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Organisms and Populations – CBSE NCERT Study Resources

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12th

12th - Biology

Organisms and Populations

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Chapter Overview: Organisms and Populations

This chapter explores the interactions between organisms and their environment, focusing on population dynamics, adaptations, and ecological principles. It covers key concepts such as habitat, niche, population attributes, and growth models, as prescribed in the CBSE Grade 12 Biology curriculum.

Organisms and Their Environment

Ecology: The study of interactions between organisms and their physical environment.

Organisms adapt to their environment through physiological, behavioral, and morphological changes. The chapter discusses:

  • Major abiotic factors: Temperature, water, light, and soil.
  • Responses to abiotic factors: Regulate, conform, migrate, or suspend.

Populations

Population: A group of individuals of the same species living in a defined geographical area.

Key attributes of a population include:

  • Birth rate (natality) and death rate (mortality).
  • Age distribution and sex ratio.
  • Population density and growth patterns.

Population Growth Models

The chapter explains two primary growth models:

  • Exponential Growth: Occurs under unlimited resources (J-shaped curve).
  • Logistic Growth: Occurs when resources are limited (S-shaped curve).

Carrying Capacity (K): The maximum population size an environment can sustain.

Life History Variations

Different species exhibit varied life history strategies:

  • r-selected species: High reproduction rate, short lifespan.
  • K-selected species: Low reproduction rate, long lifespan.

Population Interactions

Interactions among populations include:

  • Predation, competition, parasitism.
  • Commensalism and mutualism.

Competitive Exclusion Principle: Two species competing for the same resource cannot coexist indefinitely.

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
Define population density.
Answer:
Definition: Number of individuals per unit area or volume.
Question 2:
What is carrying capacity?
Answer:
Definition: Maximum population size an environment can sustain.
Question 3:
Name two density-dependent factors.
Answer:
  • Competition
  • Predation
Question 4:
Give an example of mutualism.
Answer:
Bees and flowering plants.
Question 5:
What is age pyramid?
Answer:
Definition: Graphical representation of age distribution in a population.
Question 6:
Define natality.
Answer:
Definition: Birth rate in a population.
Question 7:
Name the primary productivity measurement unit.
Answer:
g/m²/year (grams per square meter per year).
Question 8:
What is commensalism?
Answer:
Definition: One species benefits, the other is unaffected.
Question 9:
Give an example of parasitism.
Answer:
Tapeworm in human intestine.
Question 10:
Define ecological niche.
Answer:
Definition: Role and position of a species in its ecosystem.
Question 11:
What is biotic potential?
Answer:
Definition: Maximum reproductive capacity of a population.
Question 12:
Name two abiotic factors.
Answer:
  • Temperature
  • Rainfall
Question 13:
Define mortality.
Answer:
Definition: Death rate in a population.
Question 14:
What is emigration?
Answer:
Definition: Movement of individuals out of a population.
Question 15:
Define population in ecological terms.
Answer:

A population refers to a group of individuals of the same species living in a specific geographical area at a given time, capable of interbreeding and sharing resources.

Question 16:
What is the significance of age pyramids in population studies?
Answer:

Age pyramids graphically represent the age distribution of a population, helping ecologists predict growth trends (expanding, stable, or declining) and plan resource allocation.

Question 17:
Name the abiotic factors that influence organism distribution.
Answer:
  • Temperature
  • Water
  • Light
  • Soil
  • pH
  • Salinity
Question 18:
Differentiate between natality and mortality.
Answer:

Natality is the birth rate (number of births per unit population), while mortality is the death rate (number of deaths per unit population) in a given time.

Question 19:
What is carrying capacity of a habitat?
Answer:

Carrying capacity (K) is the maximum population size an environment can sustain indefinitely without degradation, based on resource availability.

Question 20:
How do predators maintain ecological balance?
Answer:

Predators control prey populations, prevent overgrazing, and promote species diversity by eliminating weaker individuals, ensuring ecosystem stability.

Question 21:
List two adaptations of xerophytes.
Answer:
  • Thick cuticle to reduce water loss
  • Deep roots to access groundwater
Question 22:
Explain commensalism with an example.
Answer:

Commensalism is a relationship where one species benefits (commensal) while the other is unaffected. Example: Orchids growing on trees for support.

Question 23:
What is the role of photoperiod in animal behavior?
Answer:

Photoperiod (day length) regulates seasonal activities like migration, breeding, and hibernation in animals by triggering hormonal changes.

Question 24:
Why are keystone species crucial for ecosystems?
Answer:

Keystone species maintain ecosystem structure (e.g., sea otters controlling sea urchins). Their loss can cause trophic cascades.

Question 25:
How does logistic growth differ from exponential growth?
Answer:

Logistic growth is S-shaped, limited by carrying capacity, while exponential growth is J-shaped, unrestricted but unsustainable in nature.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
Explain the term age pyramid with its significance.
Answer:

An age pyramid is a graphical representation of the age distribution in a population, divided into pre-reproductive, reproductive, and post-reproductive groups. It helps predict population trends like growth, stability, or decline.

Question 2:
What are r-selected species? Give one example.
Answer:

r-selected species are organisms that produce many offspring with low parental care, adapted to unstable environments. Example: Mosquitoes or Bacteria.

Question 3:
How does competition affect population growth?
Answer:

Competition for limited resources (food, space) leads to a reduction in population growth as individuals struggle to survive, often resulting in the survival of the fittest.

Question 4:
Name two density-dependent factors regulating population.
Answer:
  • Predation: Higher density attracts more predators.
  • Disease: Spreads faster in dense populations.
Question 5:
What is ecological adaptation? Provide one example.
Answer:

Ecological adaptation refers to traits developed by organisms to survive and reproduce in specific environments. Example: Camel's hump for desert survival.

