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IB ESS SL practice questions

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Question 1Foundations of environmental systems and societies

In an environmental systems and societies context, which statement best describes a system?

  1. A model that predicts the future with complete certainty
  2. A collection of interacting components that form a unified whole with emergent properties Correct answer.
  3. A single isolated variable measured repeatedly over time
  4. A list of unrelated environmental factors recorded in the field

Explanation

A system is defined by its interacting components and the boundary separating it from its surroundings, so the whole behaves differently from the parts alone. This emergent behaviour is why systems thinking is central to ESS.

Question 2Foundations of environmental systems and societies

Which pair correctly identifies a store and a flow in the hydrological cycle?

  1. Precipitation as the store and the ocean as a flow
  2. The ocean as a store and precipitation as a flow Correct answer.
  3. Groundwater as the flow and evaporation as the store
  4. Transpiration as the store and groundwater as a flow

Explanation

Stores are where matter is held for a period, such as the ocean, while flows are transfers between stores, such as precipitation. Recognising which is which is essential for drawing and interpreting system diagrams.

Question 3Foundations of environmental systems and societies

Two populations of the same species live in separate valleys with no migration between them. Which term describes this situation?

  1. Speciation
  2. Symbiosis
  3. Isolation Correct answer.
  4. Succession

Explanation

Isolation occurs when populations are prevented from interbreeding by a barrier, here the valley topography. Over long periods, isolation can allow genetic divergence and eventually speciation.

Question 4Foundations of environmental systems and societies

Which description best matches a positive feedback loop in an ecosystem?

  1. A change is amplified, pushing the system further from its original state Correct answer.
  2. A change is counteracted, returning the system towards equilibrium
  3. Energy is transferred between trophic levels without loss
  4. A population remains constant because births equal deaths

Explanation

Positive feedback amplifies a change, so the system moves further from its starting condition rather than stabilising. This contrasts with negative feedback, which dampens change and promotes equilibrium.

Question 5Foundations of environmental systems and societies

A food chain contains grass, grasshoppers, frogs and snakes. Which organism is the secondary consumer?

  1. Grass
  2. Snake
  3. Grasshopper
  4. Frog Correct answer.

Explanation

The grass is the producer, the grasshopper is the primary consumer, and the frog that eats the grasshopper is the secondary consumer. Consumer level is defined by feeding position, not by body size.

Question 6Foundations of environmental systems and societies

Which of the following is a correct statement about energy flow through ecosystems?

  1. Energy is recycled within the ecosystem by decomposers
  2. Energy is conserved in biomass with no losses
  3. Energy passes to the next level with total efficiency
  4. Energy is lost as heat at each trophic level Correct answer.

Explanation

At each transfer, a large proportion of energy is lost as heat through respiration, so less remains available to the next trophic level. Matter cycles through ecosystems, but energy flows one way and is ultimately dissipated.

Question 7Foundations of environmental systems and societies

Which process removes carbon dioxide from the atmosphere in the carbon cycle?

  1. Respiration by soil organisms
  2. Decomposition of leaf litter
  3. Photosynthesis by producers Correct answer.
  4. Combustion of fossil fuels

Explanation

Photosynthesis fixes atmospheric carbon dioxide into organic compounds in producers, acting as a major carbon sink. Respiration, combustion and decomposition all release carbon back to the atmosphere.

Question 8Foundations of environmental systems and societies

A lake receives excess nitrate from farmland runoff and develops an algal bloom. Which sequence best describes the resulting process?

  1. Immediate fish death before any algal growth occurs
  2. Oxygen increase, then algal decline, then fish recovery
  3. Nitrate removal by algae, then reduced growth of all organisms
  4. Algal growth, then oxygen depletion as algae decompose, harming fish Correct answer.

Explanation

Nutrient enrichment stimulates rapid algal growth; when the algae die, decomposers consume oxygen, causing hypoxia that harms aerobic organisms such as fish. This sequence is the classic mechanism of eutrophication.

Question 9Foundations of environmental systems and societies

Which statement correctly distinguishes a biome from an ecosystem?

