An ecosystem is the most complex level of organisation in nature.  It is made up of a community and its abiotic (nonliving and physical) environment, including climate, soil, water, air, nutrients and energy. Ecologists who try to link together and analyse the many different things mainly physical and biological activities in an environment are called system's ecologists.  Their studies often focus on the flow of energy and cycling of materials through the ecosystem.

Energy flow: Ecologists categorise the elements that make up or affect an ecosystem into 6 main parts based on flow of energy and nutrients they are:

1  The sun

2  Abiotic substances

3  Primary producer's

4. Primary consumer's

5. Secondary consumer's

6. Decomposers

The sun provides energy that primary producer's need to make food.  Primary producers mainly consist of green plants, such as grass, trees, which make food by the process of photosynthesis.  Plants also need abiotic substances, such as phosphorus, water, etc. to grow.  Primary consumers include mice, rabbits, and grasshopper’s etc. and other plant eating animals.  The predators i.e. secondary animals which include foxes, weasels etc. eat animals.  Decomposers such as bacteria and fungi, break down dead plants and animals into simple nutrients.  The nutrients go back into the soil and plants again use it.

The series of stages energy goes on through in the form of food is called a good chain.  A simple food chain would be one in which grass is a primary producer.  A primary consumer's such as rabbit eats the grass.  A hawk or a vulture may eat the rabbit, which is a secondary consumer.  Decomposing bacteria break down the uneaten remains of dead grass, rabbits and hawks as well as body water produced by them.  Most ecosystems have a variety of producer's, consumer's and decomposers, which forms an overlapping network known as food web.

Most organisms have a low ecological efficiency.  This means they are able to convert any small fractions of the available energy into stored chemical energy.  The energy remaining from the activity is burned up.

Because a lot of energy escapes as heat at each step of food chain, all ecosystems develop a pyramid of energy.  In many land ecosystem's the pyramid energy results in pyramid of biomass.

Cycling of materials

All living things are composed of certain chemical elements and compounds.  Chief among them is water, carbon, hydrogen etc. All of these materials cycle through eco-system again and again. The cycling of phosphorus provides an example of this process.  Plants take up phosphorus compounds from the soil or other animals that they eat.  Decomposers then return phosphorus to soil.

Changes in Ecosystem

In the past, the concept of balance, largely unchanging ecosystem was thought to be especially descriptive of climax communities.  But modern studies of ecosystem have altered the studies.

Conclusion based on population studies from Isle Royale point out some of this change in thinking.  For a long time, Isle Royale had neither moose nor wolf population.  Then, the first moose swam to the Island in about 1900.  The moose population reached about 3,000 by 1930, but then declined due to scarcity of plant food.

The mouse population increased again between 1948 and 1950.  But above this time, wolves made their way to the island.  As they killed the moose for food, the wolf population grew.  Ecologists pointed to Isle Royale as an example of the way in which predators can control prey and thus contributed for the development of order and stability in eco-systems.

But at beginning in mid 1960's the moose and wolf population began to fluctuate.  The eco-system began to become imbalance.  During the 1950's, when it looked as if wolves were controlling the moose.  The winter resulted in snow with a hard crust. Under these conditions the wolves could easily kill moose.

Around 1965, winter on Isle Royale returned to normal and the wolves caught fewer moose.  By the early 1980's the moose population had again become very large even though wolf population began to decline, despite the abundance of moose.

Further ecological studies indicate the changes in the food plants and nutrients may be as important in regulating the moose population as are the wolves.  As the wolves of Isle Royale, it appeared that in breeding and diseases not lack of moose - were behind the die off of the population.  Thus, it seems that predator - prey models of population control probably are oversimplifications.  Natural systems are filled with compensating mechanisms that help stabilise nature.  Hence population often need to be understood from the perspective of the entire eco-system.          

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