Population, community, ecosystem — three nested definitions
Population = a group of organisms of one species, in the same area, at the same time — all three needed. Community = all of the populations of different species in an ecosystem. Ecosystem = the community plus its environment, interacting together. Nested: population community ecosystem. Drop "one species" and you have a community; drop "interacting" and you have a habitat, not an ecosystem.
Four factors set the rate of population growth
Food supply, competition, predation and disease — the Core list, and nothing else. More food raises the growth rate; scarcity slows it or causes decline. More competition (intraspecific or interspecific) for food, space or mates slows it. More predators raise the prey death rate; fewer let prey grow faster. Disease spreads at high density, raising deaths. The last three are density-dependent, biting harder as the population grows — which flattens the sigmoid curve.
Sigmoid curve — lag, exponential, stationary, death
Lag: slow rise; individuals still adjusting (reaching maturity; microbes making enzymes), birth rate just above death rate. Exponential (log): steep, accelerating rise; resources abundant relative to a small population, birth rate far above death rate. Stationary: flat; birth rate ≈ death rate as limiting factors balance growth at the carrying capacity. Death: falls; death rate exceeds birth rate as a factor intensifies. Core: identify; Extended: explain.
Drawn from real examiner reports.
Population ≠ all organisms in an area
A population is one species only. "All the organisms living in an area" covers several species, so it defines a community and scores nothing. The mark scheme wants all three elements: one species, same area, same time. "Same time" matters too — organisms from different generations are not one population.
Flagged s22 P11/12/13 Q38 · w22 P11 Q38 — the digest records candidates writing "all organisms in an area"
Ecosystem ≠ community
A community is only the living organisms — every population of every species. An ecosystem adds the physical environment (soil, water, light, temperature) and the interactions between the two. Defining an ecosystem as "an area where organisms live" describes a habitat and misses both the environment and the word "interacting".
Lag phase ≠ no growth
The lag phase has a shallow but positive gradient — the population is still increasing, just slowly, while individuals adjust to the environment or synthesise the enzymes they need. Calling it flat, zero or "no growth" loses the mark: birth rate here slightly exceeds death rate rather than equalling it.
No sitting-specific 0610 citation in the digest for this graph-labelling error; grounded in the syllabus phase list and standard 0610 graph mark schemes.
Stationary phase ≠ death phase
Stationary = the curve is level, population roughly constant, because birth rate approximately equals death rate at the carrying capacity. Death = the curve falls, because death rate exceeds birth rate. A flat line and a declining line are different answers — "numbers have stopped rising" is stationary, not death.
No sitting-specific 0610 citation in the digest; grounded in the syllabus phase list and standard 0610 graph mark schemes.
"Increases" under-describes the log phase
For the exponential phase the mark scheme wants rapid or accelerating growth, not just "the population increases". Say the curve rises steeply because resources are abundant relative to the still-small population, so birth rate greatly exceeds death rate and each round of reproduction compounds on the last.
Naming a factor without its direction
"Predation" on its own scores nothing. State which way the growth rate moves, and why: more predators raise the prey death rate and slow growth; fewer predators let the prey population grow faster. Pair every factor you name with an increase or decrease in the rate of population growth, plus a reason.
No sitting-specific 0610 citation in the digest; grounded in the syllabus statement's limited factor list and 0610 "identify and state" conventions.
Climate and pollution are not on the list
The syllabus limits the Core factors to food supply, competition, predation and disease. Answers offering "climate" or "pollution" on their own fall outside that list. If you want to use them, link them back to one of the four named mechanisms — e.g. a drought reduces the food supply, which slows the rate of growth.
Work the four phases in order
Take lag exponential stationary death in sequence. For each, give the gradient (slow rise / steep rise / flat / fall) and the birth-rate-versus-death-rate relationship. At Extended, add the limiting factor responsible for that phase.
Identify (Core) vs explain (Extended)
"Identify" wants the phase NAMED from the graph and nothing more. "Explain" (Extended, statement 7) additionally wants the REASON for that shape, referencing a specific limiting factor where appropriate. Answer to the command word — no further.
Quote the graph, then name the phase
For "describe the shape between two named points", lead with the evidence: "between weeks 4 and 8 the curve is level". Then name the phase, then the birth/death rate relationship, then (Extended) the limiting factor that caused it. Four short steps, four marks.
Growth rate = change time
Population growth rate is the CHANGE in population size divided by the time taken — subtract the starting value first. Using the final population as the numerator is the standard error. Always give a unit, e.g. individuals per day.
Population: a group of organisms of one species, living in the same area, at the same time.
Community: all of the populations of different species living in an ecosystem.
Ecosystem: a unit containing the community of organisms and their environment, interacting together.
Nested scale: population (one species) community (all species' populations) ecosystem (community + environment + their interactions).
Define "population."
(a) Describe what is meant by a "population." [2]
(b) Describe what is meant by a "community." [1]
(c) Describe what is meant by an "ecosystem." [2]
