Organisms and Populations - Complete NEET Guide with Diagrams & Practice Questions

Introduction
Key Concepts
Important Formulas & Equations
Memory Techniques (Mnemonics)
Previous Year Questions (NEET)
Key Takeaways for Quick Revision

Introduction

Ecology is the cornerstone of understanding biology from a holistic perspective, exploring the intricate interactions between organisms and their environment. The entire Ecology unit is a high-weightage section in the NEET exam, contributing around 10-12 questions, and "Organisms and Populations" is the foundational chapter that sets the stage. You can expect 2-3 questions directly from this chapter, focusing on population growth models, age pyramids, and interspecific interactions.

This guide will break down the fundamental concepts of ecology, from how individual organisms adapt to their environment to the complex dynamics of population growth and interactions. By mastering the key terms, diagrams, and formulas presented here, you will build a strong foundation to excel in the ecology unit and boost your overall NEET score.

Key Concepts

1. Ecology: Levels of Organisation

Ecology studies interactions at four main levels of biological organisation:

Organisms: Individual living beings.
Populations: Groups of individuals of the same species living in a specific area.
Communities: Assemblages of different populations interacting in an area.
Biomes: Large terrestrial ecosystems characterized by specific vegetation and climate.

2. The Organism and Its Environment

Major Abiotic Factors

The physical and chemical factors of a habitat significantly influence organisms.

Temperature: The most ecologically relevant factor. Organisms are classified as:
- Eurythermal: Can tolerate a wide range of temperatures.
- Stenothermal: Restricted to a narrow range of temperatures.
Water: Life originated in water, and it's essential for all living organisms. Organisms are classified as:
- Euryhaline: Can tolerate a wide range of salinities.
- Stenohaline: Restricted to a narrow range of salinities.
Light: Essential for photosynthesis. The spectral quality of light is important for plants. For animals, light influences foraging, reproductive, and migratory activities.
Soil: The nature and properties of soil (composition, grain size, pH, mineral content) determine the vegetation and, consequently, the animals in an area.

Responses to Abiotic Factors

Organisms have evolved different strategies to cope with environmental stress.

Regulate: Maintain a constant internal environment (homeostasis) despite changes in external conditions. Example: Birds and mammals maintain a constant body temperature (thermoregulation).
Conform: The internal environment of the organism changes with the ambient conditions. Example: 99% of animals and nearly all plants are conformers.
Migrate: Move away temporarily from a stressful habitat to a more hospitable area and return when conditions are favorable. Example: Siberian cranes migrating to Keoladeo National Park in winter.
Suspend: Reduce metabolic activity and enter a state of dormancy during unfavorable conditions.
- Hibernation: Winter sleep (e.g., bears).
- Aestivation: Summer sleep (e.g., snails, fish).
- Diapause: A stage of suspended development in zooplankton.

Adaptations

An adaptation is any attribute of an organism (morphological, physiological, behavioral) that enables it to survive and reproduce in its habitat.

Kangaroo rat: Fulfills its water needs through internal fat oxidation and has the ability to concentrate its urine.
Desert plants (e.g., Opuntia): Have thick cuticles, stomata in deep pits (sunken stomata), and follow the CAM photosynthetic pathway to minimize water loss.
Allen's Rule: Mammals from colder climates generally have shorter ears and limbs to minimize heat loss.
High Altitude Adaptation: People living at high altitudes have a higher RBC count and increased breathing rate to compensate for low atmospheric oxygen.

3. Populations

A population is a group of individuals of the same species that live in a given geographical area, share or compete for similar resources, and can potentially interbreed.

Population Attributes

Individuals have births and deaths, but a population has rates.

Birth Rate (Natality): Per capita births.
Death Rate (Mortality): Per capita deaths.
Sex Ratio: The ratio of males to females in a population.
Age Pyramid: A plot of the age distribution (percent individuals in different age groups). The shape of the pyramid reflects the growth status of the population.

Figure: Representation of age pyramids for human population, showing expanding, stable, and declining population structures.

Population Growth

Population density (N) changes over time due to four basic processes:

Natality (B): Number of births.
Mortality (D): Number of deaths.
Immigration (I): Individuals moving into the habitat.
Emigration (E): Individuals moving out of the habitat.

Growth Models:

Exponential Growth:
- Occurs when resources (food, space) are unlimited.
- Results in a J-shaped curve when population density (N) is plotted against time (t).
- Characterized by the parameter 'r' (intrinsic rate of natural increase).
Logistic Growth:
- Occurs when resources are limited. This is a more realistic model.
- The habitat has a carrying capacity (K), which is the maximum population size it can support.
- Results in a Sigmoid (S-shaped) curve.
- Phases: A lag phase, followed by acceleration, then deceleration, and finally an asymptote when N reaches K.

Figure: Population growth curves. Curve 'a' shows exponential growth (J-shaped). Curve 'b' shows logistic growth (S-shaped), with K representing the carrying capacity.

Population Interactions

Interspecific interactions arise from the interaction of populations of two different species.

