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Organisms and Populations- CBSE Notes for Class 12 Biology

An organism is a living thing that has an organized structure, can react to stimuli, reproduce, grow, adapt, and maintain homeostasis. An organism would, therefore, be any animal, plant, fungus, protist, bacterium, or archaeon on earth. These organisms may be classified in various ways.

Organisms and Its Environment:

Ecology is the branch of biology, which studies the interactions among organisms and their physical (abiotic) environment.

The subject ecology is basically concerned with four levels of biological organisation.
These are given below:

  • Organism: It is a creature such as a plant, animal or a single-celled life form, or something that has interdependent parts and that is being compared to a living creature.
  • Population: A population is the number of living people that live together in the same place.
  • Communities: Assemblage of all the populations of different species present in an area that interact among themselves.
  • Biome: It is a large unit, which consists of a major vegetation type, associated fauna in a particular climatic zone. Tropical rainforest, deciduous forest, sea coast, deserts, etc., are the major biomes of India.
  • Environment: Ecology at organism level deals with how different organisms are adapted to their environment in terms of their survival and reproduction.

Biotic and Abiotic Factors

A biotic factor is a living organism that shapes its environment. Major Biotic factors are animals, plants, fungi, bacteria, and protists.

An abiotic factor is a non-living part of an ecosystem that shapes its environment. In a terrestrial ecosystem, examples might include temperature, light, and water.

Responses to Abiotic Factors

Different animals responds to changes in their surroundings in different ways. Some of examples are:

  • Regulation: They maintain homeostasis by ensuring constant body temperature (thermoregulation), and constant osmotic concentration (osmoregulation). For example, mammals regulate temperature by shivering in cold and sweating in heat.
  • Conformation: The internal environment of conformers changes with external environment.
  • Migration: It is a movement from stressful habitat temporarily to hospitable area and return when stressful period is over. For example, migration of birds to Keolado National Park, Rajasthan from Siberia.
  • Suspension: In this, the organisms develop mechanisms to deal with stressful situations. For example, spores (bacteria and fungi).

 

Adaptations

Adaptation to environmental dynamics is a biological process eternally operating in nature. Adaptation to environment is one of the basic characteristics of the living organisms. Living organisms are plastic and posses the inherent properties to respond to a particular environment.

Populations

Population is a group of individuals or organisms of any species living in a well–defined geographical area, at a specific time with the capability of interbreeding. For example, population of deer in a forest.

Population Attributes

Population attributes are summaries describing characteristics of the population. Formally an attribute is a function applied to the entire population and determined through the variate values on individual units.

  • Birth rate: Total number of individuals born in a given period of time.
  • Death rates: Total number of deaths in a period of time.
  • Sex Ratio: Total number of females and males per 1000 individuals.
  • Age pyramid: A plot of age distribution.

Population Growth

Population ecology is the study of how populations of plants, animals, and other organisms change over time and space and interact with their environment. The population density changes due to the following factors:

  • Natality: The number of births in a population in a given time period.
  • Mortality: The number of deaths in a population in a given time period.
  • Emigration: The number of individuals who moved to some other habitat in a certain time period.
  • Immigration: The number of individuals who have come into the habitat from elsewhere in a certain period of time.

Population Growth Models

Environmental scientists use two models to describe how populations grow over time: the exponential growth model and the logistic growth model.

  • Exponential Growth: In the limited supply of food, the population follows an exponential growth.
  • Logistic Growth: When the resources are finite and become limited sooner or later, the population growth is said to be logistic.

Life History Variation

Variation in life history traits includes timing of reproduction, size and number of offspring, number of clutches, and individual growth rates. This variation is well known within amphibian and reptile populations and most often appears associated with variation in resource availability.

Population Interactions

“Population interaction is the interaction between different populations. It refers to the effects that the organisms in a community have on one another.” There are various modes of population interaction.

  • Predation: This is a type of interaction in which an organism kills and feeds on another organism. The one who kills is known as the predator and the one who is killed is the prey.
  • Competition: This is the type of biological interaction between different animals or species in which both are harmed.
  • Parasitism: Parasitism is a type of interaction between species in which the parasite lives inside the body of another organism and cause harm to it.
  • Commensalism: In this type of interaction one organism benefits while the other is neither benefitted nor harmed.
  • Mutualism: In this type of interaction, both the species or organisms are benefitted from each other.

Organisms and Populations

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