Population

Population is a set of individuals of a particular species, which are found in a

particular geographical area.

The population that occupies a very small area, is smaller in size, such a
population is called local population. A group of such a closely related local
population is called meta-population.

Population ecology is an important area of ecology because it links ecology

to the population genetics and evolution. Natural selection operates at a
levels of population.

Population Attributes:

A population has certain attributes that an individual organism does not

have.Some of Them are given below:

1.Population Size or Density:

It is the number of individuals of a species per unit area or volume

2. Birth Rate (Natality):

It is the rate of production (birth rate) of new individuals per unit of

population per unit time.

3.Death Rate (Mortality):

It is the rate of loss of individuals (death rate) per unit time due to death or
due to the different environmental changes, competition, predation, etc.

4.Sex Ratio:

An individual is either a male or a female but a population has a sex ratio like
60% of the population are females and 40% are males.

5.Age Pyramid:

Population at any given time is composed of individuals of different ages.

When the age distribution (per cent individuals of a given age or age group)
is plotted for the population, this is called age pyramid.

The age pyramids of human population generally shows the age distribution
of males and females in a combined diagram.
The growth status of the population is reflected by the shape of the
pyramids.

(i) Growing Expanding

(ii) Stable
(iii) Declining.

Age Pyramids for Human Population

Population Growth:

The size of a population for any species is not a static parameter, it keeps
changing with time.

It depends on the following factors:

(i) Food availability

(ii) Predation pressure
(iii) Weather

The density of a population in a given habitat during a given period,

fluctuates due to the four basic processes:

(a) Natality refers to the number of births during a given period in

the population that are added to initial density.
(b)Mortality is the number of deaths in the population during a
given period.

(c) Immigration is the number of individuals of the same species that

have come into the habitat from elsewhere during the time period under
consideration.

(d)Emigration is the number of individuals of population who left the habitat

and moved elsewhere during a given period of time.
Out of these four, natality and immigration contribute an increase in
population density while mortality and emigration contribute to the decrease
in population density.

There are following two models of population growth:

1.Exponential Growth:

Availability of resources (food and space) is essential for the growth of

population. The unlimited availability results in population exponential. The
increase or decrease in population density (N) at a unit time period (t) is
calculated as (dN/dt)

2.Logistic Growth:

Practically, no population of any species in nature has unlimited resources at

its disposal. This leads to competition among the individuals and the survival
of the fittest. Therefore, a given habitat has enough resources to support a
maximum possible number, beyond which no further growth is possible.

This is called the carrying capacity (K) for that species in that habitat. When
N is plotted in relation to time t, the logistic growth show sigmoid curve and
is also called Verhulst-Pearl Logistic Growth and is calculated as

dN/dt = rN (K – N/K)

Where, N is population density at time t K is carrying capacity and r is

intrinsic rate of natural increase.

This model is more realistic in nature because no population growth can

sustain exponential growth indefinitely as there will be completion for the
basic needs.

Human population growth curve will become S-shaped, if efforts are being
made throughout the world to reduce the rate of population growth and
make it stationary

Population Interaction

Population interaction in the ecosystem occurs between the populations

which interact with one another living in a community. Population
interactions are divided into several types. There are two kinds of factors-
biotic and abiotic factors. Different kinds of population interaction affect a lot
to the abiotic factor which constitutes the ecosystem. Some of the
population interactions are Competition, Predation, Camouflage, Symbiosis,
and so on.

Population Interactions in the Ecosystem

This type of interaction occurs between different populations which affects

the organisms in a community. Interspecific interaction occurs from the
interaction of the population of two different species of organisms in the
ecosystem. There are two types of factors in the environment – biotic
(biological) and abiotic (physical) factors. There are some factors that are
important and needed for living beings to survive such as water, soil, carbon
dioxide, oxygen, and so on. Different types of population and their
interaction affects the abiotic factors in the ecosystem. Some of the major
modes of interaction between different populations are mentioned below:

Competition

This is a kind of interaction that occurs between two or more two types of
species when they compete with each other. These competitions are for
limited resources. Resources can be water, food, or prey. Competition is
categorized into different types:

 Inference Competition
 Exploitation Competition
 Apparent Competition

Predation

Predation is defined as a type of interaction between species where there is a

full dependency on one kind of species over the other for their survival. The
one which gets preyed on for food is known as the prey and the other which
feeds on the prey is known as the predator. The food chains and food webs
are based on the concept that predators is depended on other species for
food in the entire life cycle. Living organisms have their own strategies for
attacking and defending which they use to hunt weaker species. Predator
and prey are applied to plant and animal relationships. For example – a cow
feeding on grass.

Camouflage

Camouflage means ‘to disguise’. It is defined as a defense survival strategy

where a species develop structural adaptation which helps them to blend
with the environment to be safe and hide from predators. For example –
lizards, frogs, chameleons, and so on hides themselves from predators.

