Practice Questions on Food Chain & Food Web

The Sun is the primary source of energy for all ecosystems | Producers (plants) capture solar energy through photosynthesis and convert it into chemical energy | Without solar energy, no food chain can be sustained | Soil provides minerals but not energy | Water is essential for life but is not an energy source | Air (CO2) is used in photosynthesis but is not the energy source itself

Organisms that produce their own food using sunlight are called Producers or Autotrophs | They form the first trophic level and the base of every food chain | Consumers (heterotrophs) depend on producers for food | Decomposers break down dead matter and do not produce food via photosynthesis | Omnivores eat both plants and animals but cannot produce food from sunlight | Example producers: green plants, phytoplankton, cyanobacteria

Primary consumers are organisms that feed directly on producers (plants), hence they are herbivores | They occupy the second trophic level in a food chain | Carnivores feed on other animals and occupy tertiary or higher levels | Omnivores eat both plants and animals; they may sometimes act as primary consumers but are not always exclusively herbivorous | Decomposers break down dead matter and are not consumers in the traditional sense | Examples: rabbit, deer, grasshopper, cattle

A food chain must begin with a producer (Grass) and energy flows from producers to consumers | Grass → Grasshopper → Frog → Snake is the correct sequence: Grass (producer) → Grasshopper (primary consumer) → Frog (secondary consumer) → Snake (tertiary consumer) | In option A, Grasshopper → Grass is wrong as grasshoppers feed on grass, not the reverse | Option C and D both show energy flowing in the wrong direction, starting from predators | Energy always flows from lower to higher trophic levels, never in reverse

Decomposers are microorganisms like bacteria and fungi that break down dead and decaying organic matter into simpler inorganic compounds | They recycle nutrients back into the ecosystem, making them available to producers again | Lions and tigers are apex carnivores, not decomposers | Grass and trees are producers (autotrophs) | Frogs and snakes are consumers (heterotrophs) | Decomposers are sometimes called saprotrophic organisms and play a critical role in nutrient cycling

A trophic level refers to the position an organism occupies in a food chain based on its feeding relationship | Producers occupy the first trophic level; primary consumers the second; secondary consumers the third, and so on | A biome is a large ecological region characterized by its climate and vegetation | A niche is the functional role and position of an organism in its ecosystem | A habitat is the physical environment where an organism lives | The concept of trophic levels was formalized by Raymond Lindeman in 1942

A food web is an interconnected network of multiple food chains in an ecosystem | It represents the complex feeding relationships among various organisms | A single linear chain describes a food chain, not a food web | A food web includes all organisms including decomposers | It is not limited to energy flow from sun to soil; it covers all trophic interactions | Food webs provide greater ecological stability compared to simple food chains because they offer multiple energy pathways

Phytoplankton are microscopic photosynthetic organisms found in aquatic ecosystems; they are the primary producers forming the base of aquatic food chains | Rabbits are terrestrial herbivores (primary consumers) | Eagles are apex predators (tertiary/quaternary consumers) | Frogs are secondary consumers in most food chains | Phytoplankton include diatoms, green algae, and cyanobacteria | They contribute to about 50% of global oxygen production

Energy in a food chain flows unidirectionally from producers to consumers — from lower trophic levels to higher trophic levels | This flow is driven by the transfer of chemical energy stored in organic compounds | Energy does NOT flow from consumers back to producers; once lost as heat it cannot be reused | Decomposers recycle nutrients (not energy) back to the environment | The second law of thermodynamics explains that energy degrades to heat at each transfer, making flow unidirectional and non-recyclable | This is a fundamental principle distinguishing energy flow from nutrient cycling

Apex predators are at the highest trophic level (typically 4th or 5th) in a food chain | They have no natural predators in their ecosystem | The first trophic level is occupied by producers (plants) | The second trophic level is occupied by primary consumers (herbivores) | The third trophic level is occupied by secondary consumers (carnivores that eat herbivores) | Examples of apex predators: tigers, lions, eagles, sharks, orcas

The 10% Law (Lindeman’s Law) states that 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, respiration, and excretion | This law was proposed by Raymond Lindeman in 1942 based on studies in Cedar Bog Lake | 1% transfer would make ecosystems non-viable | 20% and 50% overestimate actual energy transfer efficiency | This law explains why food chains are typically limited to 4–5 trophic levels

