Populations in ecosystems (A-level only)
AQA
A-Level
A population is all the organisms of one species living in the same place at the same time that can interbreed. A community is all the populations of all the different species living in that same place at the same time, and the point of the word is that those populations interact: they compete for the same resources, eat one another, pollinate one another, parasitise one another. Community is a purely biotic term, so soil, temperature and water are not part of it. Communities are described by sampling them. Species richness counts how many different species are present; an index of diversity goes further and uses the number of individuals of each species as well, so it distinguishes a community with one dominant species from one where individuals are spread evenly.
46
Objectives
40
Exam Tips
67
Pitfalls
80
Key Terms
71
Mark Points
Subtopics in this area
Populations of different species form a community.
A community and the non-living components of its environment together form an ecosystem.
Ecosystems can range in size from the very small to the very large.
Within a habitat, a species occupies a niche governed by adaptation to both abiotic and biotic conditions.
An ecosystem supports a certain size of population of a species, called the carrying capacity. This population size can vary as a result of: the effect of abiotic factors interactions between organisms: interspecific and intraspecific competition and predation.
The size of a population can be estimated using: randomly placed quadrats, or quadrats along a belt transect, for slow-moving or non-motile organisms the mark-release-recapture method for motile organisms. The assumptions made when using the mark-release-recapture method.
Ecosystems are dynamic systems.
Primary succession, from colonisation by pioneer species to climax community.
At each stage in succession, certain species may be recognised which change the environment so that it becomes more suitable for other species with different adaptations.
The new species may change the environment in such a way that it becomes less suitable for the previous species.
Changes that organisms produce in their abiotic environment can result in a less hostile environment and change biodiversity.
Conservation of habitats frequently involves management of succession.
Students should be able to: show understanding of the need to manage the conflict between human needs and conservation in order to maintain the sustainability of natural resources evaluate evidence and data concerning issues relating to the conservation of species and habitats and consider conflicting evidence use given data to calculate the size of a population estimated using the mark-release-recapture method.
Required practical 12: Investigation into the effect of a named environmental factor on the distribution of a given species.
Populations in ecosystems (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Distinguish a population from a community by stating what each includes, using the same named habitat for both.
- Name two interactions between populations in a named community and state which population is affected in each.
- Explain why a conclusion drawn from one sampled community cannot be applied to all communities of that type.
- State the two components of an ecosystem and give a named example of each for one named ecosystem.
- Classify factors such as light intensity, predation, soil pH and interspecific competition as abiotic or biotic.
- Explain how energy flow and nutrient cycling link the living and non-living parts of one named ecosystem.
- Give one small-scale and one large-scale example of an ecosystem and identify the community and abiotic components of each.
- Explain why the boundary of an ecosystem is chosen by the investigator rather than fixed in nature.
- Suggest how the size of an ecosystem affects the sampling method and the number of samples needed.
- Define niche and contrast it with habitat using one named species.
- Describe the niche of a named species by listing at least two abiotic and two biotic conditions it is adapted to.
- Explain, in terms of competition, why two species cannot occupy the same niche in the same habitat.
- Define carrying capacity and explain why a population fluctuates around it rather than settling on it.
- Explain how one named abiotic factor changes the carrying capacity of a named ecosystem, linking the factor to survival and reproduction.
- Distinguish intraspecific from interspecific competition in a given example and predict the effect of each on population size.
- Interpret a predator-prey graph, describing the lag between the prey peak and the predator peak.
- Describe how to place quadrats randomly in a uniform habitat and how to scale a mean quadrat count up to a population estimate.
- Calculate a population size from mark-release-recapture data, writing the formula before substituting values.
- Explain how breaking each of two named mark-release-recapture assumptions would make the estimate too high or too low.
- Justify the use of a belt transect rather than random quadrats where an environmental gradient is present.
- Explain what is meant by a dynamic equilibrium in an ecosystem, using a named fluctuating population.
- Describe one short-term and one long-term process that changes the composition of an ecosystem.
- Predict the effects on at least two other populations of removing one named species from a given food web.
- Describe the sequence of primary succession from bare rock to a climax community, naming the organisms at three stages.
