Organisms interact with the biotic and a biotic factor in the environment. An organism may be affected by the changes in the environment, and the environment may be affected by the organism. The change in the environment may, in turn, have an effect on other organisms.

19.2.1 The ecosystem

Interactions in the environment result in the establishment of an organized ecological system or ecosystem. An ecosystem is not simply a random collection of organisms and environmental factors. An ecosystem is a highly organized, structured environment in which all parts exist in a delicate balance. Ecosystem can be studied at a variety of levels from individual species to the entire ecosystem.


A species is a group of organisms so similar to one an other that they can breed in nature and produce fertile off-springs. All the members of a species have similar needs, such as types of food and place of living. Be cause they have the same needs, members of a species often compete with one another for the resources.


The type of environment in which a particular species lives is its habitat. For example, the tops of the trees in a pine forest may be the habitat of a species of birds. A shallow, fast moving, cold water stream may be the habitat of a species of trout. A rotting log is an ideal habitat for various species of insects, fungi, and micro-organisms. The habitat provides the appropriate type of food, shelter, temperature, and so on that members of the species need to live (fig 19.3)

type of environment

Fig.19.3 an organism can live in a particular type of environment

Disturbing the part of an ecosystem that an organism needs to survive is called habitat destruction. Cutting down the trees in a forest is one form of habitat destruction. The destruction of habitats is a serious threat to the survival of many species.


Population is a group consisting of members of the same species that live together in the same area at the same time (fig 19.4a). For example, in 1998, 0.56 million people were living in Quetta. We can say that human population of Quetta in 1998 was 0.56 million.

Similarly, all the mango trees in a fruit garden in a particular year constitute population of mango in that fruit garden.


Populations do not live alone in their environments, in any region; many different populations share the same space. These populations interact with one another in a variety of ways. All populations that live and interact in the same environment make up a community (fig 19.4b).


An ecosystem includes all the communities that live in an area, as well as the abiotic factors in the environment (fig 19.4c). Ecosystems therefore include the water, soil, and climate in the area. A healthy ecosystem includes a wide variety of species in its community. The variety of species in an ecosystem is known as biodiversity.

If enough of a particular type of habitat is destroyed, the species that live in the habitat can die out completely. Or become extinct. When species become extinct, biodiversity in the die out completely, or become extinct. When species become extinct, biodiversity in the ecosystem is reduced.

Fig. 19.4 all the members of a species that live in an area are a population (a). All the populations in the area make up a community (b). Communities together with the abiotic factors form an ecosystem (c).

species that live in an area are a population


All the populations in the area make up a community


Communities together with the abiotic factors form an ecosystem


Where have they gone!

Natural tropical thorn forests once formed one of the most remarkable landscapes of the Indus basin plains in Pakistan. These forests extended from the foothills of the Himalayas (jhelum) to the Arabian sea (karachi). These forests (also called rakhs in the Punjab) were a source of forage, firesood, medicine and fruit. They also supported a thriving wildlife.As a consequence of habitat destruction due to land reclamation and extensive canal irrigation system, overgrazing and large scale falling for firewood, these forests have almost disappeared in the Punjab. Some dominant species of this forest system, such as salvadora oleoides (wan, peelu) can only be found in scattered remnant patches in the southern Punjab districts of bahawalpur, mazaffrgargh etc, or in old graveyards.

scattered remnant patches in the southern Punjab

Fig. 19.5 salvadora oleoides (wan, peelu)

The biosphere

The biosphere is a global ecological system comprising all of earth’s communities or ecosystems. In other words it is the largest ecosystem that comprises all the parts of the earth that support life. The biosphere reaches from the floor of the ocean to the tops of the highest mountains. All together, the biosphere is about 20 km (12.4 miles) thick. You may think that this sounds like quite distance, but the biosphere really makes up a thin layer surrounding the planet. If earth were the size of an apple, the layer that supports life would be only about as the apple’s skin.