Thus far in this book attention has been centered on the physical factors of the park as they act in apparent independence of other aspects of the environment. That is of course not the case but is only a convenient method of approaching one aspect of our subject at a time. In this section the focus will be on the relationships among the living things which occupy the park area—the biological landscape—and some of its ties to the whole environment. Such a study is what is known as ecology.

Perhaps one way of viewing the full extent of ecology, and of understanding the functions of the broad categories of the environment, is to draw an analogy with a theater. The stage and sets of the natural theater are the hills, valleys, waters, and winds of the physical landscape. All are constantly changing but give the transient and relatively short-lived human being an impression of submissiveness to the whims of more animate creatures. The actors in this theater can be represented by the flora and fauna, which exhibit the most marked changes in personality and role from season to season and year to year. Profoundly affecting both theater and actors is mankind—an actor himself—with his directorlike energies who, for a few centuries now has had a significant impact on the progression of the drama. Like the theater reviewer, who must consider all aspects of a production but devotes most of this attention to the performance of the actors, the ecologist deals with all the factors of a landscape but primarily focuses on the relationships between the plants and animals of an area.

ECOSYSTEMS

Throughout this book we have consciously used the terms flora and fauna to describe the components of the biological landscape. Seldom does one refer to the individual plant or animal which is a genetically uniform entity, or even to a local population of a certain species of organism. The reason for this is that isolated individuals or separate populations do not exist in spatial or physiological isolation. Rather, all living organism depend upon some other living organism for some part or task in its life-support cycle. Although one looks upon an apparently independent and solitary geranium in a window flower pot and may see no other living thing, there are in fact many dozens or even hundreds of different organisms at work to keep that plant producing those cheerful red blossoms. The soil alone contains many micro-organisms which are invisible to the human eye but which are nevertheless as important to the life of the geranium as the periodic watering and cultivation by the gardener. Indeed, the interdependent relationships among all living things in a portion of the earth's surface are so complex that they are dealt with as a system—an ecosystem.

Figure 12. Graph of energy storage in a vegetation succession. When energy input approximates energy output, a climax condition has been reached.

An ecosystem may range in size from a small, living-room terrarium to thousands of square miles of tropical rain forest. The most important characteristic which all have in common, however, is their ordered and effective system of physical and biological interrelationships. What must be kept in mind is that such a system of ordered relationships is not a fixed entity in which there is a standard composition of citizens. Ecosystems are what are called "open systems," which constantly change in their dimensions and constituent members. These changes are the result of arrivals and departures of various environmental factors across the indefinite boundaries of the ecosystem—thus the use of the word "open."

All ecosystems undergo evolutionary change, but the logical and theoretical goal of all these changes is an improved ecosystem which approaches a stable and balanced state of existence known as a climax community. Such a community exists when the energy and material arrivals in an ecosystem exactly balance the sum of the departing energy and mass (assuming in this atomic-nuclear age that mass and energy are not simply different forms of the same phenomenon). Although scientists disagree on the exact state of a climax community—or indeed on the philosophical question whether change ever stops—for all practical purposes the climax community is the optimum association of living things for a given set of physical conditions. It exists when the rate of change in the system becomes imperceptible to historic observation.

The process of reaching the climax stage is known as succession because individuals, populations, and associations of conditions succeed one another in waves or cycles of always diminishing energy fluctuation. An obvious example of succession is regularly opposed each year by farmers and gardeners. Assume for a moment the case of a flower bed located in a lush, green lawn of Bermuda grass. The grass of the lawn stays attractive and vigorous because all the environmental conditions favor its growth over other plants in the immediate area. The portion of the yard which is devoted to the flower bed shares the optimum growing conditions with the lawn, except that it is subjected to regular and intense periods of environmental disruption in the form of the gardener's weeding and cultivation. If perchance the gardener becomes involved in other activities which detract from his attentions, the disruptive spells of gardening become less intense and frequent, and the Bermuda grass wastes no time sending out "runners," or rhizomes, to claim the territory. First, however, will come a crop of small and large mixed annual weeds, such as dandelions or crab-grass, to try its hand as a successional stage. It may dominate for a short period but will soon share the soil's surface with the increasing mat of Bermuda rhizomes. A few days later the Bermuda cover will be complete, and the local ecosystem will be essentially stable until some major environmental change is induced by man or nature.

Within Platt National Park is an excellent example of a successional process in the Environmental Study Area east of the Nature Center. Until 1969 the Perimeter Drive extended through that portion of the park and was paved with asphalt. When the Environmental Study Area was established, the asphalt was removed, and the area was allowed to revert to natural vegetation. Grasses and annual herbs that thrive in open sunlight and with scant competition from fellow plants were the first to establish themselves. After the grasses had established some sod and contributed to the organic content and moisture-holding capacity of the soil, shrubs and juniper from the surrounding forest margins began to encroach. As these shrubs and small trees grow, they shade the sun-loving grasses which forged their first home and cause the grasses and herbs to diminish. In coming years the increase in shade and soil moisture will provide an acceptable site for the germination and growth of the large broadleaf trees of the oak-hickory forest. Two or three decades from now the oak-hickory community will complete the cycle to a local climax by shading out all the original shrubs and creating conditions favorable to the growth of a greenbrier, wild-grape, and redbud understory.

An example of succession along an abandoned roadway in the Environmental Study Area.

Plant communities and stages of succession within those communities have distinctive vertical structures which usually appear as strata in the foliage. In plant studies there are eight recognized and potentially present strata: (1) moss or lichen, (2) annual herb, (3) perennial herb, (4) low shrub, (5) tall shrub, (6) seedling tree, (7) understory tree, and (8) overstory tree.

