ORGANISMS AND POPULATIONS - part 2 12th biology notes



Population :- 
It is group of similar individual from a species in a geographical area which can interbreed, compete for similar resources Majority of organisms live in group and not in an isolated condition were they compete for similar resources, interbreed The individuals of a species living within a habitat constitute a population.

 In natural conditions, we find populations of a variety of plants, animals and micro-organisms living together within a habitat. All such populations together constitute the biological (biotic) community of the habitat. 

This chapter deals with population and community levels of organisation, population characteristics and growth, species interaction in the community, and change in community over time.



Population has its different definition at different level:- 

Human demography :- a population is a set of humans in a given area, such a village or town.
Genetics :- a population is a group of interbreeding individuals of the same species. Populations of different species are not capable of breeding with each other due to various types of biological (e.g., breeding behaviour, inability to form fertile hybrids) and physical barriers (e.g., habitat and geographical isolation).

Ecology :-

  1. a population is a group of individuals of the same species, inhabiting the same area, and functioning as a unit of biotic community. Organisms in a population undergo the same life cycle. 
  2. They experience similar ecological processes at a particular stage of the life cycle. Population Characteristics Populations are defined by a number of attributes. 
  3. To understand how population changes with time, we conside how the population density, spatial distribution of organisms, and the factors regulating population growth changed over time. 

Some of the attributes are:- 

Natality : Natality is the increase in number of individuals in a population under given environmental condition. mBirth, hatching, germination and, vegetative propagation, cause increase in number of individuals. Natality rate is the increase in individuals is expressed per unit time.

Mortality :

 Mortality is the loss of individuals due to death in a population under given environmental conditions. Mortality rate is defined as the number of individuals dying over a time-period.

Sex Ratio :

 A population has both male and female and sex ratio is defined as the ratio of one with respect to the other.

Age distribution : 

Age distribution :

A population at any given point of time has various age groups which determine its reproductive status. The three ages referred to as ecological ages in a population are pre reproductive, reproductive and postreproductive. 
The distribution of age groups in a population influences the growth of the population. Populations with more young members grow rapidly, while the declining populations have a large proportion of older individuals

Population Density :

  1.  The size of a population is represented by population density. Density is expressed as the total number of individuals present per unit area or volume at a given time. Density of plants is determined by counting the number of individuals in a predetermined sizes/area. 
  2. The density of a species is not constant and varies from time to time and from one area to another. Density is affected by many factors such as; density of plants during rainy season is more compared to dry season. 

Dispersal/ Migration: 

Dispersal/ Migration:

The majority of organisms disperse at one time or the other during their life cycles. The individuals move into (immigration) and move out of the population (emigration), and these movements affect the size of the population. Population Growth As discussed earlier, the size of a population is not static. The growth of a population is measured as an increase in its size over a period of time. Let us assume that a population having the initial size, N0, increases to size, Nt, after time interval, t. Then, the change in population size is 

Logistic or S-shaped :-

Logistic or S-shaped :-

The exponential growth cannot be sustained infinitely because not only environment is ever mchanging, food and space are limited, leading to competition between individuals. In this case only the fittest one survives. The maximum number of individuals of a population that can be sustained indefinitely in a given habitat, represents its carrying capacity (K).

When a population is supplied with a limited resource it shows an initial gradual increase in population size, followed by an exponential increase and then a gradual decline to a near constant level. 

This slow-down following the exponential phase, occurs due to increasing environmental resistance such as limited food, accumulation of harmful waste. Plotting of the rate of increase of population over time gives an S-shaped or sigmoid curve [Fig. 17.2(b)]. 

The Sshaped sigmoid growth form is represented by the following equation(also called as Verhulst-Pearl Logistic Growth), which includes an expression for environmental resistance : where dN/dt, r and N are the same as in the equation for J-shaped growth form, and (K-N)/K or 1-(N/K) stands for environmental resistance. Logistic growth curve is a more realistic representation of growth model in natural condition.



