We recognize that plants, animals, and other organisms tend to be associated in various ways with one another and also with their physical environment. For example, forests consist of populations (groups of individuals of the same species) of trees or other plants that form a plant community (unit composed of all the populations of plants occurring in a given area). The lichen and moss flora on a rock also constitute a community, as do the various seaweeds in a tidepool (Fig. 25.2). However, these communities also invariably have animals and other living organisms associated with them. It is preferable, therefore, to refer to the unit composed of all the populations of living organisms in a given area as a biotic community. Considered together, the communities and their physical environments, which interact and are interconnected by physical, chemical, and biological processes, constitute ecosystems. Some populations, communities, and ecosystems may be microscopic in extent, while others can be much larger, or even global.
Populations may vary in numbers, in density, in genetic diversity, and in the total mass of individuals. Depending on circumstances, a field biologist may investigate a population
in various ways. If, for example, a conservation organization is concerned about the preservation of a rare or threatened species, the organization may simply count the number of individuals, although this may not always be feasible. If such a count is not feasible, the organization may estimate population density (number of individuals per unit volume—e.g., five blueberry bushes per square meter). If the individuals in a population vary greatly in size or are unevenly scattered, a better estimate of the population's importance to the ecosystem may be calculated by determining the biomass (total mass of the living individuals present).
Communities are composed of populations of one to many species of organisms living together in the same location. Similar communities occur under similar environmental conditions, although actual species composition can vary considerably from one location to another. A community is difficult to define precisely, because species of one community may also occur in other communities. Furthermore, species of one community may have specific genetic adaptations to that community; therefore, if individuals are transplanted to a second, different community where the same species occurs, the transplanted individuals may not necessarily be able to survive alongside their counterparts that are themselves adapted to this second community. Individuals adapted to specific communities within their overall distribution are called ecotypes, and areas transitional between communities are called ecotones.
Analysis and classification of communities are important in the preparation of maps that form the basis of activities such as land-use planning, forestry, natural resource management, and military maneuvers.
Living organisms interacting with one another and with factors of the nonliving environment constitute an ecosystem. The nonliving factors of the environment (abiotic factors) include light, temperature, concentrations of oxygen and other gases, air circulation, fire, precipitation, rocks, and soil type. The distribution of a plant species in an ecosystem is controlled mostly by temperature, precipitation, soil type, and the effects of other living organisms (biotic factors). For example, in Mediterranean climates, such as those that occur in parts of California and Chile, nearly all precipitation occurs during the winter months, and the summers are dry. This type of climate favors spring annuals that complete their life cycles by summer and evergreen shrubs that can tolerate long periods of drought. Forests may occur in areas where melting heavy winter snowfall soaks deeply into the soil, compensating to a certain extent for the lack of summer moisture.
The distribution of plant species is influenced by the mineral content of soils. For example, serpentine soils occurring on the American west coast contain relatively high amounts of magnesium, iron, usually nickel, and chromium and low amounts of calcium. These soils often support species that are not found on nearby nonserpentine soils. Biotic interactions, such as competition for light and grazing by the animal members of the biotic community, and abiotic factors, such as mineral nutrients and available water, also influence the distribution of plant species.
The leaves and other parts of plant species that occur naturally in areas of low precipitation and high temperatures (xerophytes) generally are adapted to their particular environment through modifications that reduce transpiration. These modifications are discussed under "Specialized Leaves" in Chapter 7 and "Regulation of Transpiration" in Chapter 9. Plants of arid areas may also have specialized
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forms of photosynthesis, such as CAM photosynthesis. Similarly, plants that grow in water (hydrophytes) are modified for aquatic environments.
Ecosystems may sustain themselves entirely through photosynthetic activity, energy flow through food chains (discussed in the next paragraph), and the recycling of nutrients. Organisms, called producers, are capable of carrying on photosynthesis (e.g., plants, algae) and store energy that may be released by other organisms. Animals such as cows, caribou, caterpillars, and other organisms that feed directly on producers are called primary consumers. Secondary consumers, such as tigers, toads, and tsetse flies, feed on pri mary consumers. Decomposers break down organic materials to forms that can be reassimilated by the producers. The foremost decomposers in most ecosystems are bacteria and fungi. Note that there is not necessarily a sharp distinction between consumers and decomposers. Decomposers, for example, depend just as much as consumers on organic matter for their nutrition.
In any ecosystem, the producers and consumers interact, forming food chains or interlocking food webs that determine the flow of energy through the different levels (Fig. 25.3). In such chains or webs, each link feeds on the one below and is consumed by the one above, with photo-
Ecology synthetic organisms at the bottom and the largest non-herbivorous animals at the top. Since most non-plant organisms have more than a single source of food and are themselves often consumed by a variety of consumers, there are considerable differences in the length and intricacy of food chains or webs, but there are rarely more than six links in any given chain.
Light energy itself, which enters at the producer level, can't be recycled in an ecosystem. Only about 1% of the light energy falling on a temperate zone community is involved in the production of organic material. As the organisms at each level respire, energy dissipates as heat into the atmosphere. Some parts of organisms may not be consumed at each level of the food chain. For example, leaves that are grazed by an herbivore are broken down and much of their energy is released within the animal. However, the energy stored within the uneaten leaves of the same plant isn't released until the leaves fall from the plant and decomposers (bacteria and fungi) break them down.
