Like most living organisms, the giant sequoia does not live alone; it is but one member of a complex association of plants and animals, and its continued existence depends on its environment's physical and living components and their dynamic interactions. The study of these interactions is ecology.
The roots of ecology, a relatively young science, are a body of facts generally included in what has been popularly called natural history. But, as venerable as natural history is, its main concerns have been qualitative, while modern ecology tends more to quantify the relationships between living things and environment. The word ecology is derived from the Greek word oikos, which means "house or household," and logos, which means discourse or study of. We can divide ecology into three major aspects: (1) the components and their structure and function; (2) the dynamic energy-based activities of organisms; and (3) the changes occurring over time through the interactions of organisms and environment.
A basic concept in ecology is that of the ecosystem, which includes a given area's biological community of organisms and the interacting components of the physical environmentthe soil, atmosphere, and the phenomenon of weather. Ecosystems vary greatly in size and complexity. A pond, for example, may be considered an ecosystem, or we may consider the entire earth's surface as the dynamic system of life with its indispensable support systems of air, water, energy, and minerals. Functionally, in any ecosystem we can study either its living components or its nonliving factors. All the interacting living organisms within a given ecosystem constitute its living component or biotic community.
Within each community, each kind of organism plays a different role. Some produce food from the sun's energy; others are herbivores living upon this food, while still others consume the herbivores, and so on. Green plants are the primary producers, inasmuch as they begin the long sequence in the energy chain with its first step, the elemental fixation of solar energy. The herbivores, or animals deriving their sustenance from plants, are known as primary consumers, while the carnivores, which feed upon other animals, are called secondary and tertiary consumers. In sequoia forest communities, the giant sequoia is one of the major primary producers. The chickaree, or Douglas squirrel, is a primary consumer which feeds upon the cones of sequoias; and the marten, which preys upon the chickarees, is a secondary consumer. Upon the death of any member or portion of an individual of the biotic community, decomposers such as bacteria and fungi utilize the remains as food and eventually convert the organic matter into inorganic nutrientswater and carbon dioxideall of which is potentially recycled for use once again by the sequoia plant community's many members.
The animals of the community interact mainly along food chains where one form of life feeds upon another. All food chains start with a producer, a plant that is green or at least autotrophic, so that the producer's form and abundance greatly influence the subsequent dependent organisms in the food chain. Food chains are rarely isolated from one another because most consumers feed on many different organisms; in this manner, food chains are interwoven into food webs. The complex and varied interdependencies of food webs tend to bring stability to the community's dynamic food relationships. Simple ecosystems are more prone to greatly oscillating population numbers than complex ecosystems; thus man should be cautious in exterminating certain species within a community for he may well simplify the system and thus disrupt the dynamic balance of nature which the diversity provides.
The major interactions in the biotic community involve production and transformation of food. The pattern varies from ecosystem to ecosystem, but its basic design derives from the flow of energy and the cycling of elements and nutrient materials. The science of ecology is built on the premise that energy flows unidirectionally through the community, eventually to be lost, while nutrients or minerals are used over and over again.
The giant sequoia, capturing energy through photosynthesis, utilizes it to maintain itself and to supply two major food webs. One web involves the herbivores, namely, insects and vertebrates, which feed directly upon certain tissues of the living tree. The other major web involves organisms which decompose the dead parts of the tree fallen on the groundcone and branch debrisor the dead tree itself. Insects and fungi are important decomposers. In forest communities in general, the decomposers rather than the herbivores utilize most of the energy captured. But only as this dynamic process keeps up with the demands of the main producer can the community remain relatively unchanged. Studies suggest that the giant sequoia and its decomposers are in balance because around trees that are probably at least 2000 years old the soil is still not depleted of its vital nutrients (Zinke and Crocker 1962). The constant "rain" of twigs, cones, and branches and their subsequent decomposition by soil organisms maintain this dynamic balance by constantly returning nutrients to the soil.
The general pattern of food relationships is best explained by the pyramid of numbers concept. Simply stated, this means that green plant producers outnumber the primary consumers that feed upon them, primary consumers outnumber the predators that feed upon them, and so on up the food chain. The pyramid of numbers is based upon the pyramid of energy and is reflected also in the pyramid of biomass, or the total living (bio) weight (mass) of a certain group of organisms. For example, we measure biomass as the tons of sequoias in an acre, or pounds of insects feeding upon the sequoias. Under most conditions, the biomass of the producers is greater than the biomass of the herbivores, which is in turn greater than that of the carnivores. Thus we can observe that mountain lions are relatively few in contrast to numbers of deer, and that deer are far fewer than shrubs. What leads to these pyramids is the underlying inefficiency of energy transfer, which is roughly 10% in any step of the pyramid. At each transformation, much energy is lost, usually as heat, which the next link in the food chain cannot use. The energy pyramid always shows a decrease up the food levels from producer to consumer, but the pyramids of numbers and biomass are sometimes distorted by a surge of energy flow, which then subsides, from one level to the next.
In a forest community where most of the captured energy goes to decomposers, little activity is apparent because only a few active predators enliven the scene. Therefore, in a sequoia forest, most of the activity is in the subtle decomposition underfoot, inaudible as one walks across the forest floor. The decomposers are busy utilizing ancient sunlight to run their lives and returning to the soil the nutrients and products of photosynthesis which long ago were picked up by the roots and built up by the leaves of the giant sequoia and its associates.