- Explain how pioneer species make the abiotic environment less hostile, referring to soil formation.
- Distinguish primary from secondary succession given an unfamiliar description of a site.
- Explain how one named species at one seral stage changes a named abiotic factor.
- Predict the adaptations of the next colonising species, given the abiotic change described.
- Explain why species diversity usually increases during succession, in terms of the number of niches available.
- Explain how a later seral stage makes a habitat unsuitable for an earlier species, naming the resource competed for.
- Describe what happens to a pioneer population once shrubs and trees establish, and state where that species persists.
- Use data on the species present at different seral stages to identify which species have been excluded.
- Explain how colonising organisms change two named abiotic factors during succession.
- Interpret species richness and index of diversity data across seral stages and describe the trend.
- Suggest why biodiversity may fall between a mid-successional stage and the climax community.
- Explain why a named early-successional habitat disappears if it is not managed.
- Describe two management methods used to arrest succession and state what each removes.
- Give two distinct reasons for conserving a named habitat, going beyond protecting wildlife.
- Evaluate a claim about conservation from given data by writing at least one point for and one against, each supported by a figure.
- Identify three specific limitations of an investigation, such as sample size, absence of a control or absence of a statistical test.
- Calculate a population size from mark-release-recapture data and state one assumption that may make the estimate unreliable.
- Explain how one named sustainable practice reduces the conflict between a human need and conservation.
- Design an investigation into the effect of one named environmental factor on the distribution of a named species, stating the sampling method and the measure of abundance.
- Justify the use of a belt transect rather than random quadrats where an environmental gradient is present.
- State three conclusions about distribution from a given map or figure without going beyond the data.
- Explain why a correlation between an abiotic factor and abundance does not prove a causal relationship.
Marking Points
Key points examiners look for in your answers
- one mark for defining a population as all the individuals of one species living in the same area at the same time
- one mark for defining a community as all the populations of different species living in the same area at the same time
- one mark for stating that the populations in a community interact, with a named interaction such as competition or predation
- one mark for stating that data collected at a single site describe only that community and may not represent other communities
- one mark for describing a community using species richness and/or an index of diversity
- one mark for an ecosystem being a community, or all the populations, together with the non-living or abiotic components of the environment
- one mark for naming a correct abiotic factor such as light intensity, temperature, pH, water availability or mineral ion concentration
- one mark for stating that the biotic and abiotic parts interact
- one mark for energy flowing through the ecosystem while nutrients are cycled between organisms and the abiotic environment
- one mark for stating that abiotic conditions determine which species can survive in that ecosystem
- one mark for stating that an ecosystem may be any size provided it contains a community and the abiotic environment it interacts with
- one mark for a correct small-scale example such as a rock pool, rotting log, leaf litter or puddle
- one mark for a correct large-scale example such as a lake, woodland or ocean
- one mark for stating that smaller ecosystems are found within larger ones
- one mark for stating that ecosystems are not closed, because organisms, energy and nutrients cross the boundary
- one mark for a niche being the role or position of a species within its habitat, not simply where it lives
- one mark for naming an abiotic condition such as temperature, pH, light intensity, salinity or water availability
- one mark for naming a biotic condition such as food source, predators, competitors or pollinators
- one mark for stating that two species cannot occupy the same niche in the same habitat
- one mark for explaining exclusion, where one species outcompetes the other so that its population falls
- one mark for carrying capacity as the maximum population size that an ecosystem can support over a prolonged period
- one mark for naming an abiotic factor and linking it to survival or reproduction, for example temperature closer to the optimum increasing enzyme activity and growth
- one mark for intraspecific competition as competition between members of the same species for the same resource, becoming more intense as the population rises
- one mark for interspecific competition reducing the population size of one or both species, or excluding one species
- one mark for the predator-prey sequence in the correct order, with the predator peak lagging behind the prey peak
- one mark for random placement of quadrats using coordinates generated from random numbers, to avoid bias
- one mark for a belt transect with quadrats at regular intervals where an environmental gradient is present
- one mark for scaling up correctly, using the mean number per quadrat, the quadrat area and the total area of the habitat