A significant proportion of farmland is used to raise animal feed instead of food crops for humans. It has been estimated that when cattle graze, however, only about 10% of the energy stored by the green plants they consume is stored in animal tissue, with most of the remaining energy dissipating as heat into the atmosphere. When we eat beef, only approximately 10% of its stored energy is used by our bodies to manufacture new blood cells and otherwise sustain life. The remaining energy is lost as heat. Obviously, then, in a long food chain, the final consumer gains only a tiny fraction of the energy originally captured by the producer at the bottom of the chain.
Conversely, there is proportionately much less loss of energy between levels in short food chains. Assume, for example, that for every 100 calories of light energy falling on corn plants each day, 10 calories are stored in corn tissue. Then suppose the corn is fed to cattle. Again, only 10%, or 1 calorie, of the original energy may be stored in animal tissue. If, as secondary consumers, we eat the beef, our bodies, in turn, may use 10% of the energy available in the beef, or only 0.1% of the original energy available. If, however, we eat the corn directly, we end up with the steer's percentage, which is 10 times more of the original solar energy than we would get if we ate the steer that ate the corn. (The actual amount of energy per gram of beef is considerably higher than that found in a gram of corn.)
From this, it follows that a vegetarian diet makes much more efficient use of solar energy than one that relies heavily on meats, and where food is scarce or humans are very abundant (as in India or Ethiopia), humans may be more or less forced to favor a vegetarian diet. It also follows that in terms of the numbers of individuals and the total mass, there is a sharp reduction at each level of the food chain. In a given portion of ocean, for example, there may be billions of microscopic algal producers supporting millions of tiny crustacean consumers, which, in turn, support thousands of small fish, which meet the food needs of scores of medium-sized fish, which are finally consumed by one or two large fish (Fig. 25.4). In other words,
one large fish may very well depend on a billion tiny algae to meet its energy needs every day.
The interrelationships and interactions among the components of an ecosystem can be quite complex, but many function together in a somewhat regulatory fashion. An increase in food made available by producers can result in an increase in consumers, but the increased number of consumers and competition among them reduces the available food, which then inevitably leads to a reduction of consumers to earlier levels. While cyclical in nature, the net result is sustained self-maintenance of the ecosystem. This is the basis for the so-called Balance of Nature.
Interactions Among Plants, Herbivores, an er Organisms
While it is easy to see the total mass of consumers is largely determined by the total mass of food made available by the producers, the interactions among different producers themselves, between predators and prey, and between the decomposers and the other members of the ecosystem are usually more subtle. Many flowering plants produce substances that either inhibit or promote the growth of other flowering plants. Black walnut trees, for example, produce a substance that wilts tomatoes and potatoes and injures apple trees that come in contact with black walnut roots. Many other plants produce phytoalexins (chemicals that kill or inhibit disease fungi or bacteria), making them resistant to various diseases (see discussion under "Use of Resistant Varieties" in Appendix 2).
Conversely, some bacteria and fungi limit higher plant growth by producing various inhibitory chemical compounds. Population size in other bacteria, fungi, and flowering plants may be limited by nutritional needs and availability. The degree to which this occurs varies considerably with the organisms involved. Some of the species of the Figwort Family (Scrophulariaceae), which includes snapdragons and similar plants, have no chlorophyll, and depend entirely on their flowering plant hosts for their energy and other nutritional needs. Other related species, as well as mistletoes, do have chlorophyll but apparently also require
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supplemental food from their hosts. Still other species often parasitize the roots of certain plants but are also capable of existing independently.
Mycorrhizal fungi are intimately associated with the roots of most woody and many other plants in such a way that both organisms derive benefit (such associations, called mutualisms, are a major part of life in general; they are briefly discussed in Chapter 5). The fungi greatly increase the absorptive surface of the root, usually playing a major role in the absorption of phosphorus and other nutrients, while obtaining energy from root cells.
Thomas Belt, a naturalist of more than 100 years ago, first called attention to an association between tropical ants and thorny, rapidly growing species of Acacia (Fig. 25.5). The Acacia has large, hollow thorns at the base of each leaf and is host to ants that feed on sugars, fats, and proteins produced by petiolar nectaries and special bodies at the tip of each leaflet. The ants live within the hollow thorns and vigorously attack any other organism, from insects to large animals, that come in contact with the plant. They also kill, by girdling, any plant that touches the Acacia (girdling is discussed under "bridge grafting" in Appendix 4). Experiments have shown that when ants are removed from these Acacia species, the plants grow very slowly and usually soon die from insect attacks or from shading by other plants.
Large herbivorous animals, such as deer and moose, feed on a wide variety of plants, each differing in nutritional value. Each plant species also produces different combinations, types, and amounts of chemical compounds in addition to proteins, fats, and carbohydrates. Many of the chemical compounds may be toxic, but some animals grazing on the plants that produce these chemicals do not display symptoms of poisoning because their digestive systems are capable of breaking down the compounds and eliminating or excreting them to a limited degree. The limitations imposed by such compounds result in the consumers varying their diet, seeking familiar foods and being wary of new ones.
Some plants are dependent for their existence on predators controlling populations of grazing animals. Populations of others such as oaks may become limited in extent when squirrels and acorn-consuming birds remove acorns that would potentially become trees. All plant species have some natural defense, such as chemical compounds or structural modifications (for example, spines). If this were not so, primary consumers of all kinds from insects to elephants could soon render a species extinct. In an ecosystem, the defenses that both producers and consumers have against each other have been developed through a process of coevolution resulting from natural selection and are more or less balanced.
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