ECOLOGY (Part 2 of 3)

Competition in Communities

Competition is a characteristic of all communities. Plant roots in dry range lands compete for water. The trees of a rain forest compete for light. Crops compete for both of these as well as for nutrients. Competition is usually keen in areas where one type of community seems to overlap another. For example, a continuum between a shrub community and a marsh contains some aspects of both communities. Animals and plants trying to establish a foothold in such an overlap must cope with difficulties often non-existent in a stable community. Shrubs moving toward the marshy area must compete with other pioneer shrubs and reeds for light and nutrients.

Similarly, reeds attempting to invade the shrubby area must compete with shrubs and other reeds. This shows that competition may often be greatest among living things that have the same needs. For the same reason, competition may be extremely harsh within a species among wolves for meat or among cattle for grass, for example.

On the other hand, competition is sometimes modified through behavioural adjustments even cooperation among the members of a community. Shrubs are spaced widely on deserts. Birds nest in patterns that prevent overcrowding. Bees live together in a hive. Man can make similar adjustments, and unlike other species he can achieve cooperation by rational means. Yet human competition sometimes ends in wars, and wars frequently destroy the very things which the belligerents are striving to take away from one another.

Should it become necessary to control an undesirable species in a community, this can best be done by modifying the community. A rancher, for example, may discover that weedy annual plants are invading his native perennial pastures. His initial reaction might be to attack the weeds with chemical herbicides. This approach would be self-defeating since nature would provide the resultant bare soil with an unlimited supply of weed seed. To solve the problem ecologically, the rancher should modify the community by managing the degree and time of cattle grazing to permit normal growth of the native plant community, which would then crowd out the undesirable weeds.

Succession in Communities

A third major principle of ecology is that an orderly, predictable sequence of development takes place in any area. This sequence is called ecological succession. The successive changes produce increasingly mature communities from a barren or nearly barren start.

Succession usually culminates in a climax, a fairly stable community in equilibrium with, and limited by, climate and soil.

At one time or another virtually all land surfaces have undergone basic climatic changes and been occupied by types of plants and animals which they may no longer be able to sustain. This, however, is not what is meant by ecological succession. It is known as biotic history, extends over the vast scale of geologic time, and is deduced from fossil remains.

The future communities of an area cannot be predicted from its biotic history. Such prediction can be based only on a knowledge of ecological succession.

As soon as the first patches of soil are formed in barren areas, a series of events takes place that eventually terminates in the establishment of a climax community. This process is called primary succession. Because soil formation requires the slow weathering of rock, primary succession ordinarily spans hundreds of years. Once it begins, however, the sequence of events rarely alters. As soil formation proceeds, a succession of plants and animals appear. The last stage in this progression is the climax community.

Disclimax, an ecological community that occurs following a disturbance.

A disturbance at any point during primary succession or even at the climax can destroy the vegetation of a primary succession in whole or in part. The vegetation that follows a disturbance of this kind is called a disclimax. The disturbance can be caused by ploughing, logging, or overgrazing. When such a disturbance takes place, climate and soil are no longer the principal determinants of vegetation. The further natural growth of plants at the site of a disclimax, as contrasted with the raising of crops, is called secondary succession.

This can be completed in a few years or, at most, in decades because soil has already been formed. After secondary succession restores a balance between eroded soil and vegetation, the further development of both again becomes dependent on primary succession. Wise landowners use secondary succession to restore overgrazed range lands, cut over timber lands, and abandoned crop lands They need only protect the land from further disturbances while secondary succession heals the scars of abuse.

Changes in the community during secondary succession are rapid, because every living thing contributes to its alteration. For instance, the weeds that grow on a vacant lot produce shade and increase the soil's ability to absorb and store water. They also attract insects and birds and enrich the soil when they die and decay. The bare ground of the vacant lot is the best possible place for the pioneer sun-loving weeds to grow. Later the weeds are replaced by tree seedlings if the lot is in a forest climate, by native grasses if it is in a grasslands climate. Such changes occur until plants and animals that can make maximum use of the soil and climate are established.

The Ecosystem

A fourth key principle of ecology asserts that a community and its environment the living and the non-living constitute an ecological system, or ecosystem. Every natural community draws vital materials from its surroundings and transfers materials to it. Raw materials and decay products are exchanged continuously. Thus, in an undisturbed area basic resources are sustained, never exhausted.

