Let's say you meet a woman on an airplane on a flight from Boston to Brisbane. The two of you are seated next to each other for a couple dozen hours, and you talk the entire time — about books, politics, current events, religion, the weather, etc. You hear some of this woman's personal stories, observe the way she eats and drinks, you watch her play a game on her phone and notice that she snores when she sleeps.
By the time you get to Australia, you feel you've got a pretty good sense of who this person is, but then her entire family shows up to meet her at the airport, and immediately you learn more — and some of the assumptions you made on the plane have to be reevaluated given this new input.
Later, she invites you to visit her at home and her story broadens: The smell of her house, the taste of her drinking water, the view from her porch, the contents of her refrigerator and the setting of her thermostat speak volumes. Some of these details reinforce what you thought you already knew, and some change your mind. At some point, your investigations become, not about the woman herself, but about the whole system in which she lives.
In order to understand anything, it's helpful to understand everything — or, as much as you possibly can. In the study of ecology, the concept of an ecosystem acknowledges the fact that, as 19th century naturalist John Muir said, "When we try to pick out anything by itself, we find it hitched to everything else in the Universe."
But it's difficult to look at everything at once! And natural systems, of all the things we can investigate with science, are particularly hard to nail down. But ecologists are always trying.
In 1935, an English botanist named Arthur Tansley — strongly influenced by Danish botanist Eugenius Warming — introduced the term "ecosystem" in a paper titled "The Use and Abuse of Vegetal Concepts and Terms," published in the journal Ecology. He defined an ecosystem as "the whole system, ... including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment."
The Levels of an Ecosystem
What Tansley was trying to get at was the idea that you can look at a natural system at a bunch of different levels — and there was one level that didn't yet have a name. For instance, you could look at a wolverine — that's a single organism, just like the woman you met on the plane. But that wolverine doesn't live in a vacuum — it lives in a population of other wolverines that interact and organize themselves in specific ways (thus, an ecologist can choose to investigate wolverines at a population level). But that's not all the only way to study wolverines! Ecologists also talk about communities of living things: A wolverine doesn't just interact with members of its own species — it's an omnivore, so it eats other animals like moose and rabbit, as well as berries, roots and eggs. It gets parasites, it digs burrows that affect root systems of plants — a wolverine influences lots of living things in its home territory, and those living things affect it. Tanlsey's definition of ecosystem acknowledged that there was a level of scientific inquiry that could encompass all the organisms in the wolverine's home, in addition to the stuff that's not alive.
"The ecosystem concept ecologists now use has been refined since it was first introduced by Tansley almost a century ago," says Stephen Carpenter, a scientist in the Center for Limnology at the University of Wisconsin-Madison. "Ecosystem science studies the interactions of all the living and non-living entities in a specified place. This definition is consistent with modern concepts of energy, nutrient flow and biogeochemistry, which barely existed during Tansley's career."
The allure of the ecosystem to scientists has to do with the "system" part of the word. An ecosystem like a coral reef runs very similar software to that of the Arctic tundra where the wolverine lives, or a tropical forest. The same basic large-scale processes can apply anywhere: Organic matter decomposes and becomes nourishment for something else in a grassland or a mountain stream; nutrients like carbon, phosphorous, nitrogen and sulphur get passed around like Monopoly money everywhere — it just happens a lot faster and there's a lot more of it in a tropical rainforest than in the desert; diseases are carried along on water or air or by hapless organisms in similar ways, wherever you look; a top predator is removed from the ecosystem on a mountain top in the Andes, and the entire dynamic changes just like it would if you removed all the wolves from Yellowstone National Park in Wyoming.
An Ecosystem Is a Framework
This is to say, ecosystems are a good topic for theory — as a framework for hanging ideas about how complex natural systems work. But, while being a theoretical idea, an ecosystem is also an actual Thing — it's just a Thing without clear boundaries.
According to Eugene Odum's Fundamentals of Ecology, first published in 1953, you know you've gotten to the edge of an ecosystem when more material and energy is cycling within the boundary than crossing over it. So, a riffle in a stream cannot be an ecosystem, because although certain types of fish and aquatic invertebrates like to live in a fast, shallow section of a stream, abundant material is flowing into and out of the riffle all the time. Some might stay in it for a while, but most of it leaves pretty soon after it arrives. Even the sediment and rocks don't stay forever; when they move, it's mostly not inside the riffle, but into or out of it.
On the other hand, watersheds are classic ecosystem boundaries, but they're extremely tricky as well: The river itself is an ecosystem, because although a lot of material and energy passes in and out of it all the time (leaves and soil and dead animals fall in, terrestrial animals use the river as a grocery store), a lot is cycling within it, too. So, although the river in itself can be considered an ecosystem, it's difficult to view the river and the dry land around it as truly separate since material and energy are being exchanged across the very fluid boundary all the time, in both directions (rivers flood, after all, and deposit the nutrient-rich sediment on the land).
Ecosystems, then, are not static.
"The abiotic and biotic are essential parts of the ecosystem, and they have boundaries, albeit human-defined boundaries," says Kathleen Weathers, an ecologist at the Cary Institute of Ecosystem Studies. "And not only do ecosystems have structure and function, but these are controlled by many factors, and that ecosystems change through time."