Raymond Lindeman proposed the 10% Law (also called the Law of Trophic Efficiency) in 1942 | He studied the energy flow in Cedar Bog Lake, Minnesota | Charles Darwin proposed the Theory of Evolution by Natural Selection | Ernst Haeckel coined the term ‘Ecology’ in 1866 | Eugene Odum made significant contributions to ecosystem ecology but did not propose the 10% law | Lindeman’s work laid the foundation for quantitative ecosystem ecology

In a grassland: Grass (producer) → Rabbit (primary consumer/herbivore) → Fox (secondary consumer/carnivore) → Eagle (tertiary consumer/apex predator) | This follows the correct sequence from producers to consumers | Option B reverses the sequence (eagle cannot feed on rabbit which feeds on grass in that order) | Options C and D have incorrect energy flow direction | Grassland food chains are typically linear with grass at the base | This is one of the most cited examples in ecology textbooks (NCERT Class 10)

A food web provides more stability to an ecosystem because it has multiple alternative pathways for energy flow | If one species is removed, other food chains in the web can maintain energy transfer | A food web has multiple interlinked pathways, not a single pathway | Food webs are MORE stable than simple food chains due to redundancy | Food webs include all organisms including producers, consumers, and decomposers | Greater biodiversity in a food web generally means greater ecosystem resilience (Charles Elton’s stability-diversity hypothesis)

Biomagnification (biological magnification) is the progressive increase in concentration of persistent toxic substances at successively higher trophic levels in a food chain | This occurs because toxins are not metabolized or excreted and accumulate in fatty tissues | Increase in CO2 is a climate issue unrelated to biomagnification | Energy decrease at higher trophic levels is described by the 10% Law | Biodiversity increase is unrelated to biomagnification | Classic examples: DDT, methylmercury, PCBs accumulating in food chains

DDT (dichlorodiphenyltrichloroethane) is a classic example of biomagnification | DDT accumulates in fat tissues and increases in concentration at each trophic level — tiny amounts in water become lethal concentrations in apex predators like fish-eating birds | This led to thinning of eggshells in birds like the bald eagle, reducing reproduction | Increase of CO2 is related to global warming (greenhouse effect), not biomagnification | Ozone depletion is caused by CFCs, not related to food chains | Soil erosion is a geomorphological process unrelated to biomagnification

Secondary consumers occupy the third trophic level and feed on primary consumers (herbivores) | Primary consumers are herbivores that feed on producers (plants/first trophic level) | Tertiary consumers feed on secondary consumers and occupy the fourth trophic level | Decomposers break down dead organic matter and are not directly part of grazing food chains as feeding targets | Example: A frog (secondary consumer) feeds on a grasshopper (primary consumer) | Understanding trophic positions is essential for APPSC/TGPSC ecology questions

Phytoplankton (microscopic algae and cyanobacteria) are the primary producers in aquatic ecosystems forming the base of aquatic food chains | They perform photosynthesis using sunlight and CO2 dissolved in water | Fish are secondary or tertiary consumers, not producers | Zooplankton feed on phytoplankton and occupy the second trophic level (primary consumers) | Aquatic insects may be primary or secondary consumers depending on their diet | Phytoplankton produce approximately 50% of Earth’s oxygen and fix vast amounts of carbon

In a grassland ecosystem, the pyramid of numbers is upright because the number of organisms decreases progressively from producers to apex consumers | Grass (millions) > Grasshoppers (thousands) > Frogs (hundreds) > Snakes (tens) > Eagles (few) | An inverted pyramid of numbers is seen in tree-based or parasitic food chains where one tree supports many caterpillars | A spindle-shaped pyramid occurs in some intermediate ecosystems | A dumbbell-shaped pyramid is not a standard ecological pyramid shape | Elton’s pyramids (1927) codified this concept

