A very important corollary to the levels – of – organization concept is the principle of integrative levels, or, as it is also known, the principle of hierarchical control. Simple stated, this principle is as follows. As components combine to produce larger functional wholes in a hierarchical series, new properties emerge. Thus, as we move from organismic systems to population systems to ecosystems, new characteristics develop that were not present or not evident at the next level below. The principle of integrative levels is a more formal statement of the old adage that the ‘’whole is more than a sum of the parts’’ or, as it is often stated ,the ‘’forest is more than a collection of trees ‘’.despite the fact that this truism has been widely understood since the time of the Chinese and Greek philosophers, it tends to be overlooked in the specialization of modern science and technology that emphasizes the detailed study of smaller units on the theory that.  This is the only way to deal with complex matters.  In the real word the truth is that    although findings at any one level do aid the study of another level, they never completely explain the phenomena occurring at that level, thus , to understand and properly manage a forest we must only be knowledgeable about trees as populations, but we must also study the forest as an ecosystem.
In everyday life ecology

In everyday life we recognize the basic difficulty in perceiving both the part and the whole. When someone is taking too narrow a view,  we remark that ‘’he or she cannot see the forest for the trees’’. Technologists, in particular, have often been guilty of this kind of’’ tunnel vision’’.  Perhaps the major role of the ecologists in the near future is to promote the holistic approach to go along with the reductionist approach now so will entrenched in scientific methodology.

Perhaps an analogy will help clarify the concept of integrative levels. When two atoms of hydrogen combine with one atom of oxygen in a certain molecular configuration we get water (H20  HOH), a compound with new and completely different properties than those of its components. No matter how deeply we might study hydrogen and oxygen as separate entities we would certainly never understand water unless we also studied water. Water is an example of a compound in which the component parts become so completely bound or  ‘’inte – grated’’  that the properties of the part are almost completely  replaced by the completely different properties of the whole. There are other chemical compounds, however, in which the components partly disassociate or ionize so that the properties of the parts are not so completely submerged. Thus, when hydrogen combines with chlorine to form hydrochloric acid (HCI), the hydrogen component ionizes to a much greater extent than in water, and the properties of the hydrogen ion become evident in the acid properties of the compound. So it is with ecosystems. Some are tightly organized or integrated so that the behavior of the living components becomes greatly modified when they function together in large units. In other ecosystems  biotic components remain more loosely linked and function as semi-independent entities.  In the former cause, we must study the whole as the major parts to understand the whole, in the latter case, it is easier to understand the whole by isolating and studying the part in the traditional reductionist manner. In general, biotic systems evolving under irregular physical stress, as the desert with uncertain rainfall, are dominated by a few species while those in benign environments, sues as the moist tropics, tend to have many species with Bothe populations and nutrients showing an intense degree of symbiosis and interdependence.
Example by ecosystem

A striking example of the difference that the degree of systems integration con have on the behavior of a species component is seen in cases where insects become pests when displaced from their native ecosystems. Most agricultural pests turn out to be species that live reactively innocuous lives in their native habitat but become troublesome when the invade, or are inadvertently introduced into, a new region or new agricultural system. Thus, many pests of American agriculture come from other continems (and vice versa), as, for example, the Mediterranean fruit fly, the Japanese beetle, and the European corn borer (the list is very long). In their original habitat these species functioned as parts of well- ordered ecosystems in which excesses in reproduction or feeding rate are controlled; in new situations that lack such controls, populations may behave like a cancer that can destroy the whole system before controls can be established. As we shall note in a later chapter, one of the prices we have to pay for high crop yields is the increasing cost of artificial chemical controls that replace the disrupted natural ones.

Some attributes , obviously, become more complex and variable as we proceed from the small to the large units of nature, but it is an often overlooked fact that rates of function may become less variable. For example, the rate of photosynthesis of a whole forest or a whole corn field may be less variable than that of the individual trees or corn plants within the communities, because when one individual or species slows down, another may speed up in a compensatory manner. More specifically we can say that homeostatic mechanisms, which we may define as checks and balances (or forces and counterforce’s) that dampen oscillations, operate all along the line. We are all more or less familiar with homeostasis in the individual, as, for example, the regulatory mechanisms that keep body temperature in many fairly constant despite flucations in the environment. Regulatory mechanisms also operate at the population, community, and ecosystem level. For example, we take for granted that the carbon dioxide content of the air remains constant without realizing, perhaps, that it is the homeostatic integration of organisms and environment that maintains the steady conditions despite the large volumes of gases that continually enter and leave the air.

The phenomena of functional integration and homeostasis means that we can begin the study of ecology at any one of the various levels without having to learn everything there is to know about adjacent levels. The challenge is to recognize the unique properties of the level selected and then to devise appropriate methods of study. In everyday language this can be restated as follows: to get good answers we must first ask right questions. In subsequent chapters we will have occasion to cite examples of how man’s progress in solving environmental problems is often slowed because the wrong question is asked, or the wrong level focused upon.

As suggested in figure 1 – 1, quite different tools are needed for different levels; we do not use a microscope to study a whole ocean, a whole city, or the behavior of carbon dioxide in the atmosphere. In recent years advances in technology have expanded the scale of ecological study considerably, so that if we put our minds and money to it, appropriate measurements can be made as readily at the ecosystem level as at the individual level. Technology, of course, remains a two edged sword. Many of man’s severest problems can be traced to what might be called a ‘’careless and arrogant,’’ high energy – consuming tech – nology, which runs roughshod over human values and natural laws. However, once this self – defeating and very dangerous trend is recognized, technology can be turned around to work in the opposite direction.