An ecosystem is a self-sustaining unit of the biosphere in which living organisms (biotic components) interact with each other and with the non-living (abiotic) components of their environment (e.g., a pond, forest, grassland).

Producers (Autotrophs) — primarily green plants and phytoplankton — form the base of every food chain. They use sunlight to synthesize food through photosynthesis.

Only 10% of the energy available at one trophic level is transferred to the next trophic level. The remaining 90% is lost as heat during metabolic activities. This is called the 10% Law (Lindeman's Law).

• Biodegradable: Kitchen waste / vegetable peels / paper / cotton cloth
• Non-biodegradable: Plastic bags / DDT / polythene / glass / tin cans

The phenomenon is called Biological Magnification (Biomagnification). Harmful and non-biodegradable chemicals (e.g., DDT, mercury) accumulate in increasing concentrations as they move up the food chain because organisms at each level consume many organisms from the level below.

The ozone layer is present in the Stratosphere, approximately 15–35 km above the Earth's surface.

Chlorofluorocarbons (CFCs) — also known by the trade name Freon — are the primary compounds responsible for ozone layer depletion. Other ODS include halons, HCFCs, carbon tetrachloride, and methyl chloroform.

A food web is a network of interconnected food chains in an ecosystem. It shows all possible feeding relationships between organisms in a community and is a more realistic representation of how energy flows in nature.

• Producers (Autotrophs): Organisms that synthesize their own food using sunlight (photosynthesis) or chemicals (chemosynthesis). Example: Green plants, algae.
• Consumers (Heterotrophs): Organisms that cannot make their own food and depend on producers or other consumers for energy. Example: Herbivores (cow), carnivores (lion), omnivores (humans).

Two types of decomposers:

• Bacteria (e.g., Bacillus spp.)
• Fungi (e.g., Aspergillus, Rhizopus)

Role: Decomposers break down the complex organic compounds in dead organisms into simple inorganic substances (like CO2, H2O, minerals), which are returned to the environment for reuse by producers. This recycling of nutrients maintains the fertility of the soil and sustains life.

10% Law (Lindeman, 1942): Only 10% of the energy available at each trophic level is transferred to the next higher trophic level. The remaining 90% is used up by the organism for its own metabolic processes and lost as heat.

Calculation: Only 100 J of energy reaches the secondary consumer.

• Food Chain: A linear sequence showing the transfer of energy from one organism to another (e.g., Grass ? Grasshopper ? Frog ? Snake ? Hawk).
• Food Web: A complex network of interconnected food chains showing all possible feeding relationships in an ecosystem.

Food webs are more realistic because in nature, most organisms eat and are eaten by more than one species. A single species is rarely the exclusive prey or predator of only one other species. Food webs also show alternative pathways of energy flow that maintain ecosystem stability when one species declines.

Since only 10% of energy is transferred at each trophic level, each additional link in the food chain results in a 90% energy loss.
Example: A food chain with 3 trophic levels delivers 10× more energy to the top consumer than a 4-level chain. Therefore, eating lower on the food chain (e.g., being vegetarian) is far more efficient and can feed more people from the same agricultural land.

1. On humans: Increased UV-B radiation causes skin cancer (melanoma), accelerated skin ageing, and cataracts (clouding of the eye lens leading to blindness).
2. On ecosystems: UV radiation damages the DNA of phytoplankton (base of marine food chains), leading to collapse of aquatic food webs. It also inhibits photosynthesis in plants, reducing crop yields and disrupting terrestrial ecosystems.

• Biodegradable: Substances that can be broken down by biological agents (decomposers) into simpler, harmless compounds.
  Examples: Kitchen waste, paper, cotton, wood, animal dung.
• Non-biodegradable: Substances that cannot be broken down by decomposers. They persist in the environment for very long periods, accumulate in food chains, and cause long-term damage.
  Examples: DDT, plastic, polythene, heavy metals (mercury, lead), glass, tin.

Biological Magnification (Biomagnification): The progressive increase in the concentration of non-biodegradable toxic substances (e.g., DDT, mercury) as they pass from one trophic level to the next in a food chain.

Why it affects higher trophic levels more: Organisms at each level consume many organisms from the level below and accumulate all the toxins from their food. Since the toxins are not metabolised or excreted, they remain stored in fatty tissues. A top predator consumes thousands of prey organisms over its lifetime, accumulating toxins from all of them. Hence the concentration increases exponentially at higher trophic levels.