The pyramid of energy is always upright in all ecosystems because energy always decreases at each successive trophic level due to the 10% Law and second law of thermodynamics | The pyramid of numbers can be inverted (as in a tree-based ecosystem where one tree supports many insects) | The pyramid of biomass can be inverted (as in an aquatic ecosystem where phytoplankton biomass < zooplankton biomass at a given point in time) | The energy pyramid cannot be inverted because energy is always lost as heat and never increases up the trophic levels | This is a frequently tested concept in UPSC, APPSC, and TGPSC prelims

In aquatic (pond/ocean) ecosystems, the pyramid of biomass is inverted because phytoplankton (producers) have a very small standing biomass at any given time due to their rapid reproduction and consumption | Zooplankton that feed on phytoplankton may have higher biomass at a point in time | Grassland ecosystems have upright pyramids of biomass | Forest ecosystems have upright biomass pyramids because trees have massive biomass | Desert ecosystems generally show upright biomass pyramids | The inverted biomass pyramid in aquatic systems is possible because phytoplankton reproduce very rapidly (high turnover rate)

Decomposers (bacteria and fungi) convert complex dead organic matter (proteins, carbohydrates, fats) into simple inorganic compounds (CO2, water, minerals) through the process of decomposition | This process recycles nutrients back to the soil making them available to producers again | Decomposers do not produce energy from sunlight — that is the role of producers | They do not feed on primary consumers — that is the role of secondary consumers | They do not transfer energy to apex predators — energy flows from lower to higher trophic levels | Without decomposers, nutrients would be locked in dead matter and ecosystems would collapse

A food web does NOT involve a single species at each level — multiple species can occupy the same trophic level | A food web has multiple interlinked food chains — this IS a characteristic | A food web provides stability because of multiple energy pathways — this IS a characteristic | Energy does flow in one direction (from producers to consumers) in a food web — this IS correct | Having multiple species at each trophic level is what makes food webs complex and stable | This multi-species feature differentiates food webs from simple food chains

The grazing food chain (GFC) begins with green plants (living producers) and energy flows from producers to herbivores to carnivores | It is driven by solar energy captured through photosynthesis | The detritus food chain (DFC) begins with dead organic matter, not the grazing food chain | Herbivores are the second step (primary consumers) in the grazing food chain, not the starting point | Decomposers are the starting organisms of the detritus food chain | In terrestrial ecosystems, the grazing food chain is the dominant pathway while in forest ecosystems, the detritus food chain may be dominant

The detritus food chain (DFC) begins with dead organic matter (detritus) — fallen leaves, dead organisms, excreta | Detritivores (earthworms, millipedes, beetles) feed on this dead matter | Decomposers (bacteria, fungi) further break down the detritus into inorganic nutrients | Green plants are the start of the grazing food chain, not the detritus food chain | Herbivores are primary consumers in the grazing food chain | Sunlight initiates photosynthesis but is not the starting point of the detritus food chain | DFC plays a crucial role in nutrient cycling, especially in forest ecosystems

Ecological efficiency refers to the percentage of energy transferred from one trophic level to the next in a food chain | It is typically around 10% (Lindeman’s 10% Law) | It does not refer to the efficiency of photosynthesis alone (that is a separate concept — photosynthetic efficiency is about 1-2%) | The rate of decomposition is related to detritus food chain dynamics, not ecological efficiency | Productivity of soil is related to edaphic factors and biogeochemical cycles, not ecological efficiency | Ecological efficiency determines the number of trophic levels an ecosystem can support

Omnivores like bears can occupy multiple trophic levels because they eat both plants (acting as primary consumers at trophic level 2) and animals like fish or deer (acting as secondary or tertiary consumers at trophic level 3 or 4) | Grass is always a producer (trophic level 1) and cannot occupy other levels | Rabbits are strictly herbivores (primary consumers at trophic level 2 only) | Eagles are apex predators typically at one high trophic level | The flexibility of omnivores makes food webs more complex and resilient | Examples: humans, pigs, bears, crows

DDT concentration is highest in apex predators (top consumers) because biomagnification causes toxic substances to concentrate progressively at each trophic level | Each organism ingests all the DDT from the many organisms it eats, leading to multiplicative concentration | Producers have the lowest concentration of DDT | Primary consumers accumulate more DDT than producers but less than secondary consumers | Secondary consumers have more than primary consumers but less than apex predators | This phenomenon caused collapse of fish-eating bird populations (e.g., bald eagles, pelicans) in the USA before DDT was banned in 1972