  1. A biome contains no abiotic factors, while an ecosystem contains only abiotic factors
  2. A biome is a single habitat, while an ecosystem covers a whole continent
  3. A biome and an ecosystem are identical terms used interchangeably
  4. A biome is a large-scale community type defined by climate, while an ecosystem is a functional unit of organisms and their environment Correct answer.

Explanation

Biomes are broad regional units characterised by climate and dominant vegetation, such as tropical rainforest or tundra. An ecosystem is any functional unit in which organisms interact with their physical environment, at a range of scales.

Question 10Foundations of environmental systems and societies

In an experiment, a population of 500 individuals produces 60 births and 25 deaths over one year, with no migration. What is the approximate population change?

  1. No change, because births and deaths balance
  2. A decrease of 35 individuals
  3. An increase of 35 individuals Correct answer.
  4. An increase of 85 individuals

Explanation

Population change equals births minus deaths when migration is absent, so 60 minus 25 gives an increase of 35 individuals. Growth rate is this change divided by the starting population.

Question 11Foundations of environmental systems and societies

Which factor is a density-independent limit on a population?

  1. Competition for nesting sites
  2. Spread of an infectious disease
  3. A severe frost that kills seedlings Correct answer.
  4. Accumulation of toxic waste in a pond

Explanation

Density-independent factors affect a population regardless of its size, and an extreme weather event such as frost acts this way. Competition, disease and toxic waste intensify as density rises and are density-dependent.

Question 12Foundations of environmental systems and societies

Which statement best explains why pyramids of energy are always upright?

  1. Energy is lost at each transfer, so each level holds less than the one below Correct answer.
  2. Predators consume more biomass than producers can create
  3. Higher trophic levels contain larger organisms that store more energy
  4. Decomposers return all energy to the base of the pyramid

Explanation

Because respiration dissipates energy as heat at every transfer, the energy available at each successive trophic level is smaller than that below it. This produces an upright pyramid regardless of organism size.

Question 13Foundations of environmental systems and societies

Which human activity most directly increases the atmospheric store of carbon?

  1. Irrigating crops with river water
  2. Planting forests on degraded land
  3. Burning coal for electricity generation Correct answer.
  4. Building dams for hydroelectric power

Explanation

Combustion of fossil fuels transfers carbon stored in geological reserves into the atmosphere as carbon dioxide. Afforestation, irrigation and hydropower do not release fossil carbon in the same way.

Question 14Foundations of environmental systems and societies

A species of frog is found only on one mountain range and nowhere else on Earth. Which term applies?

  1. Keystone
  2. Invasive
  3. Endemic Correct answer.
  4. Pioneer

Explanation

An endemic species is native to and restricted to a particular geographic area, making it especially vulnerable to habitat change. Invasive, keystone and pioneer describe other ecological roles or statuses.

Question 15Foundations of environmental systems and societies

In primary succession on bare rock, which organisms typically arrive first?

  1. Lichens and mosses Correct answer.
  2. Predatory birds
  3. Large grazing mammals
  4. Climax forest trees

Explanation

Lichens and mosses are pioneer species that tolerate harsh conditions and begin soil formation on bare rock. Later species depend on the soil they create, so the sequence proceeds towards a climax community.

Question 16Foundations of environmental systems and societies

Two species of warbler feed in different parts of the same tree, reducing competition. Which concept does this illustrate?

  1. Primary succession
  2. Resource partitioning Correct answer.
  3. Mutualism
  4. Competitive exclusion

Explanation

Resource partitioning occurs when species divide a shared resource, such as feeding zones, allowing coexistence. Competitive exclusion would instead see one species eliminate the other.

Question 17Foundations of environmental systems and societies

A keystone predator is removed from a rocky shore community. Which outcome is most consistent with the keystone concept?

  1. The community becomes more diverse as predation pressure falls
  2. All other species increase equally because predation stops
  3. The community changes little because the predator was a minor component
  4. A competitor of the predator expands and reduces overall species diversity Correct answer.

Explanation

A keystone species exerts an influence disproportionate to its abundance, often by controlling a dominant competitor. Removing it can allow that competitor to expand and reduce diversity.