Decomposers break down complex dead organic matter into simple inorganic nutrients (nitrates, phosphates, CO2, water) that are returned to the soil and atmosphere.

• Without decomposers: Dead organic matter would accumulate indefinitely, nutrients locked in dead bodies would be unavailable to plants, and the planet would run out of raw materials for new life — all ecosystems would collapse.
• With decomposers: Nutrients are recycled continuously (e.g., nitrogen cycle, carbon cycle), soil fertility is maintained, and producers can keep synthesising food.

They form the essential link between the biotic and abiotic components of an ecosystem.

Role of ozone (O3): The ozone layer acts as a natural UV shield, absorbing most of the harmful ultraviolet (UV-B and UV-C) radiation from the Sun before it reaches the Earth's surface. This protects all living organisms from the damaging effects of UV rays (DNA damage, skin cancer, cataracts, immune suppression).

When ozone is depleted: The protective shield becomes thinner, allowing a greater proportion of UV-B radiation to pass through the stratosphere and reach the Earth's surface. Even a small decrease in ozone concentration (e.g., 1% drop) leads to a measurable increase in UV-B reaching Earth, causing biological harm.

Example Food Web (Grassland):

  Grass (Producer)
    |            \
  Grasshopper    Rabbit
    |               \
   Frog            Fox ------> Eagle (Tertiary Consumer)
    |
  Snake
    |
  Eagle (Tertiary Consumer)
        

Identification:

• Producers: Grass
• Primary Consumers (Herbivores): Grasshopper, Rabbit
• Secondary Consumers: Frog (eats grasshopper), Fox (eats rabbit)
• Tertiary Consumer: Eagle (eats snake and fox)
• Snake eats frogs (secondary consumer), placing it as a tertiary consumer in that chain

Energy flow (Ecological Energy Pyramid): Energy is lost at each level as heat through respiration, excretion, and incomplete digestion.

Why limited to 3–4 trophic levels: By the time energy reaches the 4th or 5th trophic level, so little energy remains (0.01% of the original) that it is insufficient to support another population of organisms. The ecological pyramid of energy therefore collapses at higher levels, making it biologically impossible to sustain a 5th or 6th trophic level in most natural food chains.

Food chain: Phytoplankton ? Zooplankton ? Small fish ? Large fish

(a) Energy at large fish level: Only 50 J of energy reaches the large fish.

(b) Highest DDT concentration: At the highest trophic level — in the Large fish (and in any organism that eats large fish, e.g., osprey or humans). DDT concentration is lowest in phytoplankton and highest in top predators (biomagnification).

(c) Fate of the 90% energy: It is used by the organism for its own metabolic activities — cellular respiration, movement, growth, reproduction, body heat maintenance — and is ultimately lost to the environment as heat energy, which cannot be reused.

Natural Formation of Ozone:
High-energy UV radiation from the Sun splits an oxygen molecule (O2) into two reactive oxygen atoms:
$\text{O}_2 + \text{UV} \rightarrow 2\text{O} \cdot$
Each oxygen atom then reacts with another O2 molecule to form ozone:
$\text{O} \cdot + \text{O}_2 \rightarrow \text{O}_3$ (Ozone)

Natural Breakdown: Ozone absorbs UV-B radiation and breaks back into O2 and O, maintaining a natural equilibrium:
$\text{O}_3 + \text{UV-B} \rightarrow \text{O}_2 + \text{O} \cdot$

How CFCs disrupt this balance:

• CFCs (e.g., CCl2F2) are chemically inert in the troposphere but rise to the stratosphere where UV light breaks them, releasing highly reactive chlorine radicals (Cl·).
• Cl· attacks ozone: $\text{Cl} \cdot + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$
• ClO then reacts with another O atom: $\text{ClO} + \text{O} \cdot \rightarrow \text{Cl} \cdot + \text{O}_2$
• The chlorine radical is regenerated and destroys another ozone molecule in a chain reaction. One chlorine atom can destroy over 100,000 ozone molecules before being inactivated.

1. Reduce — Minimising the amount of waste generated at the source.
Example: Carrying a cloth bag instead of asking for a plastic bag while shopping. This reduces plastic waste entering the environment.

2. Reuse — Using items more than once before disposing of them.
Example: Refilling glass bottles with water instead of buying new plastic bottles each time. Reusing reduces manufacturing demand and the energy cost of producing new materials.

3. Recycle — Converting waste materials into new products.
Example: Segregating paper, glass, and metal waste so they can be sent to recycling plants. Recycling reduces the need for extracting raw materials, saves energy, and prevents landfill accumulation.

