 There are different levels of organization in an ecosystem. An ecosystem consists of various levels of organization. The first level of organization in an ecosystem is a species. What is a species? Species is defined as all the organisms of same kind, which are living in an area and which can interbreed, which can live with each other and can interbreed. Next level of organization in an ecosystem is a population. All organisms of same species living in an area. Next level is community. Various populations, populations of different kinds living together in an ecosystem and then ecosystem itself. Interaction of the biotic factors in interaction with the abiotic factors and with each other. The next level is biome. Biome is various types of ecosystems present on different areas of the land on the earth, which are similar. For example, if the grassland ecosystem exists in Pakistan, then the grassland ecosystem of similar type, not of same, but similar type may be present in Australia, may be in United States and at so many other places. The ecosystems of same kind are called the biomes. We say, for example, the wetland biomes, the grassland biomes, then comes the biosphere. The last level, according to our information till now, the biosphere is the part of planet earth which supports life, living organisms. Now we just have a look on few diagrams and which are showing different levels of organization. You can see big type of parrots called macaus. These parrots are usually present in Australia, commonly called Australian parrots in Pakistan, in various types, in various zoos in different cities. These parrots are present. Parrots are of same species. Human beings, they are also of same species. But these parrots are living in different areas, maybe in one area of Australia or maybe in another area of Australia, but all of these belong to the same species. Next comes population, the human population. For example, population of Lahore, all the people which are living in Lahore is the human population of Lahore. Actually, we don't count our population of the other organism like animals, but our recent report says that in Germany, when they count their population, when they do census, then they count the human beings, the cats, population of the cats and the dogs in a city. So, human beings makes one population. For example, all of the people living in Lahore, all of the people living in Pakistan, all of the cats living in a forest, all of the dogs living in a forest. So, population is organisms of same species, living in a particular area, at a particular time. Now, we look at community, look at another diagram, which shows that different types of organisms are living together in an area. There is a water body, a pound in which there are small organisms, which are phyto and zoo plankton. These are microscopic, very small in size. The fish eats them and there is a more big fish on the other side is eating that fish which is eating the zoo and the phyto plankton. Then there are plants in which these fish are also interacting because these zoo and phyto plankton and some bacteria which is present in the sediments as you can see in the diagram. When these fish or the zoo or phyto plankton, they die, they go to these sediments and their dead bodies are degraded by these bacteria. So, the bacteria, the phyto plankton, the zoo plankton, the plants, the fish, the big fish, they are all living in the same place and they are interacting with each other. We call this a community because all the organisms of different species, for example this fish, which is eating the phyto plankton or zoo plankton, is one species of fish. The big fish, which is a carnivore fish, which belong to another species. Then the phyto plankton belong to another species, the bacteria belong to another species, all of these organisms, all of these populations are interacting with each other. We call it a community. Then have a look on an ecosystem. Example of a grassland ecosystem. Ecosystem in which the living factor are interacting with each other and they are interacting with their physical surroundings. This diagram shows that there is abiotic material, the soil. Then there are different types of plants which are growing in that soil. The temperature of environment also affect all of these organisms. Then there are producers, the plants, which makes their energy using the sunlight. Sunlight is another abiotic factor, which is providing the energy to the ecosystem through plants. Cows, for example, the deers, they are eating the producers, the plants and then they are themselves eaten by the carnivores. You can see a tiger. Tigers eats these cows or maybe the deers and this is how the complex interactions in the ecosystems they are shown. So, ecosystem is an interaction of biotic and abiotic factors in a specific area which share similar physical properties and chemical characteristics. Have a look on another picture which shows another ecosystem which have very long and large used trees and in front of these there are grasses. Let us have a look at another. This is an ecosystem which is present near to the very cold areas. You can see behind that there are mountains which are covered with snow and in front of these there is a grassland. There are grasses which are growing in this particular area. Organisms which are living in this area are specific for living in that particular environment. Then the biosphere. The picture of the globe is showing in green and light green and dark green shades all of those areas where the living organisms are present. There is a distribution of life. This whole area is called the biosphere. In organisms in relationship with their environment we use certain terminologies to describe the relationship of organisms with themselves and with their environment. The first or very first terminology which is used is called the habitat. Habitat of an organism is actually the physical location of an organism in which it normally lives. For example, you can see in the picture that a camel lives in a deserted type of area. There are two approaches which is like a desert. A tree, a large tree may be the habitat of an owl. Many trees are habitats for different types of birds. Then there is another word used niche. Niche is defined as the ecological role of an organism. Ecological role includes how its interactions with the environment. Where it will do live? What is its habitat? How it gets its food? How it gets shelter? And how it interacts with the physical surroundings? So, niche is actually the ecological role of an organism. Look at the diagram below which shows two camels. And on these two camels, two men, they are travelling. You can see that camels are moving and they are covered by their owners by a cloth. That is, their owners are actually helping them to cope with the temperature. Niche is the ecological role of the organism. That is, you can see that camel is in interaction with the human beings and they are protecting it from the harsher environments and also providing it its food. So, niche is the total ecological role of an organism. And habitat is the location of the place where an organism is living normally. Biotic and abiotic components of an