Question 18Foundations of environmental systems and societies

A system receives a pollutant at a constant rate and shows a delayed, then abrupt, change in state. Which system characteristic best explains this behaviour?

  1. A threshold and reduced resilience following cumulative pressure Correct answer.
  2. Immediate negative feedback restoring equilibrium
  3. Complete isolation from external inputs
  4. A linear response with no resilience

Explanation

Systems under cumulative pressure can cross a threshold where resilience is exceeded, producing sudden shifts after a lag. The delay occurs because the system absorbs change until the tipping point is passed.

Question 19Foundations of environmental systems and societies

Which comparison of matter and energy in ecosystems is correct?

  1. Both matter and energy are recycled indefinitely within the ecosystem
  2. Matter is recycled through biogeochemical cycles, while energy flows through and is lost as heat Correct answer.
  3. Both matter and energy leave the ecosystem permanently after one transfer
  4. Energy is recycled through food chains, while matter is lost as heat

Explanation

Matter is conserved and cycled by decomposers and other processes, whereas energy enters mainly as sunlight and is progressively dissipated as heat. This distinction underpins why ecosystems need continuous energy input.

Question 20Foundations of environmental systems and societies

In the carbon cycle, which transfer moves carbon from the atmosphere into the biomass of a forest, and by what process does it occur?

  1. Carbon moves from the atmosphere into forest biomass through respiration by decomposers.
  2. Carbon moves from the atmosphere into forest biomass through combustion of dead wood.
  3. Carbon moves from the atmosphere into forest biomass through photosynthesis by producers. Correct answer.
  4. Carbon moves from the atmosphere into forest biomass through nitrogen fixation in roots.

Explanation

Photosynthesis is the transfer that takes carbon dioxide from the atmospheric store and converts it into organic compounds stored in producer biomass, so it is the entry point of carbon into a forest food web. Respiration and combustion both release carbon back to the atmosphere rather than removing it, and nitrogen fixation does not involve carbon transfer.

Question 21Ecosystems and ecology

In an ecosystem, which term describes the total mass of living material in a given area at a given time?

  1. Biomass Correct answer.
  2. Productivity
  3. Trophic level
  4. Standing crop of energy

Explanation

Biomass is the total mass of living organisms per unit area, usually expressed as dry mass per square metre. It is a quantity of material, not a rate of production or a position in a food chain.

Question 22Ecosystems and ecology

Which process converts atmospheric nitrogen into a form usable by plants?

  1. Nitrification
  2. Nitrogen fixation Correct answer.
  3. Denitrification
  4. Ammonification

Explanation

Nitrogen fixation converts inert atmospheric N2 into ammonia or ammonium, making nitrogen available to producers. It is carried out by bacteria such as Rhizobium in root nodules and by free-living soil bacteria.

Question 23Ecosystems and ecology

Which description best matches a population rather than a community?

  1. All the individuals of one species in a defined area Correct answer.
  2. All organisms and their physical environment in a pond
  3. The role a species plays in its habitat
  4. All the different species living in a woodland

Explanation

A population is a group of individuals of the same species living in the same area at the same time. A community includes all species present, and an ecosystem adds the abiotic environment.

Question 24Ecosystems and ecology

Which type of organism occupies the first trophic level in a food chain?

  1. Producer Correct answer.
  2. Decomposer
  3. Secondary consumer
  4. Primary consumer

Explanation

Producers, mainly photosynthetic plants and algae, occupy the first trophic level because they convert solar energy into chemical energy. Consumers occupy higher levels and decomposers act across all levels.

Question 25Ecosystems and ecology

Which factor is an abiotic component of an ecosystem?

  1. Competition for light
  2. Soil pH Correct answer.
  3. Predation rate
  4. Parasitism

Explanation

Abiotic factors are non-living physical and chemical features of the environment, such as soil pH, temperature, salinity and light intensity. The other options involve interactions between living organisms.

Question 26Ecosystems and ecology

What does the term 'ecological niche' refer to?