How it helps: Together, the 3 R's reduce the volume of waste in landfills, lower pollution, conserve natural resources, reduce energy consumption, and decrease the emission of greenhouse gases.

Trophic Level: Each step or level in a food chain at which energy transfer takes place is called a trophic level. Producers form the first trophic level, primary consumers the second, secondary consumers the third, and so on.

Ecological Pyramid of Numbers (Grassland):

        Level 4 (Tertiary) :  [   Hawks/Eagles   ]  (few)
        Level 3 (Secondary):  [  Snakes/Frogs    ]
        Level 2 (Primary)  :  [  Grasshoppers    ]
        Level 1 (Producers):  [  Grass plants    ]  (millions)
        

Why always upright in grassland: In a grassland ecosystem, the number of producers (grass plants) is extremely large (millions). As we move up the food chain, each organism is larger but fewer in number. The base is always the widest (most organisms) and the apex is the narrowest (fewest). This gives an upright pyramid shape. It follows because each consumer must eat many individuals of its prey to survive.

(a) Autotrophs vs Heterotrophs:

• Autotrophs synthesize their own food from inorganic raw materials using sunlight (photosynthesis). Example: Green plants, algae.
• Heterotrophs cannot make their own food; they obtain energy by consuming other organisms. Example: Animals, fungi.

(b) Herbivores vs Carnivores:

• Herbivores eat only plants (primary consumers). Example: Cow, grasshopper, rabbit.
• Carnivores eat only other animals (secondary or higher consumers). Example: Lion, frog, eagle.

(c) Scavengers vs Decomposers:

• Scavengers feed on the bodies of dead animals (whole carcasses) without chemically breaking them down. Example: Vulture, crow, hyena.
• Decomposers break down dead organic matter chemically into simpler inorganic substances through enzymatic action. Example: Bacteria, fungi.

(a) Structure and components of an ecosystem (3 marks):
Example: A pond ecosystem.
An ecosystem consists of two main components:

1. Abiotic (Non-living) components:

• Physical factors: Light, temperature, water, wind, humidity
• Chemical factors: Nutrients in water/soil (nitrates, phosphates, CO2, O2), pH

2. Biotic (Living) components:

• Producers: Phytoplankton, algae, aquatic plants — produce food via photosynthesis
• Consumers: Zooplankton (primary), small fish (secondary), large fish/heron (tertiary)
• Decomposers: Aquatic bacteria and fungi that break down dead matter

Interaction: Biotic components depend on abiotic factors for survival. Decomposers return nutrients to the abiotic environment, which producers use — creating a cycle.

(b) Humans and biomagnification (2 marks):
Humans are omnivores and often occupy the highest trophic level (e.g., as consumers of large fish or meat). Non-biodegradable pesticides like DDT sprayed on crops enter water bodies and soil. They are absorbed by phytoplankton, then concentrated in zooplankton, then small fish, then large fish, and finally in humans who consume large fish. Since humans eat from multiple trophic levels and the toxins are fat-soluble and not excreted, their bodies accumulate the highest concentrations of these harmful chemicals.
Evidence: Studies have found measurable DDT and other organochlorine compounds in human breast milk and fatty tissue — despite bans — due to past biomagnification in food chains.

(a) Ozone depletion problem (3 marks):
Problem: The ozone layer in the stratosphere is thinning due to chemical pollutants, leading to the formation of an "ozone hole" (first detected over Antarctica). This allows excess UV-B radiation to reach Earth's surface.

Main chemicals responsible:

• CFCs (Chlorofluorocarbons) — used in refrigerators, ACs, aerosol sprays (e.g., Freon)
• Halons — used in fire extinguishers
• HCFCs, methyl chloroform, carbon tetrachloride

Mechanism: CFCs release Cl· radicals in the stratosphere, which destroy ozone in a chain reaction (one Cl atom can destroy over 100,000 O3 molecules).

Three international measures:

1. Montreal Protocol (1987): International treaty to phase out CFC production and use worldwide — most successful environmental agreement.
2. Replacement of CFCs: Use of HFCs (hydrofluorocarbons) and natural refrigerants like isobutane that do not destroy ozone.
3. Kigali Amendment (2016): Extended the Montreal Protocol to phase down HFCs (which are potent greenhouse gases).

