1. Unit I
Ecology is a study of the habitations of organisms
Some important definitions
Synecology is the study of communities, and autecology is the study of a species.
Paleoecology the ecology of fossil plants and animals
Theophrastus, a friend and associate of Aristotle, first described interrelations
between organisms and between organisms and their environment. He has, therefore
been called the first ecologist.
Ecology deals with life that includes:
(a) Every species with the lives of others; and
(b) Each species with its non-living environment as a whole and each element
or factor of that environment.
Subdivisions of Ecology
animal ecology and plant ecology.
When animals and plants are given equal emphasis, the term bioecology is often used.
Synecology is the study of communities, and autecology is the study of a species.
community ecology, the local distribution of animals in various habitats, their recognizition and
composition of community
ecosystem dynamics, the processes of soil formation, nutrient cycling,
energy flow and productivity
population ecology, the manner of population growth structure and regulation;
evolutionary ecology, the problems of niche segregation and speciation; and geographic ecology,
concerned with distribution.
2. The total physical environment of any living organism is the’ biosphere, and parts of which are of direct or
immediate importance’ to the organism constitute the ‘effective environment
Ecology is the study of the inter-relationships within ecosystems
Ecosystems are not closed systems because there is always an interflow of
matter and energy between adjacent ecosystems
The interdependence of the organism and the environment must provide the minimum
requirements’ and nothing that is dangerous. At the same time living organisms should also
be able to successfully adapt themselves to their surroundings
the ability of organisms to spread from their place of origin to another area, is a basic aspect of
population ecology and without proper means of dispersal, many animal populations would have been
eliminated over a period of time due to modifications of the environment
the microenvironment is the intimately local and immediate surroundings of the organism, the
macroenvironment is the sum total of the physical and the biotic conditions existing external to the.
Organism and its microenvironment.
the study of individual organism or species in relation to their habitat (autecology) or the study of
associations of organisms in relation to a particular area or biotope of habitat (synecology).
The term ecological valency has been used to refer to the adaptive range of both the individual and the
The niche may be better defined as the place that an animal occupies in ‘a biotic community in relation to
its food and enemies
Trophic level - relationships on the basis of food, and through successive trophic levels the
primary stock of energy as obtained by green plants is continually circulated.
The quantity of incident energy from the sun per unit area of the ecosystem and the efficiency of
conversion of this energy by other organisms is a basic principle.
Ecological energetics deals with this aspect of ecological studies. The autotrophs (plants) and
the heterotrophs (animals) utilize this energy.
The branch of ecology dealing with genetics in relation to ecology is ecological genetics.
The adaptations of animals or preference of particular organisms like insects to particular chemical
substances, come under chemical ecology
the study of soils is pedology in particular their acidity, alkalinity, humus content,
zoogeography relates to the structural and functional relations of animals in space, which form the
immediate environment of the individuals as well as the populations.
Ecosystems and their functioning
In a forest grove or woodland, for example, the larger trees the dominants influence
the environment in which all other species live.
3. forest grove or woodland, for example, the larger trees the dominants influence
the environment in which all other species live
The interrelationship (symbiosis) between the Yucca plant and the yucca
moth is a well studied example; the plant supplies food for the moth, but independent
on the moth for fertilization and perpetuation.
‘natural communities’. consist of wild, naturally occurring species of plants and animals able to
maintain themselves in the absence of man.
Artificial communities are characterized by species introduced by man or favoured by human
modification of the environment
The physical environment or the abiotic component is composed of sunlight,
atmosphere, water and soil or rock
Plants, for example; obtain their energy from sunlight, ‘their basic food from air and water, supplemented
by chemicals from’ the soil or rocks.
Plants, reflect, refract or absorb various wavelengths of sunlight, and thus modify it; by using carbon
dioxide and by giving off oxygen and water vapour they modify the atmosphere; by removing various
chemicals and by adding others they modify the soil.
Producers convert the energy received from the sun into chemical or
Consumers depend upon the producers for food. They may be herbivores,
Decomposer take the chemicals built up by the green plants and
break them into simpler forms in which they can be re-used. These include
the so-called reducer’s organisms, such as bacteria and fungi, that act in the
breakdown and decay of dead plants and animals.
ecosystems has two components:
1. Autotrophs (Self nourishing) The organisms can fix light energy and can, use
simple inorganic substances and build complex organic substances.
2. Heterotrophs (Other-nourishing) In these organisms there is utilization, rearrangement
and decomposition of complex substances into simples ones.
Each species is adapted to a particular role in the ecosystem known as its ecological niche.