A keystone species has a disproportionately large effect on its ecosystem relative to its biomass or abundance | Its removal causes dramatic disruptions in the food web and ecosystem structure | It does not necessarily have the largest population — often it has a relatively small population | It does not always occupy the first trophic level | Keystone species are not always decomposers | Classic example: Sea otters in Pacific kelp forests — they control sea urchin populations which otherwise devastate kelp; wolves in Yellowstone — their reintroduction caused a trophic cascade that restored vegetation and riverbank stability

Biomass is the total dry weight (or sometimes fresh weight) of all organisms at a particular trophic level in a given area at a given time | It is typically measured in grams per square meter (g/m²) or tonnes per hectare | Biogas is a fuel produced by anaerobic decomposition of organic matter (unrelated to trophic levels) | Bioenergy is a broad term for energy derived from biological sources | Biotic potential refers to the maximum reproductive capacity of an organism | Biomass is used to construct pyramids of biomass, which show the distribution of organic matter across trophic levels

Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) minus Respiration losses (R) by the producers | GPP is the total rate of photosynthesis including all the organic matter produced | Producers use a significant portion of GPP for their own respiration; what remains is NPP | NPP represents the energy available to primary consumers | It is NOT the total photosynthesis (that is GPP) | It is NOT energy available to secondary consumers directly (secondary consumers feed on primary consumers) | NPP is measured in g C/m²/year and is highest in tropical rainforests

Applying Lindeman’s 10% Law: Level 1 (Producers) = 10,000 kcal; Level 2 = 10,000 × 10% = 1,000 kcal; Level 3 = 1,000 × 10% = 100 kcal; Level 4 = 100 × 10% = 10 kcal; Level 5 = 10 × 10% = 1 kcal | So only 1 kcal out of 10,000 kcal reaches the 5th trophic level | 10 kcal is the energy at the 4th trophic level | 100 kcal is the energy at the 3rd trophic level | 1,000 kcal is the energy at the 2nd trophic level | This demonstrates why long food chains are energetically inefficient and why most ecosystems support only 4–5 trophic levels

An earthworm is a classic detritivore — it feeds on dead and decaying organic matter (detritus) by ingesting soil particles along with organic debris | Detritivores are different from decomposers: detritivores physically break down detritus into smaller particles while decomposers (bacteria/fungi) chemically decompose organic matter | A lion is an apex carnivore feeding on large mammals | An eagle is an apex predator feeding on fish, small mammals, and birds | Phytoplankton are producers performing photosynthesis | Other detritivores: millipedes, wood lice, dung beetles, crabs

A trophic cascade refers to the indirect effects that changes at one trophic level (typically apex predators) have on lower trophic levels, sometimes causing dramatic ecosystem-wide changes | For example, the removal of wolves (apex predators) in Yellowstone led to overgrazing by elk, which degraded vegetation and riverbanks | It is NOT about energy flow from lower to higher levels (that is standard trophic energy transfer) | It is NOT the collapse of a food chain | It is NOT about species migration | Trophic cascades can be top-down (predator effects on prey, then plants) or bottom-up (producers affecting herbivores then predators)

Producers (first trophic level) contain the maximum energy in an energy pyramid | Since energy decreases at each successive trophic level (10% Law), the base (producers) always has the most energy | Tertiary consumers (4th level) have the least energy, not the most | Secondary consumers have less energy than primary consumers | Primary consumers have less energy than producers | The energy pyramid is always upright because energy always decreases from producers to apex consumers; it can never be inverted

Tropical rainforests have the highest Gross Primary Productivity due to high solar radiation, abundant rainfall, warm temperatures, and high biodiversity year-round | Deserts have very low GPP due to water scarcity | Temperate grasslands have moderate GPP | Arctic tundra has very low GPP due to extreme cold, permafrost, and short growing seasons | Tropical rainforests cover only about 7% of land area but account for approximately 32% of global terrestrial NPP | Open oceans have low GPP per unit area but are significant due to their vast surface area

