13. organism and populations

Chapter: Organisms and Populations

1. Introduction to Ecology

Ecology: Study of interactions among organisms and between organisms and their physical (abiotic) environment.

History of Ecology:

Reiter: First used the term Ecology.

E. Haeckel: Introduced and developed the concept of ecology.

Levels of Organisation in the Living World

Living organisms show different levels of organisation, from simple to complex:

Macromolecules → Cells → Tissues → Organs → Organisms → Populations → Communities → Ecosystems → Biomes

2. Ecological Hierarchy

Ecological hierarchy: Ecological grouping of organisms into levels of increasing biological complexity.

There are four major sequential levels:

1. Organism

An individual living being.

Basic unit of ecological hierarchy.

2. Population

Organisms of the same kind (same species) inhabiting a particular geographical area.

3. Community

Several populations of different species living in a particular area.

These populations interact with one another.

4. Biome

A large regional terrestrial unit characterised by:

A specific climatic zone.

Major vegetation zones (plant communities).

Associated fauna (animal life).

There are six major groups of terrestrial biomes.

13.1 Organisms and the Environment Around Them

1. Organism and Ecology

Organismal ecology: Study of how plants and animals are adapted to their environment.

It involves studying physiology (functions of the body) to understand how organisms adapt for:

Survival — staying alive.

Propagation — multiplication or reproduction.

2. Factors Affecting Major Biomes

Earth's rotation around the Sun and the tilt of its axis cause seasonal changes.

Seasonal changes and annual variations in precipitation (rainfall and snowfall) lead to the formation of major biomes.

Major terrestrial biomes include:

Desert

Rainforest

Grassland

Tundra

3. Biomes and Habitats

Biome: A large regional area characterised by a specific climate, major vegetation and associated fauna.

Habitat: The natural place where an organism lives.

Regional and local variations within a biome produce a variety of habitats.

4. Life in Extreme Environments

Life exists even in harsh and extreme habitats, such as:

Rajasthan: Scorching deserts.

North-Eastern states of India: Perpetually rain-soaked forests.

Himalayas: High mountain tops.

13.1 Organisms and the Environment

1. Components of a Habitat

A habitat includes both:

Abiotic components: Physical and chemical factors, e.g. temperature, water, light and soil.

Biotic components: Living organisms, e.g. plants, pathogens, parasites and predators.

2. Adaptation and Natural Selection

Over time, organisms evolve adaptations through natural selection.

These adaptations improve their chances of:

Survival — staying alive.

Reproduction — producing offspring.

The adaptations help organisms survive and reproduce successfully in their habitats.

3. Important Terms and Their History

1. Ethology

Meaning: Study of animal behaviour in relation to the environment.

Hilaire (1854): Coined the term.

W. M. Wheeler (1902): Popularised the term.

2. Ecology

Meaning: Study of relationships between organisms and their surroundings.

E. Haeckel (1865): Introduced the term.

3. Bionomics

Meaning: Study of the relationship between organisms and their environment.

Lankester (1890): Coined the term.

4. Environmental Biology (Modern Ecology)

Focuses on the functional and physiological relationships between organisms and their surroundings.

G. L. Clarke (1964) and Odum (1969): Associated with its development in the 20th century.

5. Biosphere

The sum of all ecosystems on Earth.

It represents the global zone where life exists.

Quick Revision:

Ethology → Animal behaviour

Ecology → Organisms and surroundings

Bionomics → Organisms and environment

Environmental biology → Functional relationships between organisms and surroundings

Biosphere → All ecosystems on Earth

13.1.1 Habitat and Niche

1. Habitat

Definition: The place or area where a particular species lives.

It provides the environmental conditions necessary for an organism to survive and reproduce.

Abiotic factors determining the presence of organisms include:

Sunlight

Average rainfall

Annual temperature

Soil type

Topography (physical features of the land)

Examples: Pond, river, ocean.

Types of habitats:

Terrestrial: Land

Aquatic: Water

Arboreal: Trees

Aerial: Air

Microhabitat:

The immediate, small-scale surroundings of an organism.