Of the solar energy reaching the earth, only a portion will be stored by plants
often less than 1 per cent of the total-sunlight energy falling on a vegetated area is retained as chemical
energy by the green plants.
Of the total energy thus available in plant tissues for consumption by animals, usually less than 20 per
cent will be stored as chemical energy in an animal’s body tissues.
. The biosphere is that portion of the earth within which life exists. It includes all oceans and freshwater,
the lower layers of the atmosphere and the inner skin of the earths, crust the rocks and soil of the earth’s
4. Green plants capture sun energy and combine it with chemical raw materials from soil, water and air.
The food they produce supports all animal life, including the decay organisms which return it to the soil
for plant use once more. Man is a part of the biosphere and depends on its ‘ continued functioning
for his own existence.
Factors influencing Population Growth
depends upon the excess of births (natality) over deaths (mortality).
Populations, unless otherwise checked, tend to grow rapidly when there is an abundance of space and of
the materials that the species requires for its subsistence
A population growing at the maximum rate for which that species is ‘capable is said
to, be growing at its biotic potential rate.
when natality is at a maximum and mortality at a minimum operating to prevent a species from
maintaining a biotic potential rate of increase is the environmental resistance
Limits to Population Growth
Some species are intolerant of crowding and produce relatively fewer young as their habitat becomes
more fully stocked
No environment is without limits and the biosphere is limited both in extent and resources.
For plants, limits to growth would be set primarily be the extent of their ability to convert
sunlight into food energy, or by the availability of carbon dioxide in the atmosphere
Carrying capacity is a measure of the number of individuals of any species that a particular environment
Carrying capacity can be considered as having several levels:
an absolute or maximum carrying capacity capacity which is the maximum number of individuals
that can be supported by the resources of the environment at a subsistence le
a level, which is generally considered more desirable by those concerned with the health or
productivity of the species involved, termed as optimum density.
Limiting factors are often classified in two categories:
1. Density dependent
2. Density independent
A density-independent factor affects many or a few individuals without reference to the population level.
a flood is density independent since it may wipe out an entire population of a species,
climatic and atmospheric factors
Changes in rainfall and humidity, dry years or cycles and wet years or cycles, have major effects.
temperature or moisture is already near the limits of tolerance for a species
climatic and atmospheric factors tend to have their most severe effects
5. Native species are adapted to .the range of climate in the area they occupy, and although they may be
affected by climatic extremes
Certain factors of weather and climate may have catastrophic effects on populations in an area having
hurricanes, tornadoes, unusually severe floods or droughts
The abundance of natural vegetation in an area often has little relation to the inherent fertility of a soil
the organic surface layer the soil contains few of the elements needed for plant or animal nutrition, and if
the vegetation is scraped away with a bulldozer the soil that remains is often highly infertile
desert soils that support little plant life, because of lack of water, may have a rich supply of the soluble
mineral laments needed -for the support of life. When water can be made available, they will support high
levels of productivity,
Plants are classified in relation to their tolerance for dry, medium or wet conditions as xerophytes,
mesophytes and hydrophytes
The greatest number of limiting’ factors influencing plant or animal growth, abundance and distribution are
biotic in nature
The number of plant-eating animals in any area is ultimately limited by the abundance of the plants of
which the animals feed
Plant growth may be limited by competition from other plants of the same or different species each
drawing on the same reserves of soil and water
Biotic succession and land rehabilitation
Resilience of Populations and Communities
The ability of a community to recover from disturbance is a measure of its resilience.
Communities in more favorable environments tend to the resilient.
mainland species cannot reach distant islands, and in consequence, islands have developed unique,
specialized biotic communities .
Island plants may have little resistance to grazing use and may be invaded and native species displaced
by vigorous species from the mainland, as has happened with Lantana camera in Hawaii. Attempts to
develop island environments can have disastrous effects if they lead to decimation
Ecosystems are divided into terrestrial or land based ecosystems, and aquatic ecosystems in water.
divided into bio-geographical realms, .
Eurasia called the Palaearctic realm;
South and South-East Asia is the Oriental realm
6. North America is the Nearctic realm
South America forms the Neo-tropical realm
Africa the Ethiopian realm
Australia the Australian realm.
At a national or state level, this forms bio-geographic regions.
There are several distinctive geographical regions in India-
the Gangetic Plains,
the Highlands of Central India,
the Western and Eastern Ghats,
the semi-arid desert in the West, the Deccan Plateau, the Coastal Belts, and the Andaman and Nicobar
The living community of plants and animals in any area together with the non-living components of the
environment such as soil, air and water, constitute the ecosystem.