C4.1 Populations and communities

A population is one species in one area.

A population is all individuals of the same species living in a defined area at a defined time. Its size and structure change through births, deaths, immigration and emigration.

state species, boundary and time; identify demographic process; distinguish population from community.

A pond frog population can grow after breeding, shrink during drought and receive newcomers from a connected pond.

A population is not every organism in a habitat; it has a species boundary.

Estimate population size with a defined method.

Population estimates use a sample and assumptions to infer how many organisms occupy a larger area. Method choice must match whether organisms are stationary, mobile, visible or clumped.

define target; choose sample; standardise effort; extrapolate; state uncertainty and assumptions.

Quadrat counts from several random plots can estimate plant abundance across a meadow if the plots represent the meadow.

An estimate is not a census; sampling error and spatial pattern limit precision.

Random quadrats reduce placement bias.

Randomly placed quadrats give each part of a study area a known chance of selection. Repeated equal-sized quadrats allow abundance or frequency to be compared fairly.

define area/quadrat size; randomise positions; record count or cover; repeat; summarise variation.

A random-number grid selects ten meadow plots, avoiding a convenient path that might overrepresent one microhabitat.

Random placement does not guarantee a perfect sample if the number or size of quadrats is too small.

Capture–mark–recapture estimates mobile populations.

A first sample is captured, marked harmlessly and released. After mixing, a second sample is taken; the marked fraction helps estimate the total population.

N≈(first caught × second caught)/marked recaptured; check mixing, mark retention and equal catchability.

If 40 fish are marked, 50 are caught later and 10 are marked, the estimate is about 200 fish.

The calculation fails if marks are lost, animals learn the trap or the population changes between samples.

Carrying capacity is a resource-limited population size.

Carrying capacity is the population size an environment can support over time under specified conditions. Resource limits and feedback make growth slow as the population approaches it.

identify limiting resource; link density to birth/death rate; state that capacity can change.

A drought lowers available food, so the carrying capacity for grazing animals falls even if the species is unchanged.

Carrying capacity is not a fixed species constant; it depends on environment and timescale.

Negative feedback restrains population growth.

As density rises, competition, disease or predation can reduce births or increase deaths. The response opposes the change and tends to return population size toward a range.

disturbance → density-dependent response → birth/death change → correction.

Crowding increases disease transmission, lowering survival and slowing further population growth.

Not every population change is negative feedback; density-independent storms can reduce numbers without crowding.

Growth curves show changing net population rate.

A population may show lag, rapid growth and then slowing as limiting factors intensify. The slope represents net change, not simply the number of organisms present.

read slope; identify phase; connect phase to resources and density; note scale.

A newly introduced species may grow slowly at first, rise steeply when resources are abundant and flatten near capacity.

A steep curve does not mean the population is largest; it means it is changing fastest.

The sigmoid model links growth to carrying capacity.

The logistic model predicts fast growth at intermediate size and slowing growth near carrying capacity because the fraction of limiting resources falls.

identify N, r and K; explain why rate changes; compare model with data.

When N is close to K, each additional animal faces more competition, so dN/dt decreases.

A sigmoid is a model, not a guarantee: migration, seasonal resources and disturbances can break its assumptions.

Competition occurs when organisms share a limited resource.

Competition lowers access to food, space, light or mates when demands overlap. Cooperation can instead improve group performance when individuals gain from working together.

name shared resource; show effect on each participant; include cost and context.

Two plant species competing for light may each grow less tall when planted together than when grown alone.

Competition is not defined by aggression; it is a reduction in resource access caused by overlap.

A community contains interacting populations.

A community is the populations of different species living and interacting in one area. Its composition and structure depend on abiotic conditions and biotic relationships.

list populations; define boundary; identify interaction and environmental filter.

A pond community includes algae, plants, fish, bacteria and invertebrates interacting under the pond’s chemistry and light.

A community is not the same as an ecosystem: an ecosystem also includes abiotic stores and flows.

Interspecific relationships change population outcomes.

Predation, competition, mutualism, parasitism and commensalism describe how one species’ interaction affects another. The sign of an interaction can depend on conditions.

use +/−/0 effects; identify mechanism; state whether relationship is stable across contexts.

A predator reduces prey abundance but may also prevent one competitor from excluding another species.

Do not label a relationship from one observation without identifying the process and time scale.

Mutualism gives both partners a context-dependent benefit.

In mutualism, each species gains a net benefit from the interaction, often through exchange of resources or services. Benefits have costs and may change with conditions.

benefit to partner A; benefit to B; exchanged service/resource; cost or boundary.

Mycorrhizal fungi receive plant sugars while improving the plant’s access to phosphorus and water.

Mutualism is not automatically essential or permanently equal; both partners can survive without it in some settings.

Endemic and invasive describe distribution histories.

An endemic species is native and restricted to a particular region. An invasive species is introduced and spreads in a way that causes ecological or economic harm.

state native range; introduction status; spread; documented impact.

A species endemic to one island may be vulnerable to habitat loss, while an introduced predator can spread through a community lacking defences.

Non-native does not automatically mean invasive; harm and spread must be demonstrated.

Competition tests need a manipulated comparison.

To test interspecific competition, compare growth, survival or resource use with and without the competitor while controlling other variables. Replication and randomisation strengthen inference.

control competitor presence; measure response; hold conditions; replicate; interpret effect size.

Seedlings grown alone and with a neighbour under identical light can reveal whether the neighbour reduces biomass.

A difference in abundance alone does not prove competition if another environmental variable differs too.

Chi-squared tests compare observed and expected counts.

A chi-squared statistic measures how far categorical observations deviate from an expected distribution. Expected counts come from the null hypothesis, not from the observed pattern.

state null; calculate expected; compare observed–expected differences; use degrees of freedom and p-value.

If flower-colour counts differ from a proposed ratio, chi-squared can test whether the deviation is larger than sampling variation.

A significant result rejects the null pattern; it does not prove a particular cause.

Predator and prey numbers can oscillate.

Predator abundance often responds after prey abundance changes because reproduction and mortality take time. Predation can then reduce prey, producing a lagged cycle.

read peaks and troughs; identify lag; link prey to food and predator to prey availability.

Rabbits increase first, fox numbers rise later, and rabbit numbers fall before fox numbers decline from food shortage.

A correlation is not automatically a predator–prey cycle; disease, weather and migration can also drive it.

Top-down and bottom-up control start at different levels.

Top-down control begins with consumers altering lower trophic levels; bottom-up control begins with nutrients or primary production limiting higher levels. Real systems can show both.

identify direction of effect; name trophic level; trace indirect consequences through the food web.

Removing a top predator can release herbivores and reduce plant biomass, while low nitrogen can limit the same web from below.

The labels describe control pathways, not a claim that one direction always dominates.

Allelopathy and antibiotics are chemical interference.

Some organisms release chemicals that inhibit competitors or microbes. The effect depends on dose, susceptibility, transport and environmental breakdown.

producer; chemical; target; inhibitory effect; environmental condition.

A plant root exudate can suppress germination of a neighbouring species, giving the producer an advantage where the compound persists.

A laboratory inhibition zone does not prove the same ecological effect at field concentration.