Organisms And Population: Aditi Ma'am

 

Chapter 13: Organisms and Populations — 

This chapter explains how organisms live in their surroundings, how they adjust to environmental conditions, how populations grow, and how different species interact with one another.

1. Ecology

The natural world contains a huge variety of living organisms. To understand this living world, scientists study different levels of organization:

Macromolecules → Cells → Tissues → Organs → Organism → Population → Community → Ecosystem → Biome

What is Ecology?

Ecology is the study of:

• interactions between organisms, and
• interactions between organisms and their physical or abiotic environment.

The term ecology was first used by Reiter, while E. Haeckel gave proper meaning and importance to the concept. organisms and population tb

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2. Ecological Hierarchy

The chapter mainly describes four ecological levels:

Organism → Population → Community → Biome

Organism

An individual living being.

Example:
One tiger, one mango tree, one human.

Population

A group of organisms of the same species living in a particular geographical area.

Example:

All the deer living in one forest = deer population.

Community

Different populations living together and interacting.

Example:

Trees + deer + tiger + insects + birds in a forest.

Biome

A very large geographical region having a particular:

• climate,
• vegetation, and
• associated animals.

Examples:

• Desert
• Grassland
• Tropical forest
• Temperate forest
• Coniferous forest
• Arctic/alpine tundra

The graph on page 293 shows that the formation of different biomes depends mainly on temperature and rainfall. organisms and population tb

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3. Organisms and Their Environment

Different places on Earth have different environmental conditions.

Examples from the textbook include:

• hot deserts of Rajasthan,
• rain-soaked forests of North-East India,
• high Himalayan mountains.

Yet organisms survive in all these places because they develop adaptations.

Earth's movement around the Sun and the tilt of its axis create different seasons. Changes in temperature and rainfall contribute to the formation of different biomes. organisms and population tb

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4. Habitat

Definition

A habitat is the place or area where a particular species lives.

Examples:

• pond
• river
• ocean
• forest
• desert

The habitat is affected by factors such as:

• sunlight
• rainfall
• temperature
• soil
• topography

Habitats may be:

• Aquatic – water
• Terrestrial – land
• Arboreal – trees
• Aerial – air

Microhabitat

The immediate surroundings of an organism are called its microhabitat.

For example, the underside of a leaf may act as the microhabitat of a small insect. organisms and population tb

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5. Ecological Niche

The word niche was first used by J. Grinnell.

A niche means the functional role of an organism in its environment.

It explains:

• what it eats,
• where it obtains food,
• where it lives,
• how it gets shelter,
• how it interacts with other organisms.

The textbook gives a very easy comparison:

Habitat = postal address
Niche = profession/job

For example:

A woodpecker may live in a forest.
The forest = habitat.

Eating insects from tree bark and using tree holes = its niche. organisms and population tb

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6. Habitat vs Niche

Habitat	Niche
Physical place where an organism lives	Functional role of the organism
Can contain many species	Species-specific
Contains many niches	Describes one organism's role
Influenced by temperature, rainfall, soil etc.	Includes food, energy flow and interaction
Not species-specific	Species-specific

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7. Types of Ecological Niche

The textbook describes three types.

A. Spatial or Habitat Niche

Concerned with the physical space occupied by an organism.

B. Trophic Niche

Concerned with the organism's position in a food chain.

Example:

Grass → Grasshopper → Frog → Snake

Each organism occupies a different trophic niche.

C. Multidimensional or Hypervolume Niche

This considers many factors together:

• temperature
• food
• water
• predators
• competitors
• soil
• shelter, etc.

It represents the overall position of a species in different environmental conditions. organisms and population tb

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8. Fundamental and Realized Niche

Fundamental niche

The niche a species could occupy if competitors were absent.

Realized niche

The niche actually occupied by a species when competition and other environmental pressures are present.

The realized niche is therefore more realistic in nature. organisms and population tb

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9. Important Ecological Terms

Ethology

Study of animal behaviour in relation to the environment.

Ecology

Study of relationships between organisms and their surroundings.

Bionomics

Study of relationships between organisms and their environment.

Environmental Biology

Modern ecology focusing especially on functional and physiological relationships between organisms and their surroundings.

Biosphere

All ecosystems present on Earth together form the biosphere. organisms and population tb

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10. Major Abiotic Factors

The four important abiotic factors are:

Temperature + Water + Light + Soil

Abiotic means non-living environmental factors. organisms and population tb

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11. Temperature

Temperature is one of the most important environmental factors.