Lichen is a classic example of mutualistic symbiosis between algae (or cyanobacteria) and fungi | The algal component performs photosynthesis and provides nutrients (food) | The fungal component provides shelter, moisture, and mineral salts | This is NOT a relationship between two plants, two bacteria, or bacteria and fungi alone | Lichen is ecologically important as a pioneer species in primary succession on bare rocks | While lichen is not directly part of a food chain question, understanding symbiotic relationships is part of ecosystem ecology syllabus for APPSC/TGPSC

In a parasitic food chain, energy flows from a large host organism to smaller parasites, and then to even smaller hyperparasites | This is the reverse size relationship compared to the grazing food chain where organisms generally get larger going up the chain | Grass → Deer → Tiger is a grazing food chain — sizes increase | Dead matter → Fungi → Bacteria is the detritus food chain | Algae → Fish → Dolphin is a grazing/aquatic food chain | Parasitic food chains explain the inverted pyramid of numbers seen in tree-based ecosystems (one tree → many caterpillars → many more parasites)

In a nutrient cycle (biogeochemical cycle), matter/nutrients cycle continuously between biotic and abiotic components of the ecosystem | Examples: Carbon cycle, Nitrogen cycle, Phosphorus cycle, Water cycle | In food chains and food webs, energy flow is unidirectional and non-cyclic — energy cannot be recycled | In energy pyramids, energy decreases at each level and is lost as heat | The key distinction is: ENERGY flows unidirectionally and is non-recyclable; NUTRIENTS cycle continuously | This is a critical concept tested in UPSC/APPSC ecology questions — examinees often confuse energy flow with nutrient cycling

Applying 10% ecological efficiency: Producer = 500 g/m²; Primary consumer = 500 × 10% = 50 g/m²; Secondary consumer = 50 × 10% = 5 g/m²; Tertiary consumer = 5 × 10% = 0.5 g/m² | Therefore, tertiary consumer biomass = 0.5 g/m² | 5 g/m² corresponds to secondary consumer level | 50 g/m² corresponds to primary consumer level | 0.05 g/m² would be a 4th level with 10% of 0.5 | This demonstrates the dramatic energy loss across trophic levels — tertiary consumers receive only 0.1% of the original producer biomass | This calculation type frequently appears in UPSC and State PSC ecology questions

In a tree-based (parasitic) ecosystem, the pyramid of numbers is INVERTED — one large tree (producer) supports thousands of insects/caterpillars (primary consumers) which in turn support thousands of parasites | Elton’s pyramid of numbers assumes numbers decrease from producers to consumers, which fails for tree-based and parasitic food chains | In a grassland, the pyramid of numbers is correctly upright (Elton’s model works) | In a pond ecosystem, the pyramid of numbers is generally upright for producers (algae) to fish | In a desert, the pyramid of numbers is typically upright | The inverted pyramid of numbers in tree-based systems is a classic limitation of Elton’s pyramid model

The statement ‘Energy can be recycled unlike nutrients’ is INCORRECT — it is actually the OPPOSITE | Energy flow is unidirectional and NON-RECYCLABLE; energy lost as heat cannot be reused | It is NUTRIENTS/MATTER that cycle (carbon cycle, nitrogen cycle, etc.), not energy | Energy does decrease at each successive trophic level (correct) | The 10% law does apply to energy transfer (correct) | Producers do capture solar energy through photosynthesis (correct) | This reversal — energy cycles, nutrients flow unidirectionally — is a common trap question in competitive exams

This sequence shows the NUMBER of organisms at each trophic level decreasing from producers to consumers — this is a pyramid of NUMBERS | The numbers given (80,000 → 800 → 80 → 8) represent the count of individual organisms at each trophic level | A pyramid of biomass would show DRY WEIGHT (g/m²) not individual count | A pyramid of energy would show energy (kcal/m²/year) | Ecological efficiency shows percentage, not individual counts | The pyramid of numbers can be inverted in parasitic chains but is upright in this aquatic example | This type of numerical data interpretation is common in UPSC/APPSC MCQs

