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C4.1 Populations and communities

Populations and communities describe abundance, sampling, growth limits, species interactions and trophic controls that shape ecological patterns across habitats and ecosystems.

Syllabus
First assessment 2025
Topic
C4.1
Level
SL

Learning objectives

C4.1.1Populations• A population is interacting organisms of the same species in one area• Members have opportunities to interbreed and may be reproductively isolated from othersC4.1.2Population size estimation• Population size is estimated when full counts are impractical• Random sampling reduces bias; stratified and systematic sampling fit different habitatsC4.1.3Random quadrat sampling• Quadrats estimate density, frequency, cover, or abundance of sessile organisms• Random coordinates and known quadrat area support representative population estimatesC4.1.4Capture-mark-release-recapture• Capture-mark-release-recapture estimates motile animal populations• Lincoln index assumes marks persist, mixing occurs, and marking does not affect survivalC4.1.5Carrying capacity• Carrying capacity is the maximum population an environment can sustain• Limited food, space, mates, and other resources create competition near capacityC4.1.6Negative feedback control• Density-dependent factors regulate populations by negative feedback• Competition, predation, waste, and disease intensify as population density risesC4.1.7Population growth curves• Exponential growth occurs when resources are abundant and limiting factors are weak• Sigmoid growth slows as resources limit growth near carrying capacityC4.1.8Modelling sigmoid growth• Sigmoid curves can be modelled with yeast, duckweed, or other fast-growing organisms• Lag, exponential, transition, and plateau phases show changing growth rateC4.1.9Competition vs. cooperation• Intraspecific competition occurs for food, mates, space, or light• Cooperation such as social hunting or parental care can increase survival and reproductionC4.1.10Community• A community is all interacting populations in an ecosystem• Species interactions make populations interdependent within the abiotic environmentC4.1.11Interspecific relationships• Interspecific relationships occur between different species in communities• Categories include herbivory, predation, competition, mutualism, parasitism, and pathogenicityC4.1.12Mutualism as interspecific relationship• Mutualism benefits both species through exchanged resources or protection• Examples include legumes and Rhizobium, orchids and mycorrhizae, corals and zooxanthellaeC4.1.13Endemic vs. invasive species• Endemic species are native to a restricted geographic area• Invasive species can escape controls and compete with endemic species for niche resourcesC4.1.14Tests for interspecific competition• Competition is tested using lab experiments, field observations, or removal studies• Connell's barnacle study links removal experiments to fundamental and realized nichesC4.1.15Chi-squared test• Chi-squared tests association between two species from quadrat presence/absence data• Compare observed and expected counts, degrees of freedom, and critical valuesC4.1.16Predator-prey relationships• Predator-prey relationships regulate populations through density-dependent feedback• Snowshoe hare and lynx cycles show time-lagged predator and prey changesC4.1.17Top-down vs. bottom-up control• Top-down control begins with predators and cascades to lower trophic levels• Bottom-up control begins with producers or resources and affects higher trophic levelsC4.1.18Allelopathy and antibiotics• Allelopathy releases chemicals that inhibit competitor germination or growth• Antibiotics are microbial allelochemicals, such as penicillin from Penicillium

A Population Is One Interacting Species in a Defined Area

A population is all interacting individuals of the same species living in a defined area at the same time. In a sexually reproducing species, members have opportunities to interbreed.

Two groups can belong to the same species but form different populations when geography, behaviour or timing prevents regular gene flow between them.

Two groups of the same rabbit species interbreed within each group but are separated from one another by a geographic barrier.

Let Habitat Structure Choose the Sampling Design

A sample must represent the study area. Random placement reduces selection bias, but the correct design also depends on spatial structure.

Site or question Sampling design
one relatively uniform habitat random points or coordinates
two or more habitat types stratified: sample each in proportion to its area
change along an environmental gradient systematic points at fixed intervals on a transect

More repeated samples reduce random sampling error. They do not repair a systematic bias such as choosing only species-rich patches.