Ecosystems exist on many kinds of lands, in lakes, in streams, and in oceans. They are found wherever soil, air, and water support communities. The combined ecosystems of the Earth constitute the biosphere.

Ecosystems generally contain many kinds of life. A cornfield, for example, contains more than just corn. Also present are smaller plant species, insects, earthworms, and a host of soil microbes. Each of these organisms fills a specific niche each performs an essential function in the ecosystem.

The inhabitants of an ecosystem are classified as producers, consumers, and decomposers.

Green plants of any kind, whether stately oaks or tiny algae, are producers because they make their own food through photosynthesis. Animals, including man, feed on plants or on other animals and are therefore classed as consumers. Organisms that cause decay bacteria and fungi are decomposers.

Food chain, sequences in which organisms within an ecosystem feed on one another.

The sequences in which the organisms within an ecosystem feed on one another are called food chains. Usually organisms of higher biological rank feed on those of lower rank.

Ecologists group the members of any food chain into a pyramid of numbers. At the base of such a pyramid are the green plants, which are the most numerous organisms in the chain. The next level might contain first-order consumers, such as the sheep that eat the green plants. At the peak of the pyramid might be second-order consumers, such as the herdsmen who feed on the sheep. When the producers and consumers of an ecosystem die, their bodies are broken down by the decomposers into nutrients used by new plants for growth. In this manner, the food chain is perpetuated.

The biosphere seems capable of sustaining life even in the absence of consumers. Without consumers, the rate of plant growth would eventually strike a balance with the rate of decay caused by the decomposers. Hence, even if all herbivores, or plant-eaters, were absent from the biosphere, plant growth could be expected to stabilize at certain levels.

Through plant growth and decay, water and carbon, nitrogen, and other elements are circulated in endless cycles. The driving force behind these cycles is the sun. Solar energy becomes converted into food through the photosynthesis of green plants and into heat through the respiration of plants and animals.

APPLICATIONS OF ECOLOGY

Ecologists are often employed to solve serious environmental problems. Early in this century, for example, southern Ohio was ravaged by a terrible flood. The inhabitants of the area, determined to prevent a repetition of the disaster, constructed large earthen dams across the valleys north of Dayton to contain future flood waters Since the slopes of these dams consisted of gravel with an admixture of clay, they washed away easily. It was necessary to stabilize the steep slopes quickly with plant cover. Knowing which plants would grow best in such places, an ecologist recommended the scattering of alfalfa and clover seed, followed by brome grass and Japanese honeysuckle. His recommendations were followed, and dam slopes were soon covered with a fine cohesive turf. Many of the hills on neighbouring farms lacked such cover and were quickly eroded.

In the Dust Bowl region of Texas, sandy soil in dry areas blew into great dunes after the land was ploughed for wheat. Bulldozing these dunes was thought too expensive.

However, an ecologist recommended that certain plants be raised near the shifting dunes.

The plants in front of the dunes caught and held the soil, while those behind them kept the rear from blowing deeper. In a short time, wind had levelled off the high dune tops and vegetation had anchored the soil.

Ecology and Wildlife Conservation

Measures for the preservation of ducks and other migratory wild fowl are examples of ecological work with animals. When these birds grew scarce, state and federal agencies sought ways to protect them and help them reproduce. At first, laws were recommended that forbade shooting the birds in the spring when they were flying north to nest. Every female killed in the spring could mean one less brood returning in the fall. Further studies showed that many of the birds' breeding places were being destroyed when the land was drained for other uses. Some of these sites were not well-suited for the sustained growth of crops; others, where marshes and potholes once released stored water slowly, now contributed to downstream floods. Draining thus had a doubly harmful effect. Ecologists captured the endangered birds and put aluminium bands on their legs to trace their breeding places and movements. In this way it was discovered that the problem was international. As a result, the United States began to work in close cooperation with Canada and Mexico for the protection of migratory birds.

Ecologists also investigated the food habits of birds. They recognized that if proper food was unavailable, the birds would disappear even if hunting was regulated. Experts examined the stomach contents of thousands of birds from many different areas. This work led to the finding that bird food consists mainly of plant materials that thrive under natural conditions. To ensure the availability of these materials, man had to cease altering many natural communities and to stop polluting them with his wastes.