Vegetation undergoes the most obvious changes in a successional process, but it is not the only changing aspect of the ecosystem. Because of the myriad interrelationships which exist among all parts of an ecosystem, a change in one necessitates some degree of adaptation in nearly all elements, both physical and biological. Of most interest to man are the subsequent adjustments in soil character, micro-climate, and fauna. These adjustments are often so subtle and coordinated that it is difficult to ascertain which are causal factors and which are effects.

Land which has been cleared of its native climax vegetation (be it grass, shrub, or forest) by some traumatic means such as fire, cultivation, or persistent overgrazing, is immediately exposed to the full effects of the sun, wind, and rain. These are disruptive and destructive to normal soil development. The most pronounced effect is often loss of soil moisture through wind and sun evaporation followed by erosion of better developed upper horizons during strong winds and rains. Less fertile lower horizons which are thus exposed to the surface can seldom support the kind or density of vegetation that was originally present. Decreased moisture also makes the ground unsuitable for germination of many seeds or as a habitat for burrowing soil creatures that do nature's "cultivation."

In the case of the abandoned roadway, grasses were soon established, and they provided some protection from the weather. The grasses formed a root network which helped keep the soil loose, inhibited erosion, added organic matter, and held soil moisture. As larger herbs, shrubs, and juniper become established, their litter and shade will increase soil moisture and acidity, and conditions for forest plants will improve as soil conditions change.

The changes in vegetation largely induce the local expansion or shrinkage of animal habitats. The lizards, flies, and butterflies which frequent sunny areas, as well as other small herbivores—such as rabbits, mice, and birds which feed on the grasses and forbs—retreat to other domains as the forest encroaches on the old roadway. They are replaced by animals such as the salamander, the mosquito, and the oppossum, which find the shadows and moisture suitable. All the while the greatest formal migration of all has gone on totally unnoticed by size-centric human beings. It is the concurrent ebb and flow of thousands of species of insects, bacteria, fungi, mosses, and other organisms which make the ecosystem work.

Winter scene on Travertine Creek. Photo by Chester Weems.

FOOD CHAINS

Of all the interrelationships which tie an ecosystem together, perhaps none is so basic or important as the food chain. An understanding of the principles and workings of the food chain is therefore essential to the understanding of an ecosystem or landscape. The stages of energy use in the food chain are called trophic levels, levels of nourishment and processes of taking and utilizing food. It is, like the ecosystem as a whole, also an open system which can be considered a cycle for practical purposes. In that cycle the basic life-forming and energy-producing earth elements, such as calcium, phosphorus, iron, carbon, and oxygen, are repeatedly reused by one or more species of living things.

An arbitrary but convenient starting point for a discussion of the food chain and its various trophic levels is the soil. Soil is the home and site of much organic activity in its own right, but perhaps most important is its role as the storehouse of the minerals of which all living things are composed. A similar role is played by the sun, the ultimate source of all nonnuclear energy on this planet. The natural elements and minerals in the soil were once physically bound up in the crystalline structure of the rocks which form the earth's crust, but through chemical and mechanical weathering they have been released for use in the surface landscape. Once near the surface, the weathered or decomposed rock mixes with living and dead organic matter to form soil. All that is needed to release the soil's minerals for use by living organisms is water to dissolve and transport the minerals to the roots of plants. At that point the nutrients first enter the life stream and begin their journey upward through successive trophic levels, or stages in the food chain.

Figure 13. A root hair magnified to show its relationship with the soil particles and soil solution from which the root obtains essential elements for growth and reproduction. From Donahue et al., 1971.

Trophic level one is the most critical, although all have an important role in any ecosystem; its primary function is the generation of food material and oxygen, and its members are known as producers. Producers are green plants and algae which in the presence of sunlight perform photosynthesis. This is a complex reaction whereby carbon dioxide gas, taken in by the leaves from the atmosphere, and water, taken in by the roots from the soil, are converted into free oxygen gas, and a carbohydrate by certain tissues in the plants. Carbohydrates are a form of food which the plant stores in its roots, leaves, or other tissues. The carbohydrates remain in storage until needed to sustain the plant, until the plant dies, or until the plant is eaten by another organism and the stored energy is released through metabolic processes. At the same time the plant is manufacturing and storing energy in the form of carbohydrates, its own body structure is formed from the minerals of the soil. These minerals are food not in the sense that they produce energy but insofar as they are in turn digested and assimilated into the structure of the organism consuming the plant.

The successive blocks of trophic levels in the food chain are representative of consumers. These are organisms which actively consume other organisms to sustain themselves. Included in the consumer group are trophic levels two, three, and even four. Trophic level two is composed of those animals which eat the producing plants. Consumers at this level are called herbivores because they eat nothing but plants, common examples being grasshoppers, sparrows, rodents, and cattle. Trophic levels three and four are represented by consumers which are still further removed from the source of plant energy. They are secondary consumers because they live exclusively by eating herbivores or primary consumers. Consumers such as eagles or frogs which eat only flesh are known as carnivores, while some—like human beings, bears, and raccoons—transcend all consumer levels by eating both meat and vegetable matter. Members of the latter group are known as omnivores.

Figure 14. A simplified model of the food chain.

The fifth and last trophic level of the food chain is made up of all the small organisms which live upon dead organisms, either plant or animal. These organisms—bacteria, fungi, and protozoa—which cause things to "rot," are called decomposers. The decomposers are universally small creatures which immediately attack and devour any dead creature to secure food. The waste products from their activities are the various gases and waste material which give dead and decaying material a distinctive odor. In fact, decay is the physical manifestation of these decomposing organisms at work. What material they do not use in their own life-support cycles is released to the soil or atmosphere in the form of basic elements and minerals, which are soon recycled through the plants of the first trophic level. At that point the food chain is said to be complete.