Random Quadrats Turn a Fixed Area into Representative Samples

  1. Mark the site's boundaries and coordinate axes.
  2. Generate random coordinate pairs.
  3. Place the same-sized quadrat with its centre at each coordinate.
  4. Identify the target species and record the chosen response.
  5. Repeat enough times to estimate a mean and variation.
Record Best when…
density individuals can be distinguished and counted
frequency presence or absence across quadrats matters
percentage cover individuals overlap or cannot be separated easily
abundance score a defined ordinal scale is appropriate
A coordinate grid marks random quadrat centres across a habitat and shows one quadrat placed at a selected coordinate.

Scale Mean Quadrat Density to the Whole Habitat

N^=nˉ×Aa\hat N=\bar n\times\frac{A}{a}

The estimated population size equals the mean number of individuals per quadrat multiplied by total habitat area ÷ quadrat area.

Evidence Value
mean count per 0.5 m² quadrat 6.4 plants
habitat area 250 m²
estimate 6.4 × (250 ÷ 0.5) = 3,200 plants

This is an estimate, not a full count. Report sampling method, number of quadrats and variation so the uncertainty and representativeness can be judged.

Marked Animals Reveal the Fraction Sampled Again

After marked animals mix back into the population, the marked fraction in the second sample should approximate the marked fraction in the whole population.

RN≈MN^N^≈M×NR\frac{R}{N}\approx\frac{M}{\hat N}\qquad\hat N\approx\frac{M\times N}{R}

If M = 50, N = 40 and R = 10, then the estimated population size = (50 × 40) ÷ 10 = 200 animals. Fewer marked recaptures imply a larger estimated population.

Capture-mark-release-recapture sequence showing the first marked sample M, second capture N, marked recaptures R and the Lincoln estimate.

Each Lincoln Assumption Protects the Ratio

Assumption If violated… Likely effect
marked animals mix completely second sample contains too many nearby marked animals R too high → underestimate
marks persist and are recognized marked animals appear unmarked R too low → overestimate
marking does not alter survival or capture marked animals are lost or avoided R too low → overestimate
population is closed births, deaths or movement change the fractions estimate no longer describes one stable population
equal catchability some individuals dominate both samples biased in either direction

Use a harmless, inconspicuous, durable mark; allow time for mixing; recapture in the same defined area; and keep the interval short enough that demographic change is limited.

Carrying Capacity Depends on Current Environmental Limits

Carrying capacity, K, is the largest population that the current environment can sustain over time without exhausting the resources and conditions on which that population depends.

  • Plants may be limited by light, water, mineral nutrients, carbon dioxide, temperature or space.
  • Animals may be limited by food, water, mates, territory, nesting sites, disease or predation.

K is not permanently fixed. Seasonal change, disturbance, habitat management or evolution can alter the limiting environment and therefore alter carrying capacity.

A sigmoid population curve rises and then fluctuates around carrying capacity as competition, food and disease limit growth.

Density-Dependent Feedback Opposes Departure from Capacity

population density rises → competition, pathogen transmission, waste effects or predation intensify → birth rate falls and/or death rate rises → net growth falls → density moves back toward K

Factor Density-dependent? Regulatory pattern
competition, disease, predation yes effect becomes stronger as density rises
drought, frost, fire or storm usually no can change population size regardless of density

‘Negative’ describes opposition to change, not harm. The result is often fluctuation around K rather than a perfectly constant population.

A rabbit and lynx cycle represents higher prey density supporting more predators, which then lowers prey density before predator numbers decline.

Curve Shape Records How Net Population Change Is Changing

ΔN=(B+I)−(D+E)\Delta N=(B+I)-(D+E)

Population increases when births B plus immigration I exceed deaths D plus emigration E. The graph's slope shows the net rate of change.

Pattern What happens to slope? Biological conditions
exponential growth becomes progressively steeper abundant resources; weak limiting factors
sigmoid growth steepens, then becomes less steep near K density-dependent resistance increasingly offsets growth

A rising curve can still be slowing: population size increases while its slope decreases during the transition toward carrying capacity.