Question 6:
Define biotic potential.
Answer:

Biotic potential is the maximum reproductive capacity of a population under ideal environmental conditions, with no limitations on resources or survival.

Question 7:
How do abiotic factors influence population distribution?
Answer:

Abiotic factors like temperature, water, and soil determine where a species can thrive. Extreme conditions limit distribution, while optimal conditions support growth.

Question 8:
What is the role of emigration in population dynamics?
Answer:

Emigration (individuals leaving a population) reduces population density, easing resource competition and potentially stabilizing growth rates in the source habitat.

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Explain the term population density and mention any two methods to measure it in a given habitat.
Answer:

Population density refers to the number of individuals of a species per unit area or volume of a habitat. It helps in understanding the distribution and abundance of organisms.


Two methods to measure population density are:

  • Quadrant method: A square frame is placed randomly in the habitat, and the number of individuals within it is counted.
  • Mark-recapture method: Individuals are captured, marked, released, and then recaptured to estimate population size using a formula.
Question 2:
Differentiate between natality and mortality with suitable examples.
Answer:

Natality refers to the number of births per unit population in a given time, e.g., a deer population increasing due to high birth rates.


Mortality refers to the number of deaths per unit population in a given time, e.g., a decline in fish population due to predation.


Natality increases population size, while mortality decreases it.

Question 3:
Describe the logistic growth model of a population with a diagram.
Answer:

The logistic growth model describes how a population grows rapidly initially but slows down as resources become limited, eventually stabilizing at the carrying capacity (K).


Stages:

  • Lag phase: Slow growth due to few individuals.
  • Exponential phase: Rapid growth with abundant resources.
  • Stationary phase: Growth stabilizes as resources deplete.

(Diagram: S-shaped curve with labeled phases and carrying capacity.)

Question 4:
What are r-selected species? Give two characteristics and examples.
Answer:

r-selected species are organisms that reproduce rapidly in unstable environments, producing many offspring with low survival rates.


Characteristics:

  • Short lifespan and early reproduction.
  • Little to no parental care.

Examples: Mosquitoes, weeds like Parthenium.

Question 5:
Explain how mutualism differs from commensalism with examples.
Answer:

Mutualism is a relationship where both species benefit, e.g., bees pollinating flowers while obtaining nectar.


Commensalism is where one species benefits, and the other is unaffected, e.g., orchids growing on trees for support without harming them.


Mutualism is obligatory for survival, while commensalism is not.

Question 6:
Define age pyramid and interpret a triangular-shaped age pyramid.
Answer:

An age pyramid is a graphical representation of age and sex distribution in a population.


A triangular-shaped pyramid indicates:

  • High birth rates (wide base).
  • Fewer elderly individuals (narrow top).
  • Growing population, common in developing countries.
Question 7:
Define population density and explain how it is calculated.
Answer:

Population density refers to the number of individuals of a species per unit area or volume at a given time. It is a measure of how crowded or sparse a population is in a habitat.


It is calculated using the formula:
Population Density = Number of Individuals / Unit Area (or Volume)


For example, if there are 50 deer in a 10 km² forest, the population density would be 5 deer/km².

Question 8:
Differentiate between exponential growth and logistic growth in populations.
Answer:

Exponential growth occurs when resources are unlimited, leading to a rapid increase in population size. The growth curve is J-shaped.


Logistic growth occurs when resources are limited, causing the population to stabilize at the carrying capacity (K). The growth curve is S-shaped.


  • Exponential Growth: No competition, ideal conditions.
  • Logistic Growth: Competition, limited resources.
Question 9:
Explain the term age pyramid and its significance in population studies.
Answer:

An age pyramid is a graphical representation of the age and sex distribution of a population. It helps in understanding the growth trends of a population.


Significance:
1. Predicts future population growth (expanding, stable, or declining).
2. Helps in planning resources like schools, hospitals, and jobs.
3. Indicates the proportion of reproductive individuals.

Question 10:
What are r-selected species? Give two examples.
Answer:

r-selected species are organisms that produce many offspring but provide little parental care. They thrive in unstable environments.


Characteristics:
1. High reproductive rate.
2. Short lifespan.
3. Small body size.


Examples:
1. Bacteria
2. Insects like mosquitoes

Question 11:
Describe the impact of predation on population dynamics.
Answer:

Predation is a biological interaction where one organism (predator) kills and eats another (prey). It plays a key role in controlling population sizes.


Impacts:
1. Regulates prey population, preventing overpopulation.
2. Maintains ecological balance.
3. Can lead to evolutionary adaptations like camouflage in prey.

Question 12:
How do abiotic factors like temperature and water affect organisms?
Answer:

Abiotic factors are non-living components that influence the survival and distribution of organisms.


Temperature:
1. Affects enzyme activity and metabolism.
2. Determines species distribution (e.g., polar bears in cold regions).


Water:
1. Essential for physiological processes.
2. Influences habitat suitability (e.g., aquatic vs. desert organisms).

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Discuss the age pyramid of a declining population. How does it reflect demographic transitions?
Answer:
Theoretical Framework

An age pyramid with a narrow base (fewer young) and wider top (more elderly) indicates population decline, often due to low birth rates.

Evidence Analysis
  • Example: Japan’s pyramid shows aging population trends.
  • Our textbook links this to late-stage demographic transition.
Critical Evaluation

Such pyramids predict labor shortages, requiring policy interventions like immigration.

Future Implications

Similar patterns in Europe highlight the need for revised healthcare systems.

