Ecosystems contain essentially two kinds of commodities: matter (nutrients) and energy. Material nutrients cycle through the biotic and abiotic parts of the ecosystem, available for repeated use by the organisms in the ecosystem's community. These cycles of use and reuse are called biogeochemical cycles.
Energy is a different story. Energy enters an ecosystem by being used to convert low-energy carbon dioxide into high-energy carbohydrate, then passes through one or more of the organisms of the community, and is then lost to the ecosystem. Eventually, all of the energy that enters the ecosystem is lost in the form of heat.
To study and explain the movement of energy through an ecosystem, ecologists concentrate on issues of just who is eating whom. There are a variety of terms used to identify the position of a species within the eating relationships of a community.
First, we divide organisms that consume other organisms into a variety of different "vores." "Vore" means "eater." These words are generally pretty familiar. Carnivores eat pretty much only meat [carni=muscle, which is what meat is made from]. Herbivores eat pretty much only plants or plant-like materials [herbi=plant]. Omnivores (like us) eat both animal and plant material on a pretty regular basis [omni=everything]. Then there are the detrivores. They take care of all kinds of waste materials [detri=garbage].
A rather different (and often more useful) system divides all organisms into producers and consumers. The substance being "produced" or "consumed" is carbohydrate (sugars and related molecules). Producers are also often called autotrophs [auto=self; troph=eater or feeder], and consumers are often called heterotrophs [hetero=other or different]. The most familiar producers to us are the photosynthesizers [photo=light; synthesize=build]. Though most of us automatically think "plant" when we think of photosynthesis, there are many organisms which aren't technically plants which also photosynthesize. Even some bacteria photosynthesize. In fact, the majority of photosynthesis on this planet is performed by algae, not by plants.
It's generally surprising to folks to discover that not all autotrophs are photosynthesizers. Over the last few decades, the importance of chemosynthesis has become increasingly obvious. We still don't know just how significant chemosynthesis is in the biosphere overall, but we do know that it is important. Chemosynthesizers are organisms which can produce carbohydrates by using chemical energy, rather than light energy. There was a time when a biology teacher could say with confidence that all ecosystems on Earth ultimately depended on light energy. We now know that this isn't true. Entire ecosystems exist along deep, midoceanic ridge zones supported not by photosynthesis, but by chemosynthesis. None of the energy in these ecosystems comes from light. There is a very interesting and reasonably well supported suggestion that the earliest life forms on the planet may have arisen in environments like these deep sea vent regions.
Consumers come in a variety of types, typically called primary, secondary, tertiary and quaternary consumers. The differences among these different kinds of consumers are tied to exactly what they eat. Primary consumers pretty much eat only plants. Animals like rabbits, horses and cows are exclusively primary consumers. So this kind of "troph" is roughly equivalent to the herbivores described above. However, none of these trophic levels precisely corresponds to any of the "vores." The herbivores/primary consumers come the closest, but an omnivore is also sometimes a primary consumer. as is a detrivore.
Secondary consumers eat primary consumers. A lion that kills and eats a zebra is a secondary consumer. When you consume a nice, juicy T-bone steak, you are being a secondary consumer.
Tertiary consumers eat secondary consumers. If a bear eats a fish, which has been feeding on algae, the bear is functioning as a tertiary consumer. The fish was a primary consumer; the algae were producers.
Quaternary consumers eat tertiary consumers. So if that bear ate a fish which had been eating bugs which had been eating algae, the bear would be a quaternary consumer. Or if you ate the bear from the previous paragraph, you'd be functioning as a quaternary consumer.
One observation that should have leapt to your mind as you read these descriptions is that it's clear that a single organism may easily function at a variety of "trophic levels." You, for instance, can function as anything except a producer. Almost any carnivore can serve at any trophic level above primary consumer. That lion we used as a secondary consumer example could easily also be a tertiary consumer at his next meal, for instance. It's all a matter of what he's eating right now. In one meal, a human often occupies several different trophic levels. Let's say you start out with a salad (primary consumer for the lettuce, shaved carrots, cucumbers, secondary for the chopped chicken), followed by a lovely salmon steak (you're a tertiary consumer here), and two veg (primary again). A bit of cherry pie for dessert--again primary consumer.
This spotlights the big, big problem that faces any ecologist. Nothing is simple. In order to attempt to understand how ecosystems work, ecologists must simplify, sometimes to the point where they may be ignoring signficant information. Scientists attempt to make sense of the trophic relationships and energy transfers within an ecosystem via simplifications like food chains and webs and trophic pyramids.
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Updated 25 September 2004