ecosystem. Biotic components or the living components consist of animals, plants, algae, fungi, bacteria, viruses, all of the organisms which are living. And the non-living or the abiotic surroundings, the components of the environment, the ecosystem. They includes air, water, lightning, soil, rocks, all of the things which are present in an ecosystem. Now, the energy flow in an ecosystem. How energy enters in an ecosystem and how it circulates within the ecosystem is a very important topic. The energy in all of the ecosystems of the world comes through sunlight. The primary source of energy inside the ecosystem is always sunlight. There are organisms which are called producers, the plants or the photosynthetic bacteria or the algae which have capability to convert the energy which is coming through sunlight into chemical form of energy. And they can also convert the inorganic chemicals present inside the soil or inside their surroundings into organic form of chemicals. So the primary source of energy in an ecosystem is sunlight which enters in the ecosystem through plants, the producers or algae which are also the producers or the photosynthetic bacteria. All those organisms which can conduct the photosynthesis. We call them autotrophs. These are the organisms which can utilize the sunlight to make their energy and their food. We call them autotrophs. Autotrophs themselves by themselves. Trophs food, that is they can make their own food. The next level of energy flow is called the hydrotrophs or the consumers. These are the organisms which cannot carry out the photosynthesis. They need an organic source as their food to take energy from the environment. Animals for example. Animals are consumers or the heterotrophs. The heterotrophs, the animals, the consumers have to get their food primarily from the plants. The producers are the autotrophs. And among the consumers there are classes. There are groups. There are three major groups among the consumers called the herbivores, carnivores and the omnivores. Herbivores are those organisms which eat up the plants. That is, they eat the autotrophs. These herbivores, for example the deers, they eat up leaves of the plants, the cows, they eat up the grasses which are also different types of plants. Then these herbivores are eaten up by other organisms called the carnivores. For example, the lions, they eat the cows. Then the omnivores. These are organisms which get our food which consist of both animal and plant matter. For example, human beings, us. We are omnivores. We eat proteins. We eat meat. We also eat plants in the form of vegetables, beans. So we are omnivores. So consumers are divided into herbivores, the plant eaters, carnivores, the meat eaters and omnivores which eat both meat and vegetables. The pyramids in ecology. In ecology, we divide or we show these trophic levels in the form of a pyramid. This pyramid shows, as you can see in this diagram in front of you, that the producers, the plants comes at the basic level. First of all, according to energy flow, they have to produce food. They have to get the energy from sunlight and convert it into the chemical energy and the carbohydrates. Then the consumers, for example animals, have to eat up the producers. And then the third level, the decomposers. The decomposers are the organisms which live upon or which feed upon the dead organic matter. So the decomposers, when the animals and the plants, both of these die or there are some parts fall apart. For example, the fallen leaves, the fallen branches of the trees and everything that is now dead. For example, a dead animal, a dead plant. These are eaten up by the decomposers like bacteria and the fungi. And they convert them into smaller ingredients which are absorbed by the soil, which are kept inside the soil. And the producers, the plants, again can take them up into this from these simpler forms and utilize them for their life processes. So it means that this energy flow ultimately results in a cycle. At the base, you can see that producers are the larger part of the pyramid. They have to be more because they have to produce energy. They have to produce food. And then they have to provide food to the next level, consumers. Consumers, they are fewer in comparison to the producers. And then the decomposers, they are more few. They are more or less. Their number is lesser than that of consumers. So this makes an ecological pyramid. Then have a look on the next diagram. It shows you the producers, grasses at the base level. It is dividing the consumers into its ingredients, into its parts. The herbivores, you can see a rabbit, a grasshopper, a mouse which feed upon the grasses. These are called herbivores or these are also called primary consumers. That is, they are the consumers which directly have to feed upon the plants, the producers. The next level, these are called the primary carnivores or these are called the secondary consumers which feed upon the herbivores. You can see the snakes which feed upon the rats in the mice and the rats which eat upon the grasshopper. Then the upper level is the secondary carnivore. Secondary carnivore is a carnivore that eat upon another carnivore. For example, the owl which may feed upon the rat and the rat itself feed upon an insect. These are called the tertiary consumers. So primary consumers eat upon the grasses, the plants. They are also called herbivores. Secondary consumers are primary carnivores and the tertiary consumers are the secondary carnivores. And then comes the decomposers. We also use the term food chain. Usually feeding relationship between organisms exist in the form of a chain. For example, look at a diagram which shows that there is a grass which is taking the energy from sunlight for its life processes. This is eaten up by a grasshopper. Grasshopper is eaten up by a rat or a mole. The rat or mole is eaten up by an owl. This makes a food chain. Grass, grasshopper, rat and the owl. The relationship between the organisms are not as simple as a food chain. We actually simply describe a uniform or a single way, a single direction relationship between the organism by a food chain, but actually these relationships are very complex. These could not be explained by only food chains. Let us have a look on the next diagram. It shows you a food web. Actually, relationship between different traffic levels are complex because we know that there are diverse forms of organisms present in an environment. You can see that grass is present as producer. There are different types of herbivores which eat it up. The rabbits eat it, some mice eat it and some grasshoppers eat it. So there are different types of herbivores. Then there are primary carnivores. Snakes eat the moles and grasshoppers are eaten up by the big rats and owl eats up the snakes and the rats, both. So the relationship between is we can make different food chains out of this complex network. This complex network is called a food web because the more the diversity, the more complex relationship of feeding will exist between the organisms and we will not be able to explain it by the food chains and we have to make the food webs. We call this complex interaction of feeding between organisms a food web.