Each Sigmoid Phase Has a Different Growth Balance

Phase Population pattern Mechanism
lag or establishment small rise few reproductive individuals; acclimation
exponential increasingly steep rise resources abundant; births greatly exceed deaths
transition rise continues but slows competition, disease or predation strengthen
plateau fluctuates around K average gains and losses are approximately balanced

The smooth sigmoid is an idealized model. Real populations show irregular departures caused by weather, disturbance, migration and changing resource supply.

A Growth Model Needs Replication, Repeated Counts and One Changed Factor

  • Start equal cultures with a small known number of duckweed thalli or a measured yeast inoculum.
  • Use at least five replicates per condition.
  • Hold temperature, light, volume and nutrients constant unless one is the independent variable.
  • Count or measure population size at regular intervals.
  • Plot the mean against time and show variation.
Observation Interpretation
progressively steeper early rise net growth rate is increasing
curve bends toward a plateau limiting factors are strengthening
different plateau under changed nutrients the treatment changed carrying capacity
large scatter among replicates estimate is less precise; investigate uncontrolled variation

Counting duckweed area or yeast absorbance may be a proxy rather than a direct count. State what was actually measured and do not treat a smooth fitted curve as exact reality.

Estimate the Population, Then Explain Its Change

define the population → choose a representative design → sample sessile or motile organisms appropriately → calculate an estimate → test assumptions and uncertainty → follow size through time → explain slope and fluctuations using limiting factors and feedback

Evidence pattern First question to ask
quadrat estimate differs between repeats were placement, sample number and habitat structure representative?
few marked animals are recaptured is the population large, or were marks lost / mixing poor?
growth slows near a plateau which density-dependent factor is strengthening?
population drops far below K after a storm is a density-independent disturbance responsible?

A population number becomes biologically useful only when the sampling inference and the mechanism of change are both explicit.

Competition and Cooperation Can Coexist within One Population

Intraspecific interaction Immediate effect Possible fitness consequence
competition for food, light, territory or mates each competitor gains less access to a limiting resource reduced survival or reproduction
dominance hierarchy some individuals gain priority; repeated fighting is reduced lower injury cost for the group, unequal access remains
cooperative hunting individuals share pursuit and capture prey too large or fast for one hunter becomes available
parental or colony care time and energy support related young more shared genes may survive into the next generation

Members of one species overlap strongly in resource needs, so competition can be intense. Cooperation is favoured when its direct or inclusive-fitness benefit exceeds its cost in that context.

A Community Is the Interacting Biotic Part of an Ecosystem

Ecological level Includes
population one species in one area
community all interacting populations in that area
ecosystem the community plus abiotic conditions and their interactions

A change in one population can alter food, habitat, disease, pollination or competition for several others. Community structure is therefore a network outcome, not a list of species.

Plant, herbivore, predator, parasite and mutualist populations are connected in a simple community interaction network.

Interaction Signs Classify Effects on Two Species

Relationship Species A Species B Defining transfer or effect
mutualism + + each gains a resource or service
competition − − both lose access or spend resources contesting
predation + − predator kills and consumes prey
herbivory + − animal consumes plant or algal tissue
parasitism + − parasite gains resources while living on or in host
pathogenicity + − pathogen reproduces while damaging host

The signs describe effects relative to no interaction. They do not mean every individual receives the same benefit or cost, and the outcome can change with conditions.

Examples classify mutualism as positive-positive, competition as negative-negative, and predation, herbivory, parasitism and pathogenicity as positive-negative interactions.

Mutualism Persists When Both Partners Gain

In mutualism, each species receives a net benefit from the interaction under the stated conditions. The exchanged benefits may be nutrients, energy-rich compounds, protection, habitat or transport.

Partners One partner gives The other gives
legume ↔ Rhizobium carbohydrates and a root-nodule habitat fixed nitrogen compounds
orchid ↔ mycorrhizal fungus photosynthetic carbohydrates after establishment water and mineral nutrients; crucial seedling nutrition
coral ↔ zooxanthellae CO₂, mineral waste and protected habitat glucose and O₂ from photosynthesis

Mutualism is not unlimited generosity: both partners incur costs, and the relationship persists when the received benefit exceeds those costs. Dependence may be facultative or obligatory.