Temperature varies:

• from equator to poles,
• from plains to mountains,
• between seasons,
• between deserts and polar regions.

Temperature influences:

• enzymes
• metabolism
• activity
• growth
• physiology

Eurythermal organisms

Organisms capable of tolerating a wide range of temperature.

Stenothermal organisms

Organisms that can tolerate only a narrow range of temperature. organisms and population tb

Easy memory:

Eury = wide
Steno = narrow

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12. Water

Water is essential for all living organisms.

Life itself originated in water.

The availability of water strongly influences:

• survival,
• plant growth,
• productivity,
• distribution of organisms.

Desert organisms therefore require special adaptations because water is limited there. organisms and population tb

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13. Salinity

Aquatic organisms also face differences in the amount of dissolved salt in water.

Freshwater

Salinity is generally less than 5 ppt.

Sea water

Approximately 30–35 ppt.

Hypersaline lagoons

May reach around 100 ppt.

Here, ppt = parts per thousand.

Euryhaline organisms

Can tolerate a wide range of salinity.

Stenohaline organisms

Can tolerate only a narrow range of salinity.

Many freshwater organisms cannot survive in seawater and vice versa because they face osmotic problems. organisms and population tb

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14. Light

Light is extremely important because plants require it for photosynthesis.

Sunlight therefore provides the main source of energy supporting ecosystems.

Some plants growing on the forest floor receive very little sunlight because large trees block it. These plants are adapted to photosynthesize under low-light conditions.

Light also affects animals.

Changes in light duration, called photoperiod, help animals decide the timing of:

• feeding
• reproduction
• migration

Light and temperature are closely related because the Sun provides both. organisms and population tb

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15. Soil

Soil properties depend on:

• climate
• weathering

Important soil features include:

• soil composition
• grain size
• pH
• mineral composition
• water-holding capacity
• water percolation
• topography

These properties determine what type of vegetation grows in an area.

Vegetation then influences what type of animals can live there. organisms and population tb

Simple chain:

Soil → Vegetation → Animals

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16. Homeostasis

Environmental conditions continuously change.

However, organisms need relatively stable internal conditions for proper functioning.

Homeostasis

The ability of an organism to maintain a relatively constant internal environment despite changes outside the body.

It is important because enzymes and biochemical reactions function efficiently only under suitable internal conditions. organisms and population tb

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17. How Organisms Deal With Environmental Changes

Organisms generally show four responses:

Regulate → Conform → Migrate → Suspend

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A. Regulate

Some organisms maintain constant internal conditions using physiological or behavioural mechanisms.

Examples:

• birds
• mammals

They regulate:

• body temperature → thermoregulation
• salt/water balance → osmoregulation

Example:

Humans sweat when the body becomes hot.

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B. Conform

Many organisms cannot maintain a constant internal environment.

Their body conditions change according to external conditions.

Example:

If environmental temperature changes, their body temperature also changes.

Such organisms are called conformers. organisms and population tb

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C. Migrate

An organism may temporarily move away from an unfavourable area and return when conditions improve.

Example:

Many birds migrate during winter.

Migration = temporary movement to a more favourable place. organisms and population tb

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D. Suspend

Some organisms reduce their activities during stressful environmental conditions.

Dormancy

Seeds remain inactive until favourable temperature and moisture return.

Hibernation

Winter sleep.

Example given in the text: polar bear.

Aestivation

Summer sleep.

Examples:

• some snails
• some fish

Easy trick:

Hibernation → Winter
Aestivation → Summer organisms and population tb

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18. Adaptation

Definition

An adaptation is a morphological, physiological or behavioural characteristic that helps an organism survive and reproduce in its habitat.

There are three broad kinds:

• Structural/morphological adaptation
• Physiological adaptation
• Behavioural adaptation organisms and population tb

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19. Desert Plant Adaptations

Desert plants must reduce water loss.

They may possess:

Thick cuticle

Reduces water loss.

Sunken stomata

Stomata are present inside deep pits, reducing transpiration.

CAM pathway

CAM = Crassulacean Acid Metabolism

It allows stomata to stay closed during daytime.

Opuntia

In Opuntia:

• leaves are modified into spines
• flattened green stems perform photosynthesis. organisms and population tb

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20. Adaptations of Animals in Cold Regions

Allen's Rule

Mammals from cold climates generally possess shorter:

• ears
• snout
• tail
• limbs

This reduces loss of body heat.

Blubber

Aquatic mammals such as seals possess a thick layer of fat below the skin called blubber.