Question 2:
Explain the logistic growth model in populations with reference to carrying capacity. How does it differ from exponential growth?
Answer:
Theoretical Framework

The logistic growth model describes population growth that slows as it approaches the carrying capacity (K) of the environment. Unlike exponential growth, which assumes unlimited resources, this model incorporates environmental resistance.

Evidence Analysis
  • Our textbook shows the S-shaped curve, where growth rate decreases as resources deplete.
  • Example: Deer populations stabilize when food becomes scarce.
Critical Evaluation

This model is more realistic than exponential growth, as it accounts for density-dependent factors like competition.

Future Implications

Understanding this helps in wildlife conservation, such as predicting tiger population limits in reserves.

Question 3:
Analyze the impact of abiotic factors (temperature, water, light) on organism distribution. Provide examples.
Answer:
Theoretical Framework

Abiotic factors like temperature, water, and light determine where organisms can survive, as they influence metabolic rates and photosynthesis.

Evidence Analysis
  • Cacti thrive in deserts due to water conservation adaptations.
  • Example: Arctic foxes have thick fur for low temperatures.
Critical Evaluation

These factors create biogeographic zones, but organisms may adapt (e.g., camels storing water).

Future Implications

Climate change studies rely on these principles to predict species migration patterns.

Question 4:
Describe mutualism with two examples. How does it enhance ecosystem stability?
Answer:
Theoretical Framework

Mutualism is a symbiotic relationship where both species benefit, increasing biodiversity and nutrient cycling.

Evidence Analysis
  • Example 1: Bees pollinating flowers while obtaining nectar.
  • Example 2: Rhizobium bacteria fixing nitrogen for legumes.
Critical Evaluation

Such interactions reduce extinction risks, as seen in coral reefs (zooxanthellae algae).

Future Implications

Conserving mutualistic pairs is vital for agricultural sustainability (e.g., crop-pollinator networks).

Question 5:
Compare r-selected and K-selected species strategies. Which is more vulnerable to extinction?
Answer:
Theoretical Framework

r-selected species (e.g., insects) prioritize high reproduction rates, while K-selected species (e.g., elephants) focus on fewer offspring with parental care.

Evidence Analysis
  • r-strategists dominate unstable environments (mosquitoes post-monsoon).
  • K-strategists are sensitive to habitat loss (tigers).
Critical Evaluation

K-selected species face higher extinction risks due to slower recovery rates.

Future Implications

Conservation efforts prioritize K-species, as seen in Project Tiger.

Question 6:
Explain how density-dependent and density-independent factors regulate population growth. Provide examples.
Answer:
Theoretical Framework

Population regulation involves biotic (density-dependent) and abiotic (density-independent) factors. Our textbook shows that density-dependent factors like competition intensify as population size increases, while density-independent factors (e.g., climate) affect populations regardless of size.

Evidence Analysis
  • Density-dependent: Predation (e.g., tigers controlling deer populations) or disease outbreaks in crowded habitats.
  • Density-independent: Floods or droughts reducing populations uniformly.
Critical Evaluation

While density-dependent factors stabilize ecosystems via feedback, density-independent factors can cause abrupt declines. Both are critical for understanding population dynamics.

Question 7:
Analyze the role of keystone species in maintaining ecosystem stability with two examples.
Answer:
Theoretical Framework

Keystone species disproportionately impact ecosystem structure. We studied their role in trophic cascades and biodiversity maintenance.

Evidence Analysis
  • Sea otters: Their predation on sea urchins prevents kelp forest destruction.
  • African elephants: By uprooting trees, they sustain savanna grasslands.
Critical Evaluation

Loss of keystone species can collapse ecosystems, as seen in the Yellowstone wolf reintroduction case. Their conservation is vital for resilience.

Question 8:
Compare r-selected and K-selected species strategies with adaptations.
Answer:
Theoretical Framework

r-selected species prioritize high reproduction rates, while K-selected species focus on competitive efficiency in stable environments.

Evidence Analysis
  • r-selected: Mosquitoes produce numerous offspring with minimal care.
  • K-selected: Elephants have few offspring with extended parental care.
Critical Evaluation

These strategies reflect trade-offs between quantity and quality of offspring, shaped by environmental pressures.

Question 9:
Describe mutualism and its ecological significance using lichens and mycorrhizae as examples.
Answer:
Theoretical Framework

Mutualism is a symbiotic interaction benefiting both species. Our textbook highlights its role in nutrient cycling and ecosystem productivity.

Evidence Analysis
  • Lichens: Fungi-algae partnership enabling survival in harsh habitats.
  • Mycorrhizae: Fungal networks enhancing plant nutrient uptake.
Critical Evaluation

Such interactions drive coevolution and are foundational for terrestrial ecosystems. Their disruption affects biodiversity.

Question 10:
Explain how age pyramids reflect population growth trends in expanding/stable/declining populations.
Answer:
Theoretical Framework

Age pyramids graphically represent age-sex structure, indicating growth potential. We studied their interpretation in human demography.

Evidence Analysis
  • Expanding: Broad base (high youth %), e.g., Nigeria.
  • Stable: Uniform tiers, e.g., USA.
  • Declining: Narrow base (low birth rates), e.g., Japan.
Critical Evaluation

These patterns inform policy decisions on healthcare and resource allocation for sustainable development.

Question 11:
Evaluate the impact of invasive species on native biodiversity with examples.
Answer:
Theoretical Framework

Invasive species disrupt ecosystems through competition, predation, or habitat alteration. Our textbook emphasizes their threat to endemic species.

Evidence Analysis
  • Water hyacinth: Clogs Indian waterways, reducing oxygen for aquatic life.
  • Nile perch: Caused extinction of 200+ cichlid fish in Lake Victoria.
Critical Evaluation

Their control requires integrated approaches, as seen in biological control of Opuntia by cochineal insects.

