Maharashtra Board Class 12 Biology Chapter 13 Organisms and Populations PDF Download

Organisms And Populations

Natural world around us shows amazing diversity of forms and complexity of relations. To understand these, we have to study levels of organization in the living world viz. macromolecules, cells, tissues, organs, individual organism, population, communities, ecosystems and biomes.

You have already studied in school, that ecology is a study of the interactions among organisms and between the organisms and their physical (abiotic) environment. Term ecology was first used by Reiter but E. Haeckel gave substance to the term (introduced) ecology.

Ecological grouping of organisms is nothing but ecological hierarchy. There are four sequential levels with increasing complexity of ecological (biological) organizations viz, Organism, Populations, Communities and Biomes. Individual organism is the basic unit of ecological hierarchy. Organisms of same kind inhabiting a geographical area constitute population. Several populations of different species in a particular area constitute community that interact with one another in several ways. Biome constitutes a large regional terrestrial unit delimited by a specific climatic zone having major vegetation zone (plant communities) and the associated fauna. There are six major groups of terrestrial biomes. We shall explore first two levels viz, organism and populations.

13.1 Organisms And The Environment Around

Ecology at the level of organism is basically the study of animal or plant physiology which helps us to understand how the organisms are adapted to their environments, not only for their survival but also for propagation (multiplication).

You have studied in earlier classes about the rotation of earth around the Sun and the tilt of its axis, cause seasons. These seasons with annual variation in precipitation in the form of rain and snow, gives rise to formation of major biomes of the earth like desert, rain forest, grassland, tundra, etc.

Regional and local variations within each biome lead to the formation of a variety of habitats. Major biomes of earth are shown in Fig. 13.1. On the Earth, life exists even in extreme and harsh habitats like scorching deserts of Rajasthan, perpetually rain-soaked forests of North Eastern states and high mountain tops of Himalayas.

Here, we must remember that it is not only the physico-chemical (abiotic) components that make up the habitat of an organism, but the habitat also includes biotic components like plants, pathogens, parasites, and predators of the organism. We assume that over a period of time, the organism had through natural selection, evolved adaptations to optimize its survival and reproduction in its habitat.

Teacher's Note

Habitat is where an animal or plant lives. It is like your home. Different animals need different homes. A fish lives in water. A bird lives on trees.

Exam Trick

Remember: Habitat = Home. Every organism has a home. That home is called habitat. Just like you live in a house, a fish lives in water.

Points to Remember

Ecology is the study of how organisms and nature work together.
Organism is a single animal or plant.
Population is many organisms of the same type living in one place.
Biomes are large areas with certain weather and plants.
Habitat is the place where an organism lives.

Habitat And Niche

Habitat is a place or the set of environmental conditions around the organism to which it must adapt to survive and prosper. The term Niche is used to denote the functional role played by an organism in its environment (J. Grinnell 1917). Niche includes various aspects of the life of an organism like diet, shelter, etc.

A habitat defines the physical space of an organism with the other living or nonliving factors, while niche describes how that organism is linked with its physical and biological environment. In colloquial language habitat is a postal address while niche is the profession of organism.

Definition Of Habitat

Place or area where a particular species lives, is its habitat. Factors like the sunlight, average rainfall, annual temperatures, type of soil present and other abiotic (topographic) factors, affect the presence of organisms. These factors help in determining the presence of the particular type of species in the environment.

Pond, river, ocean, etc. are the examples of habitat as many organisms are found in the same place or habitat. These habitats can be arboreal, terrestrial, aerial, aquatic, etc. The immediate surrounding of an organism, sometimes also referred to as microhabitat, is an important concept to remember when working with sedentary or weakly motile organisms.

Definition Of Niche

The term niche was first time used by 'J. Grinnell'. The term ecological niche is still not well understood and is sometimes even misused.

Teacher's Note

Niche is what an organism does. It is like your job or profession. A bee's job is to collect nectar from flowers. A lion's job is to hunt deer.

Exam Trick

Remember: Habitat = Address, Niche = Job. A fish lives in water (address = habitat). The fish eats small insects (job = niche).

Points to Remember

Habitat is a place. Niche is what an organism does there.
Many species can live in one habitat.
Only one species can have one niche.
Habitat has many niches inside it.
Niche deals with food, shelter, and how organisms live.

13.2 Major Abiotic Factors

Temperature

It is the most ecologically relevant environmental factor. Average temperature on land varies from subzero levels in polar areas and high altitudes, upwards upto 50°C in tropical deserts in summer. Temperature also varies seasonally. It decreases progressively from the equator towards the poles and from plains to the mountain tops. There are some unique habitats such as hot springs (80 to 100°C) and deep-sea hydrothermal vents where average temperatures usually 400°C. Ambient temperature affects the enzyme kinetics of the cell and thus, the entire metabolism, activity and other physiology of the organism.

Only few organisms can tolerate and thrive in a wide range of temperatures (eurythermal), but, a vast majority of them are restricted to a narrow range of temperatures (stenothermal).

Teacher's Note

Some animals like the polar bear can live in very cold places. Some animals like the camel can live in very hot deserts. Different animals like different temperatures.

Exam Trick

Remember: Eurythermal = can take many temperatures. Stenothermal = can take only one type of temperature. Most animals are stenothermal. They like only their own temperature.

Points to Remember

Temperature changes from hot to cold areas on Earth.
Some animals can live in many temperatures (eurythermal).
Most animals can only live in one temperature range (stenothermal).
Temperature affects how an organism's body works.
Hot springs and deep ocean have very different temperatures.

Water

Availability of water is an important factor affecting the organisms. As we know, life on earth originated in water, its availability is so limited in deserts that only special adaptations are required to survive there. The productivity and distribution of plants are also heavily dependent on water.

Teacher's Note

Water is very important for all life. Plants need water to grow. Animals need water to drink. In deserts, there is very little water. So only special plants and animals can live there.

Exam Trick

Remember: No water = no life. Water is needed by all organisms. If there is no water, the organism cannot live.

Points to Remember

All life started in water.
Water is needed by all plants and animals.
Deserts have very little water.
Organisms in deserts have special ways to save water.
The amount of water decides what plants and animals can live in a place.

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