In 1935, Arthur Tansley, an English botanist, first coined the term ECOSYSTEM for biotic and abiotic components considered as a whole.

Eugene P. Odum, in 1983, defined  ecosystem as "any unit that includes all the organisms that function together in a given area interacting with the physical environment so that a flow of energy leads to clearly defined biotic structures and cycling of materials between living (biotic) and nonliving (abiotic) parts.
 

The place of ecosystem in the ecological hierarchy is below LANDSCAPE and above biotic COMMUNITY.

An ecosystem consists of abiotic components and biotic components. Abiotic components may include water, air, substrates (or food), and other physical and chemical factors such as temperature and pH.  Biotic components may include all interacting populations of organisms either as producers or consumers.
 

(1) Abiotic Components

Let's make a list of abiotic factors that are important to life.
 
 

Liebig’s Law of the Minimum:
 

In the context of determining which plant nutrient was most crucial for plant production, Justus Liebig, a German (1840) chemist, stated that “growth of a plant is dependent on the amount of foodstuff (nutrients) which is presented to it in minimum quantity”, or plant growth is largely constrained by the limiting factor. So it can also be called "the law of the limiting factor." Limiting factor defined: Any process (production, decomposition, energy flow or nutrient cycling)  in an ecosystem depends upon a complex of conditions or factors. Any factor that is at the lowest availability or at the highest constraint is said to be a limiting factor. Liebig's law of the minimum is often extended to any system (or case) that involves multiple factors, and is frequently used to identify crucial conditions (variables) or major causes of a problem. This "reductionistic" logic has some shortcomings: (1) interacting factors; (2) situations can change spatially and temporally; (3) the influence of unknowns; (4) 'too much of a good thing' can be bad too.

Limits of the Tolerance Concept or the Law of Tolerance:

Both too little and too much could be a limiting factor (V. E. Shelford 1913).  The success of an organism, population, or community depends on a complex of conditions; any condition that approaches or exceeds the limit of tolerance may be said to be a limiting factor.
 
 
 

--Organisms may have a wide range of tolerance for one factor and a narrow range for another.

--Organisms with wide ranges of tolerance for all factors are likely to be most widely distributed.

--When conditions are not optimum for a species with respect to one ecological factor, the limits of tolerance may be reduced for other ecological factors due to interactions among factors.
 
 

(2) Biotic Components
 

Trophic structure:

The connections of populations based on the transfer of food energy is called trophic structure.

The transfer of food from the source in primary producers (autotrophs, e.g., plants) through a series of organisms that consume and are consumed is called the food chain.  A food web is a group of interlocked food chains.

Example: We eat big fish that ate the little fish that ate the zooplankton that ate the phytoplankton that fixed the sun's energy.

Food chains are of two basic types: the grazing food chain, which, starting with green plants base, goes to grazing herbivores; and the detritus food chain, which goes from nonliving organic matter into microorganisms and then to detritus-feeding organisms and their predators.

Organisms whose nourishment is obtained from the sun by the same number of steps are said to belong to the same trophic level.  For example, green plants as primary producers are at the first trophic level, plant-eating herbivores are at the second level, and primary carnivores are at the third level.

Through these trophic structures and their interactions with abiotic components, energy flows and materials cycle.

Point #1: Primary producers form the energy base of these biotic structures by providing the total energy available for further transfers.  So studying primary production of an ecosystem is often the first step in the process of understanding an ecosystem.

Point #2: Producing one pound of meat from herbivores uses much less energy than producing one pound of meat from predators.

Point #3: Materials are recycled in the trophic process, but energy is lost to the environment as heat.