It acts as insulation and prevents heat loss. organisms and population tb

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21. Behavioural Adaptation of Desert Lizards

Desert lizards regulate their temperature using behaviour.

When cold:

☀️ They come into sunlight and bask.

When hot:

🌴 They move into shade.

Some species also burrow into sand to escape excessive heat. organisms and population tb

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22. Population

In nature, organisms usually do not live alone.

A population consists of individuals of the same species living in a defined geographical area and capable of potentially breeding with one another.

Population ecology connects:

• ecology
• genetics
• evolution
• population dynamics. organisms and population tb

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23. Characteristics of a Population

A population has properties that a single organism does not have.

Important characteristics include:

• Population size
• Population density
• Natality
• Mortality
• Sex ratio
• Immigration
• Emigration
• Age distribution
• Population growth
• Biotic potential organisms and population tb

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24. Population Size and Population Density

Population size

Total number of individuals.

Population density

Number of individuals present per unit area or space at a particular time.

It is generally represented by:

N

Sometimes simply counting individuals is not practical.

For example, instead of counting every organism, scientists may measure:

• biomass,
• animals caught per trap,
• indirect evidence.

Tiger populations may be estimated from evidence such as:

• pugmarks
• fecal pellets. organisms and population tb

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25. Natality

Natality = Birth rate

It represents the production of new individuals in a population.

Example from the textbook:

Population initially = 200 carp
New fish = 800

Birth rate:

800 ÷ 200 = 4 offspring per carp per year

Absolute natality

Births under ideal conditions with:

• plenty of food,
• water,
• no competition.

Realized natality

Actual births when environmental pressures exist.

Therefore:

Absolute natality > Realized natality organisms and population tb

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26. Mortality

Mortality = Death rate

It is the number of deaths in a population during a particular period.

It is commonly expressed as:

deaths per 1,000 individuals per year

Absolute mortality

Deaths under ideal conditions.

Realized mortality

Deaths under actual environmental conditions.

According to the textbook:

Absolute mortality < Realized mortality. organisms and population tb

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27. Sex Ratio

Sex ratio is the ratio between the numbers of the two sexes in a population.

A ratio of approximately:

Male : Female = 1 : 1

is described in the text as a common evolutionarily stable strategy.

Sex ratio can be influenced by:

• births
• deaths
• immigration
• emigration. organisms and population tb

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28. Age Distribution and Age Pyramid

Population members are divided into three age groups:

Pre-reproductive

0–14 years

Reproductive

15–44 years

Post-reproductive

45–85+ years

When these age groups are shown graphically, the diagram is called an age pyramid. organisms and population tb

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29. Types of Age Pyramid

The diagram on page 299 shows four population patterns.

Rapid growth

Example:

• Kenya
• Nigeria

There are many young individuals.

Slow growth

Example:

• United States
• Australia

Population continues increasing, but slowly.

Zero growth

Example:

• Denmark
• Italy

Birth and death patterns keep the population relatively stable.

Negative growth

Example:

• Germany
• Hungary

The younger population is relatively smaller, indicating decline according to the diagram. organisms and population tb

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30. Population Growth

Population size does not remain constant.

It changes according to factors such as:

• food availability
• predators
• weather
• births
• deaths
• immigration
• emigration. organisms and population tb

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31. Four Processes Affecting Population Size

Births (B)

Increase population.

Immigration (I)

Individuals entering the population from another place.

Deaths (D)

Decrease population.

Emigration (E)

Individuals leaving the population.

Therefore:

Nₜ₊₁ = Nₜ + [(B + I) – (D + E)]

Where:

• Nₜ = population now
• Nₜ₊₁ = population after the next time period. organisms and population tb

Easy memory:

BI increase population
DE decrease population

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32. Population Growth Models

There are two important models:

1. Exponential Growth

2. Logistic Growth

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33. Exponential Growth

When resources such as:

• food
• water
• space

are unlimited, population can grow very rapidly.

It forms a J-shaped growth curve.

The graph on page 300 shows the population initially increasing slowly and then increasing very rapidly. organisms and population tb

Example

Even slowly reproducing animals such as elephants could reach huge population numbers if:

• food was unlimited,
• space was unlimited,
• mortality was low.

But this condition usually cannot continue forever. organisms and population tb

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34. Logistic Growth

In nature, resources are normally limited.

Therefore:

Population grows → competition increases → growth slows.

Eventually, the population reaches the maximum size the habitat can support.

This is called:

Carrying Capacity (K)

The logistic growth curve has an S or sigmoid shape.

