Organism and Population Notes Class 12 Biology Chapter 11

CBSE Class 12 Organisms and Population: The study of organisms and populations is an important area of biology known as ecology. An organism is a single living individual that is capable of carrying out all basic life processes. Organisms can be unicellular, or multicellular. Whereas population referred to a group of individuals present in a geographic area. Ecology primarily focuses on four levels of biological organization: organisms, populations, communities, and biomes. Here, in this chapter 11 of NCERT Organisms and Population, we will study the first two levels of biological organization.

What is an Organism?

The organism is defined as the minimal functional unit of life. An Organism can grow, reproduce, and respond to its environment. It is made up of cells that work together to keep it alive. Organisms have specific structures and organs that enable them to perform different functions necessary for their survival. They interact with other organisms and their surroundings in complex ecosystems, forming food chains and ecological relationships. Organisms have the ability to adapt to their environment and evolve over time to increase their chances of survival.

Population

Groups of individuals from the same species that share or compete for similar resources and live together in a defined geographical area constitute a population.

The population also has the potential to interbreed. Populations are dynamic and can change over time due to factors like birth, death, immigration (individuals moving into the population), and emigration (individuals moving out of the population). They can exhibit certain characteristics and behaviors specific to their species, which may be influenced by genetic variations and environmental factors. At the population level, evolutionary changes take place through natural selection.

Population Attributes

The population have the following attributes (that individual don’t have)

Death and Birth Rate

The population has death and birth rates-

• Death Rate is the number of people per thousand who die in a particular area during a particular period of time.
• Birth Rate in a place is the number of babies born there for every 1000 people during a particular period of time.

Sex Ratio

The population has a sex ratio. The sex ratio is the number of females per 1000 males. It compares the number of females and males in a population group.

Age Pyramid

Individuals of various ages make up a population at any given moment. When the percentage of individuals of a certain age or age group is graphed for the population, it produces an age pyramid. The shape of the pyramids is an indicator of the population’s growth status, which can be (a) increasing, (b) stable, or (c) decreasing.

Size of a Population 

It indicates the status of the population in the habitat. It is measured in the form of population density, that is the number of individuals per square unit of the area. Its evaluation helps in, for example, investigating the impact of a predator in a habitat or the effect of a pesticide application in an area.

Population Growth

The population of particular species is not always constant, it changes over time. These changes are affected by numerous factors, including the accessibility of food, the existence of predators, and unfavorable weather conditions. These changes indicate whether the population is increasing or decreasing. There are four processes that affect the density of a population in a given habitat:                                                                                                           

1. Natality: Refers to the number of live births in a specific population during a certain period of time.             
2. Mortality: Refers to the number of death in a specific population during a certain period of time.                        
3. Immigration: The number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.                            
4. Emigration: The number of individuals of the population who left the habitat and went elsewhere during the time period under consideration.

Populations grow through births and immigration and decline through deaths and emigration. Population density is mainly impacted by births and deaths, while the other two factors(immigration and emigration) become significant only in specific circumstances.

Population Growth Models

These are used to analyze and predict the growth of organisms, populations, and biological systems over time. Helps to understand the dynamics of growth, the underlying factors that drive it, and how it changes over time. The two growth models explained here are: 

Exponential Growth Models 

Assumes that there are no limiting factors and that resources are unlimited. The size of the population increases at a rate that is proportional to the current size of the population, leading to a steady increase in population size over time. Graphically, represented as ‘J’ shaped.

If in a population of size N, the birth rates are represented as b and death rates as d, then the increase or decrease in N during a unit time period t (dN/dt) will be

dN/dt = (b – d) × N

Let (b–d) = r, then dN/dt = rN

The r in this equation is called the ‘intrinsic rate of natural increase’, it assesses the impact of biotic and abiotic factors on population growth.

Logistics Growth Models

Describes the population in an area having limited resources(called as nature’s carrying capacity), initially it exhibits a lag phase, followed by phases of acceleration and deceleration, finally when the population density attains the carrying capacity, it reaches an asymptote. Defines the concept of ‘survival of the fittest’.

Graphically, represented as a sigmoid curve. This type of population growth is called Verhulst – Pearl Logistic Growth. Described by the following equation:

dN/dt = rN{K-N}/K

• N = Population density at time t
• r = Intrinsic rate of natural increase
• K = Carrying capacity

Life History Variation

Population in a habitat evolves to maximize their reproductive fitness, also known as Darwinian fitness(high “r” value). Through a specific set of selection pressures, organisms adapt to achieve the most efficient reproductive strategy. The biotic and abiotic factors of habitat influence the evolution of organisms.

