2.2 Energy and biomass in ecosystems

Energy must be continually supplied, while matter enters, leaves and can be reused through cycles.

Sunlight or chemical energy enters and heat leaves; water, gases, nutrients and organisms cross the boundary. Matter cycling does not make an ecosystem closed.

A pond receives sunlight and dissolved nutrients, returns heat, and exchanges water and organisms with its surroundings.

Energy, because usable energy disperses as heat; matter can be recycled.

Energy is not recycled in the same way as nutrients.

In an ecosystem, energy is transferred and transformed—not created or destroyed—and each transfer leaves less usable energy available to the next trophic level.

The first law accounts for all energy, while the second-law idea explains usefulness: respiration and other processes disperse part of the input as heat. So energy still exists after a transfer, but the ecosystem must keep receiving new energy to maintain biological work.

A plant stores 100 kJ of light energy as biomass. A herbivore eats it; perhaps 20 kJ becomes new herbivore biomass while the rest is used in respiration or leaves as heat. Nothing vanished—the usable store became smaller.

It was transferred to respiration, movement, waste and heat; ‘lost’ means unavailable to the next trophic level, not destroyed.

Do not say energy cycles like nutrients. Matter can be reused in biogeochemical cycles; usable energy flows through and must be continually supplied.

A transfer changes location; a transformation changes form or chemical identity.

Ask whether the substance or energy form changed. Photosynthesis transforms; moving food from plant to herbivore is a transfer.

CO₂ entering a leaf is a matter transfer; photosynthesis transforms CO₂ and water into glucose and oxygen.

Transfer: location changes, but the glucose remains the same compound.

Movement alone does not make a process a transformation.

Photosynthesis uses light energy to transform carbon dioxide and water into glucose, oxygen and stored chemical energy.

CO₂ and water supply the atoms; light supplies energy. Glucose can become starch, cellulose, lipids and other biomass.

A leaf uses light to build glucose from CO₂ and water; that glucose later becomes cellulose in a growing stem.

No. Matter comes from CO₂ and water; light provides the energy to rearrange it.

Light is an energy input, not the atoms in plant biomass.

Producers make organic carbon compounds from inorganic inputs, introducing biomass and stored chemical energy into the food system.

Plants, algae and photosynthetic bacteria form trophic level 1; consumers depend directly or indirectly on their biomass.

An alga fixes CO₂ into biomass; a zooplankton eating it is a primary consumer at level 2.

No. They transform external energy and inorganic matter into organic compounds.

‘Producer’ means producer of biomass, not creator of energy.

Aerobic respiration transforms glucose and oxygen into carbon dioxide, water and usable chemical energy for cell processes.

Cells use this energy for movement, growth and repair. Respiration is a chemical process in all living organisms, including plants.

A plant respires at night and in daylight, using glucose made or stored by photosynthesis.

No. Breathing exchanges gases; respiration is the cellular chemical transformation.

Plants respire in light as well as darkness.

Respiration is not 100% efficient: some chemical energy becomes dispersed heat rather than usable chemical energy or biomass.

Heat still exists, so energy is not destroyed, but once dispersed it is unavailable to the organism for growth or transfer as biomass.

A cell uses glucose for movement; part becomes usable chemical energy and part leaves as heat to the surroundings.

Respiration spends part of it on cell work and disperses part as heat.

Heat is transformed and dispersed; it is not energy destroyed.

Every energy transformation disperses some concentrated, usable energy as heat, so ecosystem transfers are never 100% efficient.

The first law tracks total energy conserved; the second law explains why its ability to do biological work declines. They describe different aspects, not contradictions.

A plant’s stored chemical energy becomes herbivore biomass, movement and heat; only the biomass portion reaches the next level.

No. It says useful energy becomes more dispersed and less available for work.

Do not confuse conserved total energy with conserved usable energy.

Consumers obtain organic carbon from other organisms or remains; classify them by how they obtain it.

Herbivores eat producers, predators capture prey, parasites feed on hosts, scavengers eat carcasses, detritivores ingest fragments and saprotrophs digest externally.

