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LIVING ORGANISMS INTERACT

As you may recall from Chapter 3, the term "com­munity" has a far broader sense in ecology than in everyday speech. For the biologist, a community is an association of organisms of different species liv­ing and interacting together (see Focus On: The Five Kingdoms of Life for an overview of the cate­gories of organisms found in communities). Thus, you, your dog, and the fleas on your dog are all members of the same community! You could also add to the list cockroaches, silverfish, dandelions, grasses, maple trees, and much more.

Communities vary greatly in size, lack precise boundaries, and are rarely completely isolated. They interact with and influence one another in countless ways that are not always apparent. Fur­thermore, communities are nested within one an­other like Chinese boxes; that is, there arc commu­nities within communities. A forest is a community, but so is a rotting log in that forest. The log contains bacteria, fungi, slime molds, worms, insects, and perhaps even mice. The micro­organisms living within the gut of a termite in the rotting log also form a community. On the other end of the scale, the entire living world can be con­sidered a community.

Living organisms exist in a nonliving environ interactions with one another. Minerals, air, water, and sunlight are just as much a part of a honeybee's environment, for example, as the flowers that it pollinates and from which it takes nectar. Together species it contains make up an ecosystem. Although this chapter emphasizes the living community, communities and their physical environments are inseparably linked (see Focus On: Microcosms).

Every organism has its own role within the struc­ture and functions of an ecosystem; we call this role its ecological niche.

There are two aspects to an organism's ecologi­cal niche: the role the organism could play in the community and the role it actually fulfills. The niche may be far broader potentially than it is in actuality. As an analogy, a person might he capable of becoming a doctor and a lawyer, but few people manage to be both. A person's actual life style, in­cluding his or her career, is chosen from among many possibilities. Put differently, an organism is usually capable of utilizing much more of its envi­ronment's resources or of living in a wider assort

ment of habitats than it actually does. The potential ecological niche of an organism is its fundamental niche, hut various factors such as competition with other species may exclude it from part of its fundamental niche. Thus, the life style that an organism actually pursues and the resources that it actually utilizes comprise its realized niche. An example may help clarify this distinction. The little Carolina anole, native to Florida, perches on tree trunks or bushes during the day and waits for insect prey. In past years these lizards were widespread in Florida. Several years ago, however, a related species, the Cuban anole, was introduced in Florida and quickly became common, especially in urban areas. Sud­denly the Carolina anole became rare—apparently driven out of its habitat by competition from the larger Cuban lizard. Careful investigation disclosed, however, that Carolina anoles were still around but were now confined largely to the foliated crowns of trees, where they were less easily seen.

The habitat portion of the Carolina anole's fundamental niche includes the trunks and crowns of trees, exterior walls of houses, and many other locations. The Cuban anoles were able to drive Carolina anoles out from all but the tree crowns, and the latter's realized niche became much smaller as a result of this environmental competition (Fig­ure 4-3c, d). Because all natural communities con­sist of numerous species, many of which compote to some extent, the complex interactions among them produce the realized niche of each.

Competitive Exclusion

When two species are very similar, as arc the Caro­lina and Cuban anoles, their fundamental niches may overlap. However, no two species can occupy the same niche in the same community indefi­nitely, because competitive exclusion eventually occurs. In this process, one species is excluded from a niche by another as a result of competition be­tween species (interspecific competition). Al­though it is possible for two different species to compete for a single resource without being total competitors, two species with absolutely identical ecological niches cannot coexist. Coexistence can occur, however, if the overlap in the two species' niches is reduced, in the lizard example, direct competition between the two species was reduced as the Cuban anole competitively excluded the Carolina anole from most of its former physical habitat until the only place that remained open to it was the tree.

Competition between different species, then, determines a species' realized niche. The initial evi­dence for this came from a series of experiments

conducted by the Russian biologist A. F. Cause in 1934- In one study Cause grew two species of Paramecium (a type of protozoa), P. Aurelia and the larger P. caudatum (Figure 4-4)- When the two were grown in separate test tubes, each species quickly increased its population to a high level, which it maintained for some time thereafter. When the two were grown together, however, only P. Aurelia thrived; P. caudatum dwindled and even­tually died out. Under different sets of culture con­ditions, P. caudatum prevailed over P. Aurelia. Cause interpreted this to mean that one set of con­ditions favored one species, and a different set fa­vored the other. Because the two species were simi­lar, given time one or the other would eventually triumph at the other's complete cost.

Competitive exclusion of a wild mouse (Mus musculus) population by voles (small rodents with short tails) apparently occurred in California dur­ing the 1960s. The aggressive voles ate much of the mice's food supply, and the voles' continual prox­imity may have also disturbed the mouse popula­tion in other ways. The wild mice exhibited less vigor and a lower reproductive rate that eventually resulted in their local extinction.

Apparent contradictions to the competitive exclusion principle sometimes occur. In Florida, for instance, native fish and introduced (non-native) cichlid fish seem to coexist in identical niches. Similarly, in the same area botanists have observed closely competitive plant species. Although such situations seem to contradict the concept of com­petitive exclusion, the realized niches of these or­ganisms may differ significantly in some way that scientists do not yet understand.

Limiting Factors

The factors that actually determine an organism's realized niche can be extremely difficult to identify. For this reason the concept of the ecological niche is largely abstract, although some of its dimensions can be experimentally determined. Whatever envi­ronmental variable tends to restrict the realized niche of an organism is called a limiting factor. What factors actually determine the realize niche of a creature? An organism's niche is basi­cally determined by the sum of its structural, physi­ological, and behavioral adaptations. Such adapta­tions determine, for example, the tolerance an organism has for environmental extremes. If any feature of its environment lies outside the bounds of its tolerance, then the organism cannot live there.

Most of the limiting factors that it has been possible to investigate are simple variables such as the mineral content of soil, temperature extremes, amount of precipitation, and the like. Such investi­gations have disclosed that any factor that exceeds an organism's tolerance for it or is present in quan­tities smaller than the minimum required by the organism limits the occurrence of that organism in a community. By their interaction, such factors help to define an organism's realized niche.

The concept of limiting factors was originated in the 19th century by the agricultural chemist J von Liebig, who propounded what is now called the law of the minimum. As amended in 1913 by V. E. Shelford, the law of the minimum holds that the growth of each organism is limited by whatever essential factor is in shortest supply or is present in harmful excess.

This consideration applies throughout the life cycle of an organism. For instance, although adult blue crabs can live in almost fresh water, they can­not become permanently established there because their larvae cannot tolerate fresh water. Similarly, the ring-necked pheasant, a popular game bird, has been introduced widely in North America but does not survive in the southern United States. The adult birds do well, hut the eggs cannot develop properly in the high southern temperatures.

As a result of more recent studies of limiting factors, ecologists now understand that von Liebig viewed limiting factors much too narrowly. He understood, rightly, that an excess of one limiting factor cannot make up for the deficiency of an­other. But what von Liebig didn't realize is that for several factors, their interactions collectively restrict its realized niche more severely than would be expected from simple addition of the effects of the individual limiting factors.

We have seen that an organism's ecological niche takes into account all aspects of that organism’s existence. Now we examine Coevolution and symbiosis two biological factors that strongly influence an organism’s niche.