Question 12:
Explain the concept of population interactions with suitable examples. Discuss how these interactions influence the ecosystem dynamics.
Answer:

In an ecosystem, different species interact with each other in various ways, which are collectively termed as population interactions. These interactions can be beneficial, harmful, or neutral for the species involved. The major types of interactions include:

  • Mutualism: Both species benefit. Example: Lichens (association of fungi and algae). The fungi provide shelter, while algae perform photosynthesis.
  • Competition: Both species are harmed as they compete for the same resources. Example: Trees in a forest competing for sunlight and nutrients.
  • Predation: One species (predator) benefits by consuming the other (prey). Example: Tiger (predator) hunting deer (prey).
  • Parasitism: One species (parasite) benefits at the expense of the other (host). Example: Tapeworm in the human intestine.
  • Commensalism: One species benefits, while the other is unaffected. Example: Orchids growing on trees for support.

These interactions play a crucial role in maintaining ecosystem balance. For instance, predation controls prey populations, preventing overgrazing, while mutualism enhances biodiversity. Such dynamics ensure energy flow and nutrient cycling, making ecosystems sustainable.

Question 13:
Describe the logistic growth model of a population with a graph. How does it differ from exponential growth? Explain the factors affecting carrying capacity.
Answer:

The logistic growth model describes how a population grows rapidly initially but slows down as it approaches the carrying capacity (K) of the environment. The graph of logistic growth is an S-shaped (sigmoid) curve, showing three phases:

  • Lag phase: Slow growth due to fewer individuals.
  • Log phase: Rapid exponential growth as resources are abundant.
  • Stationary phase: Growth stabilizes as the population reaches carrying capacity.

In contrast, exponential growth (J-shaped curve) occurs when resources are unlimited, leading to unchecked population expansion, which is unrealistic in nature.

Factors affecting carrying capacity include:

  • Resource availability: Food, water, and space limit population size.
  • Predation and disease: Natural checks that prevent overpopulation.
  • Competition: Intra- and inter-species competition reduce growth rates.
  • Environmental conditions: Climate changes or disasters can alter K.

Understanding logistic growth helps in conservation efforts, agriculture, and managing invasive species by predicting population limits.

Question 14:
Explain the concept of population interactions with suitable examples. Discuss how these interactions influence the ecosystem stability.
Answer:

Population interactions refer to the relationships between different species in an ecosystem, which can be beneficial, harmful, or neutral. These interactions play a crucial role in maintaining ecological balance and biodiversity.

Here are the major types of population interactions with examples:

  • Mutualism (+/+): Both species benefit. Example: Lichens (algae and fungi), where algae provide food via photosynthesis, and fungi offer shelter and minerals.
  • Competition (-/-): Both species are harmed. Example: Lions and hyenas compete for the same prey.
  • Predation (+/-): One species benefits, the other is harmed. Example: Tiger (predator) hunting a deer (prey).
  • Parasitism (+/-): One benefits, the other is harmed. Example: Cuscuta (dodder plant) deriving nutrients from a host plant.
  • Commensalism (+/0): One benefits, the other is unaffected. Example: Orchids growing on trees for support without harming them.

These interactions influence ecosystem stability by:

  • Regulating population sizes (e.g., predation controls prey numbers).
  • Promoting species diversity (e.g., mutualism supports coexistence).
  • Maintaining energy flow and nutrient cycling (e.g., decomposers break down dead matter).

Without these interactions, ecosystems would become imbalanced, leading to overpopulation of certain species or collapse of food chains.

Question 15:
Explain the concept of population interactions with suitable examples. Discuss how these interactions influence the ecosystem structure and functioning.
Answer:

In an ecosystem, different species interact with each other in various ways, known as population interactions. These interactions can be beneficial, harmful, or neutral for the species involved. The major types of interactions include:

  • Mutualism: Both species benefit. Example: Lichens (association of fungi and algae), where fungi provide shelter and algae perform photosynthesis.
  • Competition: Both species are harmed as they compete for the same resources. Example: Lions and hyenas competing for prey.
  • Predation: One species (predator) benefits by consuming the other (prey). Example: Tiger (predator) and deer (prey).
  • Parasitism: One species (parasite) benefits at the expense of the other (host). Example: Tapeworm in the human intestine.
  • Commensalism: One species benefits while the other is unaffected. Example: Orchids growing on trees for support.

These interactions play a crucial role in maintaining the ecosystem balance. For instance, predation controls prey populations, preventing overgrazing. Mutualism enhances biodiversity, while competition drives natural selection. Such interactions ensure energy flow and nutrient cycling, making ecosystems sustainable.

Additionally, these relationships influence species distribution and abundance, shaping the structure of ecological communities. For example, the absence of a predator can lead to an explosion of prey populations, disrupting the ecosystem. Thus, understanding population interactions is vital for conservation efforts and maintaining ecological harmony.

Question 16:
Explain the concept of population interaction with suitable examples. Discuss how mutualism differs from commensalism, highlighting their ecological significance.
Answer:

Population interaction refers to the effects that organisms of different species have on one another in an ecosystem. These interactions can be beneficial, harmful, or neutral, depending on the species involved. Examples include mutualism, commensalism, predation, parasitism, and competition.


Mutualism is a type of interaction where both species benefit. For example, lichens represent a mutualistic relationship between fungi and algae. The fungi provide shelter and moisture, while the algae perform photosynthesis, supplying nutrients. Another example is the relationship between bees and flowering plants, where bees get nectar, and plants get pollinated.