A woodlouse ingests dead leaves as a detritivore; a fungus releases enzymes and absorbs dissolved products as a saprotroph.

Scavenger; classify the feeding route, not the animal’s appearance.

Detritivores ingest fragments; saprotrophs digest outside the body.

A food chain starts with producers and follows organic matter and energy from food source to consumer.

The arrow means ‘is eaten by’ or ‘matter flows to’, not ‘attacks’. Primary consumers eat producers; later consumers follow.

Oak → caterpillar → tit → hawk: each arrow points to the organism receiving the food.

Caterpillar → tit, because organic matter moves from food to eater.

Do not point arrows from predator toward prey.

Trophic levels track stages of consumer transfer; decomposers receive dead matter and waste from many levels, so they do not fit one fixed level.

Producers are level 1 and primary consumers level 2; decomposers transform material from several levels and return inorganic nutrients.

A fungus can decompose dead leaves from producers and carcasses from consumers, linking several trophic pathways.

Usually no; show their links to dead material from multiple levels.

Omitting decomposers from a simple chain does not mean they are absent.

Energy and organic matter narrow between trophic levels because food may be unharvested, uneaten, unabsorbed or respired rather than stored as new biomass.

Uneaten and egested material can enter detrital pathways, but it does not become the consumer’s biomass. Absorbed energy spent in respiration leaves mainly as heat.

A herbivore ignores some leaves, eats some, excretes some and respires some; only the remainder becomes growth.

It was eaten but not absorbed, so it can support detrital consumers rather than new herbivore biomass.

Faeces are not respiratory heat; they are unabsorbed material.

Net productivity is gross productivity minus respiratory loss: NP = GP − R.

The remainder is new biomass available for growth, reproduction and potential harvest. Productivity is a rate, so keep area and time units.

GP 1,000 kJ m⁻² yr⁻¹ minus respiration 300 gives NP 700 kJ m⁻² yr⁻¹.

No; report area and time, such as 700 kJ m⁻² yr⁻¹.

Standing biomass is a store, not productivity, which is a rate.

Because each transfer leaves less net biomass, eventually too little usable energy remains to support another trophic population.

Respiration, uneaten food and unabsorbed material reduce transfer. ‘About 10%’ is an order-of-magnitude guide, not a fixed law.

If 1,000 units reach producers and only a fraction becomes herbivore biomass, several further transfers leave little support for a top predator.

No; prey abundance and production constrain the whole chain.

Ten percent is not a universal fixed efficiency.

A food web joins overlapping chains; arrows show matter and energy from food to consumer, and omnivores can occupy different levels in different paths.

When one prey changes, follow each link. Generalists may switch food; specialists may fall sharply, and indirect effects can travel to the prey’s food.

If one insect declines, a specialist bird loses food while a generalist shifts to berries; reduced grazing may let plants increase.

No. Its level depends on which food path is being traced.

A web cannot be read as one single chain.

Biomass at a trophic level can be estimated by collecting representative samples and drying them to constant mass. Removing water makes dry mass a practical approximation of organic matter because water is the main inorganic component of most living samples.

Record sampled area or volume, fresh mass and repeated dry masses until further heating causes no change. Scale the mean dry mass to the habitat only when sampling is representative. Energy content can be estimated by combusting a known dry mass and measuring heat transferred to a known mass of water.

If 2.0 g of dry plant material heats 100 g of water by 8°C, the measured heat gain is mass × specific heat capacity × temperature change; divide by 2.0 g to report energy per gram. Heat lost to apparatus and incomplete combustion make the experimental value an underestimate.

Wet mass is not directly comparable biomass because water content varies. Use dry mass per unit area for standing biomass and energy per unit dry mass for combustion results, with units and uncertainty stated.

Do not treat biomass as energy; a gram of tissue does not reveal how quickly it was produced or consumed.

The shape of a pyramid only makes sense after identifying what is measured and the units used.

Numbers may describe organisms, biomass, energy or productivity. Compare like with like, then explain a narrowing or widening level using transfer and loss.

