Biological Communities

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An ecosystem is a complete community of interdependent organisms as well as the inorganic components of their environment; by contrast, a biological community is just the living members of an ecosystem. Within the study of biological communities there are a great number of complexities involved in analyzing the relationships between species as well as the characteristics of specific communities. Yet many of the concepts applicable to biological communities as a whole also apply to human communities in particular, and this makes these ideas easier to understand. For example, the competitive urge that motivates humans to war (and to less destructive forms of strife in the business or sports worlds) may be linked to the larger phenomenon of biological competition. Indeed, much of the driving force behind the development of human societies, as it turns out, has been biological in nature.


The Biosphere, Ecosystems, and Ecology

An ecosystem is a community of interdependent organisms along with the inorganic components of their environmentair, water, and the mineral content of the soiland 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.

Introduction to Biological Communities

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 nicheeven 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.

Evaluating Biological Communities

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 biomasspotentially burnable energyproduced 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.)



The word competition typically brings to mind images of a basketball court or football field, on one hand, or a Wall Street trading floor or board-room, on the other. In biological terms, however, competition is the interaction among organisms of the same or different species vying for a common resource that appears in a limited supply relative to the demand. Put another way, this means that the capability of the environment to supply resources is smaller than the potential biological requirement for those resources.

Scarcity of resources relative to the need for them is one of the governing facts of human life, reflected in such common expressions as "There is no such thing as a free lunch." In fact, humans have spent most of their history in a modified form of biological competition, war. Furthermore, it may well be that our modern predilection for sports or business competition is simply a matter of transforming biological competition into a more refined form. In any case, competition prevails throughout the world of living things.


For example, plants often compete for access to a limited supply of nutrients, water, sunlight, and space. Intraspecific competition occurs when individuals of the same species vie for access to essential resources (later we look at intraspecific competition between humans), or for mating partners, whereas interspecific competition takes place between different species.

Individuals of the same species have virtually identical resource requirements: for example, all humans need food, water, air, and some protection from the natural elements. For this reason, whenever populations of a species are crowded together, intraspecific competition is intense. This also has been illustrated by experiments involving laboratory mice, which become increasingly brutal to one another when confronted with severely diminished resources. When intraspecific competition occurs in dense populations, the result is a process known as self-thinning, characterized by the mortality (death) of those individuals less capable of surviving coupled with the survival of individuals that are more competitive. If this sounds like the "survival of the fittest," an idea associated with evolution that became the justification for a number of nefarious social movements and activities (see Evolution), it is no accident.

Ideas related to intraspecific competition influenced the English naturalist Charles Darwin (1809-1882) in developing his theory of evolution by means of natural selection. Intraspecific competition is an important regulator of population size and can make the species as a whole more fit by ensuring that only the hardiest individuals survive. Likewise, interspecific competition, or competition between species, plays a strong role in shaping ecological communities. Furthermore, competition between species is also an agent of natural selection.


Environmental changes that affect a biological community may change the competitive relationships within it, leading to interesting results. During the early 1950s, a fungal pathogen (a disease-carrying parasite in the form of a fungus) known as chestnut blight, or Endothia parasitica, ended the dominance of the American chestnut in the eastern United States. Up to that time, the American chestnut (Castanea dentata ) had been the leading species in the canopy, or uppermost layer, of the deciduous (prone to seasonal shedding of leaves) forests in the region. Thanks to the chestnut blight, it was as though the winner had been disqualified from a race, meaning that all the runners-up changed their standings.

By being relieved of the stresses associated with competition, other trees were allowed to become more successful and dominant in their habitat. They took advantage of this change to fill in the canopy gaps left by the demise of mature chestnut trees. In the same way, if a wildfire, storm, or other stress disturbs a mature forest, plants that previously have been suppressed by the higher-canopy trees find themselves with much greater access to such resources as light, moisture, and nutrients. As a result, they thrive.

Given the distasteful aspects of biological competition, such as the destruction of the "weak" in favor of the "strong" (actually, less adapted and more adapted are much more accurate terms in this context), one might wish for a situation in which no competition exists. Indeed, there are such situations in nature, but they are far from pleasant. In such biomes as the arctic tundra, for instance, competition is low, but this is not because all nature lives in happiness and harmony; instead, organisms face such powerful environmental stresses from the local climate that competition is not the most significant factor limiting populations.


Creatures on the tundra face little stress from competition, but a great deal of stress in the form of very short growing seasons, thin soil, limited ground cover, low average temperatures and rainfall, high winds, and so on. If the density of individual plants of the tundra is decreased experimentally by thinning, the residual plants do not thrive as a result, as they might in a less harsh climate. In the case of the tundra, it is not competition that constrains their productivity, and therefore the reduction of potential competition does little to improve conditions for the organisms that survive.