Its phases shown on page 300 are:

Lag phase → Log/Exponential phase → Diminishing growth → Stationary phase

At the stationary phase, population is near the carrying capacity. organisms and population tb

The textbook calls this the:

Verhulst-Pearl Logistic Growth

Because natural resources are limited, logistic growth is considered more realistic than unlimited exponential growth. organisms and population tb

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35. Population Interactions

No species normally lives completely alone.

Organisms interact with organisms of:

Same species

Called intraspecific interaction.

Different species

Called interspecific interaction. organisms and population tb

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36. Symbols Used for Population Interactions

+ = benefited

– = harmed/inhibited

0 = unaffected

The textbook classifies interactions into different types. organisms and population tb

Interaction	Species A	Species B
Neutralism	0	0
Competition	–	–
Amensalism	–	0
Mutualism	+	+
Commensalism	+	0
Protocooperation	+	+
Parasitism	+	–
Predation	+	–

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37. Neutralism

Neither species significantly affects the other.

0 / 0

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38. Mutualism

Both species benefit.

+ / +

The relationship is described as highly interdependent.

Example: Lichen

Lichen contains:

• a fungus
• an alga/cyanobacterium

Both live together and benefit.

The diagram on page 302 shows the fungal and algal components of a lichen. organisms and population tb

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39. Plant–Animal Mutualism

Plants often depend on animals for:

• pollination
• seed dispersal

Animals receive food such as:

• nectar
• pollen
• fruits

So both benefit.

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40. Co-evolution

Sometimes two interacting species evolve together.

Example shown on page 302:

Hummingbird + flower

The characteristics of the flower and its pollinator may become closely linked during evolution.

This is called co-evolution. organisms and population tb

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41. Competition

Competition occurs when organisms compete for a resource.

Both are negatively affected.

– / –

They may compete for:

• food
• water
• space
• shelter

Example in the textbook:

At shallow creeks near Mumbai:

Flamingos and resident fish compete for zooplankton.

Competition may also occur even when resources are abundant because the presence of one species may reduce the feeding efficiency of the other. organisms and population tb

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42. Gause's Competitive Exclusion Principle

It states that:

Two closely related species competing for the same limited resources cannot coexist indefinitely.

Eventually, the competitively inferior species may be eliminated. organisms and population tb

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43. Resource Partitioning

Species may avoid competition by using a resource differently.

Example:

Two species may feed:

• at different times,
• in different places,
• on different parts of the same resource.

This allows them to coexist.

This mechanism is called resource partitioning. organisms and population tb

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44. Amensalism

One species is harmed while the other is unaffected.

– / 0

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45. Parasitism

In parasitism:

Parasite = benefits (+)
Host = harmed (–)

Parasites obtain food or shelter from their hosts.

Many parasites become host-specific, meaning they can live on only particular hosts. organisms and population tb

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46. Adaptations of Parasites

Parasites may show adaptations such as:

• loss of unnecessary sense organs,
• suckers or adhesive organs,
• loss of digestive system,
• very high reproductive capacity.

Their life cycle can be complex and may involve:

• intermediate hosts
• vectors. organisms and population tb

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47. Example: Plasmodium

The textbook mentions Plasmodium vivax.

It requires a mosquito as a vector for transmission from one host to another. organisms and population tb

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48. Ectoparasites

Parasites that live on the outside surface of the host are called ectoparasites.

Examples:

• lice on humans
• ticks on dogs
• ectoparasitic copepods on marine fishes. organisms and population tb

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49. Cuscuta

Cuscuta is a parasitic plant.

During evolution it lost:

• normal leaves,
• chlorophyll.

Therefore, it obtains nutrition from its host plant. organisms and population tb

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50. Brood Parasitism

In brood parasitism, one bird lays its eggs in the nest of another bird.

The host bird:

• incubates the eggs,
• may raise the parasite's young.

Example:

Asian koel → Common Indian crow

Koel eggs resemble the host's eggs in:

• colour
• size

This reduces the chance of the host recognising and removing them. organisms and population tb

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51. Effects of Parasites on Hosts

Parasites may:

• reduce survival,
• slow growth,
• reduce reproduction,
• weaken the host,
• make it easier for predators to attack,
• sometimes cause host death. organisms and population tb

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52. Predation

In predation:

Predator benefits (+)
Prey is harmed (–)

Examples:

Tiger → Deer

But the textbook explains that predation is broader than this.

A sparrow eating seeds is also considered a form of predator-prey interaction in this context. organisms and population tb

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53. Importance of Predators

Predators are extremely important for ecosystem stability.

