I UNIT 10 ECOSYSTEM :BASIC CONCEPTS ' Structure 10.1 Introduction Objectives 10.2 Ecology 10.3 Ecosystem and its Structure 10.3.1 The Biotic Component 10.3.2 The Abiotic Component 10.4 Functions of an Ecosystem 10.4.1 Food-Chain nnd Food-Web 10.4.2 Trophic Levels 10.4.3 Energy, Sources of Energy andEnergy How 10.4.4 Biogeochemical Cycles 10.5 The Concept of Habitat and Ecological Niche 10.6 The Concept of Homeostasis 10.7 Types of Ecosystems 10.8 Summary 10.9 Key Words 10.10 Further Readings 10.1 1 Answers to SAQs 10.1 INTRODUCTION The purpose of this unit is to make clear some of the basic concepts in ecology. After defining what is ecology, we will also outline the fundamental structure of an ecosystem. We will study how each ecosystem has an abiotic (non-living) and a biotic (living) component and how there is a continuous interaction among these two components. In each ecosystem there are producer organisms and consumer organisms. Producers generate food with the help of solar energy while all the consumers are directly or hldirectly dependent on these producers, for food. There are different kinds of food-chains and we will outline some examples in this unit. Energy passes along the food-chain in each ecosystem - it is a one-way traffic. We will, however, note that the various minerals continuously circulate between the abiotic and biotic components of the ecosystem. These are the so called biogeochemical cycles. We will then try to follow the concept of ecological niche and also the concept of homeostasis. Objectives I After studying this unit, you should be able to describe the components of an ecosystem, elaborate upon the biotic and abiotic components and the interaction between them, explain the food-chains, food-web and energy flow in an ecosystem, outline various biogeochernical cycles, and explain the concept of ecological niche as well as the concept of homeostasis. 10.2 ECOLOGY Ever since the time of Greek philosophers, there is considerable written material that is definitely ecological. Man must have been using his ecological knowledge since time immemorial. This must have been important for his survival in the wild environment. Even today many tribal people all over the world are using this ecological knowledge and living happily in the wilderness. With the tremendous increase in population, every one including you, must understand ecology more thoroughly. This is essential for our survival and well being. The word ecology was recently coined by a Gerrnan biologist Emst Haeckel, in 18.69. This word has a Greek root oikos meaning 'house'. Thus, an ecologist actually.studiesthe organisms 'at home'. For the sake of definition we will call ecology as the study of relationship of organisms to their environment or surrounding. It really inclu&sThe study of the structure and function of nature. Right here you must note that human being is also another organism and study of his environment is equally part of ecology. Broadly speaking, ecology can be subdivided into two major branches (1) autecology, and (2) synecology. When you are studying one organism and its relationship with the surrounding environment, you are doing autecological study. Study of groups of organisms which are associated together as a unit is the realm of synecology. For example, you are interested in a particular snake and you want to find out where it lives, what it eats, how many eggs does it lay, etc., you are pursuing autecology. On the other hand, if you are interested in a particular river, the kind of terrain it is flowing through, the kinds of fishes and other animals as well as plants that are found in the river, the inter-relationshipof plants-animals and physicochemical factors of water, etc., then the objective is synecological. Both these aspects are important but the synecological studies give total picture of the situation. Of course, much efforts and time are also required to complete synecological work. The autecological approach is relatively straightforward but by no means easy or simple. Study of ecology is becoming more and more important as we are modifying the environmpt surrounding us. Here, the word environment is used broadly to denote everything that surrounds us -that is it includes other living organisms as well as non-living things. This and subsequent units will give you sufficient idea about the basics of ecology and its usefulness in day to day planning for economic development. 