Commensalism, on the other hand, is an interaction where one species benefits, and the other is neither helped nor harmed. An example is orchids growing on trees—the orchids get support and sunlight, while the tree remains unaffected.


The key difference between mutualism and commensalism lies in the ecological impact:

  • In mutualism, both species gain an advantage, enhancing survival and reproduction.
  • In commensalism, only one species benefits, while the other remains neutral.

Ecologically, mutualism promotes biodiversity and ecosystem stability, as interdependent species thrive together. Commensalism, while less impactful, still contributes to species distribution and niche specialization.

Question 17:
Explain the concept of population interactions with suitable examples. Discuss how mutualism differs from commensalism in an ecosystem.
Answer:

Population interactions refer to the relationships between different species in an ecosystem, which can be beneficial, harmful, or neutral. These interactions play a crucial role in maintaining ecological balance.

Mutualism is a relationship where both species benefit. For example, lichens represent a mutualistic association between fungi and algae. The fungi provide shelter and moisture, while the algae perform photosynthesis and supply food.

Commensalism, on the other hand, is a relationship where one species benefits, and the other is neither harmed nor benefited. An example is the orchid growing on a mango tree. The orchid gets support and sunlight, but the tree is unaffected.

Key differences between mutualism and commensalism:

  • Mutualism involves reciprocal benefits, while commensalism benefits only one species.
  • Mutualism is essential for the survival of both species, whereas commensalism is not obligatory.
Question 18:
Describe the logistic growth model of a population with a graph. Explain the factors that influence the carrying capacity of a habitat.
Answer:

The logistic growth model describes how a population grows rapidly initially but slows down as it approaches the carrying capacity (K) of the habitat. The graph is S-shaped (sigmoid curve), showing three phases:
1. Lag phase: Slow growth due to few individuals.
2. Log phase: Exponential growth due to abundant resources.
3. Stationary phase: Growth stabilizes as resources become limited.

Factors influencing carrying capacity:

  • Availability of resources: Food, water, and space determine how many individuals a habitat can support.
  • Predation and competition: Higher predation or interspecific competition can reduce K.
  • Environmental conditions: Climate changes or natural disasters can alter K.
  • Disease: Outbreaks can decrease population size, affecting K.

For example, in a forest, the deer population stabilizes when food (grass) becomes scarce, demonstrating the logistic growth model.

Question 19:
Explain the concept of population interactions with suitable examples. How do these interactions influence the ecosystem?
Answer:

In an ecosystem, different species interact with each other in various ways, known as population interactions. These interactions can be classified into several types based on their effects on the interacting species:

  • Mutualism: Both species benefit. Example: Lichens (association of fungi and algae). The fungi provide shelter, while the algae perform photosynthesis.
  • Competition: Both species are harmed as they compete for the same resources. Example: Lions and hyenas competing for prey.
  • Predation: One species (predator) benefits, while the other (prey) is harmed. Example: Tiger (predator) and deer (prey).
  • Parasitism: One species (parasite) benefits, while the other (host) is harmed. Example: Tapeworm in the human intestine.
  • Commensalism: One species benefits, while the other is unaffected. Example: Orchids growing on trees for support.

These interactions play a crucial role in maintaining the balance of the ecosystem. They regulate population sizes, influence species distribution, and contribute to biodiversity. For instance, predation controls prey populations, while mutualism enhances survival chances for both species. Such interactions ensure energy flow and nutrient cycling, making ecosystems sustainable.

Question 20:
Describe the logistic growth model of a population with a graph. How does it differ from exponential growth?
Answer:

The logistic growth model describes how a population grows when resources are limited. Unlike exponential growth, which assumes unlimited resources, logistic growth considers carrying capacity (K)—the maximum population size an environment can sustain.

The growth curve is S-shaped (sigmoid) and consists of three phases:

  • Lag phase: Slow growth as the population adapts to the environment.
  • Log phase: Rapid growth due to abundant resources.
  • Stationary phase: Growth slows as the population reaches carrying capacity.

Mathematically, the logistic growth equation is:
dN/dt = rN (K - N)/K
where dN/dt = population growth rate, r = intrinsic growth rate, N = population size, and K = carrying capacity.

In contrast, exponential growth follows the equation:
dN/dt = rN
and produces a J-shaped curve, as seen in populations with unlimited resources (e.g., bacteria in a lab).

Logistic growth is more realistic because it accounts for environmental resistance (e.g., competition, predation). The graph shows how population growth stabilizes near K, ensuring long-term survival.

Question 21:
Explain the concept of population interactions with suitable examples. Discuss how mutualism differs from commensalism in ecological communities.
Answer:

Population interactions refer to the relationships between different species in an ecosystem, which can be beneficial, harmful, or neutral. These interactions play a crucial role in maintaining ecological balance. Examples include:

  • Mutualism: Both species benefit. Example: Lichens (algae and fungi), where algae provide food via photosynthesis, and fungi offer shelter.
  • Commensalism: One species benefits, while the other is unaffected. Example: Orchids growing on trees for support without harming the host.

The key difference lies in the outcome for the involved species. In mutualism, both gain, whereas in commensalism, only one benefits, and the other remains unaffected. These interactions influence biodiversity and ecosystem stability.

Question 22:
Describe the logistic growth model of a population with a graph. How does carrying capacity influence population dynamics?
Answer:

The logistic growth model describes how a population grows rapidly initially but slows down as it approaches the carrying capacity (K) of the environment. The graph is an S-shaped (sigmoid) curve with three phases:


1. Lag phase: Slow growth due to limited individuals.
2. Exponential phase: Rapid growth with abundant resources.
3. Stationary phase: Growth stabilizes as resources deplete.