They:

1. Control prey population

Without predators, prey population may increase excessively.

Example from the text:

If frogs disappear → locust population may increase → crops can be destroyed.

2. Help biological control

Predators can be used to control agricultural pests.

3. Maintain species diversity

Predators can reduce excessive competition among prey species. organisms and population tb

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54. Invasive Species

An organism introduced into a new geographical area may spread rapidly when its natural predators are absent.

Such organisms can become invasive species.

The textbook gives zebra mussels in North America as an example. organisms and population tb

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55. Why Predators Cannot Overexploit Their Prey

If predators kill too many prey:

Prey population falls → prey may disappear → predator loses food → predator population also falls.

Therefore, predator and prey populations influence each other. organisms and population tb

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56. Anti-predator Adaptations

Prey species develop mechanisms to protect themselves.

Examples:

Camouflage

Colour or body pattern helps an organism blend into the environment.

Speed

Prey may escape predators by running or swimming quickly.

The textbook describes this continuous improvement between predator and prey as an:

Evolutionary arms race

Predator develops better hunting methods while prey develops better defence mechanisms. organisms and population tb

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57. Monarch Butterfly Defence

The Monarch butterfly contains a chemical that makes it unpleasant to predators.

The butterfly obtains the chemical during the caterpillar stage by feeding on a poisonous plant.

Its warning colour also helps predators recognise it.

This is illustrated on page 304. organisms and population tb

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58. Defence in Plants Against Herbivores

For plants, herbivores act like predators.

Plants therefore develop defences.

Structural defence

Examples:

• thorns in Acacia
• spines in Cactus

Chemical defence

Some plants produce chemicals that:

• make herbivores sick,
• interfere with digestion,
• stop feeding,
• interfere with reproduction,
• may even kill the herbivore.

Example: Calotropis

It produces poisonous cardiac glycosides.

The textbook also mentions defensive plant chemicals such as:

• nicotine
• caffeine
• quinine
• strychnine
• opium. organisms and population tb

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59. Commensalism

In commensalism:

One species benefits (+)
Other species is unaffected (0)

So:

+ / 0

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Example 1: Orchid and Mango Tree

An orchid growing on the branch of a mango tree receives:

• support,
• better access to light.

The mango tree is neither helped nor harmed.

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Example 2: Cattle Egret and Cattle

As cattle move through grass, they disturb insects.

The cattle egret catches these insects.

Therefore:

Egret = benefits
Cattle = unaffected

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Example 3: Clownfish and Sea Anemone

The clownfish lives among the stinging tentacles of the sea anemone.

It receives protection from predators.

According to the textbook, the sea anemone is not significantly benefited or harmed.

The diagram on page 305 shows both the cattle-egret and clownfish examples. organisms and population tb

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60. Entire Population Interaction Table — Easy Revision

Interaction	Result	Simple Meaning	Example
Neutralism	0, 0	Neither affected	No significant effect
Competition	–, –	Both suffer	Flamingo & fish
Amensalism	–, 0	One harmed	Other unaffected
Mutualism	+, +	Both benefit	Lichen
Commensalism	+, 0	One benefits	Cattle egret & cattle
Protocooperation	+, +	Both benefit	Positive association
Parasitism	+, –	Parasite benefits	Cuscuta / lice
Predation	+, –	Predator benefits	Tiger & deer

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61. Important Formula

Remember this formula:

Nₜ₊₁ = Nₜ + [(B + I) – (D + E)]

Where:

B = Birth
I = Immigration
D = Death
E = Emigration

Increase population:

Birth + Immigration

Decrease population:

Death + Emigration

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62. Four Responses to Environmental Stress

Remember:

R – C – M – S

R – Regulate
C – Conform
M – Migrate
S – Suspend

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63. Four Main Abiotic Factors

Remember:

T – W – L – S

T – Temperature
W – Water
L – Light
S – Soil.

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65. Important Days Given in the Chapter

The last page also lists:

World Environment Day — 5 June

World Population Day — 11 July

World Earth Day — 22 April

World Ozone Day — 16 September

The instrument used to measure the height of forest trees is called a hypsometer. organisms and population tb

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Quickest Way to Remember the Whole Chapter

Think of the chapter in this order:

ORGANISM → ENVIRONMENT → ABIOTIC FACTORS → ADAPTATION → POPULATION → POPULATION GROWTH → POPULATION INTERACTIONS

That single sequence covers almost the entire logic of Chapter 13: Organisms and Populations.