Introduced Does Not Automatically Mean Invasive

Term Meaning
native occurs in an area without human introduction
endemic native and restricted to a particular geographic area
alien / introduced moved outside its native range by human activity
invasive introduced, spreads rapidly and causes ecological harm

In Britain, introduced grey squirrels can digest acorns earlier and use a wider diet than red squirrels. Their niches overlap, so more grey squirrels compete for hazelnuts, pine cones and habitat while also carrying squirrelpox that strongly affects red squirrels.

Escape from predators or pathogens, rapid reproduction and broad resource use can expand an introduced species' realized niche. The native competitor may then lose resources and occupy a smaller realized niche.

Competition Needs Evidence Stronger Than Co-Occurrence

Approach What it can show Main limitation
field observation / random sampling species distributions are associated shared abiotic conditions may cause the pattern
laboratory comparison performance changes when species are grown alone versus together simplified conditions may not transfer to the field
field removal or addition one species changes when the other is manipulated difficult controls and other field changes remain

Strong evidence predicts a response, manipulates the proposed competitor, measures the response, uses controls and replication, and rules out alternative explanations such as desiccation or substrate.

One species doing better where another is absent suggests competition; it does not by itself prove that competition caused the distribution.

Connell Revealed a Niche Hidden by Competition

Observation or manipulation Inference
Semibalanus absent high on shore even when competitors are removed desiccation restricts its fundamental niche
Chthamalus normally occupies upper shore its observed realized niche is narrow
removing Semibalanus lets Chthamalus survive lower down Semibalanus competition excludes Chthamalus from part of its fundamental niche

The fundamental niche is the range tolerated without biotic exclusion. The realized niche is the range actually occupied after competition and other biotic interactions.

Natural barnacle zonation is compared with Semibalanus removal, after which Chthamalus expands downward into the middle shore, revealing a broader fundamental niche.

Expected Counts Describe No Association

For two species scored present or absent in random quadrats, the null hypothesis H₀ states that their distributions are independent: there is no association between presence of one and presence of the other.

200 quadrats Bell present Bell absent Row total
ling present 89 45 134
ling absent 31 35 66
column total 120 80 200

E=row total×column totalgrand totalE=\frac{\text{row total}\times\text{column total}}{\text{grand total}}

Expected both present = (134 × 120) ÷ 200 = 80.4. Keep expected values unrounded during the calculation so rounding does not distort χ².

Chi-Squared Measures Departure from Independence

χ2=∑(O−E)2E\chi^2=\sum\frac{(O-E)^2}{E}

Cell O E (O−E)²/E
both present 89 80.4 0.920
ling present, bell absent 45 53.6 1.380
ling absent, bell present 31 39.6 1.868
both absent 35 26.4 2.802
sum 6.97

For a 2 × 2 table, df = (2−1)(2−1) = 1. At p = 0.05 the critical value is 3.84. Because 6.97 > 3.84, reject H₀: the species show a statistically significant association.

Association is not proof of competition or causation. The species may respond to the same abiotic factor or interact in another way; biological evidence must identify the mechanism.

Predator Peaks Follow Prey Peaks

more hare → more food raises lynx survival and reproduction after a delay → more lynx increases hare mortality → fewer hare reduce food for lynx after a delay → lynx decline → reduced predation allows hare recovery

The predator curve lags behind the prey curve because births, maturation and deaths take time. Predation is density-dependent, but weather, vegetation and other predators can also affect the real cycle.

Snowshoe hare population peaks precede lynx peaks; more hare supports more lynx, and more lynx then lowers hare numbers.

Top-Down and Bottom-Up Control Begin at Opposite Ends

Control Initial change Example cascade
top-down predator abundance or predation fewer predators → more herbivores → fewer producers
bottom-up nutrients, habitat or primary production fewer nutrients → fewer producers → fewer herbivores → fewer predators

Identify where the disturbance begins, then follow its direct resource or predation effects one trophic level at a time. Many communities contain both controls, but one may dominate under particular conditions.