Carrying capacity is the maximum population size an environment can sustain. It limits growth by regulating resources like food and space. When a population exceeds K, competition increases, leading to mortality or migration, restoring balance. This model reflects real-world scenarios better than exponential growth.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
A study in the Western Ghats recorded a decline in the endemic frog species due to climate change-induced habitat fragmentation. Analyze the ecological consequences and suggest mitigation strategies.
Answer:
Case Deconstruction

The decline of endemic frogs disrupts food chains, as they control insect populations and serve as prey for higher trophic levels. Habitat fragmentation isolates populations, reducing genetic diversity.

Theoretical Application
  • Restore corridors between fragmented habitats to enable migration.
  • Implement captive breeding programs to preserve genetic diversity.
Critical Evaluation

Our textbook shows similar cases like the Golden Toad extinction. Climate adaptation strategies, such as creating microhabitats, could mitigate further losses.

Question 2:
In a density-dependent population, lemmings exhibit cyclical boom-bust patterns. Explain the regulatory mechanisms and compare them with logistic growth models.
Answer:
Case Deconstruction

Lemming cycles are driven by predation and food scarcity at peak densities, aligning with density-dependent factors like competition and disease.

Theoretical Application
  • Logistic growth models predict stabilization at carrying capacity, but lemmings overshoot due to delayed feedback.
  • Examples include snowshoe hares and lynx in our textbook.
Critical Evaluation

Unlike idealized models, real populations face stochastic events (e.g., harsh winters), causing deviations. Monitoring these cycles helps predict ecosystem stability.

Question 3:
Coral reefs exhibit mutualistic symbiosis with zooxanthellae. Describe how ocean acidification disrupts this relationship and impacts reef biodiversity.
Answer:
Case Deconstruction

Zooxanthellae provide corals with photosynthesis products, while corals offer shelter. Acidification reduces calcium carbonate availability, weakening coral skeletons.

Theoretical Application
  • Bleaching events increase as symbionts are expelled under stress.
  • Fish diversity declines due to habitat loss, as seen in the Great Barrier Reef.
Critical Evaluation

Our textbook emphasizes cascading effects: 25% of marine species rely on reefs. Reducing CO2 emissions is critical to mitigate acidification.

Question 4:
A grassland ecosystem shows increased primary productivity but declining pollinator diversity. Discuss the paradox using trophic cascade theory.
Answer:
Case Deconstruction

High productivity from invasive plants may outcompete native flora, reducing pollinator food sources. This disrupts plant-pollinator networks.

Theoretical Application
  • Trophic cascades occur when pollinator loss affects seed dispersal, altering vegetation structure.
  • Examples include European honeybees displacing native bees.
Critical Evaluation

Our textbook shows monocultures boost productivity but harm biodiversity. Sustainable agriculture must balance yield with ecosystem services.

Question 5:
A study in the Western Ghats observed that endemic frog species declined by 40% due to habitat fragmentation. Analyze the ecological consequences and suggest conservation strategies.
Answer:
Case Deconstruction

The decline of endemic frog species disrupts food chains, as frogs control insect populations and serve as prey. Habitat fragmentation isolates populations, reducing genetic diversity.

Theoretical Application
  • Restore corridors to reconnect fragmented habitats, as studied in our textbook.
  • Implement buffer zones to minimize human interference, like in Silent Valley National Park.
Critical Evaluation

Conservation must prioritize endemic species, as they are irreplaceable. Evidence from the Western Ghats shows habitat restoration can reverse declines.

Question 6:
A farmer observed allelopathic effects of Parthenium on wheat crops. Explain the biochemical basis and propose two sustainable farming practices to mitigate this.
Answer:
Case Deconstruction

Parthenium releases allelochemicals like phenolics, inhibiting wheat germination. Our textbook confirms these compounds disrupt root growth.

Theoretical Application
  • Crop rotation with resistant species like sorghum, which suppresses Parthenium.
  • Organic mulching to block allelochemical diffusion, as practiced in Punjab.
Critical Evaluation

Sustainable methods reduce herbicide dependence. Field trials in Rajasthan show sorghum rotations cut Parthenium biomass by 60%.

Question 7:
Data shows zooplankton populations in a lake shifted from Daphnia to Cyclops after industrial effluents increased. Link this to trophic cascades and water quality.
Answer:
Case Deconstruction

Effluents likely elevated toxins, favoring toxin-tolerant Cyclops. Daphnia decline reduces algae grazing, worsening eutrophication.

Theoretical Application
  • Daphnia’s loss disrupts trophic cascades, as seen in Bengaluru lakes.
  • Cyclops dominance indicates pollution, similar to Yamuna River studies.
Critical Evaluation

Zooplankton shifts are bioindicators. NCERT data correlates Cyclops abundance with BOD levels above 8 mg/L.

Question 8:
Compare r-selected and K-selected species strategies using the examples of mice and elephants. How do these adapt to environmental stochasticity?
Answer:
Case Deconstruction

Mice (r-selected) produce many offspring with low care, while elephants (K-selected) invest in few offspring. Our textbook contrasts their survival rates.

Theoretical Application
  • Mice thrive in unstable environments (e.g., crop fields) by rapid reproduction.
  • Elephants rely on stable habitats (e.g., forests), as seen in Periyar Reserve.
Critical Evaluation

Environmental stochasticity favors r-strategists during disturbances. Long-term data shows elephant populations crash faster in droughts.

Question 9:
A study in the Western Ghats observed that endemic frog species declined by 40% after invasive Lantana camara altered soil pH. Analyze the interspecific competition and niche displacement mechanisms involved.
Answer:
Case Deconstruction

The invasive Lantana camara acidifies soil, disrupting the microhabitat for endemic frogs. Our textbook shows such abiotic stress reduces breeding success.

