Components of Ecosystems
An ecosystem is made up of two basic components
INFORMATION SHEET
I) Abiotic components
II) Biotic components
I) Abiotic Components
These include the non-living parts of the environment. These are chemical factors such as air, water, soil
and the basic elements and compounds of the environment.
Abiotic factors are broadly classified into three categories:
• Climatic factors include the climatic regime and physical factors of the environment like light,
humidity, atmospheric temperature, wind, etc.
• Edaphic factors are related to the structure and composition of soil, including its physical and
chemical properties, like soil and its types, soil profile, minerals, organic matter, soil water, soil
organisms.
• Inorganic substances refer to substances such as water, carbon, sulphur, nitrogen, and
phosphorus. Organic substances like proteins, lipids, carbohydrates, humic substances also fit
into this category.
II) Biotic Components
It comprises the living parts of the environment (plants and animals), which includes the association of a
number of interrelated populations belonging to different species in a common environment. The
populations are that of animal community, plant community and microbial community. Biotic community
is distinguished into autotrophs, heterotrophs, and saprotrophs.
Trophic Levels of the Biotic Environment
A trophic level is a feeding level of one or more organisms in a food web (i.e., producer, primary
consumer, secondary consumer, etc).
• Autotrophs are also called producers. These are photosynthetic green plants and some species of
bacteria, such as cyanobacteria, that carry out photosynthesis to produce food for other animals. They
synthesize high-energy complex organic compounds (food) from inorganic raw materials with the help of
sunlight. Autotrophs form the basis of any biotic system. In terrestrial ecosystems, the autotrophs are
mainly the rooted plants. In aquatic ecosystems, floating plants called phytoplankton and shallow water
rooted plants called macrophytes are the dominant producers.
• Heterotrophs are also called consumers. Heterotrophs are organisms deriving their nutritional
requirements from complex organic substances. They are animals feeding on other organisms. There are
several different trophic levels of heterotrophs. Heterotrophs can be classified into three general categories:
• Herbivore: An organism that consumes only plant material (i.e., moose, beaver, grasshopper)
• Carnivore: An organism that consumes other species of animals (i.e., wolf, hawk, shark)
• Omnivore: An organism that consumes plant material and other species of animals (i.e., bear,
raccoon, humans)
• Primary Consumer: First consuming organism in a food chain (a herbivore or an omnivore);
occupies the second trophic level (i.e., rabbits)
• Secondary Consumer: Second consuming organism in a food chain (a carnivore or an omnivore);
occupies the third tropic level.
• Saprobes are also called decomposers or reducers. They break down the complex organic compounds of
dead matter (of plants and animals). Decomposers do not ingest their food. Instead, they secrete digestive
enzymes into the dead and decaying plant and animal remains to digest the organic material. Enzymes act
upon the complex organic compounds of the dead matter. Decomposers absorb a part of the decomposition
products for their own nourishment, and release unused minerals back into the environment (soil). Released
minerals are reused as nutrients by the plants (producers).
Factors Affecting Consumers
There are a number of factors which can affect the consumers in the food web.
Bioaccumulation refers to an increase in concentration of a pollutant from the environment to the first
organism in a food chain.
Biomagnification is the process whereby substances (i.e., toxic compounds and chemicals) collect in the
bodies of organisms in progressively higher concentrations towards the top of a food chain. It refers to
increase in concentration of a pollutant from one link in a food chain to another.
Bioaccumulation can have significant impacts on consumers in the food web, because of the increase in
toxicity of the chemical involved, as it is passed on from one trophic level to the next.
Together, these phenomena mean that even small concentrations of chemicals in the environment can
find their way into organisms in high enough dosages to cause problems. In order for biomagnification to
occur, the pollutant must be:
1.
2.
3.
4.
long-lived
mobile
soluble in fats
biologically active
If a pollutant is short-lived, it will be broken down before it can become dangerous. If it is not mobile, it
will stay in one place and is unlikely to be taken up by organisms. If the pollutant is soluble in water it will
be excreted by the organism. Pollutants that dissolve in fats, however, may be retained for a long time. It
is traditional to measure the amount of pollutants in fatty tissues of organisms such as fish. In mammals,
we often test the milk produced by females, since the milk has a lot of fat in it and because the very young
are often more susceptible to damage from toxins (poisons). .
An example of biomagnification is the increased biomagnification of the chemical insecticide DDT in
species of organisms. DDT stands for dichloro diphenyl trichlorethane. It is a chlorinated hydrocarbon,
which is a class of chemicals which often fit the characteristics necessary for biomagnification. DDT has a
half-life of 15 years, which means that if you use 100 kg of DDT, it will break down as follows:
Year Amount Remaining
0
100 kg
15
50 kg
30
25 kg
45
12.5 kg
60
6.25 kg
75
3.13 kg
90
1.56 kg
105
0.78 kg
120
0.39 kg
This means that 100 years after releasing 100kg of DDT into the environment, there will still be over 1.5kg
of DDT in the environment. If it does bioaccumulate and biomagnify, much of the DDT will be in the
bodies of organisms. Overally, DDT is not very toxic to humans, but it is highly toxic to insects (which is
why it is used as an insecticide). Because it could be safely handled by humans, it was extensively used
shortly after its discovery just before WW II. During the war, it was used to reduce mosquito populations
and thus control malaria in areas where US troops were fighting (particularly in the tropics). It was also
used on civilian populations in Europe, to prevent the spread of lice and the diseases they carried. After
the war, DDT became popular not only to protect humans from insect-borne diseases, but to protect
crops as well. As the first of the modern pesticides, it was overused, and soon led to the discovery of the
phenomena of insect resistance to pesticides, bioaccumulation, and biomagnification. By the 1960s,
global problems with DDT and other pesticides were becoming so pervasive that they began to attract
much attention.
The figure shows how DDT becomes concentrated in the tissues of organisms representing four successive
trophic levels in a food chain.
The concentration effect occurs because DDT is metabolized and excreted much more slowly than the
nutrients that are passed from one trophic level to the next. So DDT accumulates in the bodies (especially
in fat). Thus most of the DDT ingested as part of gross production (at the bottom of the trophic level; by
primary consumers) is still present in the net production (at the top of the trophic level; by secondary and
tertiary consumers) that remain at that trophic level. This is why the hazard of DDT to nontarget animals is
particularly acute for those species living at the top of food chains.
For example,



Spraying a marsh to control mosquitoes will cause trace amounts of DDT to accumulate in the cells
of microscopic aquatic organisms, the plankton in the marsh.
In feeding on the plankton, filter-feeders, like clams and some fish, harvest DDT as well as food.
(Concentrations of DDT 10 times greater than those in the plankton have been measured in
clams.)
The process of concentration goes right on up the food chain from one trophic level to the next.
Gulls, which feed on clams, may accumulate DDT to 40 or more times the concentration in their
prey. This represents a 400-fold increase in concentration along the length of this short food chain.
There is abundant evidence that some carnivores at the ends of longer food chains (e.g. ospreys, pelicans,
falcons, and eagles) suffered serious declines in fecundity and hence in population size because of this
phenomenon in the years before use of DDT was banned (1972) in the United States.
Adapted December 2011 from <http://www.tutorvista.com/content/biology/biology-iv/ecosystem/ecosystem-components.php> and
<http://www.marietta.edu/~biol/102/2bioma95.html>.