If levels contain 1,000 kJ m⁻² y⁻¹, 100 and 10, the pattern shows roughly tenfold energy loss at each transfer—not tenfold fewer animals.

Only that measured biomass changes; you cannot call it an energy pyramid without energy units and time.

Never infer trophic efficiency from a picture alone; the variable and units control the interpretation.

Bioaccumulation increases a substance within one organism; biomagnification increases its concentration at higher trophic levels.

A persistent, fat-soluble pollutant can enter faster than an organism excretes it. Predators then consume many contaminated prey, concentrating the pollutant through the food chain.

Small fish each retain a little mercury; a tuna eating many fish can end with a higher tissue concentration than any single prey fish.

Bioaccumulation: the change is within one organism over time, not across trophic levels.

Biomagnification is not simply pollution becoming stronger in water; it is a trophic-level concentration pattern.

Microplastics are small plastic particles that can absorb or adsorb persistent pollutants such as PCBs from surrounding water. When organisms ingest contaminated particles, the plastic and associated chemicals can enter trophic pathways.

Plankton or filter feeders may ingest particles, predators eat many contaminated prey, and persistent chemicals that are absorbed and poorly excreted can bioaccumulate and biomagnify. Transfer depends on particle size, polymer, pollutant binding, gut residence and whether the chemical desorbs into tissue.

Contaminated microplastics ingested by zooplankton can be transferred to small fish and then larger predators. Finding particles in predators demonstrates transfer, but showing toxic impact requires tissue concentration, dose and biological-effect evidence rather than particle presence alone.

Microplastics do not automatically magnify every attached chemical. Distinguish physical particle transfer from chemical uptake, compare exposure routes such as food and water, and avoid claiming causation from detection alone.

A high predator concentration does not prove predators created the pollutant; it reflects accumulation and food-web transfer.

Human actions change where energy is used and where matter is stored, moved or lost from ecosystems.

Harvest removes biomass, dams redirect water and burning transfers stored chemical energy to heat and gases. Track the pathway rather than labelling an action simply good or bad.

Fertiliser can raise crop production, but runoff moves nitrogen to a lake and can increase algal growth and oxygen demand.

Both: nitrogen moved into the lake, while extra algal growth changed energy flow and respiration.

Matter is not destroyed by harvesting; it is relocated, transformed or exported from the system.

Burning fossil fuels raises carbon dioxide but warming and co-pollutants can reduce primary productivity; deforestation removes producer biomass and transpiration; urbanization replaces productive habitat and fragments food webs; agriculture redirects production into harvest and can export nutrients through runoff. For each activity, trace the changed storage or flow, its effect on photosynthetic capacity or trophic links, and any displaced impact rather than assuming one universal outcome.

Autotrophs build organic matter from inorganic carbon; heterotrophs obtain organic matter by consuming or absorbing it.

Light or chemical reactions supply energy for autotrophic carbon fixation. Heterotrophs transfer that stored chemical energy through feeding, detritus or parasitism.

A green plant is an autotroph; a fungus digesting leaf litter is a heterotroph, even though neither needs to hunt an animal.

Autotroph: its carbon comes from inorganic CO₂; the energy source can be chemical rather than light.

Autotroph does not mean ‘uses sunlight’; it refers to carbon source, not energy source.

Both groups fix inorganic carbon, but photoautotrophs use light energy whereas chemoautotrophs obtain energy by oxidising chemicals.

The carbon source determines ‘auto’; the energy source determines ‘photo’ or ‘chemo’. This distinction predicts where an organism can live.

Algae at the surface use light; sulfur-oxidising bacteria near a dark vent can fix CO₂ using chemical energy.

Chemoautotroph: chemical energy plus inorganic carbon.

Dark does not automatically mean heterotrophic; inspect the carbon and energy sources separately.

Primary productivity is the rate at which producers fix energy into new organic matter.

It is a rate, so area and time matter. Light, nutrients, temperature and water can limit fixation; high standing biomass does not necessarily mean high current productivity.

A mature forest may hold large biomass but add little each year, while a young fertilised crop can have faster annual production.

Carbon fixed per area per time directly measures the rate.