Every organism have evolved over a long period of time to be fit and reproduce maximally. Not all species employ the same reproductive strategy and thus differ in life history. Pacific Salmon Fish, Bamboo- reproduce only once in their life time Most Birds and mammal- reproduce more than once through life time Oysters, pelagic fish- produce large number of small sized off spring Birds and mammals- produce small number of large sized offspring



Any species on this earth is not capable of living on its own. Members of the biotic community in an area are dependent on one another for food, space, reproduction and protection. Food interactions are reflected in food chains, which comprises plants, animals and micro-organisms. Plants and animals have well established their interdependence for reproduction. 

It can be best illustrated by the following examples:- Both flowers and insects have developed and evolved through the geological time scale in a way leading to their interdependence. Flowers offer nectar to insects as a reward for pollination. 
  • -Birds and mammals help in the dispersal of seeds and fruits.
  • -Cuckoo laying eggs in the nest of other birds
  • -Gall wasps embedding their eggs deep into the tissues of plants to ensure hatching and protection

Species Interactions Interspecific interactions arises from the interaction between populations of species in a community and it can be broadly categorised into positive (beneficial) and negative (inhibition) interactions, and zero for neutral interactoin depending upon the nature of effect on the interacting organisms  Interactions between Populations of Different Species in the Biotic Community

i) Predation : An interaction between species which involves killing and consumption of prey is called predation. Predator, is the species which eats and the Prey is the one consumed. 

Herbivores such as Deer, is also a predator because it consumes plants, or a sparrow consuming the seed. Predation acts as a pipeline of energy transfer from one tropic level to the other. Predation also helps to keep check on population. Tiger, the predator, keeps check on deer (prey) population. 

When the predator population increases, it consumes more prey, thereby reducing the prey population. Natural predation can also be put to good use in agricultural practices. Biological control method is a method were a natural predator is used to control the pest population infesting the crop. 

 When a foreign species of plant or animal is introduced in a new area, they grow very efficiently as compared to the local species because they lack competitor and natural enemies/predator and thus become invasive. 

When prickly pear cactus was introduced in Australia it spread rapidly on the millions of hectare of land. It was broughtunder control with the introduction of a natural predator (a moth). To counter predation, particularly the animals develop certain protective mechanisms to avoid the enemies.

Camouflage : some species of insects and frogs are cryptically colored to avoid detection. Butterflies and moths have colours matching the flower colours to make their detection difficult. Praying mantis and leaf insect 

 which even mimic the shapes of leaves and branches, thus keeping it safe in the hiding from the predator. Plants produce secondary metabolites which makes it distasteful or poisonous thus saving it from herbivory from animals.

ii) Competition: Interaction between two species, where both suffer adverse effects, is known as competition. Competition occurs when two species compete for the same resources, such as space, light and nutrients, etc.

Competition is basically of two types :

(i) interspecific, and (ii) intraspecific. 

Interspecific competition occurs between individuals of two different species occurring in a habitat. Intraspecific competition occurs between individuals of the same species. Intraspecific competition is more intense than interspecific competition since the same species are very similar and thus they compete more fiercely.

Competition according to Darwin is a key to organic evolution, since competition leads to survival of the fittest. Gause’s Competitive Exclusion Principle states that two closely related sharing and competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated in time. 

The Abingdon tortoise in Galapagos island was completely eliminate by the introduction of goats on the island due to efficient browsing habit of the goat. Another aspect of Competition is competitive release, in which when one competitor species is removed, there is an abrupt increase in the number of the other competitor due to the release of competitive pressure.

Resource partitioning :

Resource partitioning

To overcome competition and continuing battle nature has evolved a scheme were 
two or more species can co-exist while sharing the same resource. MacArthur showed that five closely related bird species on a tree was able to co-exist by having different foraging habits.

iii) Parasitism : Parasitism, is an interaction were the species smaller in size (the parasite) lives in or on the larger species (host) from which it obtains food and shelter. Parasites affect the population growth of hosts, shorten the life cycle, weaken the host, and reduce the reproduction rate of the host organism.

