By Robert Hazen, George Mason University
Organisms never occur in isolation. One of the great principles of biology is that all individuals are part of ecosystems, which are complex communities of organisms and their physical environment; they are intricate collections of living and non-living components. Ecosystems are amazingly varied, but there are six characteristics common to all of them.
Ecological Communities
First of all, all ecosystems are dependent on both living and non-living parts. The physical and chemical environment defines the non-living portions of an ecosystem. The environment includes the weather and the climate; it includes the nature of the local rocks and the soils—the temperature and the salinity of local bodies of water, for example, and so forth and so on.
All of the different species, then, that interact in an ecosystem form an ecological community. Ecological communities may include a great variety of plants and animals, but they always include a host of microbes, one-celled organisms. They have to include organisms that convert sunlight, or some other form of energy, into food.
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Flow of Energy
The second characteristic of all ecosystems is that they all require energy. Energy flows through an ecosystem according to the laws of thermodynamics. The flow of energy through an ecosystem, through the organisms, is called the ‘food web’. Every organism has to obtain energy, either directly from its environment or by eating other organisms.
The concept of the trophic level defines a hierarchy of energy producers and consumers in every ecosystem. A trophic level includes all the organisms in an ecosystem that get their energy from the same source. We have plants, for example, which get their energy from the Sun by photosynthesis. They constitute the first trophic level; they are the energy producers in most ecosystems.
However, we also have those deep-ocean environments, where microbes that use the chemical energy of rocks are the primary producers. In those ecosystems, it’s the microbes that are the primary, or the first, trophic level. The second trophic level includes herbivores; that includes all the animals that eat plants. Then come carnivores that eat herbivores, in the third trophic level. We also have bacteria and scavengers in many ecosystems that eat dead organisms, and they form yet another trophic level.
At each trophic level, most of the available energy goes unused. Plants use only a few percent of the Sun’s radiant energy that comes down and hits them. Herbivores harvest perhaps only 10 percent of the energy available in plants; and carnivores, similarly, use maybe only 10 percent of the energy of the animals that they eat. The ratio of biomass of plants to herbivores to carnivores is typically about 100:10:1. This is why large carnivores are relatively rare: they get much less energy than the herbivores, and the herbivores get less energy than the original plants.
Recycling of Matter
The third characteristic of all ecosystems is that matter is constantly recycled; the atoms are used over and over again. This is just like any geochemical cycle. The atoms and molecules aren’t destroyed; they’re just reused, recycled.
Let us illustrate this idea by the history of a carbon atom. We can pass the carbon atom around from person to person, and see where it goes. A carbon atom could start as a gas molecule, for example, a molecule of CO2. That CO2 molecule could be taken up by a plant and used to construct a blade of grass. We might have a cow come along, and it eats that blade of grass, and the carbon atom is used to make some of the milk that the cow produces. We might drink that milk in our breakfast coffee, and that carbon might now be part of our tissues. Indeed, that carbon atom could remain in our body for the rest of our life; but eventually, we’re going to die, and that carbon atom is going to return to Earth, and it will continue its endless cycle, to be used over and over again.
Ecological Niche
The fourth characteristic of every ecosystem is that every organism in an ecosystem occupies an ecological niche. An ecological niche represents a specific strategy for obtaining energy and atoms from the environment.
If we have an insect that lives off the sap of bark in a specific kind of tree, that’s an ecological niche. A worm that scavenges in shallow soil has its own ecological niche; so does a plant that thrives under the shade of a larger bush. Every organism has to compete for resources in its ecological niche. In general, therefore, an ecosystem doesn’t support two species in identical ecological niches.
Ecologists were puzzled once when they found two very similar species of birds living in the exact same tree in a forest; this is an example of violating the idea of ecological niches being occupied by single species. When the ecologists looked more closely, they found that one bird lived in the lower branches and ate one kind of insect, and the other species lived in the high branches and ate a different kind of insect. So, these were separate ecological niches, even though they were in the same tree.
Stability and Disruption in Ecosystems
The fifth characteristic of ecosystems: populations of different species achieve a balance in a stable ecosystem. This situation is a consequence of the fact that matter and energy are limited resources; they have to be shared by all the individuals in any ecosystem. While species’ populations may vary with changes in weather, or food supplies, from year to year, the relative size of populations usually remains quite similar from year to year.
The sixth and last of these characteristics is that a change in environment, or the introduction or loss of a species, can disrupt an ecosystem. This disruption can be gradual, as in a climate change, or it can be quite sudden and quite dramatic.
Common Questions about Ecosystems and their Six Characteristics
All the different species that interact in an ecosystem form an ecological community. Ecological communities may include a great variety of plants and animals, but they always include a host of microbes, one-celled organisms.
All ecosystems require energy. The flow of energy through an ecosystem, through the organisms, is called the food web.
A change in environment, or the introduction or loss of a species, can disrupt an ecosystem. This change can be gradual or sudden.