- one mark for the correct mark-release-recapture expression, first sample multiplied by second sample divided by the number marked in the second sample
- one mark for any two valid assumptions, such as marked individuals mixing randomly, marking not affecting survival or behaviour, marks not being lost, and no births, deaths or migration between samples
- one mark for dynamic meaning constantly changing, with populations fluctuating rather than remaining constant
- one mark for energy flowing through the ecosystem and being lost as heat, so a continuous input of light energy is needed
- one mark for nutrients being cycled between organisms and the abiotic environment
- one mark for a named mechanism of change, such as seasonal variation in an abiotic factor, predator-prey cycling or succession
- one mark for negative feedback holding a population close to its carrying capacity
- one mark for primary succession beginning on bare rock or newly formed land where there is no soil
- one mark for naming a pioneer species, such as lichen on rock or marram grass on sand, adapted to harsh abiotic conditions
- one mark for pioneers eroding the rock and their dead remains being decomposed to form humus and therefore soil
- one mark for the soil retaining water and mineral ions so that larger plants can establish
- one mark for the climax community as the final stable community, determined largely by the climate
- one mark for organisms changing the abiotic environment, with the change named, such as soil formation, greater water retention, more mineral ions, reduced wind exposure or shade
- one mark for that change making conditions less hostile so species with different adaptations can now survive
- one mark for naming an adaptation of a species at a later seral stage, such as deeper roots or greater height
- one mark for a greater number of niches leading to increased species diversity
- one mark for increased biomass, soil depth or stability as succession proceeds
- one mark for the new species changing the abiotic environment in a way that disadvantages the earlier species, for example by reducing light intensity through shading
- one mark for identifying the competition as interspecific and naming the resource competed for
- one mark for the earlier species being outcompeted, so its population falls and it is excluded from that habitat
- one mark for stating that the earlier species survives elsewhere rather than becoming extinct
- one mark for linking this loss to the change in species composition seen at the climax community
- one mark for naming an abiotic factor that becomes less hostile, such as soil water content, mineral ion concentration, temperature range, wind exposure or pH
- one mark for the mechanism, such as dead organisms being decomposed by saprobionts to form humus that retains water and releases mineral ions
- one mark for stating that more species are adapted to, or can tolerate, the new conditions
- one mark for increased biodiversity expressed as species richness and/or an index of diversity; note that species richness alone counts species present but omits evenness, so an index of diversity gives a fuller comparison
- one mark for recognising that diversity can fall again where a closed climax canopy shades out earlier species
- one mark for stating that without management succession would continue towards the climax community
- one mark for naming a management method such as grazing, mowing, burning, coppicing or scrub clearance
- one mark for explaining that the method removes larger competing plants so that light still reaches ground level and early-successional species are not outcompeted
- one mark for conserving species and biodiversity, or conserving habitats and niches, as a reason for the management
- one mark for a second distinct reason, such as a source of medicines or timber, reduced erosion, reduced climate change, or tourism
- State a point supporting the claim, quoting specific data from the provided source.
- State a point against the claim, quoting specific data, to provide a balanced evaluation.
- Identify a limitation of the evidence, such as the absence of a control, limited sample size, or lack of a statistical test.
- Name a sustainable practice, such as selective felling, coppicing, fishing quotas, or ecotourism.
- Identify the specific human need conflicting with conservation, such as agriculture, timber, or employment.
- Calculate population size using the mark-release-recapture formula and state an assumption, such as no migration or no loss of marks.
- Name the environmental factor being investigated and state suitable apparatus to measure it accurately.
- Specify a belt transect with quadrats placed at regular intervals when sampling along an environmental gradient.
- Specify random sampling using randomly generated coordinates when comparing areas without an environmental gradient.
- State a valid measure of abundance, such as percentage cover or frequency, ensuring repeats are taken to calculate a mean.
- State clearly that a correlation between the environmental factor and species distribution does not establish causation.
Examiner Tips
Expert advice for maximising your marks
- 💡Learn population, community, habitat, niche and ecosystem as one set and check which one the stem is actually asking for.
- 💡In a data question, name the community exactly as the stem names it, for example the fish community at that river site.
- 💡If asked to criticise a conclusion about a community, look first for one site, one date or one season.
- 💡Learn the exact terminology for fill-the-gap questions, as precise wording is required.