Features that make a successful parasite:-

 Loss of unnecessary sense organ
 Presence of adhesive organs or suckers for clinging to the host
 Absence of digestive system
 High reproduction rate
 Presence of one or two intermediate host to facilitate para sitisation (human liver fluke: snail and fish; Malaria parasite:- mosquito)
 Tendency to co-evolve with the immune system of the host species to overcome them
 specialised structures to reach or invade the host A parasite can be

Ectoparasites :- The parasites which remain outside the host are called ectoparasites, or external parasites e.g.,
  •  - ticks, mites and lice
  • - copepods on the body of marine fish
  • - Cuscuta (dodder), Loranthus Viscum, (Mistletoe) and Rafflesia 

Endoparasite :- They live inside the body of the host organism. Malarial parasite inside the human body.

Brood Parasitism :- it is shown by birds. Cuckoo (Koel) lays egg in the nest of crow. During the course of evolution the egg of parasite bird resembles the egg of the host, thus avoiding the detection and ejection from the nest.

iv.) Commensalism : It is a relationship between two species where one species is benefited, while the other neither gets any benefit, nor is adversely affected under normal conditions. Some species live inside the body of larger species in order to protect themselves from the enemies and adverse environment.

Examples :-
 Sucker fish and shark (Fig. 17.6). The sucker fish attaches itself to the shark surface with the help of its dorsal fin, which is modified into a hold fast. The sucker fish is dispersed from one place to other getting better food supply and protection from predators. However, the shark is neither benefited nor harmed by the sucker fish.

 Epiphytes (e.g., mosses, ferns, orchids, money plant) growing on a mango trees gets better light exposure, without harming the tree.

 Egret forage close to the grazing cattle, because while moving the cattle stir the insect out form the bush on which the birds feed.

 Interaction between sea anemone and clown fish. The clown fish gets protection due to the stinging tentacle of clown fish while staying away from it.

 Barnacles attached to the back of the whale.
v.) Mutualism: An association of two species were both species are benefited, is called as mutualism.

 Algae and fungi exhibit mutually beneficial relationship in lichens (Fig. 17.4). The fungi provide protection to algae, and the latter prepares food for the fungi. In this condition there is a close physical association between the individuals of a pair of species and thus called as symbiosis (living together)

 Mycorrhizae are mutualistic relationship between fungi and roots of about 80 per cent of higher plants. The fungus helps in mineral nutrition of the plants with which they are associated and obtains, in turn, carbohydrates from plants.

 Plants and animals shows an amazing mutual relation. The insects help plants in pollination and seed dispersal. In turn they are rewarded with food in the form of nectar, pollen grains, and fruits. This is case which occurs generally. But sometimes the animal steal food without aiding pollination. To overcome this the plant and animal co-evolve. Co-evolution is the tightly linked evolution of the pollinator and the flower.

Examples for co-evolution:-


 Fig tree flower can be pollinated by its partner wasp species and no other species. The wasp uses the fig fruit for laying egg (oviposition) and nutrition of young larvae. The wasp pollinated fig while in  search of a suitable site for laying eggs.

 The Orchid, Ophyrus utilizes sexual deceit to get itself pollinated by a bee species. The male bee reaches the flower, as one of its petal resemble the female bee in size, shape and color and “pseudo copulates” with the flower. When it pseudo copulates with the other flower pollination takes place 

Based on interaction mutualism may be:- 

Obligate mutualism- species are completely dependent upon each other. Nitrogen fixing bacteria (Rhizobium) living in root nodules of legumes, where the bacteria, deriving nutrition from the host plant, fix atmospheric nitrogen and make it available to the plants. In the case of coral reef, coelenterates and algae live in obligate relationship. Bacteria in the gut of some animals (cattle) help in cellulose digestion. 

Facultative mutualism- one species may survive even in the absence of the other partner species. Sea anemone, which gets attached to the shell of hermit crab
 The sea anemone grows on the back of the crab, providing camoufl
age and protection (the sea anemone has stinging cells) and, in turn, the sea anemone is transported for reaching new food sources. This type of mutualism is also called proto-cooperation.

ORGANISMS AND POPULATIONS part 1 notes click now 

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