Statement I is CORRECT — greater food web complexity (more species and connections) provides more alternative energy pathways, increasing ecosystem resilience and stability | Statement II is CORRECT — keystone species have disproportionate effects; their removal triggers trophic cascades (e.g., wolf removal in Yellowstone) that destabilize the ecosystem | Statement III is INCORRECT — most food webs have only 4–5 trophic levels; adding more levels is energetically unsustainable due to the 10% Law (only 0.01% of energy would reach a 5th level from producers) | Therefore only I and II are correct | This multi-statement format is very common in UPSC CSE Prelims and APPSC/TGPSC Group I exams

BOD is highest in the decomposition zone (also called the septic zone) of a polluted water body | In this zone, decomposers (aerobic bacteria) work intensively to break down organic pollutants, consuming large amounts of dissolved oxygen | This creates very low oxygen levels, making the water hypoxic or anoxic | In the clean water zone, BOD is low and dissolved oxygen is high | In the recovery zone, BOD decreases as organic matter is depleted and oxygen levels begin to recover | The relationship between BOD and decomposer activity in food chains is part of aquatic ecology and water pollution topics in environmental science

The pyramid of energy is always upright because energy is always lost as heat at each trophic level due to metabolic activities (respiration, movement, excretion), following the second law of thermodynamics | Less energy is always available at higher trophic levels, making the pyramid wider at the base | Producers are NOT always more numerous than consumers — in tree-based systems, one tree supports thousands of insects (inverted numbers pyramid) | Energy CANNOT be recycled; lost heat energy cannot re-enter the food chain — this irreversibility ensures the energy pyramid is always upright | Biomass does NOT increase with each trophic level — it generally decreases (except in inverted biomass pyramids) | The energy pyramid overcomes the limitations of number and biomass pyramids

Minamata disease (Japan, 1950s–1960s) occurred through the pathway: Industrial methylmercury discharge → Water → Phytoplankton → Zooplankton → Fish → Humans | Mercury biomagnified through the aquatic food chain, reaching lethal concentrations in fish consumed by humans and cats | The Chisso Corporation factory discharged methylmercury into Minamata Bay | Soil → plants → humans is a terrestrial pathway not involved in Minamata disease | Air → plants → cattle → humans was not the pathway for Minamata | Soil → earthworms → birds was not the Minamata pathway | Minamata disease is recognized internationally as a landmark case of industrial pollution and biomagnification through food chains

When humans eat plants, they receive energy from the second trophic level (about 10% of solar energy captured by plants) | When humans eat meat (herbivorous animals), they receive only 1% of the original solar energy (10% × 10% = 1%) due to two energy losses | Shorter food chains are therefore far more energy-efficient for human nutrition | The argument is not about disease or pathogens — it is purely about trophic energy loss at each level | Shorter chains do NOT have less biodiversity — that is incorrect | Decomposers are not more active in shorter chains — their activity depends on organic matter availability | This concept is important for food security discussions in India related to sustainable agriculture

This is a classic example of a TROPHIC CASCADE — specifically a top-down trophic cascade where the reintroduction of an apex predator (wolf) had cascading effects on lower trophic levels, indirectly transforming the entire ecosystem | Wolves reduced elk numbers and changed elk behavior (elk avoided open areas), allowing vegetation to recover along riverbanks | Recovered vegetation stabilized riverbanks, reducing erosion and attracting beavers and songbirds | Ecological succession refers to sequential change in community composition over time (e.g., bare rock → lichens → moss → shrubs → trees) | Biomagnification is the accumulation of toxins at higher trophic levels | Competitive exclusion (Gause’s Principle) states that two species competing for identical resources cannot coexist | Yellowstone’s wolf reintroduction (1995) is the most cited trophic cascade example globally

Shorter food chains (fewer trophic levels) are generally MORE vulnerable to disturbance because the loss of any single species can collapse the chain | Longer and more complex food webs have greater redundancy — multiple species at each trophic level can fulfil similar roles, providing resilience | Ecosystem resilience is NOT simply determined by the number of apex predators alone | Simpler food chains are actually LESS resilient, not more, because they lack alternative energy pathways | This concept is central to conservation ecology — protecting food web complexity is key to ecosystem resilience | In APPSC and TGPSC ecology sections, questions on food web stability and disturbance are commonly asked in the context of biodiversity conservation