Interaction	Species A	Species B	Example
Mutualism	+	+	Lichens (Fungus + Algae), Mycorrhizae (Fungus + Roots)
Competition	-	-	Barnacles (Balanus and Chathamalus)
Predation	+	-	Tiger and Deer, Starfish (Pisaster) and invertebrates
Parasitism	+	-	Cuscuta on hedge plants, Human liver fluke
Commensalism	+	0	Orchid on a mango tree, Cattle egret and grazing cattle
Amensalism	-	0	Penicillium secreting penicillin, killing bacteria

Predation (+, -): Essential for energy transfer across trophic levels and for keeping prey populations under control (e.g., cactus-feeding moth controlling prickly pear cactus in Australia).
Competition (-, -): Occurs when closely related species compete for the same limited resources.
- Gause's Competitive Exclusion Principle: Two species competing for the same resources cannot coexist indefinitely; the inferior one will be eliminated.
- Resource Partitioning: Coexistence can be promoted if species avoid competition by using different resources or foraging patterns (e.g., MacArthur's warblers).
Parasitism (+, -): The parasite depends on the host for food and lodging. Brood parasitism (e.g., Cuckoo laying eggs in a Crow's nest) is a special case.
Commensalism (+, 0): One species benefits, the other is unaffected. Example: Clownfish living among the stinging tentacles of a sea anemone.
Mutualism (+, +): Both species benefit. Plant-pollinator interactions are a classic example, often involving co-evolution (e.g., fig tree and wasp). Some orchids, like Ophrys, employ "sexual deceit" for pollination.

Important Formulas & Equations

Population Density Change: Nt+1 = Nt + [(B + I) – (D + E)]
- Nt: Population density at time t
- B: Births, I: Immigration
- D: Deaths, E: Emigration
Exponential Growth Rate: dN/dt = rN
- dN/dt: Rate of change in population size
- r: Intrinsic rate of natural increase (r = b - d)
- N: Population size
Integral Form of Exponential Growth: Nt = N₀e^(rt)
- Nt: Population density after time t
- N₀: Population density at time zero
- e: The base of natural logarithms (≈ 2.71828)
Logistic Growth Rate: dN/dt = rN((K-N)/K)
- K: Carrying Capacity
- (K-N)/K: Represents environmental resistance.

Memory Techniques (Mnemonics)

Population Interactions Signs:
- Mutualism: M for "More for both" (+, +)
- Competition: C for "Conflict" (-, -)
- Commensalism: "Come On, I'm neutral" (+, 0)
- Amensalism: "Ah, man! I'm harmed, you're neutral" (-, 0)
Growth Curves:
- J-shaped: J for "Jubilation" or "Joyride" (unlimited resources).
- S-shaped: S for "Struggle" or "Stable" (limited resources, reaches stability at K).
Responses to Abiotic Factors: Remember the four M's of stress escape - Migrate, Suspend (dorMancy), and the two main strategies - Regulate vs. conforM.

Previous Year Questions (NEET)

Q1. Which of the following is not an attribute of a population? (NEET 2020) a) Natality b) Mortality c) Species interaction d) Sex ratio

Explanation: Natality (birth rate), mortality (death rate), and sex ratio are all attributes of a population. Species interaction, such as predation or mutualism, occurs between two different populations and is an attribute of a community. Answer: (c) Species interaction

Q2. The principle of Competitive Exclusion was stated by: (NEET-II 2016) a) MacArthur b) Verhulst and Pearl c) C. Darwin d) G.F. Gause

Explanation: G.F. Gause stated the Competitive Exclusion Principle, which posits that two species competing for the same limiting resources cannot coexist at constant population values. Answer: (d) G.F. Gause

Q3. An association of individuals of different species living in the same habitat and having functional interactions is: (NEET 2015) a) Population b) Ecological niche c) Biotic community d) Ecosystem

Explanation: A biotic community is an assemblage of populations of different species living in the same area and interacting with one another. A population consists of individuals of a single species. Answer: (c) Biotic community

Q4. A sedentary sea anemone gets attached to the shell lining of a hermit crab. The association is: (NEET 2013) a) Commensalism b) Amensalism c) Ectoparasitism d) Symbiosis

Explanation: This is an example of protocooperation, a type of mutualism (+,+). The sea anemone gets transport and leftover food, while the hermit crab gets camouflage and protection from the anemone's stinging cells. Since both benefit, it falls under the broad category of Symbiosis (which includes mutualism). Answer: (d) Symbiosis (often used interchangeably with Mutualism in such contexts).

Key Takeaways for Quick Revision

Ecology studies interactions at four levels: organism, population, community, and biome.
Population attributes (birth/death rates, sex ratio, age distribution) are unique to the group, not the individual.
Age pyramids (expanding, stable, declining) visually represent the growth status of a population.
Exponential growth (J-curve) occurs with unlimited resources, while logistic growth (S-curve) occurs with limited resources and is constrained by the carrying capacity (K).
Population interactions are key drivers of community structure. Know the six main types and their signs (+, -, 0).
Competition leads to either exclusion (Gause's Principle) or coexistence (Resource Partitioning).
Mutualism often involves co-evolution, as seen in many plant-pollinator relationships like the fig and wasp.

Organisms And Population