An ecosystem is a community of interdependent organisms along with the inorganic components of their environment—air, water, and the mineral content of the soil—and a biome is an ecosystem, such as a tundra, that extends over a large area. All living organisms are part of a larger system of life-forms, which likewise interact with large systems of inorganic materials in the operation of a still larger system called Earth. (The concepts of ecosystem, biosphere, and biome are discussed, respectively, in Ecosystems and Ecology, The Biosphere, and Biomes.)
Note the importance of the distinctions between living and nonliving and organic and inorganic. Although in popular terms, organic means anything that is living as well as anything that was once living, along with their parts and products (bones, leaves, wood, sap, blood, urine, and so on), in fact, the scientific meaning of the word is both much broader and more targeted. A substance is organic if it contains carbon and hydrogen, and thus organic materials also include such items as plastics that have never been living.
The study of the relationship between living things and their environment, pioneered by the German zoologist Ernst Haeckel (1834-1919) and others, is called ecology. Though the world scientific community was initially slow to accept ecology as a subject of study, the discipline has gained increasing respect since the mid-twentieth century. This change is due to growing acceptance for the idea that all of life is interconnected and that the living world is tied to the nonliving, or inorganic, world. On the other hand, there is also the gathering awareness that certain aspects of industrial civilization may have a negative impact on the environment, an awareness that has spurred further interest in the study of ecology.
The term biological community refers to all the living components in an ecosystem. A slightly different concept is encompassed in the word biota, which refers to all flora and fauna, or plant and animal life, in a particular region.
For the biological community to survive and thrive, a balance must be maintained between consumption and production of resources. Nature provides for that balance in numerous ways, but beginning in the late twentieth century students of ecology in the industrialized world have become more and more concerned with the possible negative impact their own societies exert on Earth's biological communities and ecosystems.
It should be noted, however, that nature itself sometimes replaces biological communities in a process called succession. This process involves the progressive replacement of earlier biological communities with others over time. Coupled with succession is the idea of climax, a theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession. (See Succession and Climax for more about these subjects.)
Whereas climax and succession apply to broad biological communities, a niche refers to the role a particular organism or species plays within that community. Though the concept of niche is abstract, it is unquestionable that each organism plays a vital role and that the totality of the biological community (and, indeed, the ecosystem) would suffer stress if a large enough group of organisms were removed from it. Furthermore, given the apparent interrelatedness of all components in a biological community, every species must have a niche—even human beings.
An interesting idea, and one that is somewhat similar to a niche, is that of an indicator species. This is a plant or animal that, by its presence, abundance, or chemical composition, demonstrates a particular aspect of the character or quality of the environment. Indicator species, for instance, can be plants that accumulate large concentrations of metals in their tissues, thus indicating a preponderance of metals in the soil. This metal, in turn, could indicate the presence of valuable deposits nearby, or it could serve as a sign that the soil is being contaminated. (See Food Webs for more about indicator species.)
Another concept closely tied to the concept of niche is that of symbiosis. The latter refers to a biological relationship in which (usually) two species live in close proximity to one another and interact regularly in such a way as to benefit one or both of the organisms. Symbiosis may exist between two or more individuals of the same species, as well as between two or more individuals representing two different species. The three principal varieties of symbiosis are mutualism, in which both participants benefit; commensalism, in which only one participant benefits, but at no expense to the other participant; and parasitism, in which one participant benefits at the expense of the other. These subjects are covered in much greater depth within the essays on Symbiosis and Parasites and Parasitology.
A few billion years ago, Earth's oceans and lands were populated with just a few varieties of single-cell organisms, but over time increasing differentiation of species led to the development of the much more complex ecosystems we know now. Such differentiation is essential, since the life forms in a particular region must adapt to that biome, whether it be forest or grassland, desert or aquatic environments, mountain setting or jungle.
Diversity is a measure of the number of different species within a biological community, while complexity is the number of niches within it. Put another way, the complexity of a community is the number of species that could exist in it. Abundance is the measure of populations within individual species; thus, if a biological community has large numbers of individuals, even if it is not diverse in species, it is still said to be abundant.
During its brief summer growing season, the arctic tundra has vast numbers of insects, migratory birds, and mammals, and thus its abundance is high, whereas its diversity is low. On the other hand, a rain forest might have several hundred or even a thousand different tree species, and an even larger number of insect species, in only a few hectares, but there may be only a few individuals representing each of those species in that area. Thus, the forest could have extremely high diversity but low abundance of any particular species. Needless to say, the rain forest is likely to have a much greater complexity than the tundra, meaning that it is theoretically likely to contain far more species.
Another way to evaluate ecosystems is in terms of productivity. Productivity refers to the amount of biomass—potentially burnable energy—produced by green plants as they capture sunlight and use its energy to create new organic compounds that can be consumed by local animal life. Once again, a forest, and particularly a rain forest, has a very high level of productivity, whereas a desert or tundra ecosystem does not.
Food web (in contrast to the more popular, but less correct term, food chain) is the designation preferred by scientists to describe the means by which energy is transferred through a biological community. Within the food web are various stages, called trophic levels, that identify the position of various organisms in relation to the organisms they consume and the organisms that consume them.
Green plants that depend for their nourishment on photosynthesis, or the biological conversion of electromagnetic energy from the Sun into chemical energy, are primary producers. Herbivores, or plant-eating creatures, are primary consumers, whereas the animals that eat herbivores (whether carnivores or omnivores) are secondary consumers. The largest carnivores and herbivores are usually not prey for any other creatures, but when they die, they, too, will be consumed by detritivores, or scavengers, as well as decomposers, such as bacteria and fungi.
The second law of thermodynamics, one of several laws governing energy and the systems in which that energy is applied, holds that in each energy transfer some energy is lost. In the case of food webs, this means that much of the energy in each trophic level is unavailable to organisms at the next level. This, in turn, means that each successive trophic level generally has far fewer members than the prey on which they feed. While there might be thousands of primary producers in a particular community, there might be only a few top predators, including humans. (See Food Webs for more on these subjects.)