Important for sedentary (stationary) or weakly motile (less mobile) organisms.

2. Niche

Definition: The functional role played by an organism in its environment.

J. Grinnell (1917): Associated with the concept of ecological niche.

Includes different aspects of an organism's life, such as:

Diet (food habits)

Shelter

Interactions with other organisms

Relationship with the physical environment

3. Habitat vs Niche

Habitat = Address: Where an organism lives.

Niche = Profession: How an organism lives and functions in its environment.

3. Types of Ecological Niches

There are three types of niches:

A. Spatial or Habitat Niche

Refers to the physical space occupied by an organism.

Describes where an organism lives within its habitat.

B. Trophic Niche

Based on the trophic level (feeding position) of an organism in a food chain.

Depends on how an organism obtains its food and energy.

C. Multidimensional or Hypervolume Niche

Considers multiple environmental factors, both biotic and abiotic.

These factors together form a theoretical space called a hypervolume niche.

It describes the position of an organism along different environmental gradients (changes in environmental conditions).

4. Fundamental and Realised Niches

A. Fundamental Niche

The range of environmental conditions and resources a species could use in the absence of competitors.

Rarely exists fully in nature because organisms face competition.

B. Realised Niche

The actual range of conditions and resources used by a species in the presence of competitors.

More realistic than the fundamental niche.

5. Key Abiotic Factors

The main abiotic (non-living) factors that influence a habitat are:

1. Temperature — surrounding environmental temperature.

2. Water — availability of water.

3. Light — intensity and duration of light.

4. Soil — type and properties of soil.

13.2 Major Abiotic Factors

Abiotic factors are the non-living physical and chemical components of the environment that affect organisms.

1. Temperature 🌡️

One of the most ecologically relevant environmental factors.

Temperature variations:

Polar regions and high altitudes: Below 0°C.

Tropical deserts in summer: Up to 50°C.

Hot springs: 80–100°C.

Deep-sea hydrothermal vents: Around 400°C 

Temperature decreases:

From the equator towards the poles.

From plains towards mountain tops.

It also varies seasonally.

Effects of temperature:

Affects enzyme kinetics (rate of enzyme-controlled reactions).

Influences metabolism, activity and physiological functions.

Types of organisms based on temperature tolerance:

Eurythermal: Can tolerate a wide range of temperatures.

Stenothermal: Can tolerate only a narrow range of temperatures.

2. Water 💧

Water is essential for life; life on Earth originated in water.

Water availability affects:

Survival of organisms.

Plant productivity and distribution.

Deserts have limited water; organisms require special adaptations to survive there.

Water-related factors in aquatic habitats:

Chemical composition of water.

pH of water.

Salinity (concentration of dissolved salts).

Salinity in different water bodies:

Freshwater (streams, lakes and rivers): Less than 5 ppt.

Seas and oceans: 30–35 ppt.

Hypersaline lagoons: Up to 100 ppt.

ppt = parts per thousand.

Types of organisms based on salinity tolerance:

Euryhaline: Tolerate a wide range of salinities.

Stenohaline: Tolerate only a narrow range of salinities.

Why can't many freshwater animals survive in seawater, and vice versa?

They face osmotic problems due to differences in salt concentration between their body fluids and the surrounding water.

3. Light ☀️

Importance in plants:

Light is essential for photosynthesis.

Photosynthesis captures solar energy, which supports the entire ecosystem.

Some herbs and shrubs growing under tall forest trees are adapted to perform photosynthesis under low-light conditions.

Importance in animals:

Daily (diurnal) and seasonal changes in light intensity and duration provide cues for:

Foraging (searching for food).

Reproductive activities.

Migration.

Note: Sunlight is the source of both light and heat, so light availability on land is closely related to temperature.

4. Soil

Soil properties depend on climate and weathering processes (breakdown of rocks).

Important soil characteristics:

Soil composition.

Grain size.

Percolation (movement of water through soil).

Water-holding capacity.

pH.

Mineral composition.

Topography (physical features of the land).

