Life on the Earth: NCERT Solutions (2026)

This difference highlights two fundamental laws of ecology:

• One-Way Energy Flow: Energy enters an ecosystem from a single external source: the Sun. Green plants (producers) capture solar energy and convert it into chemical food. When herbivores eat these plants, and carnivores eat the herbivores, energy moves up the chain. At each step, roughly 90% of the energy is lost as heat due to metabolic processes and respiration (the 10% Rule). This lost heat escapes into space and can never flow backward or be reabsorbed by plants. Hence, energy flow is strictly linear and one-way.
• Cyclic Nutrient Movement: The chemical elements that make up living matter (like Carbon, Nitrogen, and Hydrogen) are finite and cannot be continuously created or lost into space. When plants and animals grow, they absorb these nutrients from the soil, water, and air. When they die, decomposers (like fungi and bacteria) break down their organic tissue, returning these exact chemical minerals right back to the soil and atmosphere. This allows new plants to absorb them again, creating an endless, closed loop.

Memory Trick: Energy Escapes (Linear/One-way); Nutrients Never leave (Cyclic/Circular).

If nitrogen-fixing bacteria (such as Rhizobium or Azotobacter) were completely destroyed, it would trigger a massive collapse across the biosphere:

• The Problem: Although nitrogen gas makes up 78% of our atmosphere, plants and animals cannot absorb or use it directly in its gaseous form because it is chemically inert.
• The Breakdown: Without these specialized bacteria, there would be no way to convert atmospheric nitrogen into usable forms like nitrates. Plants would quickly develop severe nutrient deficiencies, leaving them unable to manufacture vital proteins, chlorophyll, or DNA.
• The Impact: As plant growth grinds to a halt, herbivores would starve from a lack of food, which would soon cause carnivore populations to collapse as well. The entire food web would disintegrate, severely threatening life on Earth.

(This serves as a structured sample layout based on common Indian school garden ecosystems for your practical files)

1. Ecosystem Map Flow:

2. Functional Groups Table:

(a) The process by which water vapor is released from the leaves of plants into the atmosphere is called:
(i) Evaporation   (ii) Condensation   (iii) Transpiration   (iv) Precipitation

Correct Answer: (iii) Transpiration.
Explanation: Transpiration is the biological evaporation of water from plant stomata, playing a big role in driving local rain cycles.

(b) Which biome is characterized by dense, multi-layered evergreen canopies and year-round heavy rainfall?
(i) Tundra Biome   (ii) Desert Biome   (iii) Tropical Rainforest Biome   (iv) Temperate Grassland Biome

Correct Answer: (iii) Tropical Rainforest Biome.
Explanation: Located near the equator, these biomes receive continuous high solar energy and rainfall, supporting thick, layered evergreen forests.

A biogeochemical cycle is the pathway through which a chemical substance moves through both the biotic (living) and abiotic (non-living like atmosphere, water, soil) compartments of Earth.

Why they are absolutely vital:

• Constant Resource Availability: The Earth does not receive new supplies of raw materials like carbon, nitrogen, or oxygen from space. These cycles ensure that existing elements are continuously recycled, keeping them available for generations of living organisms.
• Pollution Mitigation: Cycles naturally regulate and clean the environment. For instance, the carbon cycle pulls dangerous excess carbon dioxide out of the air through forest capture and ocean absorption.
• Ecosystem Stability: If even a single step in a cycle gets blocked, it creates immediate bottlenecks—such as soil losing its fertility or toxic gases building up in the air—which can destabilize entire ecosystems.

The 10% Law (introduced by Raymond Lindeman) states that when energy moves from one trophic level to the next higher level, only about 10% of that energy is successfully stored as organic biomass.

• Where does the other 90% go? Most of the energy is lost as metabolic heat during daily activities like respiration, movement, and digestion, while the rest is lost as undigested waste.
• Example Calculations: If primary producers generate 10,000 Joules of energy from sunlight:
  • Primary Consumers (Herbivores) get only 1,000 Joules (10%).
  • Secondary Consumers (Carnivores) get only 100 Joules (10%).
  • Tertiary Consumers (Apex predators) get a mere 10 Joules (10%).
• Impact on Chain Length: Because the energy pool shrinks so rapidly at each step, very little usable energy is left after 4 or 5 trophic levels to support another population. This energy drain places a strict natural limit on how long a food chain can grow.

