C4.1Theme C — Interaction and interdependence
Populations and communities — IB Biology HL Revision Notes & Topical Questions
10 min read 12 parts Lesson 9 of 11 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
C4.1 Populations and communities — IB BIOLOGY HL notes: Explain how population size is estimated and limited, how populations grow, and how species interact within communities.Population = a group of organisms of the same species living in an area at one time, which interact and …
- Nature of science
- Frame quadrat
- Lincoln index: population size
- Carrying capacity (K)
- Biotic factors
- Population density
Part 1 of 9Populations and sampling, quadrat sampling
Population
Population a group of organisms of the same species living in an area at one time, which interact and breed together.
- A population separated from others cannot breed with them, so no gene exchange occurs.
Counting
- Counting a total count is possible only for small areas or large species, so sampling is used in larger habitats.
Sampling
Sampling measuring small samples that represent the whole population.
| Type | Method | Advantage |
|---|---|---|
| Random | Sample positions chosen at random, for example random co-ordinates on a grid | Avoids bias from the person sampling, best for uniform areas |
| Systematic | Sample points at fixed intervals, for example along a transect | Avoids missing parts of the habitat and shows the effect of features such as a river |
- Nature of science
- sampling inevitably causes sampling error, the difference between estimated and true population size.
- Missing an empty area overestimates and missing a crowded area underestimates, because individuals are rarely evenly distributed.
- Good design, the right sampling type and a large sample reduce error.
- Authors must report methods so readers can judge error.
Frame quadrat
Frame quadrat a square frame placed in the study area, used for sessile organisms.
- A 1 m2 quadrat suits small organisms such as herbaceous plants or limpets, and 400 m2 areas suit trees (marked by string).
- Quadrats can be placed randomly or systematically.
Data recorded
- Presence or absence.
- Frequency, the number of individuals in the quadrat.
- Abundance on the ACFOR scale (abundant, common, frequent, occasional, rare, none).
- Percentage cover, which is easier when the quadrat is divided into small squares.
Analysis
- Analysis mean and standard deviation.
- Standard deviation measures spread around the mean.
- A small value means results are close to the mean with little variation, and a large value means they are widely spread.
- It indicates how evenly a population is distributed.
Part 2 of 9Estimating motile populations and carrying capacity and limiting factors
- Capture–mark–release–recapture
1
Catch as many individuals as possible, count them and mark them harmlessly, for example with non-toxic paint.
2
Release them and let them mix randomly with the population.
3
After enough time, catch a second large sample.
4
Count the marked and unmarked individuals.
Lincoln index
Lincoln index population size
(M × N) ÷ R.
(M × N) ÷ R.
- M is the number marked in the first sample.
- N is the total number in the second sample.
- R is the number of marked individuals recaptured.
Assumptions
- Marked individuals mix fully with the population.
- Marking does not affect survival, for example by making them more visible to predators.
- Marks stay visible.
- Population size is constant, with no major births, deaths or migration.
Carrying capacity (K)
Carrying capacity (K) the maximum number of individuals of a species an ecosystem can support, seen where a growth curve levels off.
Abiotic factors
- Light availability limits plant photosynthesis.
- Temperature affects the rate of metabolic reactions.
- Soil mineral availability limits proteins and chlorophyll.
Biotic factors
- Biotic factors energy spent on competing, avoiding predators or fighting disease is not available for growth and reproduction.
- Competition for resources.
- Predation.
- Disease.
Part 3 of 9Density-dependent and independent factors, predator–prey relationships and population control
Population density
Population density the number of individuals per unit area.
Density-dependent factors have a greater effect at high density.
- Pathogens and pests spread faster.
- Competition increases.
- Predators are attracted to dense prey populations.
Density-independent factors have the same effect at any density, such as a flood or other natural disaster.
Negative feedback
- Negative feedback density-dependent factors keep a population at or below K and make it fluctuate around K.
- Above K, disease and competition reduce survival and reproduction, so the population falls.
- Below K, the same factors ease, so survival and reproduction rise and the population increases.
Positive feedback
- Positive feedback in a population, more individuals give more reproduction and further increase, until a density-dependent factor limits growth.
- Predator–prey cycle
1
Predators increase when more prey are available.
2
Prey decrease as predators increase.
3
Predators decrease as prey decrease.
4
Prey increase as predators decrease, and the cycle repeats.
- The Canada lynx and snowshoe hare are a classic example.