Effects:

These characteristics determine the vegetation of an area.

Vegetation, in turn, influences the types of animals found there.

5. Homeostasis

Abiotic factors show diurnal (daily) and seasonal variations.

Organisms develop adaptations to survive these changes.

Many species have evolved mechanisms to maintain a relatively constant internal environment.

Homeostasis: Maintenance of a stable internal environment despite changes in external conditions.

It helps biochemical reactions and physiological functions proceed at suitable rates.

6. Responses of Organisms to Environmental Changes

Organisms respond to environmental changes in different ways.

A. Regulate

Some organisms maintain a relatively constant internal environment through physiological and behavioural changes.

They regulate factors such as body temperature and osmotic concentration.

Examples: All birds and mammals.

Thermoregulation: Regulation of body temperature.

Osmoregulation: Regulation of water and salt balance.

B. Conform

Most plants and many animals cannot maintain a constant internal environment.

Their internal conditions change with the external environment.

Examples:

Body temperature changes with ambient temperature.

In aquatic animals, the osmotic concentration of body fluids may change with that of the surrounding water.

Such organisms are called conformers.

Quick Revision

Eurythermal → Wide temperature tolerance.

Stenothermal → Narrow temperature tolerance.

Euryhaline → Wide salinity tolerance.

Stenohaline → Narrow salinity tolerance.

Homeostasis → Stable internal environment.

Regulators → Maintain internal conditions.

Conformers → Internal conditions change with the environment.

13.2 Major Abiotic Factors 

C. Partial Regulation

Some organisms can regulate their internal environment within a limited range of external conditions.

Beyond this range, they behave as conformers.

If stressful conditions are temporary or localised, organisms may migrate or suspend their activities.

D. Migration

Organisms temporarily move away from a stressful habitat to a more favourable area.

They return when the stressful period is over.

Example: Many birds migrate long distances during winter to find more favourable conditions.

E. Suspension of Activities

Organisms reduce their activities to survive unfavourable conditions.

In plants:

Seeds survive stressful periods through dormancy (a state of greatly reduced metabolic activity).

They germinate when moisture and temperature become favourable.

In animals:

Hibernation: Winter dormancy or reduced activity to survive cold conditions.

Example: Polar bear (as given in the textbook).

Aestivation: Reduced activity during hot, dry summer conditions.

Examples: Some snails and fish.

13.3 Adaptation

1. Definition

Adaptation: A morphological, physiological or behavioural attribute that enables an organism to survive and reproduce in its habitat.

Organisms cope with environmental variations through:

Morphological adaptations: Changes in body structure.

Physiological adaptations: Changes in body functions.

Behavioural adaptations: Changes in behaviour, such as migration.

2. Adaptations in Desert Plants 

Thick cuticle: Reduces water loss from leaf surfaces.

Sunken stomata: Stomata present in deep pits reduce water loss through transpiration.

CAM pathway (Crassulacean Acid Metabolism):

Stomata remain closed during the daytime.

Helps conserve water.

Example: Opuntia

Leaves are modified into spines.

Flattened green stems perform photosynthesis.

3. Adaptations in Cold Climates

Allen's Rule

Mammals in colder climates generally have shorter:

Snouts

Ears

Tails

Limbs

These features help reduce heat loss.

Blubber in aquatic mammals

Seals have a thick layer of fat called blubber beneath the skin.

Blubber acts as an insulator, reducing body heat loss in polar seas.

4. Behavioural Adaptations in Desert Lizards 🦎

Desert lizards regulate their body temperature through behaviour:

When body temperature falls: They bask in sunlight to absorb heat.

When environmental temperature rises: They move into shade.

During extreme heat: Some burrow into sand to escape high temperatures.

Quick Revision

Migration → Temporary movement to a favourable habitat.

Dormancy → Reduced metabolic activity during unfavourable conditions.

Hibernation → Winter inactivity.

Aestivation → Summer inactivity.

CAM → Water conservation in desert plants.

Allen's Rule → Shorter body extremities help reduce heat loss in colder climates.