Ecologists are very interested in the subject of just who is eating whom in any ecosystem. Yes, some eat plants, some eat other animals, and some eat both. But a plant-eater probably doesn't eat just any plants. Like most of us, animals are typically relatively choosy about just what they'll accept for dinner.
As with so many aspects of ecology, this issue gets more complicated the closer you look. Scientists frequently talk about food chains and food webs. These two ways of looking at the trophic ("eating") relationships within an ecosystem are related to each other.
Food chains dissect out single trails of eating relationships. These are very simplistic, but can be very revealing as well. Here's a little representation of a food chain:
Here we have a plant species being consumed by an herbivorous fish species, which is being eaten by a carnivorous fish species which is being devoured by yet another carnivorous fish species. In eco-jargon, a producer, eaten by a primary consumer, eaten by a secondary consumer, eaten by a tertiary consumer
Of course, this is a very simplistic way of looking at the eating relationships in almost any ecosystem. It's a very rare species which eats only one thing. Most herbivores eat a variety of different kinds of plants; most carnivores eat a variety of different kinds of animals. And then there are omnivores, who happily munch away on either. So though this food chain shows us a very interesting set of specific predator to prey relationships, it's far from an adequate diagram of the whole set of such relationships within an ecosystem.
A better attempt at that holistic examination of a community's trophic interactions would be a food web. It's virtually inpossible to really construct the complete food web of any ecosystem--there are simply too many species, and too many organisms to sort out--but nevertheless, a food web can tell us a great deal about the ecosystem.
Here's a small demonstration of a food web:
In this little depiction, we have several plant and fish species. Following the arrows, which point from eaten to eater, we can see that the actual trophic relationships among the creatures in this ecosystem are much more complex than our little food chain could show. Each of the fish is seen to have several different food choices. And some of the predators are occasionally prey. In some cases, which species consumes which may depend upon who successfully sneaks up on whom.
If you can imagine something like this diagram, but including all of the dozens and dozens of species in a typical ecosystem, you can begin to get an appreciation of the overwhelming complication of the tasks that ecologists attempt to perform.
As mentioned above, food chains and food webs serve the purpose of allowing us to examine the specific, species-to-species relationships among the organisms in an ecosystem. While this is a very important way to understand the trophic structure of the community, it is also important to understand the larger picture. Ecologists are very interested in the distribution of energy and biomass among the different components of the trophic structure. To see how this is done, read the essay on Trophic Pyramids.
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