- Real food webs are more complex, with other predators, prey and factors.
| Control | Meaning | Example |
|---|---|---|
| Top-down | Population limited by predators or herbivores | Snowshoe hare limited by lynx; plants limited by herbivory |
| Bottom-up | Population limited by resources | Lynx limited by hare numbers; plants limited by light |
Top-down cascade
- Top-down cascade fewer foxes can mean more rabbits and less grass growth, and food chains link to others, so effects are complex.
Dominance
- Dominance one control usually dominates,
- such as bottom-up nutrient control in coastal seagrass,
- and overfishing can switch the system to top-down control.
Non-lethal effects
- Non-lethal effects predators alter prey behaviour, diet and where prey spend time, which changes ecosystem structure.
Part 4 of 9Allelopathy and antibiotic secretion
Competition strategies
- Competition strategies species may raise their own survival, as camouflage does, or lower the survival of a competitor with harmful chemicals.
- Harmful chemicals are secondary metabolites, unlike primary metabolites that are essential for survival.
Allelopathy
Allelopathy secretion of harmful secondary metabolites that damage other organisms.
- Plants secrete them from roots into soil, release harmful gases through stomata, or store them in leaves that release them when they fall and decay.
- Garlic mustard produces sinigrin, which reduces seed germination and root growth of other plants.
- Bracken ferns release toxins into the soil and from decaying fronds.
- Himalayan balsam is thought to secrete allelochemicals that limit other plants, which may help it to be invasive along UK waterways.
Antibiotic secretion
- Antibiotic secretion it is allelopathy in microorganisms.
- Penicillin comes from Penicillium, and some bacteria also secrete antibiotics.
- Antibiotics prevent cell wall formation or inhibit protein synthesis in competitors, so the producer survives and reproduces more.
Part 5 of 9Population growth curves
Sigmoid growth curve
- Sigmoid growth curve it is seen in new or recovering populations,
- such as Antarctic fur seals at Cape Shirreff, recovering after 1800s hunting and measured by pup counts.
- Exponential (logarithmic) phaseno limiting factors, and growth rate increases with the number of individuals.
- Transition phaselimiting factors act, such as competition and predators, and growth slows while the population still rises.
- Plateau (stationary) phasedeath rate equals birth rate at the carrying capacity, with slight fluctuation.
Nature of science
- Nature of science the curve is an idealised model, useful for testing predictions but not a perfect picture of complex ecosystems,
- and the seal population soon declined because of climate change.
Exponential growth
- Exponential growth growth is proportional to the number of individuals.
- It is tested by plotting population size on a logarithmic y-axis against time on a linear x-axis, where exponential growth gives a straight line.
- Log scales display values over several orders of magnitude, and numbers on a log10 scale are powers of 10.
Part 6 of 9Interspecific relationships in communities
Community
Community multiple populations of different species living and interacting in the same area, with all groups of organisms included,
- and a community interacting with its abiotic environment forms an ecosystem.
Interspecific
Interspecific interactions between different species.
| Interaction | Description | Examples |
|---|---|---|
| Herbivory | Feeding on a plant | Cattle graze grass, sea turtles eat seagrass, bees take nectar |
| Predation | Catching and eating an animal or a recently dead one | Dolphins eat fish, lions hunt zebra, red kites eat roadkill |
| Interspecific competition | Different species compete for the same resources | Oak and beech trees, lions and hyenas, red and grey squirrels |
| Mutualism | Both species benefit | Pistol shrimp and goby fish, oxpeckers removing parasites |
| Parasitism | A parasite lives in or on a host and harms it | Mistletoe on trees, fleas on mammals |
| Pathogenicity | An infectious microorganism causes disease in a host | Mycobacterium tuberculosis, the fungus causing Dutch elm disease |
Part 7 of 9Mutualism examples
Root nodules of legumes (Fabaceae such as peas, beans and clover)
- Bacteria such as Rhizobium in spherical nodules fix nitrogen, converting nitrogen gas to ammonia and then nitrates.
- Plants use nitrates for proteins and nucleic acids.
- The bacteria receive carbohydrates from the plant's photosynthesis.
Mycorrhizae
- Mycorrhizae fungal hyphae interact with plant roots.
- Hyphae greatly increase the root surface area, so more water and mineral ions are absorbed.
- The fungus receives organic compounds such as glucose.
- Orchid seeds may get nutrients for germination from mycorrhizae.
