Organisms and Populations Class 12 Notes Biology Chapter 13


  • Ecology is a study of interactions between living organisms and its physical or abiotic or non-living environment.


  • Organisms are the basic unit of study in ecology.
  • Ecology is basically concerned with biological interactions at four levels. These levels are organism level, population level, community level and biome level.
  • The smallest level of organisation is an individual of species and the largest level of organisation is biosphere.
  • Habitat is a specific place or locality where an organism lives.
  • The key elements that lead to so much variation in the conditions of different habitats can be subdivided into two components –
    • Abiotic or nonliving or physical components – Includes all the physical components, such as soil, temperature, water, light, etc.
    • Biotic or living or biological components – Includes all the living organisms.
  • Temperature is the most ecologically relevant environmental factor ranging from below zero degree in polar regions and greater than 50°C at high altitudes in arid or deserts during summer.
  • Majority of animals are cold-blooded or poikilotherms or ectotherms which do not have constant body temperature. Change in the temperature of environment affect mostly to poikilotherms. Eg. reptiles, fish and amphibians.
  • Mammals and birds are homeotherms or warm-blooded animals with a constant body temperature irrespective of the environmental temperature.
  • Many poikoilotherms undergoes an inactive phase, during winter season, called hibernation.
  • Some animals also undergo a period of dormancy, called aestivation, during a drought or dry season.
  • A few organisms, called eurythermal, can tolerate and thrive very well in a wide range of temperature. However, some organisms, are called stenothermal, and are restricted to a narrow range of temperature.
  • Water is the most important factor influencing the life of organisms. It covers about 73% of Earth’s surface.
  • Aquatic organisms having wide range of salt tolerance are called euryhaline.
  • Many freshwater animals cannot live for long in sea water and vice versa because of the osmotic problems, they would face.
  • Light is an important abiotic factor. Life is not possible on earth without the presence of sun light.
  • Plants produce food through photosynthesis, a process which is only possible when sunlight is available as a source of energy. Light is also essential for transpiration and flowering, and sometimes in germination of seeds.
  • Light loving plants are known as heliophytes while shade loving plants are called sciophytes.
  • Soil is the loose surface material consisting of inorganic particles and organic matter that covers most of the land surface.
  • Soil provides the structural support & the source of water and nutrients for plants used in agriculture.
  • Various characteristics of the soil such as soil composition, grain size and aggregation determine the percolation and water-holding capacity of the soil. These characteristics along with parameters such as pH, mineral composition and topography determine to a large extent the vegetation in many areas.
  • The organism should try to maintain the constancy ofits internal environment, a process called homeostasis, by monitoring changes in the external and internal environment.
  • The term homeostasis was coined in 1930 by the physician Walter Cannon.
  • Biome is a major complex biotic community which is characterized by distinctive plant and animal species and maintained under the climatic conditions of the region.
  • The various possible ways the organisms cope up with the changing environment: regulate, conform, migrate, and suspend.
  • Some organisms are able to maintain homeostasis by physiological (sometimes behavioural also) means which ensures constant body temperature, constant osmotic concentration, etc.
  • About 99% of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature changes with the temperature of the surrounding environment. In aquatic animals, the osmotic concentration of the body fluids changes with that of the osmotic concentration of water. These animals and plants are simply conformers.
  • Migrate is a mechanism where the organism moves away temporarily from the stressful habitat to a more hospitable area and returns when the conditions are favorable.
  • Suspend is a mechanism employed by bacteria, fungi, lower plants and some animals to survive unfavorable conditions.
  • Under unfavorable conditions, many zooplankton species present in lakes and ponds, and certain insects are known to enter diapause, a stage of suspended development.
  • Adaptation is any ability of an organism that enables an organism to survive and reproduce in its habitat.
  • On the basis of various mode of adaptation plants are grouped as xerophytes (grow in dry habitat), hydrophytes (grow in water), heliophytes (growing in bright light), halophytes (growing in saline habitat), and sciophytes (growing in partial shades).


  • A population is a group of individuals of the same species within a community. It can also be defined as the total number of species of other class of organisms in a defined area or habitat.
  • Population growth refers to change in the size of a population-which can be either positive or negative-over time, depending on the balance of births and deaths. If there are many deaths, the world’s population will grow very slowly or can even decline. Population growth is measured in both absolute and relative terms.
  • The nature of population is determined by various factors such as density, birth rate or natality, death rate or mortality, emigration, immigration and sex ratio.
  • There are mainly two types of population growth curves – J-shaped or exponential growth curve, and S-shaped or logistic growth curve.
  • J-shaped growth curve is a curve on a graph that records the situation in which, in a new environment, the population density of an organism increases rapidly in an exponential or logarithmic form, but then stops abruptly as environmental resistance or some other factors suddenly becomes effective.
  • S-shaped or logistic growth curve is type of growth curve is shown by the yeast growth under laboratory conditions.
  • The S-shape (sigmoid) curve is generated when a population approaches the environment’s carrying capacity.
  • It has five phases –
    • Lag phase
    • Positive acceleration phase
    • Exponential phase
    • Negative acceleration phase
    • Stationary phase

Population Interaction

  • Some species of insects and frogs are cryptically-coloured to avoid being detected easily by the predator. This is called camouflage.
  • Amensalism is a relationship in which one species is harmed whereas the other is unaffected.
  • Competition is defined as a process in which the fitness of one species is significantly lower in the presence of another species.
  • Competition is of two types – intraspecific (between individuals of same species) & inter specific (between individuals of different species).
  • Parasitism is a relationship between two species in which one benefits on the host (harm) of the other. It is always an one side relationship for the parasite which is always benefitted from the host.
  • The interaction in which one species benefits and the other is neither harmed nor benefited is known as commensalism.
  • The examples showing commensalism are orchids which grow as epiphytes on mango or other fruit trees.
  • Orchids are benefited by getting a shelter, while the tree is neither benefited nor harmed.
  • Mutualism interaction is a positive reciprocal relationship between two different species.
    • It confers benefits on both the interacting species.
    • For examples : Lichens represent an intimate mutualistic relationship between a fungus and green algae.
  • Predation is an interaction between members in which one population adversely affects the other by direct attack (capture, kill and eat) but is nevertheless dependent on other. The former is called predators & the latter is called prey.
  • Predation also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species.
  • Biological control is a method of reducing or eliminating plant pests by introducing predators or micro-organisms that attack the targeted pests but spare other species in the area.

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