Organisms and Populations: CBSE Class 12 Biology Chapter 11 Revision Notes

Ecology is the scientific study of the complex interactions between living organisms and their abiotic (physico-chemical) and biotic (other species) environments. Biological organization spans multiple levels, but ecology primarily focuses on organisms, populations, communities, and biomes. Understanding population ecology is critical because it links environmental factors to evolution and natural selection, which operate at the population level. This note explores how populations are measured, how they grow under various resource conditions (exponential vs. logistic models), and the diverse interspecific interactions—ranging from mutualism to competition—that shape biological communities.

1. Levels of Biological Organisation

• Complexity is investigated at various levels: macromolecules, cells, tissues, organs, individual organisms, populations, communities, ecosystems, and biomes.
• Ecology specifically focuses on four levels:
• Organisms: The individual unit.
• Populations: Groups of individuals of the same species in a defined area.
• Communities: Groups of interacting populations.
• Biomes: Large-scale regional systems.

2. Population Attributes

A population has specific characteristics that an individual organism does not possess.

Comparison: Individual Organism vs. Population Attribute

Attribute	Individual Organism	Population Attribute
Birth/Death	An individual is born or dies.	Birth and Death Rates: Referred to as per capita changes in numbers.
Sex	An individual is male or female.	Sex Ratio: The percentage of males vs. females in the group.
Age	An individual has a specific age.	Age Distribution: The proportion of different age groups (Pre-reproductive, Reproductive, Post-reproductive).

Key Term: Age Pyramid

• Graphical representation of the age distribution of a population.
• The shape of the pyramid indicates growth status:
• Growing: Broad base (high young population).
• Stable: Even distribution.
• Declining: Narrow base.

3. Population Density (N)

Population density reflects the status of a species in its habitat. It can be measured in several ways depending on the species:

• Total Number: Most common but sometimes difficult to determine (e.g., millions of bacteria).
• Biomass or Per Cent Cover: More meaningful for species with huge roles but low numbers (e.g., a single large Banyan tree vs. many small grasses).
• Relative Density: Number of individuals caught per trap (e.g., fish in a lake).
• Indirect Estimation: Tiger census based on pug marks and fecal pellets.

4. Population Growth

The size of a population changes over time based on four basic processes:

• Natality (B): Number of births added to the initial density.
• Mortality (D): Number of deaths.
• Immigration (I): Number of individuals entering the habitat from elsewhere.
• Emigration (E): Number of individuals leaving the habitat.
• Growth Equation: N_t = N₀ e^rt
• Population increases if (B + I) > (D + E).
• Natality and Mortality are the most influential factors under normal conditions.

5. Population Growth Models

Comparison: Exponential Growth vs. Logistic Growth

Feature	Exponential Growth	Logistic Growth
Resource Condition	Unlimited resources.	Limited resources.
Curve Shape	<< /strong>strong>J-shaped curve.	Sigmoid (S-shaped) curve.
Equation	dN/dt = rN.	dN/dt = rN (K - N)/K.
Realism	Less realistic; resources eventually become limiting.	More realistic as it accounts for carrying capacity.
Key Parameter	Intrinsic rate of natural increase (r).	Carrying Capacity (K): Maximum number a habitat can support.

6. Interspecific Interactions

Interactions between two different species can be beneficial (+), detrimental (-), or neutral (0).

Types of Interspecific Interactions

Interaction Name	Species A	Species B	Example(s)
Mutualism	+	+	Lichens (Algae/Fungi), Mycorrhizae (Fungi/Roots), Fig-Wasp.
Competition	-	-	Abingdon tortoise and Goats; Flamingoes and Fish for zooplankton.
Predation	+	-	Tiger and Deer; Sparrow eating seeds; Starfish Pisaster.
Parasitism	+	-	Human liver fluke, Ticks on dogs, Cuscuta on hedge plants.
Commensalism	+	0	Orchid on Mango branch; Barnacles on Whales; Cattle Egret and Cattle.
Amensalism	-	0	One species harmed, other unaffected.

7. Detailed Interaction Mechanisms

A. Predation

• Energy Transfer: Acts as a "conduit" to transfer energy to higher trophic levels.
• Control: Predators keep prey populations under check to prevent ecosystem instability.
• Defense: Prey evolve defenses like camouflage (frogs/insects), thorns (Acacia/Cactus), or distasteful chemicals (Monarch butterfly).

B. Competition

• Gause's Competitive Exclusion Principle: Two closely related species competing for the same limited resources cannot coexist; the inferior one is eliminated.
• Resource Partitioning: Species avoid competition by choosing different feeding times or foraging patterns (e.g., Warblers on the same tree).

C. Parasitism

• Adaptations: Loss of unnecessary sense organs, suckers to cling to host, and high reproductive capacity.
• Brood Parasitism: Parasitic birds (like the Koel) lay eggs in the host's nest (Crow) for incubation.

Frequently Asked Questions (FAQs)

1. What is the difference between birth rate and birth in ecological terms?
   
   An individual has a birth (an event), whereas a population has a birth rate, which is the per capita increase in numbers over a specific time. For example, if 8 new plants are added to 20 existing plants, the rate is 0.4 offspring per plant per year.
   
   
2. Why is the logistic growth model considered more realistic than the exponential model?
   
   In nature, resources like food and space are finite and eventually become limiting. The logistic model accounts for this by including Carrying Capacity (K), the maximum population size a habitat can sustainably support.
   
   
3. How do plants defend themselves against herbivores (predators)?
   
   Plants use morphological defenses like thorns (Cactus) and chemical defenses like producing cardiac glycosides (Calotropis). They also produce substances like nicotine and caffeine to discourage grazers and browsers.