Ecology Fundamentals: Ecosystems and Food Chains

Ecology studies relationships between organisms and their environment.

Ernst Haeckel introduced the term in his 1866 work. Ecology includes physical conditions as well as relationships between living organisms.

A population contains members of one species living in an area. A community includes interacting populations of different species.

An ecosystem includes the community and its non-living environment. A pond includes fish, algae, microbes, water, light and dissolved nutrients.

Misconception: an ecosystem is only a list of plants and animals. Exchanges of energy and matter with the physical environment are part of the definition.

Many species can share one habitat. A niche describes how an organism uses resources and interacts with its surroundings.

Biotic factors include living organisms and their interactions. Predators, competitors and food organisms are examples.

Abiotic factors include temperature, light, water availability and chemical conditions. Soil contains both non-living material and living organisms.

A pond fish needs suitable oxygen, temperature and food. Changing any limiting condition can affect its survival.

The biosphere includes Earth’s living organisms and the regions where life occurs. Life interacts with land, water and air.

• Lithosphere: Earth’s rigid outer layer, including the crust and uppermost mantle.
• Hydrosphere: Earth’s water in liquid, solid and gaseous forms.
• Atmosphere: the surrounding envelope of gases.

Organisms occur in oceans, soils and some underground environments. Life is unevenly distributed rather than equally abundant everywhere.

Dry air near Earth’s surface contains approximately 78% nitrogen and 21% oxygen. Venus and Mars have carbon-dioxide-dominated atmospheres.

Oxygen-producing photosynthesis helped transform Earth’s atmosphere. That statement concerns oxygen; photosynthesis does not directly explain the entire nitrogen inventory.

Producers make organic substances from inorganic sources. Green plants, algae and cyanobacteria commonly use sunlight through photosynthesis.

Some producers instead obtain energy from chemical reactions. Sunlight therefore powers most familiar food webs, rather than every ecosystem.

Consumers obtain organic food from other organisms. Herbivores eat plant material, while carnivores eat animals.

Decomposers such as many fungi and bacteria chemically break down dead organic material. Fungal enzymes act outside the organism before nutrients are absorbed.

Detritivores ingest dead organic material and fragments. Earthworms help fragment material that microbes then break down further.

Decomposers release nutrients into forms producers can use. Recycling matter does not mean recycling all the original usable energy.

A food chain shows one route by which organic matter and energy pass through feeding. Arrows usually point from food to the organism eating it.

Grass → goat → human is a possible feeding chain. Grass produces organic matter; the goat and human obtain food from the preceding organism.

Do not infer a chain merely because three organisms share an area. Each arrow must represent a possible feeding relationship.

A consumer’s trophic level depends on the food eaten. A primary consumer feeds on producers; a secondary consumer feeds on primary consumers.

A tertiary consumer feeds on secondary consumers. Carnivores are not all automatically secondary consumers.

Humans, bears and crows can be omnivores. An omnivore can feed at more than one trophic level.

A food web combines several connected food chains. Most organisms interact with more than one food source or consumer.

A grasshopper may be eaten by a frog or a bird. A bird may also eat seeds or other insects.

Changing one population can affect several linked populations. Alternative feeding routes can sometimes buffer the effect.

The size and direction of a disturbance depend on the particular web. Removing one species does not always cause collapse.

Misconception: a web is just a longer chain. A web represents branching and overlapping relationships.

A grazing food chain begins with living producers. Herbivores eat those producers, and predators eat consumers.

A detritus food chain begins with dead organic material and waste. Detritivores and decomposers obtain resources from that material.

A fallen leaf may be fragmented by an earthworm and decomposed by microbes. Detritivore consumers can themselves be eaten by predators.

The grazing and detritus pathways are connected. Organisms from several trophic levels eventually contribute dead material or waste.

A detritus pathway transfers energy as well as recycling nutrients. Decomposition is not an energy-free recycling process.

Organisms use energy in respiration and maintenance. Some material is not eaten, and some consumed material is not assimilated.

