Quick Revision Notes: Class 12 Biology

Chapter: Organisms and Populations

Ecology: The branch of science that studies the interactions among organisms and between the organism and its physical (abiotic) environment.
Organism: The basic unit of ecological study.
Population: A group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding.
Community: A collection of many populations of different species living together in a particular area and interacting with one another.
Biome: A large regional unit characterized by a major vegetation type and associated fauna (e.g., Desert, Rain Forest, Tundra).

Ecological conditions are shaped by key abiotic factors:

Temperature (तापमान): The most ecologically relevant environmental factor. Eurythermal vs. Stenothermal.
- Eurythermal (प्रuruतापी): Organisms that can tolerate a wide range of temperatures.
- Stenothermal (तनुतापी): Organisms restricted to a narrow range of temperatures.
Water (जल): Life on earth originated in water and is unsustainable without it. Productivity and distribution of plants are heavily dependent on water.
- Euryhaline: Tolerate wide range of salinities.
- Stenohaline: Restricted to a narrow range of salinities.
Light (प्रकाश): Plants need light for photosynthesis. Photoperiodic requirements dictate flowering in many plants. Animals use diurnal and seasonal variations in light intensity and duration as cues for timing foraging, reproductive, and migratory activities.
Soil (मृदा): The nature and properties of soil depend on the climate, the weathering process, whether soil is transported or sedimentary, and how soil development occurred. Grain size and aggregation determine percolation and water-holding capacity of the soils.

How do organisms cope with stressful external environments?

Homeostasis (समस्थैतिकी): Maintaining a constant internal environment despite fluctuating external conditions.
Cope / Response Mechanisms:
1. Regulate (विनियमन): Maintain homeostasis by physiological (and sometimes behavioural) means (e.g., birds and mammals - endotherms).
2. Conform (अनुरुप रहना): Body temperature or osmotic concentration changes with the ambient environment (e.g., majority of animals and plants - ectotherms).
3. Migrate (प्रवास): Organisms move temporarily from a stressful habitat to a more hospitable area and return when stress is over (e.g., Siberian birds in Keoladeo National Park).
4. Suspend (निलंबन):
  - Spores/Cysts: Bacteria, fungi, and lower plants produce thick-walled spores to survive unfavorable conditions.
  - Hibernation (शीतनिष्क्रियता): Winter sleep (e.g., Polar bears).
  - Aestivation (ग्रीष्मनिष्क्रियता): Summer sleep (e.g., some snails and fish).
  - Diapause: A stage of suspended development seen in zooplankton.

Morphological, physiological, or behavioural attribute that enables an organism to survive and reproduce in its habitat.

Kangaroo rat: Internal oxidation of fat water source; lacks sweat glands; concentrates urine.
Opuntia (Desert plants): Leaves reduced to spines (reduced transpiration); photosynthesis done by flattened stems (phylloclade); CAM pathway.
Allen's Rule: Mammals from colder climates tend to have shorter ears and limbs to minimize heat loss.
Altitude Sickness: At high altitudes, body compensates for low atmospheric pressure (low oxygen) by increasing red blood cell production, decreasing binding capacity of hemoglobin, and increasing breathing rate.

Unlike an individual, a population has certain characteristics:

Birth Rate (Natality): Number of births in a given period in the population that are added to the initial density.
Death Rate (Mortality): Number of deaths in the given period.
Sex Ratio: Ratio of males to females in a population.
Age Pyramids: The plot of age distribution (percentage of individuals of a given age or age group) resulting in shapes: Expanding, Stable, and Declining.
Population Density ($N$): Number of individuals of a species per unit area or volume.

The size of a population keeps changing depending on food availability, predation pressure, and weather.

$$\text{Formula: } N_{t+1} = N_t + [(B + I) - (D + E)]$$

Exponential Growth (घातीय वृद्धि):
- Occurs when resources are unlimited.
- Equation: $\frac{dN}{dt} = rN$
- Integral form: $N_t = N_0 e^{rt}$
- Where, $N_t =$ Population density after time $t$, $N_0 =$ Population density at time zero, $r =$ Intrinsic rate of natural increase, $e =$ Base of natural logarithms ($2.71828$).
- Curve shape: 'J'-shaped.
Logistic Growth (संभीव वृद्धि):
- Occurs when resources become limiting (Verhulst-Pearl Logistic Growth).
- Equation: $\frac{dN}{dt} = rN \left(\frac{K - N}{K}\right)$
- Where, $N =$ Population density at time $t$, $r =$ Intrinsic rate of natural increase, $K =$ Carrying capacity (वहन क्षमता).
- Curve shape: Sigmoid ('S'-shaped).

Type of Interaction	Species A	Species B	Name of Interaction	Example
Mutualism	$+$	$+$	Both species benefit	Lichens, Mycorrhizae, Pollination
Competition	$-$	$-$	Both species are harmed	Gause's Competitive Exclusion
Predation	$+$	$-$	One benefits, one is harmed	Tiger and Deer, Sparrow and Seed
Parasitism	$+$	$-$	One benefits, one is harmed	Cuscuta on hedge plants, Ticks on dogs
Commensalism	$+$	$0$	One benefits, one unaffected	Orchid on mango branch, Barnacles on whales
Amensalism	$-$	$0$	One harmed, one unaffected	Penicillium inhibiting bacteria

Competitive Exclusion Principle (गॉस का प्रतिस्पर्धी अपवर्जन सिद्धांत): States that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will eventually be eliminated.
Resource Partitioning: Two competing species can co-exist by avoiding competition, e.g., MacArthur's warblers foraging on different parts of the same tree.
Brood Parasitism: Parasitic birds laying eggs in the nest of its host and letting the host incubate them (e.g., Cuckoo/Koel laying eggs in Crow's nest).