🌸Chapter 11
Organisms & Populations
(Biology Notes)
1. Introduction
- Ecology: The study of interactions
between organisms and their environment.
- Population: A group of individuals of
the same species living in a defined area at a given time.
- Focus: Population structure,
dynamics, and adaptations.
Key NCERT Keywords: Ecology, Population, Population density, Birth rate, Death
rate, Dispersion, Age structure.
2. Characteristics of a Population
Populations have specific characteristics that can be
measured:
2.1. Population Size (N)
- Definition: The total number of
individuals of a species in a defined area at a given time.
- Example: 150 deer in a forest patch.
2.2. Population Density (D)
- Definition: The number of individuals per
unit area or volume.
- Formula:
D=Number of individuals (N)Area or volume (A or V)D
= \frac{\text{Number of individuals (N)}}{\text{Area or volume (A or
V)}}D=Area or volume (A or V)Number of individuals (N)
- Types:
- High density → Crowded
population
- Low density → Scattered
population
2.3. Population Dispersion /
Distribution
- Definition: The pattern of spacing among
individuals in a population.
- Types of Dispersion:
1.
Clumped: Individuals occur in groups (common
in nature, e.g., elephants, schools of fish).
2.
Uniform: Individuals are evenly spaced
(e.g., penguins, territorial animals).
3.
Random: No specific pattern (e.g.,
dandelions, forest trees).
Diagram for dispersion:
Clumped: *
* * *
Uniform: *
* * *
Random: *
* * *
2.4. Age Structure
- Definition: Distribution of individuals of
different age groups in a population.
- Importance: Helps predict population
growth trends.
- Age Groups:
- Pre-reproductive → Young, not breeding yet
- Reproductive → Breeding adults
- Post-reproductive → Old, no longer breeding
Example of age pyramid:
- Expanding population → Broad
base (many young)
- Stable population → Uniform
width
- Declining population → Narrow
base
3. Population Growth
3.1. Types of Population Growth
1.
Exponential
Growth (J-shaped curve)
o Population grows rapidly
under ideal conditions.
o Formula:
dNdt=rN\frac{dN}{dt} = rNdtdN=rN
o r → intrinsic rate of natural increase
o Example: Bacteria in lab cultures.
2.
Logistic
Growth (S-shaped curve)
o Growth slows as population reaches carrying
capacity (K) of environment.
o Formula:
dNdt=rN(1−NK)\frac{dN}{dt} = rN
\left(1-\frac{N}{K}\right)dtdN=rN(1−KN)
o Carrying capacity → Maximum
population size environment can support.
Diagram: J-shaped vs S-shaped curve
3.2. Factors Affecting Population
Growth
- Birth Rate (Natality): Number of births per 1000
individuals per year.
- Death Rate (Mortality): Number of deaths per 1000
individuals per year.
- Immigration: Individuals entering a
population.
- Emigration: Individuals leaving a
population.
Population Growth Equation:
ΔN=(B−D)+(I−E)\Delta N = (B - D) + (I - E)ΔN=(B−D)+(I−E)
Where,
B = Births, D = Deaths, I = Immigration, E = Emigration
4. Population Interactions
4.1. Intraspecific Competition
- Definition: Competition among individuals
of the same species.
- Example: Plants competing for
sunlight, animals competing for mates.
4.2. Interspecific Competition
- Definition: Competition between different
species for the same resource.
- Outcome: Competitive exclusion or
resource partitioning.
- Example: Lions and hyenas competing for
prey.
4.3. Other Interactions
- Predation: One species (predator) kills
and eats another (prey).
- Parasitism: One species (parasite)
benefits, host is harmed.
- Mutualism: Both species benefit.
- Commensalism: One benefits, other unaffected.
- Amensalism: One harmed, other unaffected.
5. Adaptations of Populations
- Physiological Adaptations: Internal body functions to
survive (e.g., camel stores water in hump).
- Morphological Adaptations: Physical structures help
survival (e.g., cactus spines).
- Behavioral Adaptations: Behavioral changes (e.g.,
nocturnal activity in desert animals).
6. Population Regulation
- Populations do not grow
indefinitely.
- Regulating Factors:
- Density-dependent factors: Affect population based on
density (e.g., food, disease, predation).
- Density-independent factors: Affect population regardless
of density (e.g., floods, fire, climate).
Example:
- High density → More disease →
Population decreases
- Low density → Less disease →
Population increases
7. Summary Table of Population
Characteristics
|
Characteristic |
Definition |
Example |
|
Population size |
Total individuals in area |
150 deer in a forest |
|
Population density |
Individuals per unit area |
20 trees per hectare |
|
Dispersion |
Spatial distribution pattern |
Clumped, uniform, random |
|
Age structure |
Distribution in age groups |
Expanding, stable, declining |
|
Growth pattern |
Increase in population over time |
Exponential (J), Logistic (S) |
|
Interactions |
Relationships with other species |
Predation, competition, mutualism |
8. Important Diagrams for Board/NEET
1.
Types
of Population Dispersion:
Clumped, Uniform, Random
2.
Population
Growth Curves:
J-shaped, S-shaped
3.
Age
Pyramid:
Expanding, Stable, Declining
9. Important NCERT Definitions
- Population: Group of individuals of same
species in a given area.
- Population Density: Number of individuals per unit
area/volume.
- Dispersion: Pattern of spacing among
individuals.
- Carrying Capacity (K): Maximum population size an
environment can sustain.
- Exponential Growth: Rapid growth under ideal
conditions.
- Logistic Growth: Growth limited by resources,
forming S-shaped curve.
10. Quick Tips for NEET & Board
- Remember formulas: D = N/A,
ΔN = (B-D) + (I-E)
- Draw diagrams neatly; they
carry marks.
- Learn examples for each
interaction and adaptation.
- Relate population concepts
to human populations (urbanization, resource use).
Reference:
- NCERT Class 12 Biology,
Chapter: Organisms and Populations, Page Nos: 156–166