Only part of one trophic level’s production becomes production at the next. Much usable energy ultimately disperses as heat.

The 10% rule is a convenient school approximation. Under that model, about 90% is not transferred into the next level’s production.

Actual transfer efficiency varies among organisms and ecosystems. The entire remaining fraction is not necessarily lost immediately as heat.

Suppose producers supply 10,000 kJ of production during a stated interval. Applying 10% transfer gives 1,000 kJ, then 100 kJ, then 10 kJ.

Use matching areas and time intervals for each comparison. This example illustrates an assumption, rather than measured universal efficiency.

Limited transfer helps restrict food-chain length. Food chains usually have three to five levels, depending on the source. That is a common pattern, rather than an absolute limit.

Energy enters ecosystems and changes form as organisms use it. Energy remains conserved, but increasingly disperses as less usable heat.

A continuing external energy supply supports ecosystem activity. Decomposers cannot turn all dispersed heat back into food energy.

Carbon, nitrogen and other elements move between living organisms and non-living reservoirs. Their atoms can be used repeatedly.

An ecosystem can still gain or lose nutrients through water, air or harvesting. Cyclic nutrient processes do not make every local ecosystem closed.

Energy flows; matter cycles is the useful distinction. Avoid saying energy disappears, because physical energy conservation still applies.

An ecological pyramid compares trophic levels using a specified quantity. Numbers, standing biomass and production are different quantities.

A numbers pyramid counts organisms. One tree can support many insects, so the producer bar need not be widest.

A biomass pyramid measures living mass present at a particular time. Aquatic producer biomass can be smaller than consumer biomass.

Rapid phytoplankton growth and turnover can support consumers despite a small standing stock. A snapshot of biomass does not measure production over time.

An energy pyramid compares production per area and time. It is upright when the trophic levels use consistent boundaries and units.

Misconception: an inverted biomass pyramid means energy increases up the chain. Standing biomass and energy transfer answer different questions.

Biodiversity includes genetic diversity, species diversity and ecosystem diversity. Species richness counts species, rather than measuring every aspect of biodiversity.

Different species can respond differently to drought, pests or other disturbances. Their responses can help maintain community production when one species declines.

Greater plant diversity has increased temporal stability of production in grassland experiments. Resistance to disturbance and recovery afterward are different measures.

More species do not guarantee that every individual population becomes more stable. The result depends on the function, disturbance and community being measured.

Bioaccumulation is chemical buildup within an organism through exposure over time. Uptake can exceed elimination.

Biomagnification is increasing chemical concentration across feeding levels. Certain persistent contaminants can become more concentrated in predators than their prey.

Persistence alone does not guarantee biomagnification. Uptake, storage, metabolism and elimination also matter.

Some persistent organic pollutants illustrate the risk. Food choice and exposure determine a person’s chemical burden.

Humans are not invariably the top consumer in every food web. Human contamination is not automatically the highest among all species.

Misconception: fewer organisms at the top means less contaminant in each individual. Concentration and total population quantity are different measurements.

Biodegradation uses biological activity to break down materials. Vegetable scraps and suitable paper can provide substrates for decomposition.

Biodegradable does not mean instant breakdown. Temperature, moisture and oxygen affect the process.

Composting manages decomposition to produce a useful soil amendment. Suitable moisture and aeration support the microbes involved.

Vermicomposting uses suitable worms together with microbial activity. Earthworms fragment and process organic material.

An earthworm’s muscular gizzard helps grind ingested material. Worm species and conditions affect whether vermicomposting works well.

Glass is not normally decomposed by compost microbes. Keep unsuitable material out of a compost system.

An evolutionary adaptation is an inherited feature associated with survival or reproduction in particular conditions. Adaptations develop through population changes across generations.

Acclimatisation is an individual’s adjustment to changed conditions. It can occur without changing the population’s inherited traits.

With time at high altitude, the body can make more red blood cells. This individual adjustment differs from inherited high-altitude adaptations.

Penguins have feathers and insulating fat rather than fur.