Theoretical Application
  • Competitive exclusion: Invasive plants outcompete native flora, reducing food sources.
  • Niche shift: Frogs may relocate to suboptimal habitats, increasing predation risk.
Critical Evaluation

Data aligns with Gause’s principle, as frogs show population bottlenecking. Similar effects were noted in Australia with cane toads displacing native species.

Question 10:
In a tiger reserve, camera traps revealed higher predation pressure on herbivores near waterholes during droughts. Explain the density-dependent factors and resource partitioning observed.
Answer:
Case Deconstruction

Drought concentrates herbivores near limited water, increasing their aggregation density. We studied how this elevates tiger encounter rates.

Theoretical Application
  • Density-dependent mortality: Predation scales with prey density (e.g., Ranthambore data).
  • Temporal partitioning: Herbivores like chital shift activity to night.
Critical Evaluation

This mirrors Kruger National Park findings where lions exploited drought-induced prey clusters. NCERT emphasizes such top-down regulation.

Question 11:
Coral reefs show zooxanthellae expulsion when SST exceeds 31°C. Discuss the symbiotic breakdown and its cascading effects on trophic levels.
Answer:
Case Deconstruction

Thermal stress disrupts mutualism between corals and zooxanthellae, causing bleaching. Our textbook links this to photosynthetic decline.

Theoretical Application
  • Primary producer collapse: Coral death reduces carbonate deposition.
  • Trophic cascade: Fish diversity drops by 60% (Great Barrier Reef data).
Critical Evaluation

Similar patterns were observed in Caribbean reefs post-1998 El Niño. NCERT confirms obligate symbiosis vulnerability.

Question 12:
A grassland ecosystem showed 30% lower net primary productivity after prolonged monoculture farming. Evaluate the ecological succession and soil degradation processes.
Answer:
Case Deconstruction

Monocultures deplete soil micronutrients like zinc, reducing plant biomass. We studied how this halts secondary succession.

Theoretical Application
  • Biodiversity loss: Single-crop systems lack facilitation species.
  • Erosion: Bare soil increases runoff (Punjab case study).
Critical Evaluation

Data matches Iowa’s corn belt degradation. NCERT states detritivore populations decline in such simplified ecosystems.

Question 13:

A group of ecologists studied a population of deer in a forest. They observed that the deer population showed a sharp decline during a particular season. Upon investigation, they found that a new predator had entered the forest. Based on this scenario, answer the following:

  • What could be the possible reasons for the decline in the deer population?
  • How does the introduction of a new predator affect the ecological balance of the forest?
Answer:

The decline in the deer population could be due to:

  • Predation pressure: The new predator may have increased hunting, reducing deer numbers.
  • Competition: The predator might have disrupted the deer's food sources or habitat.

The introduction of a new predator affects the ecological balance by:

  • Altering the food chain, as the predator may prey on other species too.
  • Causing a trophic cascade, where changes at one trophic level affect others, e.g., overgrazing by deer may decrease if their numbers drop.

This scenario highlights the importance of population dynamics and species interactions in maintaining ecosystem stability.

Question 14:

In a wetland ecosystem, researchers noticed that the population of frogs increased significantly after a period of heavy rainfall. However, after a few months, the population stabilized. Analyze this situation and answer:

  • What biotic and abiotic factors could have contributed to the initial increase in the frog population?
  • Why did the population stabilize later?
Answer:

The initial increase in the frog population could be due to:

  • Abiotic factors: Heavy rainfall created more breeding sites (puddles, ponds) and increased humidity, favoring frog reproduction.
  • Biotic factors: Abundance of food (insects) and fewer predators due to temporary habitat changes.

The population stabilized later because:

  • Carrying capacity was reached, limiting resources like food and space.
  • Predators adapted to the new frog population, restoring ecological balance.

This demonstrates how environmental factors and species interactions regulate population growth in ecosystems.

Question 15:
A population of Panthera tigris (Bengal tiger) in a national park shows a decline due to habitat fragmentation. Ecologists observed reduced genetic diversity and increased inbreeding.

(a) Explain how habitat fragmentation leads to reduced genetic diversity.
(b) Suggest two conservation strategies to mitigate this issue.

Answer:

(a) Habitat fragmentation divides large populations into smaller, isolated groups, restricting gene flow.
This leads to:

  • Limited mating opportunities, increasing inbreeding depression
  • Loss of allelic diversity due to genetic drift in small populations

(b) Conservation strategies:

  • Wildlife corridors: Connect fragmented habitats to restore gene flow
  • Assisted migration: Introduce genetically diverse individuals to boost variation

Note: Corridors also reduce human-wildlife conflict, while migration requires careful screening to avoid ecological imbalance.

Question 16:
A study on Rattus rattus (house rat) in urban areas showed exponential population growth until resources became limited. The curve later stabilized.

(a) Identify the growth model observed and draw its curve.
(b) List two density-dependent factors that could stabilize the population.

Answer:

(a) The growth follows the logistic model:
1. Initial exponential phase (J-shaped curve)
2. Slowing growth as resources deplete
3. Stabilization at carrying capacity (K) (S-shaped curve)

Diagram description:
Y-axis: Population size
X-axis: Time
Curve starts steep, then asymptotes at K-line

(b) Density-dependent factors:

  • Competition for food/shelter increases mortality
  • Spread of diseases due to close contact

Note: These factors maintain ecological balance by preventing overshoot of K.