  1. The functional role of a species and its use of resources Correct answer.
  2. The physical location where an organism lives
  3. The maximum population a habitat can support
  4. The total number of species in a habitat

Explanation

A niche is the functional role of a species in a community, including its habitat use, food sources, activity times and tolerance ranges. The physical location alone is the habitat, not the niche.

Question 27Ecosystems and ecology

Which pair of organisms shows a mutualistic relationship?

  1. A tapeworm absorbing nutrients in a host gut
  2. A tick feeding on a dog
  3. A lion preying on a zebra
  4. Mycorrhizal fungi supplying phosphorus to a plant while receiving sugars Correct answer.

Explanation

Mutualism benefits both partners: the fungus delivers mineral nutrients such as phosphorus while the plant supplies carbohydrates from photosynthesis. The other options describe parasitism or predation, where one partner is harmed.

Question 28Ecosystems and ecology

Which term describes the variety of species present in an ecosystem?

  1. Species richness Correct answer.
  2. Genetic diversity
  3. Ecological succession
  4. Habitat diversity

Explanation

Species richness is the number of different species present in a defined area. Genetic diversity refers to variation within a species, and habitat diversity to the range of habitats available.

Question 29Ecosystems and ecology

Energy is lost at each trophic level. Which statement best explains the typical 10% transfer between levels?

  1. Most energy is lost as heat through respiration and in uneaten or undigested material Correct answer.
  2. Most energy is converted to biomass in the next level
  3. Energy is destroyed when organisms are eaten
  4. Energy transfer is limited only by the number of predators

Explanation

Only a small fraction of ingested energy becomes new consumer biomass. The rest is lost as heat from respiration, in excretion, or in parts that are not eaten, so roughly 10% passes to the next level.

Question 30Ecosystems and ecology

A food chain has a producer level of 40 000 kJ m-2 yr-1. Using the standard 10% transfer efficiency, what energy reaches the secondary consumer?

  1. 40 kJ m-2 yr-1
  2. 40 000 kJ m-2 yr-1
  3. 4 000 kJ m-2 yr-1
  4. 400 kJ m-2 yr-1 Correct answer.

Explanation

Energy passes through two transfers to reach the secondary consumer: 40 000 x 0.1 gives 4 000 at the primary consumer, then 4 000 x 0.1 gives 400 kJ m-2 yr-1. Each step loses about 90% of the energy.

Question 31Ecosystems and ecology

Which change would most likely increase the carrying capacity of a grassland for a grazing herbivore?

  1. Planting additional palatable grass species Correct answer.
  2. A season of unusually low rainfall
  3. Introducing a new predator to the area
  4. A disease outbreak reducing the herbivore population

Explanation

Carrying capacity is set by the resources available in the environment. Adding palatable grass increases food supply, raising the maximum population the habitat can sustain. The other options reduce resources or increase mortality.

Question 32Ecosystems and ecology

Two bird species feed on insects in the same tree but one hunts on upper branches and the other on lower branches. What does this illustrate?

  1. Resource partitioning Correct answer.
  2. Primary succession
  3. Competitive exclusion
  4. A symbiotic relationship

Explanation

Resource partitioning occurs when species divide a shared resource, here by feeding at different heights, which reduces direct competition and allows coexistence. Competitive exclusion would mean one species is eliminated.

Question 33Ecosystems and ecology

In a predator-prey cycle, why does the predator population usually peak after the prey population?

  1. Prey populations are controlled only by abiotic factors
  2. Predators convert prey directly into prey biomass
  3. Predators always reproduce faster than prey
  4. Predator numbers rise only after increased prey provides more food, then fall as prey is depleted Correct answer.

Explanation

Predator growth lags behind prey abundance because predators need time to find food and reproduce. Once predators reduce the prey, food shortage then drives the predator population down, producing the lagged cycle.

Question 34Ecosystems and ecology

Which of these is a density-independent factor affecting a population?

  1. Food shortage
  2. A severe frost Correct answer.
  3. Spread of an infectious disease
  4. Accumulation of toxic waste

Explanation

Density-independent factors act regardless of population size, and a severe frost kills individuals whether the population is dense or sparse. The other factors intensify as density rises, so they are density-dependent.