Emperor penguins also huddle to reduce heat loss. Do not describe every penguin as uniformly white or give all penguin species the same cold-weather behaviour.

A biome is a broad ecological grouping associated with climate and characteristic vegetation. Classifications can differ, so check how many biomes a source lists.

Terrestrial examples include forests, grasslands, deserts and tundra. Forests themselves include tropical, temperate and boreal types.

Temperature and rainfall strongly influence vegetation. Soil, seasonal patterns and disturbance also affect the communities present.

Savannas combine a grass-dominated layer with scattered trees.

Chaparral is the Mediterranean-type biome, with dry summers and wetter winters.

Regional names do not identify uniform vegetation. A country can contain several biomes, so exam options need their exact geographic scope.

Freshwater ecosystems include rivers, streams, lakes and ponds. Marine ecosystems include oceans and coral reefs.

Salinity is a useful distinction, but water movement and light also matter. Streams differ from still lakes in flow and oxygen conditions.

Light availability limits photosynthesis at depth. Organisms in deeper habitats depend on other energy pathways or food arriving from elsewhere.

Estuaries mix river water with seawater. Their salinity can vary with tides and freshwater input.

Aquatic is a broad category, rather than one uniform habitat. A shallow pond and the deep ocean support different communities.

Photosynthesis incorporates carbon dioxide into organic substances. Respiration and decomposition return carbon to inorganic forms.

Combustion releases carbon from fuels and other organic material. Oceans also exchange carbon dioxide with the atmosphere.

Oxygen-producing photosynthesis releases oxygen derived from water. Respiration, combustion and oxidation consume molecular oxygen.

Oxygen atoms also occur in water, carbon dioxide, minerals and biological molecules. The oxygen cycle therefore includes more than atmospheric oxygen gas.

A reservoir stores an element; a flux moves it between reservoirs. Reservoir sizes and transfer rates are different measurements.

Gaseous cycles have substantial atmospheric or oceanic reservoirs. Sedimentary cycles involve rocks and sediments as major reservoirs.

Phosphorus weathering releases phosphate from rocks into soil and water.

Most plants cannot use atmospheric nitrogen gas directly. Nitrogen fixation converts N₂ into usable nitrogen compounds.

Certain bacteria and archaea perform biological fixation. Rhizobia fix nitrogen in legume root nodules.

Some cyanobacteria also fix nitrogen. Do not assume every cyanobacterium has that ability.

Lightning and industrial processes provide additional routes to fixed nitrogen. Fixation is different from plants absorbing nitrate or ammonium.

Ammonification releases ammonium during breakdown of organic nitrogen. Nitrification oxidises ammonium through nitrite toward nitrate.

Plants assimilate available nitrogen into organic substances. Animals obtain nitrogen by feeding.

Denitrification converts nitrate toward gaseous nitrogen under suitable conditions. The gas returns to the atmosphere.

Nitrogen makes up about 78% of dry air. Its abundance does not remove the need for fixation and recycling.

Ecosystems are dynamic. Population sizes respond to resources, weather, predators, competitors and disturbance.

Predators can influence herbivore populations. Their effects depend on prey availability and other interactions.

A stable function does not require every population to remain constant. Stability can mean resistance, recovery or low variation over time.

Succession is change in community composition through time. Primary succession begins where developed soil is absent.

Secondary succession follows disturbance where soil remains.

Species alter light, soil and resource availability as communities develop. Those changes can affect which species establish later.

Succession is not a guarantee of one fixed final community. Climate, repeated disturbance and human activity can redirect the sequence.

Energy flow is directional, while nutrients participate in cycles. Limited transfer of usable energy constrains food-chain length.

A genuine chain needs an actual feeding relationship at each arrow. An ecosystem includes its organisms and their physical surroundings.

Match regional vegetation questions to the stated location and classification.

These six questions come from earlier papers on this topic:

• CDS-II 2023, Q47
• CDS-II 2021, Q13
• NDA-NA-I 2018, Q91
• UPSC CSP 2015, Q84
• NDA-NA-II 2016, Q59
• CAPF (ACs) 2019, Q95