Question 17:
A group of ecologists studied a population of deer in a forest ecosystem. They observed that the deer population showed a sharp decline during a particular season. Upon investigation, they found that the decline was due to a shortage of food resources and an outbreak of a parasitic infection. Based on this case, answer the following:

(a) Identify the density-dependent factors affecting the deer population.
(b) Explain how these factors regulate the population size.
Answer:

(a) The density-dependent factors affecting the deer population in this case are:

  • Shortage of food resources: As the population grows, competition for limited food increases, leading to starvation.
  • Parasitic infection: Higher population density facilitates the spread of diseases due to closer contact among individuals.

(b) These factors regulate population size by:

  • Food shortage: Limits the carrying capacity of the environment, causing mortality or reduced reproduction.
  • Parasitic infection: Increases mortality rates as the disease spreads more easily in dense populations, reducing population size.
These mechanisms ensure the population stabilizes around the ecosystem's carrying capacity.

Question 18:
In a wetland ecosystem, two species of birds, Species A and Species B, were observed feeding on the same type of fish. Over time, Species A started feeding in shallow waters while Species B preferred deeper waters. Analyze this case and answer:

(a) What ecological concept does this scenario demonstrate?
(b) How does this behavior benefit both species?
Answer:

(a) This scenario demonstrates the concept of resource partitioning, where competing species coexist by utilizing different parts of the same resource.

(b) This behavior benefits both species by:

  • Reducing competition: By feeding in different zones (shallow vs. deep waters), they avoid direct competition for the same fish.
  • Enhancing survival: Each species can exploit a specific niche, ensuring access to sufficient food without interference.
  • Promoting biodiversity: Coexistence is maintained, preventing the exclusion of one species by the other.
This adaptation is a key mechanism for species coexistence in ecosystems.

Question 19:
A group of ecologists studied a population of deer in a forest ecosystem. They observed that the deer population showed a sharp decline during a particular season. Upon investigation, they found that the decline was due to a shortage of food resources and an outbreak of a parasitic disease. Based on this case, answer the following:

(a) Identify the density-dependent factors affecting the deer population.
(b) Explain how these factors regulate the population size.
Answer:

(a) The density-dependent factors affecting the deer population in this case are:

  • Shortage of food resources: As the deer population grows, competition for limited food increases, leading to starvation and reduced reproduction.
  • Outbreak of parasitic disease: Higher population density facilitates the spread of diseases, as parasites find more hosts to infect.

(b) These factors regulate the population size by:

  • Food shortage: Limits the carrying capacity of the environment, causing mortality or migration of deer.
  • Parasitic disease: Increases mortality rates and reduces reproductive success, stabilizing the population.
These mechanisms ensure the population does not exceed the ecosystem's resources, maintaining ecological balance.

Question 20:
In a coastal ecosystem, two species of barnacles, Chthamalus and Balanus, were observed occupying different vertical zones on rocky shores. Chthamalus is found in the upper intertidal zone, while Balanus dominates the lower zone. Answer the following based on this case:

(a) What ecological concept explains this distribution pattern?
(b) How does this concept help in understanding species coexistence?
Answer:

(a) The distribution pattern is explained by the concept of competitive exclusion and resource partitioning. Balanus outcompetes Chthamalus in the lower zone due to its superior competitive ability, forcing Chthamalus to occupy the upper zone where Balanus cannot survive.

(b) This concept helps in understanding species coexistence by:

  • Niche differentiation: Each species adapts to a specific part of the habitat, reducing direct competition.
  • Resource partitioning: The species utilize different resources (space, in this case), allowing them to coexist without one eliminating the other.
This demonstrates how natural selection promotes biodiversity by minimizing competition.

Question 21:
A group of researchers studied a population of deer in a forest ecosystem. They observed that the deer population showed a significant decline during the winter months. Based on your understanding of population ecology, explain the possible reasons for this decline and how such fluctuations impact the ecosystem.
Answer:

The decline in the deer population during winter can be attributed to several factors related to population ecology:

  • Resource limitation: During winter, food availability (like grass and shrubs) decreases due to snow cover, leading to starvation and reduced reproductive success.
  • Harsh weather conditions: Extreme cold can increase mortality rates, especially among young and weak individuals.
  • Predation pressure: Predators like wolves may find it easier to hunt deer in winter due to limited hiding places and slower movement of deer in snow.

Such population fluctuations impact the ecosystem by:

  • Altering the food web, as predators may switch to alternative prey.
  • Affecting plant populations due to reduced grazing pressure, leading to changes in vegetation structure.
  • Influencing nutrient cycling, as fewer deer mean less dung for decomposers.

Understanding these dynamics helps in conservation planning and maintaining ecological balance.

Question 22:
In a pond ecosystem, the population of algae suddenly increased, leading to a decrease in dissolved oxygen levels. Fish and other aquatic organisms began dying. Analyze this situation using the concept of population interactions and suggest measures to restore the ecosystem.
Answer:

The sudden increase in algae population, known as an algal bloom, is often caused by eutrophication due to excessive nutrients (like phosphates and nitrates) from agricultural runoff or sewage. This leads to:

  • Reduced sunlight penetration, affecting submerged plants.
  • Decreased dissolved oxygen levels as algae die and decompose, causing hypoxia.
  • Death of fish and other organisms due to oxygen depletion and toxin release by certain algae.

This scenario involves population interactions like:

  • Competition: Algae outcompete other plants for light and nutrients.
  • Predation: Zooplankton populations may initially increase by feeding on algae but later decline due to oxygen shortage.

To restore the ecosystem:

  • Reduce nutrient input by controlling fertilizer use and treating sewage.
  • Introduce biomanipulation by adding algae-eating fish or zooplankton.
  • Aerate the water to increase oxygen levels.

Such measures help maintain ecological balance and prevent recurring blooms.

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