Question 35Ecosystems and ecology

Which sequence correctly describes primary succession on bare rock?

  1. Grasses, lichens, shrubs, pioneer forest
  2. Shrubs, climax forest, mosses, lichens
  3. Climax forest, shrubs, grasses, pioneer lichens
  4. Pioneer lichens, mosses and grasses, shrubs, climax forest Correct answer.

Explanation

Primary succession begins on substrate with no soil, where pioneer species such as lichens and mosses weather rock and build soil. Later, grasses, shrubs and eventually a climax community establish as soil depth and nutrients increase.

Question 36Ecosystems and ecology

Why is gross primary productivity (GPP) always greater than net primary productivity (NPP)?

  1. GPP is measured only at night
  2. NPP includes energy lost in decomposer respiration
  3. NPP equals GPP minus the energy used by producers in respiration Correct answer.
  4. GPP excludes energy captured by photosynthesis

Explanation

NPP is the energy remaining after producers use some of their fixed energy for their own respiration, so NPP equals GPP minus producer respiration. GPP is the total energy fixed by photosynthesis.

Question 37Ecosystems and ecology

A pyramid of biomass may be inverted in a marine ecosystem. Which explanation is most valid?

  1. Marine consumers have no respiratory losses
  2. Energy transfer in water exceeds 100%
  3. Phytoplankton reproduce very rapidly and are consumed quickly, so a small standing biomass supports a larger consumer biomass Correct answer.
  4. Biomass pyramids can never be inverted

Explanation

Phytoplankton have a very high turnover rate, so their standing biomass at any moment is small even though their production over time is large. That production can support a larger standing biomass of zooplankton, producing an inverted pyramid.

Question 38Ecosystems and ecology

Which statement best explains why decomposers are essential to nutrient cycling in an ecosystem?

  1. They fix atmospheric nitrogen into nitrate
  2. They convert organic matter into inorganic nutrients that producers can absorb Correct answer.
  3. They transfer energy directly to secondary consumers
  4. They increase the total energy entering the ecosystem

Explanation

Decomposers break down dead organic matter and release inorganic nutrients such as nitrates and phosphates back into the soil. These nutrients are then available for uptake by producers, closing the nutrient cycle.

Question 39Ecosystems and ecology

Which feature of an ecosystem most directly increases its resilience to disturbance?

  1. High species diversity and functional redundancy Correct answer.
  2. A single dominant producer species
  3. A short food chain with few links
  4. Low genetic variation within populations

Explanation

Diverse communities with functional redundancy can maintain key processes when some species decline, because other species can fill similar roles. This buffering raises resilience to disturbance.

Question 40Ecosystems and ecology

Which method gives the most direct estimate of the population size of a mobile animal species in a defined area?

  1. Measuring net primary productivity
  2. Recording soil nutrient levels
  3. Counting the number of species present
  4. Mark-recapture sampling Correct answer.

Explanation

Mark-recapture estimates abundance by marking a sample, releasing it, then using the proportion of marked individuals in a later sample to estimate total population size. It suits mobile animals that cannot be counted directly.

Question 41Ecosystems and ecology

In a mark-recapture study, 80 animals are marked and released. A later sample of 100 contains 16 marked individuals. What is the estimated population size?

  1. 800
  2. 1 600
  3. 500 Correct answer.
  4. 160

Explanation

Using the Lincoln index, population size equals (marked first x total second) divided by marked recaptured: (80 x 100) / 16 = 500. The proportion of marked animals in the second sample estimates the proportion in the whole population.

Question 42Ecosystems and ecology

Two species of Paramecium grown together in a laboratory culture compete for the same food. One eventually eliminates the other. Which concept does this demonstrate?

  1. Density-independent regulation
  2. Resource partitioning
  3. Mutualism
  4. Competitive exclusion principle Correct answer.

Explanation

The competitive exclusion principle states that two species competing for the same limiting resource cannot coexist indefinitely, and the better competitor excludes the other. Here, identical food requirements lead to elimination of one species.

Question 43Ecosystems and ecology

A lake receives heavy nitrate runoff from farmland. Which sequence of events is most likely?