One or two factors such as water quality and quantity, or temperature, or topography, often dominate the physical environment of a given community. Through the long evolutionary process of natural selection, forms of life have become adapted to varying habitats, each having major unique characteristics such as the dryness of deserts or salty wetness of salt marshes. Ecology divides the factors which affect life into two major categories, the climate and substrate. The climate is determined by great world forces which shape the movement of air masses and winds, the precipitation (its timing and amount), and the duration of hot or cold periods, all of which are functions of solar energy. The substrate may be the dominant factor such as in the marine environment, but usually on land it merely modifies the climatic effects. For example, high mountain ranges such as the Sierra Nevada profoundly influence the climate to the east, inducing a great desert in its rain shadow. On the western slope of the Sierra, climatic conditions combine with soils and topography to produce a favorable habitat for the giant sequoia.
Only a short distance from native groves in the Sierra, limiting factors apparently restrict the spread of the sequoia (see "Soil Moisture Availability"). At the lower elevations, the factor may well be the amount of available soil moisture or the high temperatures, while at higher elevations winter cold may be critical (Beetham 1962; Rundel 1969). Each species of organism can tolerate extremes of physical conditions only to certain limits. Thus, the maximum and the minimum values of a specific condition such as soil moisture are very important in the survival of a species. Average conditions are considered in ecology, but the extremes which an organism can tolerate mainly determine the size and distribution of the population of that organism. This law of tolerance applies as well to all factors upon which the organism depends, so that an organism with a wide tolerance range for most factors may be severely limited by having a narrow tolerance range for just one factor. The law of the minimum is another fundamental idea: just as a chain is only as strong as its weakest link, so an organism's presence and numbers are often determined by the needed ingredient that is in the shortest supply. Organisms with a wide tolerance range for all the necessary factors tend to be widely distributed.
In general, the early reproductive stages of an organism have the narrowest ranges of tolerance. The narrower this range, the more offspring that organism tends to produce. A single giant sequoia may produce several million seeds during its lifetime, yet most will not germinate, and only a few of the resulting seedlings will survive to become mature trees.
An additional ecological concept is the carrying capacity of the total environment, i.e., that the environment's resources can sustain a certain density. Thus, on an acre of Sierran soil there may be about 15 sequoias more than 3 ft tall. The conditions prevalent in this grove, then, permit an average number of 15 sequoias per acre, and this figure is the density value for that particular population. Other groves may sustain as many as 35 per acre under a different set of circumstances. The carrying capacity may vary from grove to grove or from time to time in the same area. Most populations of organisms seem to have reached the optimum number that can survive in the particular area in which they are found.
A very significant phenomenon is the periodic invasion of new species of plants in most plant communities. Although changes of this sort may be due to several phenomena, the one of greatest relevance is known as ecological succession. Successional changes occur when one assemblage of plants alters the soil and light regimes to such an extent that the plants' own progeny cannot compete successfully with the more tolerant invaders, which replace them. The different stages, named seres, vary greatly from one climate to another. In climates where rainfall is up to 30 inches per year or more trees tend to dominate the later seral stages, replacing herbaceous plants and shrubs. Inasmuch as plants are the foundation of food webs, the kinds of animal life likewise change with each new assemblage of plants. Periodic interruptions by such phenomena as fire tend to return the community structure to an earlier seral stage in which there is often an abrupt invasion of shrubs, herbs, or shade-tolerant trees typical of that particular stage.
A prolonged period without disturbances produces a long-enduring community which terminates the process of change. This terminal stage is called the climax community; it comprises an assemblage of plants that can tolerate the conditions which they themselves create and can reproduce their kind in full shade and root competition. Soils develop to maturity concurrently and, barring climatic changes, the climax species can hypothetically continue to reproduce without end.
Because of variations in topography and soil, a given locale may exhibit several long-established interspersed plant associations which may appear as a variety of climax communities. These variations are due mainly to differences in the substrate and not to the general climate of the region. Thus forests of trees growing in deep soils may be adjacent to shrubby communities growing on rocky substrates or next to a wet meadow of small herbaceous plants. The subdivisions of forested areas are sometimes called subclimaxes, and it is often assumed that even the rocky areas and meadows will eventually give way to the forest community of that particular locale.
Before the advent of western civilization, forested climax communities were probably less common than they are today. We know that fire came frequently to most forests, favoring certain plants and discouraging others. The maintenance of such a community is often called a fire climax. Some ecologists place the giant sequoia, whose reproduction is favored by fire, in this category. The larger sequoias, while affected adversely by fires, resist heat better than the other trees of their community, and many have persisted through numerous fires, standing today in forests which have largely progressed to the climax state. Such relicts are unique among plants, being able to persist for more than 3000 years and, in a sense, standing ready to replenish their kind when the proper conditions arise. Fire is the most widespread and frequent agent producing an environment favorable to the revitalization of the sequoia forest community. And so succession is repeatedly set back and the sequoia is favored over most other species until it produces conditions of too much shade and leaf litter for its offspring to tolerate.
The giant sequoia and its associate plants form, in the Sierra Nevada, a fascinating mosaic, unique among all forest communities on earth.
Last Updated: 06-Mar-2007