Symbiosis

Symbiosis in the Greek language means to live together. There are many
species which are depended on each other for survival. These types of
species are depended on each other for food. The different types of
symbiosis in population interaction are :

Parasitism – It is an interaction in which one organism feeds off another

organism. An organism that feeds on other species is known as a parasite
and the prey species are known as the host. This is a case of one-sided
symbiosis. Parasites multiply and cause harm to the hosts.

Mutualism – It is a kind of interaction that occurs between two or more

types of species of organisms. In these types of interactions, each type of
species benefits in some or other way from another other species. It is a
common type of ecological interaction.

Commensalism – This is a special kind of interaction where two or more

species are dependent on each other for food and survival and both get
benefitted, without harming each other.For eg., cattle egrets and livestock,
birds following army ants, barnacles and whales, etc. all exhibit
commensalism.

Ecosystem Patterns, Components; Productivity and Decomposition

Introduction

The ecosystem is defined as a self-sustaining and self-regulatory structural

and functional unit of the biosphere comprising of a community of living
beings and a physical environment both interacting and exchanging
materials among them. A.G Tansley firstly coined the term ecosystem in
1935.

Types of Ecosystem

 Terrestrial ecosystem: it is a land-based ecosystem

Forest ecosystem: it comprises various plants, animals, and microorganisms

that live in coordination with abiotic factors. The major function of the forest
ecosystem is to maintain the temperature of the earth
  Grassland ecosystem: this ecosystem is dominated mostly by grasses
and herbs, e.g. savannah grassland
 Tundra ecosystem: this ecosystem is found in cold areas where rainfall
is scarce, and no trees are present. Most of the time in a year these are
covered with snow
 Dessert ecosystem: this type of ecosystem shows warmer days and
colder nights, with little rainfall
 Aquatic Ecosystem: these ecosystems are found in the body of water.
These are further grouped into:
 Freshwater Ecosystem: this type of aquatic ecosystem are devoid of
salt content, e.g. streams, wetlands, rivers, lakes, and ponds
 Marine Ecosystem: this ecosystem contains salt and shows greater
biodiversity than freshwater ecosystems, e.g. oceans and seas

Components of Ecosystem

Every ecosystem has both structural and functional components. Ecosystem

refers to an open system that receives input in form of solar energy, and the
flow of this energy is unidirectional from producers to consumers. The
ecosystem is divided into two distinct components:

Biotic Component: this component of the ecosystem includes all the living
members of the ecosystem such as:

Producers: these are also known as autotrophs. Producers help in the fixation
of light energy, use of simple inorganic substances, and synthesis of complex
organic substances. As the members of autotrophs can convert light energy
to chemical energy these are referred to as producers. E.g algae, bryophytes
and vascular plants

Consumers: they are also known as heterotrophs. Their main function is the
intake, rearrangement, and utilisation of complex inorganic materials.
Members involved are known as consumers as they can consume organic
food

Consumers are further divided into

Primary consumers: they generally refer to herbivores. They rely on plants

for their food

Secondary consumers: the members of this group are dependent on primary

consumers, they can either be carnivores or omnivores
Tertiary consumers: they depend on secondary consumers for food. They are
omnivores

Quaternary consumers: they depend on tertiary consumers. They are present

only in some food chains. Quaternary consumers are generally present at the
top of the food chain, having no natural predators

Decomposers: since they feed on dead bodies of organisms and organic

wastes of living organisms, they come under saprotrophs. The members of
this group secret digestive enzymes to digest the organic matter. As they can
remove the dead bodies of organisms they are known as reducers. Because
of their small size, they are also known as micro consumers. E.g. bacteria,
fungi

Abiotic Components: it includes all the non-living components such as light,

wind, soil, temperature, and water.

Patterns of Ecosystem

The interaction of biotic components with abiotic components leads to a

physical pattern that is the characteristics for each type of ecosystem such
as:

 Species composition: it refers to the identification and enumeration of

plants and animals species of the ecosystem. Based the geography,
topography, and climate species composition varies greatly. Areas with
maximum species diversity occurs in coral reefs, and tropical rainforest

 Stratification: refers to the vertical distribution of various species

occupying different levels, e.g. canopy trees and under-story trees at
the top

Productivity

It refers to the rate of biomass production, per unit area over some time by
plants. The rate at which sunlight gets accumulated by producers for the
synthesis of organic compounds, which is expressed in terms of Kcal per
metre square per year. It includes the following types:
  Primary productivity: it refers to the amount of biomass produced per
unit area over some time by plants during photosynthesis. It is further
classified into two types:

 Gross primary productivity (GPP): refers to the rate of production of

organic matter by green plants per unit area in unit time during
photosynthesis

 Net primary productivity (NPP): a definite amount of GPP is utilised by

plants in respiration. After respiration, a considerable amount of energy
is left which is stored as organic matter in producers in unit time and
area is NPP. NPP is the only available biomass for consumption to
heterotrophs

NPP = GPP – R

The annual NPP of the whole biosphere is about 170 billion tons of organic
matter, of this about 70 percent is pre-occupied by the surface, about 55
billion tons indicates the productivity of oceans. Rest about 115 billion tons is
made available for land productivity.