10.3 ECOSYSTEM AND ITS STRUCTURE To study ecology, one has to take a s&er unit ipd this unit is called as ecosystem. Ecosystem is a basic unit of ecology. For example, Grassland ecosystem, Forest ecdsystem River ecosystem, Lake ecosystem. Ecosystem is defined as structural and functional unit of ecology. This means, each such system has a defdte structure and each smcture associated with an ecosystem has a definite role to play in the functioning of an ecosystem. Each ecosystem is made up of living organisms and their surrounding non-living components. These two parts of an ecosystem are therefore, called as living (biotic) and non-living (abiotic) components. These two parts do not stand-3iisolation, however, the biotic component requires abiotic components. Actually, many minerals are continuously moving from abiotic world to biotic world and again back to abiotic world. Living organisms are also continuously modifying their surrounding non-living environment, and as we shall see later, help the process of ecological succession. Interdependence of organisms (within themselves) and on abiotic world can be studied by carefully observing the plants and animals around your place. Look, for example, how nodules on the roots of legumes harbor bacteria which 'fix' nitrogen for the plant. See how the type soil and moisture content decide what kindof vegetation can grow in a particular area. Ecosystem is thus, essentially a biotic community in continuous interaction with its physical environment. It is a dynamic entity. Taking a large, aslronautical view, the entire biosphere can be seen as a gigantic single ecosystem. We will actually see later that some materials like water, carbon dioxide, etc., are circulated globally. By biosphere we mean that portion of the earth and its atmosphere which supports life. All the different ecosystems that we mentioned earlier are then parts of the biosphere. Later, we will be analysing some of these ecosystems in details to have a look at their abiotic and biotic components. Let us fast study some basics about these two components. 10.3.1 The Biotic Component The biotic component is, of course, all the living organisms present in an ecosystem. Some of these organisms are large while some are tiny. Considerable number of organisms are so 'midute that these cannot be seen with a naked eye. So bacteria, fungi, small algae and plants, shrubs and trees, insects and birds, all are parts of the biotic component. Man is also a part of this vastassemblage of organism He is the only organism, however, who can modify vast areas of his environment within very short time. Apart from the strength of his Ecosystem :Basic Concepts muscles, he alone can make tools and machines and use them Biotic component is further subdivided into two broad groups on the basis of their trophic (nutritional) status. These groups are : 1) Autotrophic organisms which, as their name implies, are 'self nourishing' organism~, In short, these organisms can produce their own food. These organisms are generally, ' chlorophyll possessing plants, though of course, other pigments are also useful. Thus, all green plants mainly are autotrophs and since they use solar energy they are also called as photoautotrophs. Some bacteria can survive on the energy generated through chemical reactions alone and are known as chemoautotrophs. 2) Heterotrophic organisms which, as their name indicates, are 'other-nourishing'. This means they depend on others for nourishment as they cannot prepare their own food. You can also call autotrophic organisms as producers because they produce food. Similarly, heterotrophs can also be identified as consumers as they consume food generated by the producers. Once again all plants are producers and all animals (and other organisms that do not manufacture their own food) are consumers. Sometimes the consumers are grouped into two - (a) macro-consumers, and (b) micro-consumers. The former includes all animals while the latter category includes bacteria and fungi that decompose organic matter produced by living organisms as well as remains of dead organisms. To introduce yet anotber terminology, we can call the organisms that feed directly on plants as herbivores, while those that feed on animal flesh as carnivores. What is important to note is that the biotic component of an ecosystem always consists of producers and consumers or autotrophs and heterotrophs. Each ecosystem will, of course, be different. Each will have its own producers and consumers. No two ecosystems will ever be absolutely similar as far as their biotic component is concerned. Each has'a unique composition. For example, a pond will have algae and other aquatic plants as producers whilei grassland has various grasses and shrubs as producers. 10.3.2 Abiotic Component The abiotic component of an ecosystem is everything else other than the living organisms.'~( is an assemblage of organic and inorganic substances present in an ecosystem. The various : climatic factors that affect the ecosystem functioning are also included in the abiotic component. Once again you will note that each ecosystem is unique in possessing a set of different abiotic factors. Substances like water, arbo on dioxide, phosphates, carbonates, nitrates, etc. and elements like carbon, nitrogen gas, sulphur, phosphorus, etc, are found in the ecosystems.These are called as inorganic substances.