(b) Ill effects of increased UV radiation (2 marks):
(i) On human health:

• Increased incidence of skin cancer (particularly malignant melanoma)
• Cataracts and other eye disorders; weakening of the immune system

(ii) On ecosystems:

• UV radiation destroys phytoplankton in oceans, which form the base of the marine food chain — threatening global fisheries.
• Damages the DNA of plants and inhibits photosynthesis, reducing crop productivity and disrupting terrestrial food chains.

1. Trophic level: Vultures feed at the secondary or tertiary consumer level (they eat dead herbivores and carnivores alike). They are scavengers — they feed on dead animals (carrion) but do not chemically decompose them like true decomposers.
2. Disruption of food web: Vultures are keystone scavengers. Their loss means:
   • Dead animal carcasses accumulate, serving as breeding grounds for pathogens.
   • Feral dog and rat populations increase (filling the scavenger niche), leading to more rabies, plague, and other zoonotic diseases.
   • The entire decomposition/nutrient cycling network is disrupted.
3. Biomagnification of diclofenac: Even if diclofenac concentration in one cattle carcass is low, vultures eat many carcasses over their lifetime. Being non-biodegradable, the drug accumulates in vulture tissues. As the drug is not excreted, its concentration increases with each meal — a classic example of biomagnification at the scavenger level.
4. Two measures to protect vultures:
   • Ban diclofenac in veterinary use and replace it with meloxicam (shown to be safe for vultures) — India banned veterinary diclofenac in 2006.
   • Establish vulture safe zones and breeding centres (captive breeding programs) to rehabilitate and release vultures, as done by the Bombay Natural History Society (BNHS).

1. Why plastic is non-biodegradable: Plastic is made from synthetic polymers (long-chain hydrocarbons) that microorganisms cannot break down enzymatically. In the environment, UV light and physical weathering break plastic into smaller and smaller fragments (microplastics), but it is never chemically converted into harmless substances — it persists for hundreds to thousands of years.
2. Marine food chain showing biomagnification:
   Phytoplankton (low conc.) ? Small fish (medium conc.) ? Tuna/Shark (highest conc.)
   The highest concentration of microplastics and associated toxins is found in the Tuna/Shark (top predator level).
3. Health risk for the fisherman: By regularly eating large predatory fish, the fisherman accumulates high concentrations of microplastics and toxic chemicals (dioxins, PCBs, heavy metals) in his body tissues. This increases the risk of cancer, hormonal disruption (endocrine disruption), immune disorders, and neurological damage. The phenomenon is Biological Magnification (Biomagnification).
4. Two solutions:
   • Individual level: Refuse single-use plastics; carry reusable bags, bottles, and containers to reduce plastic entering waste streams and eventually oceans.
   • Policy/Government level: Implement and enforce a complete ban on single-use plastics; fund ocean clean-up initiatives and invest in plastic waste management infrastructure and biodegradable alternatives.

1. The ozone hole: The "ozone hole" refers to a region of abnormally thinned ozone layer (not an actual hole) over Antarctica. It was first observed (and confirmed) by British scientists in the 1980s, appearing each Antarctic spring (September–November) when returning sunlight activates CFC-based reactions on polar stratospheric clouds.
2. Chemical mechanism of CFC destruction of ozone:
   CFC (e.g., CCl2F2) exposed to UV in stratosphere releases Cl·: $\text{CF}_2\text{Cl}_2 + h\nu \rightarrow \text{CF}_2\text{Cl} \cdot + \text{Cl} \cdot$
   Cl· reacts with ozone: $\text{Cl} \cdot + \text{O}_3 \rightarrow \text{ClO} \cdot + \text{O}_2$
   ClO· reacts with O atom: $\text{ClO} \cdot + \text{O} \cdot \rightarrow \text{Cl} \cdot + \text{O}_2$
   The Cl· is regenerated (chain reaction). One Cl atom can destroy ~100,000 ozone molecules.
3. Two consequences if ozone depletion had continued:
   • Massive increase in skin cancers and eye cataracts in humans and animals worldwide.
   • Collapse of phytoplankton populations in the Southern Ocean, devastating the marine food chain and threatening global fisheries and the ocean carbon cycle.
4. Two CFC alternatives and why safer:
   • HFCs (Hydrofluorocarbons) — contain no chlorine, so they do not release Cl· radicals and do not deplete the ozone layer. (Note: They are being phased down due to being potent greenhouse gases — Kigali Amendment.)
   • Natural refrigerants (e.g., isobutane, ammonia, CO2) — do not contain halogens and are either naturally present in the environment or have very low global warming potential, making them safe for the ozone layer.