Organisms And Population – NEET NOTES

1. Biological Organisation

Biological organization refers to levels at which life exists, from individuals to the biosphere.
Organism → basic unit of study in ecology.
Population → group of individuals of same species in a given area.
Community → different populations interacting in an area.
Ecosystem → community + abiotic factors.
Biome → large ecological areas with specific climate and species.
Biosphere → global sum of all ecosystems.

2. Organism and Its Environment

Every organism interacts with its environment and other organisms.
Two components of environment:
Biotic (living): other organisms (predators, prey, competitors).
Abiotic (non-living): temperature, water, light, soil.
The environmental factors determine:
Where the organism can live.
How abundant it is.

3. Major Abiotic Factors

a. Temperature

Most important ecological factor.
Affects:
Metabolism
Growth
Reproduction
Survival
Range of tolerance:
Eurythermal organisms: tolerate wide temperature range (e.g. cat, dog).
Stenothermal organisms: tolerate narrow range (e.g. polar bear).
Enzymes and membrane fluidity are temperature-sensitive.
Tropical regions have more biodiversity due to stable temperature.

b. Water

Essential for life – major component of protoplasm.
Availability affects:
Distribution of plants and animals.
Aquatic organisms face:
Problems of osmotic balance.
Freshwater animals: gain water, risk of cell bursting.
Marine animals: lose water, risk of dehydration.
Desert organisms adapt to water scarcity (e.g. kangaroo rat – produces concentrated urine).

c. Light

Essential for:
Photosynthesis in autotrophs.
Influences photoperiodism – flowering, breeding, migration.
Light intensity, duration, and quality affect plants.
Some plants are shade-tolerant.
Algae in deep oceans need light for photosynthesis.

d. Soil

Determines vegetation and thus animals in an area.
Factors:
Soil composition (minerals)
Grain size, water-holding capacity
pH, topography
Affects types of microbes and plants growing in it.

4. Responses to Abiotic Factors

Organisms deal with abiotic factors in 4 ways:

a. Regulate

Maintain constant internal environment (homeostasis).
Examples:
Humans maintain body temperature (sweat in summer, shiver in winter).
Birds and mammals are thermoregulators.
Freshwater fish regulate internal salt concentration.
Energetically expensive strategy.

b. Conform

Cannot regulate internal environment.
Internal conditions change with external environment.
Examples:
Most animals (e.g. invertebrates) and plants are conformers.
Frog’s body temperature changes with surroundings.
Advantage: less energy required.
Limitation: narrow tolerance; cannot survive in extreme conditions.

c. Migrate

Temporary movement to more favorable environment.
Examples:
Birds migrate from Siberia to Keoladeo National Park in winter.
Humpback whales migrate to warmer waters for breeding.

d. Suspend

Enter a dormant or inactive state to avoid stress.
Examples:
Hibernation: Bears in cold regions.
Aestivation: Snails/frogs in summer.
Spore formation: Bacteria in unfavorable conditions.
Diapause: Insects halt development (e.g. silk moth larvae).

5. Adaptations

a. What is Adaptation?

Any feature that enhances survival and reproduction.
Can be:
Morphological (body structure)
Physiological (body function)
Behavioral (actions)
Examples:
Thick fur in polar bear.
CAM photosynthesis in cacti to conserve water.
Fat stored in camel’s hump.
Desert lizards regulate body temperature by basking/sheltering.

b. Allen’s Rule

Warm-blooded animals in cold climates have shorter ears, limbs, and tails to minimize heat loss.
Example: Arctic fox has smaller ears than desert fox.

c. Altitude Sickness

Occurs due to low oxygen at high altitudes.
Symptoms: nausea, fatigue, heart palpitations.
Body copes by:
Increasing red blood cell production.
Breathing rate increases.
Enhancing oxygen-carrying capacity.

d. Adaptations to Cold Stress

Thick fur/fat layer (polar bear, arctic seals).
Small body surface area (to reduce heat loss).
Migration or hibernation.
Plants may have antifreeze proteins or compact leaves.

e. Behavioral Responses

Actions done to cope with environmental changes.
Examples:
Desert lizards bask in the sun to warm up and hide in shade to cool down.
Animals burrow to avoid temperature extremes.
Birds change migratory patterns as per seasons.

6. Population

6.1 Population Definition

A population is a group of individuals of the same species, living in a defined geographical area, sharing or competing for similar resources, and interbreeding.
Example: All banyan trees in a park, or all deer in a forest.

6.2 Population Attributes

Characteristics of populations that are not shown by individuals. a. Birth Rate (Natality):
- Number of births per individual per unit time.
- E.g., 20 births per 1000 → birth rate = 0.02.
b. Death Rate (Mortality):
- Number of deaths per individual per unit time.
- E.g., 10 deaths per 1000 → death rate = 0.01.
c. Sex Ratio:
- Number of females per 1000 males.
- Important for determining potential reproductive output.
d. Age Structure:
- Proportion of individuals in pre-reproductive, reproductive, and post-reproductive age groups.
- Influences population growth trends.

