We are all aware that the kinds of organisms to be found in both rural and urban areas in a particular part of the world depend not only on the local conditions of existence – that is, hot or cold, wet or dry – but also on geography. Each major land mass as well as the major oceans have their own special fauna and flora. Thus, we expect to see kangaroos in Australia but not elsewhere; or hummingbirds and cacti in the new world but not in the old world. And the different continents are the original home of different races of human beings and different kinds of domesticated plants and animals. The fascinating story of adaptive radiation is considered in more detail in other volumes of the modern biology series that deal with animal and plant diversity. From the standpoint of the overall structure and function of ecosystems, it is important only that we realize that the biological units available for incorporation into system vary with the geographical region. The word taxa is a good term to use in this connection when we wish to speak of orders, families, genera, and species without wishing to designate a particular taxonomic category. Thus, we can say that both local environment and geography play a part in determining the taxa of an ecosystem. As already indicated, the type and level of energy plays an important role in determining the kinds as well as the numbers of organisms present. As will be discussed later, the biotic community itself may play an important role in this regard.

       What is not always so well understood is that ecologically similar, or ecologically equivalent, species have evolved in different parts of the gloge where the physical environment is similar. The species of grasses in the temperate, semiarid part of Australia are largely different from those of a similar climatic region of north America, but they perform the same basic function as producers in the ecosystem. Likewise, the grazing kangaroos of the Australian grasslands are ecological equivalents of the grazing bison (or the cattle that have replaced them) on north American grasslands since they a similar functional position in the ecosystem in a similar habitat. Ecologists use the term habitat to mean the place where an organism lives, and the term ecological niche to mean the role that the organism plays in the ecosystem the habitat is the ‘’address’’ so to speak, and the niche is the ‘’progression’’. Thus, we can say that the kangaroo, bison, and cow, although not closely related taxonomically, occupy the same niche when present in grassland ecosystems.

       In recent years professional ecologists have become intensely interested in quantifying the ecological niche in terms of a set of conditions within which each kind of organism can operate (the fundamental niche) or does operate (the realized niche). In this manner ‘’niche width’’ and ‘’niche overlap’’ between two or more kinds of organisms can be compared. The reason for such interest stems from the discovery that the way in which taxa divide up available space, energy, and resources has a profound influence on the evolution of structure and behavior and on the origin and extinction of species. We will touch but briefly on these matters in this book, but if you wish to read more we suggest you start with the review by Whittaker, Levin, and root (1973) and the book entitled geographical ecology by MacArthur (1972)

Man, of course, has a considerable influence on the taxonomic composition of many ecosystems, not only urban but remote ones in which he may but a minor inhabitant. We might think of his efforts to remove or introduce species as a sort of ecosystem surgery; sometimes the surgery is planned, but too often it is accidental or inadvertent where the alteration involves the replacement of one species with another in the same niche, or the filling of an unoccupied niche, the overall effect on the function of the ecosystem may be neutral or beneficial. Thus, when Midwestern prairies were converted to agricultural fields the native prairie chicken was unable to adapt to the altered environment but the introduced ring – necked pheasant, which had become adapted to the agro – ecosystem in Europe (partly, at least, through artificial selection by man) has thrived in the altered in the altered landscape. As far as the hunter is concerned the ‘’game bird niche’’ has been more than adequately filled by the introduction. Too often, how – ever, the introduced species become pests, creating serious environmental problems. Especially grave problems often result with domesticated plants and animals ‘’ escape’’ back to nature and become severe pests because of the absence of both artificial or natural controls. Damage caused by weeds and feral2 animals to crops, watersheds, forests, and lakes can be extremely costly in terms of diverting energy away from human use. on some of the Hawaiian islands, feral goats have had a more severe impact on soil, and fauna that has man’s plow and bulldozers. Detrimental impact by man on his environment is not confined to industrialized societies nor to the twentieth century. overgrazing and other types of overexploitation of solar – powered nature have contributed to the downfall of many early civilizations.

        Species vary greatly in the rigidity of their niches. Same species may function differently that is , occupy different niches – indifferent  habitats  or  geographical  regions. the  case of the coral, as discussed in the previous section, is probably a good illustration. Man, himself, is another good example. In some regions man’s food niche is that of a carnivore (meat eater), while in other regions it is that of a herbivore (plant eater); in mosts cases man is omnivorous (mixed feeder). Man’s role in nature, as well as his whole way of life and cultural development can be quite different according to the major energy source on which he depends ofr food.

         Species vary, of course, in the breadth of their niche. Nature has its specialists and its generalists. There are insects, for example, that feed only on one special part of one species of plant, other species of insects may be able to live on dozens of different species of plants. Among the algae there are species that can species that can function either as autotrophy  or as heterotrophy; other species are obligate autotrophy only. Although more study is needed, it would seem that the specialists are often more efficient in the use of their resources and, therefore, often become very successful (that is, abundant) when their resources are in ample supply. On the other hand, the specialists are more vulnerable to changes, such as might result from marked environmental or biological upheavals or the exhaustion of the resource. Since the niche of nonspecialized species tends to be broader, they may be more adaptable to changes, even though never so locally abundant. Most natural ecosystems seemto have a variety of species, including both specialists and generalists.