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  3. Ecosystems in and out of Balance A) It is known that ecosystems have a structure consisting of producers (green plants whic

Ecosystems in and out of Balance A) It is known that ecosystems have a structure consisting of producers (green plants whic

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问题     Ecosystems in and out of Balance
    A)  It is known that ecosystems have a structure consisting of producers (green plants which use light energy to produce living matter from non-living matter in the environment), consumers (all the animals which feed directly or indirectly on the green plants), and decomposers (分解者) (bacteria and fungi that change the dead organic material back into simple raw materials which can then again be used by the producers). This structure, which is necessary to maintain the flow of energy and nutrients through the system, consists of the interactions between hundreds or even thousands of different kinds of plants, animals, and microbes which grow, reproduce and die in a never-ending cycle.
    B)  At this point, we might ask ourselves, since ecosystems consist of interactions between reproduction populations, what prevents one species(kind) of organism from reproducing in such numbers that it overcomes and eliminates other species? In other words, what are the factors that maintain the structure, or balance, of that ecosystem? To answer the question we must first recognize that the idea of ecosystem balance is relative. In fact, ecosystems are always changing and adjusting. The relative degree of balance is the main factor in determining the rate of change. A well-balanced system will change very slowly, perhaps unnoticeably, in the course of direct human experience. An unbalanced system changes more or less rapidly; the greater the imbalance, the more rapid the change. It follows that if a change is made in one or more of the factors that affect balance, the ecosystem itself will also change.
    C)  One way of viewing ecosystem change and balance is in terms of the two factors: biotic potential and environmental resistance. The biotic potential of a species is its capacity for reproducing itself; in a general sense it is the combination of all the factors that permit its kind to become more numerous. Birth rate is an obvious factor, but it is only one. Given favorable conditions, every species has a biotic potential to increase its population. For example, a pair of frogs has the biotic potential to produce several hundred offspring in one season and each offspring in turn could potentially produce several hundred more.
    D)  The fact that populations in nature generally do not "explode" in numbers is not due to limits in biotic potential, but to a second factor, environmental resistance. The environmental resistance facing each species is the combination of all the factors that limit the survival of its members. These factors are similar for both plants and animals. Thus, there is a relationship between biotic potential and environmental resistance. If the environmental resistance for a species is less than its biotic potential, its members will increase; if the environmental resistance is greater than biotic potential, then its numbers will decrease. In a stable ecosystem, the biotic potential of each species is evenly balanced by environmental resistance.
    E)  Now let’s look at ourselves as a species in relation to ecosystem balance. Modern scientists believe that humankind, like other animals, evolved through millions of years of changes and adaptations to the environment. Despite this similarity with other creatures, however, the evolution of humankind differs from that of other species in one important and unique way. In other species evolution has led to specialization, both in the species abilities and in its place within the environmental structure. For example, the giraffe is marvelously adapted to grazing on treetops but, as such, it is also specialized and thus restricted to grazing on trees and shrubs (灌木). The same is true for countless other species. For humankind the reverse is true. Our evolution had led to a very generalized capability. Our highly developed intelligence and ability to make and handle tools mean that we can do virtually anything. Rather than evolving into a specialized role in balance with natural enemies, competitive species and environmental factors, humans evolved in such a way that we are capable of moving into every environment on Earth and even into space. Said another way, we see in humankind a tremendous imbalance between biotic potential and environmental resistance. The result is the rapidly increasing world population, frequently referred to as the population explosion. Further, to support our growing population, natural ecosystems are being increasingly displaced by human habitations, agriculture, and other human-supporting activities.
    F)  From the viewpoint of evolutionary history, the spreading of human beings over the earth can be looked at as a natural process, the latest of many waves of change that have occurred since the earliest beginnings of life on this planet. Unfortunately,  there is nothing in ecological or evolutionary theory to support the view that humans will be the last or even a long-lasting "wave". In fact, there are many indications that the human wave, at least that of humans in a technological society, may be relatively short-lived. Why is it so? First the rate and degree of many changes being brought about by humans are extreme. Previous evolutionary changes have occurred over the course of many millions of years. Thus the slow process of readaptation and development of new species more or less balanced extinctions, and ecosystems remained in relative balance throughout the course of change. In contrast, the significant changes brought about by humans have occurred in only the last 200 years.
    G)  The result is that extinctions are occurring at a distressingly fast rate, a rate which is more than likely to increase in the future. Changes in the biosphere are occurring so rapidly that we have no way of accurately predicting the outcome. Many ecologists are concerned that the basic balances within the biosphere will be so altered that all life on earth, including human life, will be disrupted.
    H)  Second, there is danger in the simplicity of the human ecosystem. Based as it is on relatively few species of agricultural crops and animals, the human ecosystem is inherently unstable. Agricultural production is only tenuously (脆弱地) balanced by the massive use of powerful chemicals to control pests, and these chemicals are causing ecological upsets that frequently make pest problems even worse. Also, plant scientists warn that an outbreak of crop disease for which we do not have a cure could wipe out a significant portion of the world food supply in one season.
    I)  Third, there are many indications that present human expansion is resulting in overgrazing of vast areas of the earth’s surface. Overgrazing occurs when plant-eating populations expand to the point when they eat the vegetation faster than it can produce and, consequently, destroy it.
    J)  In conclusion, there is no way that the human species can avoid facing the ultimate checks and balances that apply to other species and ecosystems. Fortunately, however, ecological realities need not be ignored. We as humans do have the unique evolutionary traits of exceptional intelligence and technological capability. We have the potential to use these traits to make adjustments in our societies and lifestyles to live within ecological limits and in balance with the rest of the biosphere.

There are no absolutely balanced ecosystems since they are always changing.
选项
答案B
解析 由题干中的ecosystems和changing定位到B)段第三、四句。同义转述题。由定位句可知，生态平衡是一个相对的概念。事实上，生态系统总是处于不断变化和调整当中。题干中的no absolutely对应定位句中的relative。故选B)。
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