Although both biomes are dominated by grass landscapes, they feature distinct regional differences:

A Biodiversity Hotspot is a biogeographic region that holds an exceptionally high concentration of unique native species found nowhere else on Earth (endemism), but is facing severe, immediate threats of destruction. To qualify as a hotspot, a region must have lost at least 70% of its original natural habitat.

Prominent Indian Hotspots:

1. The Western Ghats: A mountain chain running parallel to India's western coast, packed with rare amphibians, plants, and bird species.
2. The Eastern Himalayas: The lush montane forests of northeastern India, famous for their rich plant diversity and endangered wildlife like the Red Panda.

Forests serve as the primary land-based anchors of the carbon cycle. Cutting them down disrupts this balance in two major ways:

• Loss of Carbon Sinks: Living trees act as major "carbon sinks," absorbing huge amounts of carbon dioxide (CO₂) from the air to build their wood and leaves through photosynthesis. Deforestation removes these natural filters, leaving more greenhouse gases to accumulate in the atmosphere.
• Instant Carbon Release: When forests are cleared, the logs are often burned or left to rot. This releases all the carbon stored inside the wood back into the air as CO₂. This sudden surge traps extra solar heat, driving global warming and climate change.

Answer: Endemic species are native plants or animals that live exclusively in one specific geographic area and are not found naturally anywhere else in the world (e.g., the Lion-tailed Macaque in India's Western Ghats).

Answer: A food chain is a single, linear path showing who eats whom in an ecosystem. In contrast, a food web is an interconnected network of multiple food chains, reflecting the complex, realistic feeding choices available in nature.

Answer: Since organisms cannot directly use inert atmospheric nitrogen gas (N₂), the nitrogen cycle relies on a multi-stage microbial process to convert it into usable nutrients:

1. Nitrogen Fixation: Soil bacteria (like Rhizobium in legume root nodules) or lightning strikes break down atmospheric nitrogen gas, turning it into ammonia.
2. Nitrification: Specialized soil bacteria (Nitrosomonas and Nitrobacter) convert this ammonia first into nitrites and then into highly soluble nitrates, which plants can easily absorb through their roots.
3. Assimilation: Plants use these absorbed nitrates to manufacture essential organic compounds like proteins and nucleic acids. Herbivores then eat these plants to build their own body proteins.
4. Denitrification: When plants and animals produce waste or die, decomposers break down the organic matter to release ammonia. Eventually, deep-soil anaerobic bacteria (Pseudomonas) convert these nitrogen compounds back into inert nitrogen gas (N₂), releasing it back into the atmosphere to complete the cycle.

Answer: Roughly 1% of solar energy is successfully captured and converted into chemical energy by primary producers.

Answer:

1. Modified Leaves: Leaves shrink into sharp spines or needles to drastically reduce the surface area available for water loss via transpiration.
2. Thick Waxy Coatings: Stems and pads are covered with a heavy, waterproof waxy layer to seal in moisture.
3. Deep Root Networks: Plants grow extensive, deep root systems into distant underground water reserves.

(i) What type of ecological impact is illustrated by the removal of wolves from this ecosystem?
Ans: This case illustrates a Trophic Cascade, where removing a top predator triggers a damaging chain reaction that disrupts lower trophic levels and changes the physical landscape.

(ii) How does this case prove that predators are vital for maintaining structural stability inside a food web?
Ans: Predators keep herbivore populations in check, preventing overgrazing. This protects native plant life, prevents soil erosion, and maintains a stable habitat for other species across the ecosystem.

1. Why are wetlands called "biodiversity checkpoints" or natural filters?
Ans: They naturally filter out toxic agricultural pollutants like excess nitrates and phosphates while providing a rich, supportive habitat for a wide variety of species.

2. Identify two major environmental problems caused by destroying regional wetlands.
Ans: Draining wetlands causes frequent flash flooding during heavy rains and destroys vital nesting habitats for migratory birds.

3. How does paving over wetlands disrupt the local hydrological cycle?
Ans: It replaces absorbent soil with waterproof concrete, stopping rain from soaking into the ground. This lowers groundwater levels and increases dangerous surface runoff.