- 💡Name a specific abiotic factor such as light intensity rather than a vague one such as weather.
- 💡If a question asks how a factor affects an ecosystem, follow it through to a named population.
- 💡State the scale you are working at, then name the community and one abiotic factor at that scale.
- 💡In sampling questions, link the size of the area to the number of samples needed.
- 💡When asked to suggest why an estimate is unreliable, area sampled and number of quadrats are usually available marks.
- 💡If you write altitude, depth or distance up a shore, immediately add the abiotic factor that changes along it.
- 💡Use adapted to explicitly, because the specification wording ties the niche to adaptation.
- 💡In distribution questions, match each species to the conditions given in the stem before writing the explanation.
- 💡Describe a fluctuating graph in the order prey then predator, quoting figures and units from the axes.
- 💡Answer explain questions about abiotic factors as a chain: factor, effect on the organism, effect on survival or reproduction, effect on population size.
- 💡Always label competition as intraspecific or interspecific; the label itself often carries the mark.
- 💡Write the mark-release-recapture formula before substituting; a correct expression can be credited even if the arithmetic then fails, and round to a whole organism.
- 💡If asked why the estimate is too high, look for marks being lost or marked animals dying, since both reduce the recapture count.
- 💡Say what you would record in each quadrat, such as percentage cover, rather than just saying use a quadrat.
- 💡Use fluctuates around the carrying capacity rather than stays the same when describing a stable population.
- 💡When a question introduces a change, such as adding a predator or warming a river, trace it through at least two further populations.
- 💡Quote figures from the graph when describing the size of a fluctuation.
- 💡Answer succession questions as a repeating chain: species present, change to the abiotic environment, new species now able to survive.
- 💡Use the terms pioneer species, seral stage and climax community by name.
- 💡Check whether soil is already present before deciding between primary and secondary succession.
- 💡Name the abiotic factor that changed and then name the adaptation the next species has; both are separate marks.
- 💡In explain answers, keep to a three-step chain: organism, abiotic change, new species able to survive.
- 💡If the question gives a table of seral stages, quote the stage names from it.
- 💡Write outcompeted for, then name the resource; the resource earns the mark, not the word competition.
- 💡Explain both directions of the same change: it favours one species and disadvantages another.
- 💡Use data on species present at each stage to say which species have disappeared.
- 💡Quote the index of diversity when data are supplied, and say what a higher value means about the community.
- 💡Give the abiotic factor and the direction of change, for example soil water content increases.
- 💡Link every abiotic change to the species that can now survive.
- 💡A suggest two reasons question needs two genuinely different reasons; check the wording of the question and the number of marks available before deciding how many distinct points to give.
- 💡Always link the management method back to halting or deflecting succession.
- 💡Where local people are mentioned in the stem, land use and livelihoods are usually creditable points.
- 💡Set out evaluate answers under 'For' and 'Against' headings to ensure both sides are covered before adding data limitations.
- 💡Write out the mark-release-recapture formula before calculating, and always round the final answer to a whole number of organisms.
- 💡When asked for conclusions about distribution, give one specific statement per species and tie each to a feature shown on the provided map or figure.
- 💡Name the specific statistical test used (e.g., Spearman's rank correlation) and state exactly what it tests for, rather than just saying 'do a stats test'.
