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An ecosystem is generally defined as a community of organisms living in a particular environment and the physical elements in that environment with which they interact. But where does one particular ecosystem end and another begin? An ecosystem can be as small as a field or as large as the ocean, depending on the scale that the researcher is examining. The borders of an ecosystem may be clear, such as a pond; other borders may be less easy to define, such as grassland that gradually changes into brush lands.
Just as there is an immense diversity of individual species on the planet, so is there a rich diversity of ecosystems, from the icy arctic zones to tropical forests lush with plants and animals. Even the depths of the oceans, once thought to be barren, are now known to be teeming with living microorganisms and other life. There is much that remains to be discovered. Biologists do not know with any certainty how many species there are or even why some areas, such as the tropics, are richer in biological diversity than others.
It is known that human activities, mostly unintended, threaten biodiversity by altering habitats and introducing non-native species. There is a general agreement on the importance of protecting biological diversity, especially since humans depend on the services provided by living organisms and ecosystems. There is less agreement on the best approaches to conservation and how to balance preservation of habitats while meeting human needs.
Food Chains and Food Webs
The source of all food is the activity of autotrophs, mainly photosynthesis by plants.
They are called producers because they can manufacture food from inorganic raw materials.


The food chain consists of four main parts:
  1. The Sun :-
    It provides the energy for everything on the planet.
  2. Producers :-
    These include all green plants. These are also known as autotrophs, since they make their own food. Producers are able to harness the energy of the sun to make food. Ultimately, every (aerobic) organism is dependent on plants for oxygen (which is the waste product from photosynthesis) and food (which is produced in the form of glucose through photosynthesis). They make up the bulk of the food chain or web.
  3. Consumers :-
    In short, every organism that eats something else is a consumer. They include herbivores, carnivores, parasites, and scavengers. Primary consumers are the herbivores, and are the second largest biomass in an ecosystem. The animals that eat the herbivores make up the third largest biomass, and are also known as secondary consumers. This continues with tertiary consumers, etc.
  4. Decomposers :-
    These are mainly bacteria and fungi that convert dead matter into gases such as carbon and nitrogen to be released back into the air, soil, or water. Fungi and other organisms that break down dead organic matter are known as saprophytes. Even though most of us hate mushrooms or molds, they actually play a very important role. Without decomposers, the earth would be covered with trash. Decomposers are necessary since they recycle the nutrients to be used again by producers.
Each level of consumption in a food chain is called a trophic level.
Most food chains are interconnected. Animals typically consume a varied diet and, in turn, serve as food for a variety of other creatures that prey on them. These interconnections create food webs.
Food webs show how organisms living in an ecosystem depend on one another to obtain the nutrients and energy they need to live. For example, a food web for an oak tree is given. Caterpillars eat the tree’s leaves for nourishment; beetles live on the tree and eat the bark; woodpeckers eat beetles living on the tree; jays and squirrels eat the acorns; and the oak tree makes its own food with energy from the sun through the process of photosynthesis. The web becomes more complex as additional organisms are added to it.


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