It is interesting to observe what happens in a tundra environment if the intensity of environmental stress is artificially and experimentally alleviated by enclosing an area under a greenhouse and by fertilizing it with nutrients. Such experiments have been performed, and the results are fascinating: under these more favorable environmental conditions, competition actually increases, resulting in a biological community not unlike that of a more hospitable biome.

Biological Communities and Civilizations

In his best-seller Guns, Germs, and Steel: The Fate of Human Societies, ethnobotanist Jared M. Diamond showed that local biological communities are among the leading determinants of the success or failure of human civilizations. The book had its beginnings, he wrote, during his many years of work with the native peoples of New Guinea. One day, a young man put a simple question to him: why do the societies of the West enjoy an abundance of material wealth and comforts, while those of New Guinea have so little?

The question may have been simple, but the answer was not obvious. As a scientist, Diamond refused to give an answer informed by the politics of the Left or Right, which might have blamed the problem, respectively, on western exploitation or on the failures of the New Guineans themselves. Instead, he approached it as a question of environment, and the result was his thought-provoking analysis, contained in Guns, Germs, and Steel.


As Diamond showed, the places where agriculture was born were precisely those blessed with favorable climate, soil, and indigenous plant and animal life. Of course, it is no accident that civilization was born in the societies where agriculture first developed. Before a civilization can evolve, a society must become settled, and for that to happen, it must have agriculture.

Agriculture came into existence in four places during a period from about 8000 to 6000 b.c. In roughly chronological order, these locations were Mesopotamia, Egypt, India, and China. All were destined to emerge as civilizations, complete with written language, cities, and organized governments, between about 3000 and 2000 b.c.

In the New World, by contrast, agriculture appeared much later and in a much smaller way. The same was true of Africa and the Pacific Islands. In seeking to find the reasons why this happened, Diamond noted a number of factors, including geography. The agricultural areas of the Old World were stretched across a wide area at similar latitudes. This meant that the climates were not significantly different and would support agricultural exchanges, such as the spread of wheat and other crops from one region or ecosystem to another. By contrast, the landmasses of the New World or Africa have a much greater north-to-south distance than they do east to west, resulting in great differences of climate.


Today such places as the American Midwest support abundant agriculture, and one might wonder why that was not the case in the centuries before Europeans arrived. The reason is simple but subtle, and it has nothing to do, as many Europeans and their descendants believed, with the cultural "superiority" of Europeans over Native Americans. The fact is that the native North American biological communities were far less diverse than their counterparts in the Old World. Peoples of the New World successfully domesticated corn and potatoes, because those were available to them. But they could not domesticate emmer wheat, the variety used for making bread, when they had no access to that species (it originated in Mesopotamia and spread throughout the Old World).

The New World also possessed few animals that could be domesticated either for food or for labor. Every single plant or animal that is a part of human life today had to be domesticatedadapted in such a way that it becomes useful and advantageous for humansand the range of species capable of domestication is far from limitless. In fact, it is safe to say that all major species capable of being domesticated have been, thousands of years ago. The list of animals that can be domesticated is a short one, much shorter than the list of animals that can be tamed. A bear, for instance, can be captured, or raised from birth in captivity, but it is unlikely that humans would ever be able to breed bears in such a way that their wild instincts all but disappeared and they became reliable, useful companions.

The animals that helped make possible the development of farms, villages, and ultimately empires in the Old Worldcows and oxen, horses and donkeys, sheep and so onwere absent from the New World. (Actually, horses had once existed in the Americas, but they had been destroyed through overhunting, as discussed in the context of mass extinction within the Paleontology essay.) Many Indian tribes domesticated some types of birds and other creatures for food, but the only animal ever adapted for laborby the most developed civilization of the pre-Columbian Americas, the Incawas the llama, which is too small to carry heavy loads.


The Europeans' advantage over the Native Americans derived ultimately from the ecological complexity of their biological communities compared with those of the Native Americans. This was also true of the "biological communities" they could not see, and of which people were unaware in 1500: the world of microorganisms, or the "germs" in the title of Diamond's book.

The native peoples of the New World had no natural resistance to smallpox or a host of other diseases, including measles, chicken pox, influenza, typhoid fever, and the bubonic plague. As with many plants and animals of the Old World, they simply had no exposure to these microorganisms. In the Old World, however, close contact with farm animals exposed humans to diseases, as did close contact with other people in crowded, filthy cities. This exposure, of course, killed off large numbers in such plagues as the Black Death (1347-1351), but those who survived tended to be much stronger and possessed vastly greater immunities. Therefore, the vast majority of Native American deaths that followed the European invasion were not a result of warfare, enslavement, or massacre of villages (though all of these occurred as well), but of infection. (See Infection and Infectious Diseases for more on these subjects.)