  1. Nitrate removal, reduced decomposition, oxygen increase
  2. Algal bloom, increased decomposition, oxygen depletion, death of fish Correct answer.
  3. Decreased algal growth, increased oxygen, recovery of fish
  4. Oxygen depletion, algal bloom, increased fish growth, clear water

Explanation

Excess nitrate causes eutrophication: rapid algal growth forms a bloom, which then dies and is decomposed by bacteria that consume dissolved oxygen. The resulting hypoxia kills fish and other aerobic organisms.

Question 44Ecosystems and ecology

Why does only a small fraction of the energy in a primary consumer's food become secondary consumer biomass?

  1. Most energy is converted into new biomass at each step
  2. Energy is lost as heat in respiration and in excretion and uneaten parts, so little remains for growth Correct answer.
  3. Energy is conserved perfectly between trophic levels
  4. Secondary consumers photosynthesise to supplement their energy

Explanation

According to the second law of thermodynamics, energy transfers are inefficient: much is dissipated as heat during respiration, and further energy is lost in excretion and in material that is not consumed. Only a small proportion becomes new consumer biomass.

Question 45Ecosystems and ecology

A climax community is described as stable, yet succession can still occur after a disturbance. Which interpretation is most consistent with ecological theory?

  1. Succession always proceeds in one direction and cannot be reversed
  2. Climax communities never change once established
  3. A climax community is a dynamic steady state that may be reset to earlier stages by disturbances such as fire or storms Correct answer.
  4. Climax communities have no nutrient cycling

Explanation

The climax community is best viewed as a dynamic equilibrium in which species composition fluctuates around a stable state. Disturbances can reset succession to an earlier seral stage, after which succession proceeds again.

Question 46Ecosystems and ecology

In an ecosystem, which term describes the total mass of living material in a given trophic level at a particular moment?

  1. Standing crop Correct answer.
  2. Ecological pyramid
  3. Net primary productivity
  4. Gross primary productivity

Explanation

Standing crop is the biomass present in a trophic level at one point in time, expressed as mass per unit area. It is a snapshot measure, so it differs from productivity, which is a rate of energy or biomass generated over time.

Question 47Ecosystems and ecology

Which process returns carbon locked in dead organic matter to the atmosphere as carbon dioxide?

  1. Transpiration by leaves
  2. Nitrogen fixation by bacteria
  3. Photosynthesis by producers
  4. Respiration by decomposers Correct answer.

Explanation

Decomposers such as bacteria and fungi break down dead organic matter and release carbon dioxide through their own respiration. This closes the carbon cycle by making carbon available to producers again.

Question 48Ecosystems and ecology

In the carbon cycle, which store holds the largest quantity of carbon on Earth?

  1. Surface ocean water
  2. The atmosphere
  3. Living biomass
  4. Sedimentary rocks and fossil fuels Correct answer.

Explanation

The lithosphere, especially carbonate rocks and fossil fuel deposits, contains by far the greatest carbon store. Fluxes between the atmosphere, oceans and biomass are small compared with this geological reservoir.

Question 49Ecosystems and ecology

A farmer adds a legume crop to a rotation. Which biogeochemical change most directly explains the benefit to the following cereal crop?

  1. Nitrogen-fixing bacteria in root nodules raise soil nitrate availability Correct answer.
  2. The legume's deep roots draw potassium to the surface
  3. Decomposers release phosphorus from the legume roots
  4. Legumes increase soil oxygen and speed up nitrification

Explanation

Legumes form symbiotic associations with nitrogen-fixing bacteria, converting atmospheric nitrogen into ammonia that is later nitrified to nitrate. The next crop therefore has more available nitrogen.

Question 50Ecosystems and ecology

Two species of warbler feed on insects in the same conifer forest, but one forages high in the canopy and the other low. This separation best illustrates which concept?

  1. Resource partitioning Correct answer.
  2. Trophic cascade
  3. Competitive exclusion
  4. Primary succession

Explanation

Resource partitioning occurs when species divide a shared resource, here vertical feeding space, reducing direct competition. It allows coexistence where complete overlap might otherwise lead to exclusion.