A trophic chain shows top-down effects beginning with predators and bottom-up effects beginning with nutrients and producers.

Allelochemicals Suppress Competitors at a Distance

Allelopathy is chemical inhibition of another organism. An allelopathic plant releases an allelochemical that reduces a competitor's germination, photosynthesis or growth.

Release route How a neighbour is exposed
root exudate chemical diffuses through soil and is absorbed by roots
volatile release from leaves gas reaches nearby shoots
decomposing litter compounds enter soil as leaves break down

Broccoli residues can inhibit later brassica seedlings, so crop rotation reduces carry-over effects. Dense bracken stands release compounds that help exclude other plants and can reduce local diversity.

Poor growth beside a plant does not prove allelopathy; shading, water use and mineral competition must be controlled or measured.

An Antibiotic Can Be an Ecological Competition Chemical

Microbes also compete chemically. Penicillium releases penicillin, which inhibits susceptible bacteria nearby and reduces their competition for space and nutrients.

On an agar plate, a clear zone of inhibition around the fungal colony shows that a diffusible substance prevents bacterial growth. Zone size also depends on diffusion and dose, so it is not a direct measure of ecological success by itself.

Humans use antibiotics as medicines, but their original biological role is an interaction among organisms in a community.

Broccoli roots release allelochemicals that inhibit a nearby seedling, while Penicillium produces a clear inhibition zone among Staphylococcus colonies.

Explain Community Change with Signs, Mechanism and Evidence

Question What a strong explanation identifies
Who interacts? populations and whether each gains or loses
What is transferred or limited? food, habitat, nutrients, protection, disease or a chemical
Where does change begin? individual pair, predator level, producer/resource level or introduced species
How does it propagate? competition, feedback, trophic cascade or niche restriction
What supports causation? manipulation, control, replication and a predicted response
What supports association? representative sampling, expected counts and a statistical decision

Use observations to find a pattern, statistics to judge whether it is unlikely under a null model, and experiments or field manipulations to test the proposed mechanism.

Community structure emerges from many population interactions. Naming a relationship is only the start; explanation requires its mechanism, direction of effect, feedback through the network and the limits of the evidence.

Populations exam focus

1 mark

State the biological term for a group of organisms of the same species living in an area.

Population size estimation

6 marks

Discuss how wild fish populations are assessed to ensure sustainable fishing practices.

Random quadrat sampling

3 marks

Describe a method that can be used to measure the diversity of herbaceous plants in one of the green areas at different distances from the main road.

Capture-mark-release-recapture exam focus

6 marks

Explain the technique used to estimate the population size of a named species of organism that is able to move.

Carrying capacity

7 marks

Explain why populations that have grown exponentially reach a maximum size, rather than continue to grow.

Negative feedback control

2 marks

Explain the change in numbers of the cactus moth throughout the study period.

Population growth curves

6 marks

Discuss the factors affecting population growth that can result in an exponential growth curve.

Competition vs. cooperation

2 marks

Compare and contrast the frequency of monopoly and fighting when there is a change from two trays to one tray of food.

Community exam focus

3 marks

Compare and contrast the community structure within and outside the marine protected area.

Interspecific relationships

3 marks

Evaluate whether crop yield may be improved through enhancement of pollinator richness.

Mutualism as interspecific relationship

3 marks

Analyse the relationship between plants and their pollinators.

Endemic vs. invasive species

6 marks

Discuss, giving an example, the possible effects of the introduction of an alien species into an ecosystem.

Tests for interspecific competition

3 marks

Explain the results shown in this experiment.

Chi-squared test

3 marks

Outline how chi-squared can be used to test for an association between the distributions of the two species.

Predator-prey relationships

3 marks

Describe the effect of removing trout on frog density in Upper and Lower LeConte Lakes.

Top-down vs. bottom-up control

3 marks

Explain how nutrients can have a positive or negative bottom-up effect on seagrass.

Allelopathy and antibiotics

3 marks

Explain how a named plant can reduce competition by allelopathy.