6.3 Age Pyramids

Graphical representation of age structure in a population. a. Expanding Pyramid:
- Broad base, more young individuals.
- Indicates rapid growth.
- Example: India.
b. Stable Pyramid:
- Bell-shaped; all age groups roughly equal.
- Indicates zero or slow growth.
- Example: USA.
c. Declining Pyramid:
- Narrow base; fewer young individuals.
- Indicates negative growth.
- Example: Japan.

6.4 Population Density

Number of individuals of a species per unit area or volume.
Helps ecologists understand how crowded or sparse a population is.
Absolute Density – actual head count.
Relative Density – indirect estimation (e.g., pug marks, faeces).
Crude Density = Total population / Total area.
Ecological Density = Population / Habitable area.

6.5 Population Growth

The change in population size and composition over time.
Influenced by:
- Natality (births)
- Mortality (deaths)
- Immigration (individuals coming in)
- Emigration (individuals leaving)
Formula:
Nt = N₀ + (B + I) – (D + E)
- Nt = Population at time t
- N₀ = Initial population
- B = Births, D = Deaths
- I = Immigration, E = Emigration

7. Growth Models

7.1 Exponential Growth

Occurs when resources are unlimited.
Population grows at a rapid rate and forms a J-shaped curve.
Formula:
dN/dt = rN
- r = intrinsic rate of natural increase
- N = population size
- dN/dt = rate of change in population over time
Example: Bacteria doubling every 20 minutes.
Not sustainable in nature due to resource limitation.

7.2 Logistic Growth

More realistic model as it includes limited resources.
Follows S-shaped or sigmoid curve.
Formula:
dN/dt = rN(K – N)/K
- K = carrying capacity (maximum population an area can support)
Population growth slows as it nears carrying capacity due to resource shortage.

8. Life History Variation

Different organisms adopt different reproductive strategies.
High number of offspring, low survival:
- Example: Insects, oysters.
- Reproduce in large numbers, but little to no parental care.
Few offspring, high survival:
- Example: Humans, elephants.
- Fewer offspring but with parental care, longer life span.
Strategies depend on:
Environmental stability
Predation pressure
Resource availability

9. Population Interactions

When different species live in the same habitat, they interact in various ways.

Interaction	Species A	Species B	Example
Mutualism	+	+	Bee and flower
Competition	–	–	Flamingo and fish (zooplankton)
Predation	+	–	Lion and deer
Parasitism	+	–	Tapeworm in human
Commensalism	+	0	Cattle egret and cattle
Amensalism	–	0	Penicillium killing bacteria

10. Parasitism

One organism (parasite) benefits; the other (host) is harmed. a. Endoparasites
- Live inside the host body (e.g., tapeworm, Plasmodium).
- Highly specialized; complex life cycles.
b. Ectoparasites
- Live on the external surface (e.g., lice, ticks, leech).
c. Brood Parasitism
- One bird lays eggs in the nest of another species.
- Host raises the young.
- Example: Koel lays eggs in crow’s nest.

11. Predation

One organism (predator) kills and consumes another (prey).
Maintains ecological balance:
- Controls prey population.
- Promotes species diversity.
- Removes weak and sick individuals.
Prey Adaptations:
- Camouflage (leaf insect)
- Warning coloration (poison dart frog)
- Toxins (monarch butterfly)
- Spines/thorns (cactus)

12. Competition

Interaction where both species are harmed due to competition for resources. a. Intraspecific – same species (more intense) b. Interspecific – different species
Gause’s Principle (Competitive Exclusion):
- Two species with identical niches cannot coexist indefinitely.
- One will outcompete the other.

13. Mutualism

Both species benefit from the interaction.
Examples:
- Mycorrhiza: Fungi + plant roots (fungi get sugars; plant gets nutrients).
- Lichens: Algae + fungus.
- Clownfish and sea anemone: Clownfish gets shelter; removes parasites from anemone.
- Pollination mutualism: Bees, butterflies, birds pollinate flowers in return for nectar.

14. Most Important Topics from Organisms and Populations for NEET 2025

Population Growth Models – Exponential and Logistic with equations.
Population Interactions Table – All six types with examples.
Adaptations – Allen’s Rule, Altitude Sickness, Cold Stress.
Responses to Abiotic Factors – Regulate, Conform, Migrate, Suspend.
Predation and Prey Adaptations
Parasitism (including brood parasitism and examples)
Competition and Gause’s Principle
Population Attributes – Birth rate, Death rate, Age Pyramid
Life History Variation – r and K strategies
Carrying Capacity and Real-World Logistic Growth