Common Mistakes
Pitfalls to avoid in your exam answers
- including soil, temperature or water in the definition of a community, which describes an ecosystem instead
- using the word population for a mixed group containing several species
- concluding that results from one sampled site apply to every community of that type, when the mark scheme credits the opposite point
- describing a community by species richness alone when the index of diversity also needs the number of individuals of each species
- treating habitat and community as interchangeable words
- naming predators, competitors or pathogens as abiotic factors
- writing the vague phrase the environment instead of the non-living components of the environment
- stating that nutrients flow through and energy is recycled, which reverses both
- listing soil or water as members of the community
- using habitat and ecosystem as if they were the same term
- assuming an ecosystem has to be large, and so refusing to accept a rock pool as one
- calling a small ecosystem a habitat when the question specifically asked for an ecosystem
- treating the boundary of an ecosystem as fixed in nature rather than chosen by the investigator
- naming a small ecosystem without identifying both its community and its abiotic components
- forgetting that a larger area needs more samples before an estimate is valid
- defining a niche as where an organism lives, which is the habitat
- giving different altitudes or depths as the abiotic factor, which is insufficient without naming temperature, light intensity or oxygen concentration
- claiming two species sharing a food source must share a niche, when the niches differ in feeding height, timing or prey size
- classifying predators or competitors as abiotic conditions
- assuming the outcompeted species must become extinct rather than being excluded from that habitat
- writing competition for food without saying whether the competition is intraspecific or interspecific
- stating that predators kill all the prey, rather than reducing prey numbers until predators starve
- treating carrying capacity as a fixed property of the species rather than of that ecosystem
- saying the population stops growing because it has reached the carrying capacity, with no mechanism given
- drawing or describing predator and prey peaks as coinciding
- throwing a quadrat over the shoulder and calling the placement random
- using random quadrats where there is a clear gradient, or a transect where there is none, so the pattern is missed or invented
- forgetting to divide by the quadrat area when scaling up, so the estimate is wrong by a whole factor
- inverting the mark-release-recapture formula and dividing by the first sample instead of the number recaptured carrying marks
- giving only the assumption that population size does not change, and missing that marks must persist and must not affect predation
- reading dynamic as unstable, and so claiming that populations in a stable ecosystem do not change at all
- stating that energy is recycled, when energy is transferred and lost as heat while nutrients are recycled
- describing a fall in one population without following the effect through to the species that depend on it
- treating succession as the only way an ecosystem can change
- describing an equilibrium as a fixed population size rather than a mean about which numbers fluctuate
- calling colonisation of a cleared field or a burnt wood primary succession, when soil remains and it is secondary succession
- saying pioneers make soil without mentioning erosion of the rock and decomposition of dead organisms
- assuming every climax community is woodland, when climate can give grassland, tundra or dune heath
- listing seral stages in order with no abiotic change to explain why the sequence happens
- describing pioneers as the first plants to evolve rather than the first to colonise
- writing that species evolve into the next stage, when the new species colonise from elsewhere
- crediting climate rather than the organisms for the change between one seral stage and the next
- confusing more suitable for other species with more suitable for the species already there
- describing the change without naming the abiotic factor that changed
- assuming every earlier species survives alongside the later ones
- writing that pioneer species die out or become extinct, rather than being outcompeted and excluded from that area
- saying the new species kills the earlier species, when the mechanism is competition for a resource
- omitting the named resource, so the competition mark is not earned
- describing intraspecific competition when the two species are different
- assuming the climax community must still contain all the earlier species
- writing that the environment improves without naming a single abiotic factor
- treating biodiversity as species richness only, ignoring the number of individuals of each species; correction: species richness omits evenness, so use an index of diversity for fuller comparisons
- claiming diversity always reaches its maximum at the climax community
- describing decomposition without naming saprobionts or humus
- confusing a less hostile abiotic environment with reduced competition, when competition usually intensifies
- stating that conservation means leaving a habitat completely undisturbed, which would destroy an early-successional habitat
- naming a management method without saying which stage of succession it prevents
- writing to save the animals instead of naming conserved species, habitats or niches
- confusing conservation, which is active management to maintain a habitat, with preservation, which is leaving it untouched
- giving the same reason twice in different words when the question asks for two reasons
- Answering an evaluate question with supporting points only; correction: always provide evidence both for and against the claim.
- Stating that results are significant or not significant without context; correction: specify what differs, the direction of the difference, and reference a statistical test if provided.
- Treating standard deviation as a statistical test; correction: standard deviation shows the spread of data around the mean, whereas a statistical test determines the probability that a difference is due to chance.
- Describing conservation as stopping all human use; correction: define conservation as managing human use sustainably.
- Concluding that the measured factor causes the distribution; correction: state that other abiotic or biotic factors may change along the same transect and influence distribution.
- Using random quadrats where there is an obvious environmental gradient; correction: use systematic sampling (a belt transect) to observe changes along a gradient.
- Taking a single reading of the environmental factor; correction: take multiple repeat readings at each sampling site to calculate a reliable mean.