Population Interaction 

Two different species of population interact with each other to form interspecific interactions, that could be beneficial, detrimental, or neutral to one of the species or both. Interaction between the two species can be classified as:   

1. Competition (both species suffer),                 
2. Predation and Parasitism (one benefits and the other suffers),                                     
3. Commensalism (one benefits and the other is unaffected),                                        
4. Amensalism (one is harmed, other unaffected) and
5. Mutualism (both species benefit).

Predation

Predation acts as a natural mechanism through which energy fixed by plants is transferred to higher trophic levels in an ecosystem. It also keeps the prey population under control, for example, biological control methods used in agriculture pest control are based on this method. Predators contribute to the preservation of species diversity by lessening the level of competition among prey species.

Predators are prudent in nature. To minimize the effects of predation, prey species have developed various defense mechanisms. For example, The Monarch butterfly is highly distasteful to its predator, some insects use camouflage. 

Plants have developed various defense mechanisms (mechanical and chemical) to protect themselves from their predators (herbivores). For example, thorns on the cactus, many plants produce and store poisonous substances.

Competition

Competition can be described as a phenomenon in which the presence of one species has a notable negative impact on the fitness of another species, as measured by its intrinsic rate of increase (also known as “r”). 

With limited resources the competitively superior species will eliminate the other species. For example, with the introduction of goats on the Galapagos island Abingdon tortoise became extinct within a decade. For the same resources, the totally unrelated species can also compete.

Another form of competition is competitive release. If a species is limited to a small geographic region due to the presence of a more competitive species, removing the competitive species through experimental means can result in a significant expansion of the limited species’ range. Gause’s ‘Competitive Exclusion Principle’ competing for the same resources the closely related species can not co-exist indefinitely, the inferior one will be eliminated.

Parasitism

Parasites negatively impact the host’s survival, growth, and reproductive capabilities, and reduce the population density. Parasites have a complex life cycles, involving one or two intermediate hosts or vectors. Special adaptations have been adopted by parasites like the presence of adhesive organs or suckers to cling on to the host, loss of digestive system, and high reproductive capacity. Many of the parasites are host specific. Parasitism negatively impact the host’s survival, growth, and reproductive capabilities, and reduce the population density.

Types of Parasitism:

• Ectoparasites: Feed on the external surface of the host organism, for example, ticks on dogs and licks on humans.
• Endoparasites: These are those that live inside the host body at a different sites (liver, kidney, lungs, etc.,)
• Brood parasitism in birds: The eggs of the parasitic bird resemble the host’s egg in size and color. The parasitic bird lays its eggs in the nest of the host bird and depends on the host to incubate them. Example cuckoo(host) and crow(parasite

Commensalism

Commensalism  is an interaction in which one species is positively impacted while the other is unaffected – neither benefiting nor being harmed. For example, cattle egret (benefits) living in close association with grazing cattle (unaffected), clown fish (benefits) living among sea anemones (unaffected).                           

Mutualism

Mutualism interaction provides benefits to both the interacting species. For example, Lichens(Fungi + Algae), the mycorrhizal (fungi +roots of higher plants).

Mutualism can also be seen in plant and animal relationships. Plants provide animals with nectar and fruits, and in turn, animals help plants in pollination and dispersal of seeds.

Plant-animal interactions often involve the co-evolution of the mutualists. For example, the interaction between a fig tree and female wasp. Fig tree gets pollinated with wasp’s help and wasp gets nectar(food) and a site for laying eggs.

FAQs on Organism and Population

Q1: What are the Levels of Biological Organization in the Ecosystem?

Answer:

There are four levels of biological organization in the ecosystem.

1. Individual
2. Population
3. Community
4. Ecosystem

Q2: Define Population.

Answer:

A group of individuals of the same species living and interbreeding within a given area for the continuation of the species.

Q3: What are the attributes of the population?

Answer:

Attributes of the population are:

1. Death rate and birth rate.
2. Sex ratio
3. Population density
4. Age distribution

Q4: Name some of the recently extinct species in India.

Answer:

Some of the extinct species are:

1. Tusked Elephant
2. Megalochelhys Atlas
3. Stegodon
4. Sivatherium