 Assalamu alaikum. In today's lecture, we are going to talk about the ecosystems. What are the ecosystems? What are their different properties? Then, we are also going to talk about the basic components of an ecosystem. And then, we are going to talk about some examples of different ecosystems from the world and from Pakistan. We start with our today's lecture. Ecosystem. What is an ecosystem? Ecosystem is an area in which the living factors or the biotic factors are living in interaction with each other and with their physical and chemical environment, that is the abiotic factors. Usually, ecosystems consist of a specific type of climate in which all the biotic factors, that is the living components are well settled in their environment, in that environment and the physical factors support the presence of a particular type of life in that area. So, ecosystem is an area, a specific part of the world where a biotic community, we can say the biotic factors are living in interaction with each other and with their physical and chemical environment, that is the abiotic factors. Now, there are many types of ecosystems, mainly divided into two major categories. Aquatic and terrestrial. Aquatic. Aquatic ecosystems are those which are present in the waters, in the oceans, in the seas and sometimes lakes, small lakes, very large lakes, pounds because we know that a very major part of the world consists of water. So, water supports different kinds of life. Then comes the terrestrial ecosystems. Terrestrial ecosystems are the ecosystems present on land. As we know that land is also diverse. We have different types of lands on mountains, in the valleys, the deserts, those areas which are covered with snow all the year like the Arctic area and so on. So, terrestrial ecosystems, they are also very diverse in their forms. So, ecosystems if we widely divide them, we can divide them into two categories. Aquatic, present in water, terrestrial, present on land. Now, we are going to talk about basic components of an ecosystem. What are the basic components of an ecosystem? If we widely divide the components of an ecosystem, we can divide them into two. The biotic components, the living factors or in other words the living organisms and the abiotic factors or the non-living factors. For example, water, temperature, air, light, the biotic factors include the plants, the animals, the bacteria, the viruses, the fungi, the algae, all of the living organisms. So, we divide the components of an ecosystem into biotic and abiotic, living and non-living. Now, first we talk about the biotic components, biotic, the living components, living components of an ecosystem. Living components, when they are further divided into subgroups, we divide them according to the energy flow. We know that all the energy which is entering into an ecosystem is coming through sunlight. Sunlight brings the energy and the plants or some other organisms which can carry out a specific process called photosynthesis, can use the sunlight energy and convert it into chemical form of energy and food, food in the form of carbohydrates. This food and this energy then flows into whole of the living part of ecosystem. From plants to animals for example, and from animals to fungi, the other decomposers. We divide the biotic components by this process we called the trophic levels, that is the feeding levels, different feeding levels in an ecosystem. Because we know that feeding or energy flow, this is the major property of life. According to the trophic levels, we divide organisms, the living organisms into three major categories, the producers, the consumers and the decomposers. Producers for example, plants, algae, consumers for example, the animals and the decomposers, the fungi and the bacteria. Producers, the plants, algae, few bacteria, some protesta, unicellular organisms, we call them producers because they have a capability to convert the sunlight energy by the photosynthesis process and another process called Calvin cycle into the chemical form of energy that is utilized by the living organisms for their life processes. And they can also convert in organic carbon molecules and make organic carbon molecules for their own use and for the use by the consumers. We make energy pyramids or food pyramids in the ecosystem. The slide shows that the producers are at the base of the energy pyramid and the food pyramid. Producers, the plants, they are much more in their numbers because they have to produce food and of course, they also have to sustain themselves for producing more food. At the next level comes the consumers, the animals for example, they have to consume the food produced by the producers. So, their number is less than that of producers because they have to eat upon producers and the producers have to sustain for supporting the animal life form. If they eat up all the producers, then what will happen? They will die. So, consumers, their number is less than that of producers. Then comes the third part, the decomposers. These are very important. Some fungi and some bacteria, they are decomposers. They decompose the organic matter. For example, the dead bodies of plants and animals, they also decompose or break down the, for example, for different parts of organisms body. For example, the fallen branches of the trees, the fallen leaves of the trees, as I have told you, the leaves of the trees fall and their small branches also fall. And they, under the same tree and under that tree, make a whole bed on the ground. They slowly become a part of the soil. And here, fungi and some bacteria, which break them down and convert them into very small parts, and ultimately, make them into different types of organic and inorganic compounds. These compounds can be used as plants for their food. Plants cannot directly use their fallen parts. Instead, they can use the small parts of bacteria that are converted into small parts, which can be used as compounds. So, decomposers have a very important role. The role of decomposers is actually to convert the dead organic matter, maybe a fallen part of a plant, maybe the dead body of an animal or a plant. And they convert it into smaller molecules, smaller compounds, which could be utilized by the producers, plants, again through the soil. And then, they become part of the, somehow part of the plants body. And then, it comes back to the consumers and then to the decomposers and back to the producers. So, a cycle continues like this. So, the producers, they make food and they convert the sun, the energy coming through sunlight into chemical energy and they convert inorganic carbon molecules into the organic carbon molecules. Without the producers, no life. Plants are necessary for supporting life. Also, photosynthetic bacteria are necessary for life. The algae, they are also required for life. You look at another pyramid. This pyramid shows a further leveling of different types of organisms. We can see at the base, there is a grass producers. They utilize the sunlight, make carbohydrates, make synergy. These are eaten up by the herbivore animals, the animals which actually eat upon the plants. Then, we also call them primary consumers. Then, there are certain next level, the organisms which eat upon the primary consumers, the herbivores, which eat the herbivores. For example, cows eats grass and lion eats the cows. Grass is producer, cow is primary consumer, lion is secondary consumer. Sometimes, there are tertiary consumers. We also can call them secondary carnivores, which eat upon other carnivores. For example, there are rats or mice or frogs or sometimes grasshoppers which eat up the grass. Then, a mouse, for example, eats upon that grasshopper. A snake eats upon the mouse and an eagle eats upon the snake. So, an eagle is actually eating a snake which has already eaten a mouse and a mouse had eaten a grasshopper and grasshopper had eaten grass. So, the eagle is a tertiary consumer. We also can call it a secondary carnivore. That is the carnivore which is eating another carnivore. In ecosystems, lot many chains and pyramids like this do exist. These are actually relationships between different feeding levels. We call them pyramids because their shape, when we make them in the form of numbers, they are in the form of a pyramid. That is, producers more. They have to support a consumer life, the herbivores. Then comes carnivores. Carnivores number is fewer than the herbivores. Then comes the secondary carnivores. Their number is also less than that of the primary carnivores and so on. So, these are the ecological pyramids. The relationship between the biotic factors according to their feeding levels.