Even the practice of cannibalism in such remote locations as the New Guinea highlands is, according to Diamond, a consequence of a relatively limited biological community. In the past, westerners assumed that only very "primitive" societies engaged in cannibalism. However, events have shown that, when faced with starvation, even people from European and European-influenced civilizations may consume human flesh in order to survive.

For instance, in 1846 members of the Donner party, making the journey west across North America, resorted to eating the bodies of those who had died in the perilous crossing. Much the same happened in the 1970s, when a plane carrying Uruguayan athletes crashed in the Andes, and the survivors lived off the flesh of those who had died. Though their upbringing and cultural norms may have told them that cannibalism was immoral or at the very least disgusting, their bodies told them that if they did not consume the only available food, they would die.

Whereas these circumstances were unusual and temporary, peoples in some parts of the world have been faced with a situation in which the only sources of protein provided within the biological community are ones whose consumption seems repugnant from the western viewpoint. Discussing the New Guinea highlands, Diamond noted that the area is virtually bereft of protein sources in the form of large, nonhuman mammals. Nor is bird life sufficient to support the local populace, and marine food sources are far away. For this reason, natives are prone not only to cannibalism but also to another culinary practice that most westerners find appalling: eating bugs, worms, grubs, caterpillars, and other creepy-crawly creatures.

WHERE TO LEARN MORE The Digital Biology Project (Web site). <>.

DeLong, J. Bradford. "Review of Diamond ," Guns, Germs, and Steel (Web site). <>.

"Designing a Report on the State of the Nation's Ecosystems." U.S. Geological Survey, Biological Resources Division (Web site). <>.

Diamond, Jared M. Guns, Germs, and Steel: The Fates of Human Societies. New York: W. W. Norton, 1997.

Living Resources and Biological Communities (Web site). <>.

Miller, Kenton, and Laura Tangley. Trees of Life: Saving Tropical Forests and Their Biological Wealth. Boston: Beacon Press, 1991.

Nebel, Bernard J. Environmental Science: The Way the World Works. Englewood Cliffs, NJ: Prentice Hall, 1990.

NMITA: Neogene Marine Biota of Tropical America (Web site). <>.

Patent, Dorothy Hinshaw. The Vanishing Feast: How Dwindling Genetic Diversity Threatens the World's Food Supply. San Diego: Harcourt Brace, 1994.

Plant Communities of California (Web site). <>.

Quinn, John R. Wildlife Survivors: The Flora and Fauna of Tomorrow. Blue Ridge Summit, PA: TAB Books, 1994.



A measure of the degree to which an ecosystem possesses large numbers of particular species. An abundant ecosystem may or may not have a wide array of different species. Compare with complexity.


Energy derived from biological sources that are used directly as fuel (as opposed to food, which becomes fuel). Examples of bioenergy include wood or manure that can be burned. Usually, petrochemicals, such as petroleum or natural gas, though they are derived from the bodies of dead organisms, are treated separately from forms of bioenergy.


The living components of an ecosystem.


The increase in bioaccumulated contamination at higher levels of the food web. Biomagnification results from the fact that larger organisms consume larger quantities of foodand, hence, in the case of polluted materials, more toxins.


Materials that are burned or processed to produce bioenergy.


A large ecosystem, characterized by its dominant life-forms.


A combination of all living things on Earthplants, animals, birds, marine life, insects, viruses, single-cell organisms, and so onas well as all formerly living things that have not yet decomposed.


A combination of all flora and fauna (plant and animal life, respectively) in a region.


The upper portion or layer of the trees in a forest. A forest with a closed canopy is one so dense with vegetation that the sky is not visible from the ground.


A meat-eating organism, or an organism that eats only meat (as distinguished from an omnivore).


A theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession.


The range of ecological niches within a biological community. The degree of complexity is the number of different species that theoretically could exist in a given biota, as opposed to its diversity, or actual range of existing species.


Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.


A chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the biosphere, this often is achieved through the help of detritivores and decomposers.


Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers; however, unlike decomposerswhich tend to be bacteria or fungidetritivores are relatively complex organisms, such as earthworms or maggots.


A measure of the number of different species within a biological community.


The study of the relation ships between organisms and their environments.


A community of interdependent organisms along with the inorganic components of their environment.


The flow of energy between organisms in a food web.


A plant-eating organism.


A plant or animal that, by its presence, abundance, or chemical composition, demonstrates a particular aspect of the character or quali ty of the environment.


The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.


A term referring to the role that a particular organism plays within its biological community.


An organism that eats both plants and other animals.


A disease-carrying parasite, usually a microorganism.


The biological conversion of light energy (that is, electro magnetic energy) from the Sun to chemical energy in plants.


Green plants that depend on photosynthesis for their nourishment.


The amount of bio-mass produced by green plants in a given biome.


The progressive replacement of earlier biological communities with others over time.


Various stages within a food web. For instance, plants are on one trophic level, herbivores on another, and so on.