in each pool. At the very base are bacteria, which support phytoplankton and cytoplankton, which support single-celled animals, which support fly larvae. All of it relies on nutrients delivered by drowning bugs.
"You've got four or five trophic levels in a pitcher plant, just like you've got four or five trophic levels in a lake," said Ellison.
Fly larvae are the top-level predator in the pitcher, the analogues of terrestrial tigers or wolves. They're what ecologists call a "keystone" species, who control the abundance every other species, but require a habitat of sufficient size to support those other creatures.
That dynamic is a basic tenet of ecology, but when Ellison and Gotelli quantified it in a 2008 Public Library of Science Biology paper, "it was the first time anyone anyone had actually done the experiment to show the relative importance of habitat size and the presence or absence of top predators on controlling the abundance of all the organisms in a complete food web," said Ellison.
He and Gotelli have also used pitcher plants to study the effects of nutrient overloading. A high-density insect hatch can produce a nutrient surplus comparable to that caused in larger waters by fertilizer runoff or sewage overflows. In both cases, the transition from a rich, multi-level system to one that's oxygen-starved and algae-dominated is the same.
The latest frontier of the pair's research is the dynamics of growth and decomposition, or what ecologists call the "green" and "brown" food webs. "One of the questions that's percolating up in ecology is how you link these," said Ellison. "It's hard to study soil and figure out the pathways of nutrients and energy."
Their description of these processes in pitcher plants, published last year in Ecology, is "the first good example of how you link the green and brown webs, and we can do it experimentally," said Ellison.
The University of Florida's Holt said that some ecosystem processes might be scale-dependent, emerging only at certain absolute sizes. But he thinks other pitcher plant processes — predator-prey interactions, mutually beneficial species, the effects of disturbance — are found across ecosystems.
"Everything that happens in a pitcher plant happens at a larger scale," said Holt. "There's a tremendous amount of information in there."
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Images: 1. Dendroicablog/Flickr 2. University of Karlsruhe
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