Productivity is not the same as biomass; one is a rate, the other is an amount present.

A field estimate can harvest replicate plots of known area at two times, dry samples to constant mass and calculate change in biomass per area per time; controls or corrections are needed for grazing, death and movement. A laboratory estimate can expose matched photosynthesizing samples to defined light or nutrient conditions and measure biomass, oxygen or carbon-dioxide change. Report productivity in units such as kg C m⁻² yr⁻¹ and distinguish the rate from standing biomass.

Consumers gain secondary production from ingested organic matter. Gross secondary productivity, or assimilated food, equals ingestion minus faecal waste; net secondary productivity is the biomass retained for growth and reproduction after respiratory losses.

Use GSP = I − F and NSP = GSP − R, keeping area and time units when the values are rates. Faeces remain undigested and unabsorbed, whereas respiration uses assimilated material and releases energy mainly as heat.

A caterpillar ingests 100 kJ, loses 60 kJ in faeces and respires 30 kJ. GSP = 100 − 60 = 40 kJ assimilated; NSP = 40 − 30 = 10 kJ retained as new consumer biomass.

Do not call all ingested food productivity. State whether the question asks for assimilation/gross secondary productivity or net new biomass, show each subtraction, and include the given units.

Food ingested is not automatically growth; assimilation and retention must be separated from waste and respiration.

Net primary productivity is the energy stored by producers after their own respiration: NPP = GPP − R.

Gross primary productivity counts all fixation. Producers use some fixed energy to maintain cells; the remainder becomes biomass available to consumers and decomposers.

If GPP is 500 and plant respiration is 180 g C m⁻² y⁻¹, NPP is 320 g C m⁻² y⁻¹.

It falls to 260; more fixed carbon is spent by producers before it can enter the food web.

NPP is not GPP plus respiration; respiration is the deduction from gross fixation.

Maximum sustainable yield is the largest harvest rate that can continue without reducing the population’s long-term ability to replace itself.

Recruitment and growth vary with population size and environment. A target is sustainable only when harvest stays within replenishment and uncertainty is allowed for.

A fishery adds 800 tonnes in a year but removes 900; the 100-tonne deficit means that quota is not sustainable.

No. The safe harvest rate must fall with replenishment, or the stock declines.

MSY is not the biggest catch ever recorded; it is a continuing rate under stated conditions.

In ESS, the maximum sustainable yield of a producer or consumer system is bounded by its net primary or net secondary productivity—the biomass added after the organism's respiratory costs. Harvest above that replenishment reduces the productive store. Because productivity varies with stock size and environment, a precautionary quota should remain below an uncertain estimate rather than treating MSY as a permanent number.

Eating lower on a food chain usually yields more food energy because fewer transfers lose energy as heat and waste.

A crop eaten directly skips livestock conversion losses. The benefit depends on land, feed, species and production method, so it is a tendency rather than a universal number.

If plants provide 10,000 kJ and each transfer retains 10%, direct eating accesses about 10,000 kJ, while one livestock step leaves about 1,000 kJ.

It removes a trophic transfer, so more producer energy becomes available per unit input.

‘Lower trophic’ does not mean every plant food has identical impact; compare the actual system and resources.

Ecological efficiency is the fraction of energy at one trophic level transferred into production at the next.

Use efficiency = output production ÷ input production × 100. The remainder covers respiration, waste, uneaten material and other losses.

A herbivore level produces 80 kJ from 1,000 kJ of plant production: efficiency = 8%.

15% (12 ÷ 80 × 100); use the immediately preceding level as input.

Do not divide by the original producer input when calculating a single transfer; that gives a multi-step yield instead.

Each energy transfer disperses some usable energy as heat, increasing entropy and leaving less available for work.

Energy is conserved, but its concentration and usefulness change. Respiration, friction and decomposition spread energy into the surroundings.

A 100 kJ food input may yield 20 kJ of new biomass and roughly 80 kJ dissipated through metabolism and waste.

No. It remains energy, but is more dispersed and less available to drive another trophic transfer.

The second law does not contradict conservation of energy; it limits how much remains concentrated and useful.