- Some orchids cannot photosynthesise and are heterotrophs that rely on fungi digesting dead matter,
- and the orchid benefits the fungus only on its death.
Zooxanthellae in hard corals
- Coral polyps (phylum Cnidaria) have soft bodies and tentacles with stinging cells (nematocysts) containing zooxanthellae.
- Polyps secrete calcium carbonate for the skeleton and give the algae shelter.
- The algae photosynthesise and supply carbon compounds such as carbohydrates.
Part 8 of 9Invasive and endemic species, testing for interspecific competition
Invasive species
Invasive species a non-native (alien) species that harms the environment it is introduced to.
- Humans cause most cases by knowingly trading species (garden plants, zoo animals or pets that escape), unknowingly transporting them (rats on ships) or introducing them as biological control.
Success
- Success in a new ecosystem they lack natural predators and competitors, so numbers rise rapidly.
- They outcompete native species that occupy a similar niche.
- They may be successful predators that cause prey to decline.
- They introduce new diseases to which natives have no immunity.
- Grey squirrels from North America (1800s) largely outcompeted UK red squirrels, which are recovering in some places.
Endemic species
Endemic species species found in one place only, such as the Scottish crossbill, so local extinction is total extinction.
- Australia's endemic species evolved in isolation and are hit by invasive European red foxes (introduced in the 1800s for hunting) and domestic cats (probably brought on ships).
- Over 10% of Australia's endemic mammals have gone extinct since European settlement, with small mammals most at risk.
- Some species survive only on islands that foxes and cats have not reached.
Competitive exclusion
- Competitive exclusion the weaker competitor is pushed from its fundamental niche into a realised niche, or becomes locally extinct.
Test
- Test removing a competitor and observing a change in distribution suggests competition,
- while no change suggests another factor; it indicates but does not prove competition.
- In the laboratory, culture species alone and together, for example bacteria, and measure population size or colony distribution.
- In the field, take random quadrats recording presence or absence of each species,
- then remove one species from a small area and measure the effect on the other.
Nature of science
- Nature of science hypotheses can be tested by experiments and observations.
- Laboratory tests are controlled and small-scale, so only the independent variable changes, but organisms may behave differently from nature.
- Field tests are realistic and large-scale, but other factors cannot be controlled and they are hard to replicate.
Part 9 of 9Chi-squared test for association
Association
- Association species distributions may show positive association (symbiosis), negative association (competition) or independence.
- (no interaction, with apparent patterns due to chance)
Use
- Use random quadrat sampling with the chi-squared test checks whether observed and expected results differ significantly.
- Null hypothesisno association between the distributions of the two species.
- Alternative hypothesisthere is an association, positive or negative.
Method
Chi-squared test for association
Build a contingency table of quadrats with one, both or neither species.
Calculate row, column and overall totals.
Calculate expected values: E
Calculate expected values: E row total × column total ÷ overall total.
Subtract E from O for each cell.
Square each difference.
Divide each squared difference by E.
Add all values to get χ2
Σ (O − E)2 ÷ E.
Find degrees of freedom
(columns − 1) × (rows − 1).
Use the probability level of 0.05 (5%).
Read the critical value from a table for the degrees of freedom (3.84 for 1 degree of freedom at 0.05).
Compare the χ2 value with the critical value.
- Interpretation
- If χ2 is larger than the critical value, the association is significant and the null hypothesis is rejected.
- If it is equal or smaller, the result is not significant and the null hypothesis is accepted.
- At p = 0.05 there is only a 5% probability that the result is due to chance.
Key terms
| Term | Meaning |
|---|---|
| Carrying capacity | the maximum population an ecosystem can support. |
| Lincoln index | (M × N) ÷ R, an estimate of population size from recaptures. |
| Density-dependent factor | a factor whose effect depends on population density. |
| Mutualism | an interaction in which both species benefit. |
| Allelopathy | release of harmful chemicals to reduce competitors. |
| Invasive species | a non-native species that harms its new environment. |
| Endemic species | a species found in only one place. |
| Chi-squared test | a test of whether observed and expected values differ significantly. |
| Top-down control | limitation of a population by predators. |
Avoid these mistakes
1
Random sampling avoids all error — sampling error still occurs and falls with larger samples.
2
Density-independent factors keep populations near carrying capacity — density-dependent factors do this by negative feedback.
3
Competition always involves visible fighting — shared resource use is competition too.
4
A significant chi-squared value proves the cause of an association — it shows the association is unlikely to be by chance only.
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