 Secondary productivity: it is the rate of assimilation of food energy by

consumers, thus it is the energy available at consumers’ level for
transfer to the next trophic level

Decomposition

It is the physical and chemical breakdown of complex organic matter(i.e.

detritus) into inorganic substances like carbon dioxide, water, and nutrients,
this process is known as decomposition. E.g. of best decomposer is
earthworm that helps in the breakdown of complex organic matter as well as
in the loosening of the soil. The detritus comprises the remains of dead
plants such as leaves, barks, flowers, and dead remains of animals, including
their fecal matter, all of these form the raw material for decomposition.
Decomposition completely disposes of the whole detritus. The
biogeochemical are recycled by decomposition and create space for newer
generations of organisms. There are two types of detritus: above-ground
detritus includes (leaf, dried plants parts, remains of animals, etc.) and
below-ground detritus includes (mainly dead roots, also remains of
underground dead animals).

Decomposition is controlled by the chemical composition of detritus and

some climatic factors. The rate of decomposition is slower if it is rich in lignin
and chitin, it is quicker if it contains nitrogen and water-soluble substances
like sugars. There are about five processes involved in decomposition these
includes:

 Fragmentation: is the process in which detritus is broken down into

smaller particles due to the action of detritivores such as (termites,
earthworms), detritus is ground completely when it passes through the
digestive tracts of animals. This causes an increase in the surface area
of detritus particles

 Catabolism: decomposers like bacteria and fungi excrete digestive

enzymes over detritus that change the insoluble complex organic
substances into simple, organic substances and inorganic compounds.
A small section of broken-down food is taken up by decomposers and is
immobilised by them

 Leaching: the catabolised and fragmented particles contain a lot of

water-soluble nutrients that are inorganic. These nutrients are
subjected to get dissolved in water by percolating deep into the soil
and thus get precipitated in the process of leaching

 Humification: it leads to the accumulation of a dark coloured

amorphous substance known as humus, which is rich in lignin and
cellulose. It undergoes decomposition at a slow rate and is highly
resistant to microbial action. Humus is generally colloidal. It also acts
as a reservoir of nutrients and is helpful in the maintenance of soil
moisture and aeration
  Mineralisation: humus further gets degraded by some other microbes
and releases inorganic nutrients that occur via the process of
mineralisation. It involves both non-minerals and minerals from organic
matter. It is a slow process as it involves trapping in of humus and
immobilisation in decomposers

 Aerobiosis: activity of decomposer organisms are better in presence of

aerobic conditions because decomposition in oxygen is a requiring
process. Whereas anaerobiosis leads to piling up of detritus and it also
reduces the rate of decomposition

Energy Flow

All living organisms rely on producers for food, either directly or indirectly.
Energy flows in a unidirectional manner from the sun to producers and then
to consumers. Photosynthetically active radiation (PAR) is important for
plants to synthesize food. Animals, which obtain their food from plants, are
known as consumers. The process of energy transfer through consumption is
represented by food chains, where energy flows from producers to
consumers.

There are two types of food chains:

 Grazing Food Chain (GFC): This starts with living plant material and
progresses through herbivores to carnivores.
 Detritus Food Chain (DFC): This begins with dead organic matter and
consists of decomposers, which break down dead materials into
simpler substances.

The natural connections among food chains create a complex food web.

Each organism occupies a specific position in the food chain known as a

trophic level, and each trophic level has a specific amount of living material
at a given time, referred to as the standing crop. This is measured as the
biomass of living organisms or the number of organisms in a unit area.

The number of trophic levels in a grazing food chain is limited, as energy

transfer follows the 10 percent law, meaning that only 10 percent of energy
is transferred to each trophic level from the one below it. In a GFC, the
possible trophic levels include: producer, herbivore, primary carnivore and
secondary carnivore.

Ecological Pyramids

An ecological pyramid is a graphical representation of various ecological

parameters—such as number, biomass and energy arranged in different
trophic levels of a food chain. Producers are placed at the base, herbivores in
the middle and carnivores at the top. Ecological pyramids can be upright,
inverted or spindle-shaped.

There are three common types of ecological pyramids:

1.Pyramid of Number: This pyramid shows the number of individuals per unit
area at different trophic levels, with producers at the base and consumers at
successively higher levels. It is generally upright. However, in the case of a
large tree, the pyramid of number can be inverted because the number of
insects feeding on the tree exceeds the number of trees.

2.Pyramid of Biomass: This pyramid represents the biomass at various

trophic levels. It is usually upright, except in aquatic food chains that involve
short-lived organisms like plankton. In the sea the pyramid of biomass can be
inverted because the biomass of fish can exceed that of phytoplankton.

3.Pyramid of Energy: This pyramid graphically represents the amount of

energy captured by different trophic levels per unit area. The pyramid of
energy is always upright and cannot be inverted. This is because energy is
always lost as heat during each transfer from one trophic level to the next
(e.g., during feeding, digestion, assimilation and respiration).