A population is all the individuals of one species living in a given area at one time. When conditions are favourable populations grow; when resources run short or predators increase they stabilise or shrink. GCSE biology exams ask you to explain two characteristic growth curve shapes — the J-curve and the S-curve — and the factors that produce each.
What is population growth?
Population growth depends on the balance between additions and losses:
Population change = (Births + Immigration) − (Deaths + Emigration)
When births outpace deaths (and immigration exceeds emigration), the population grows. The rate and shape of this growth depend on whether limiting factors are acting.
What is exponential (J-shaped) growth?
When a population has unlimited resources — plenty of food, space, and no predators or disease — it grows exponentially. Each individual reproduces and its offspring reproduce, creating a compounding effect that produces a J-shaped curve.
Key features of the J-curve:
- Growth rate increases over time (the curve steepens)
- There is no upper limit in the idealised model
- In reality, J-shaped growth only occurs briefly before resources are depleted
Worked example:
Suppose a bacterial population doubles every 20 minutes starting with 100 cells:
| Time (min) | Population |
|---|---|
| 0 | 100 |
| 20 | 200 |
| 40 | 400 |
| 60 | 800 |
| 80 | 1,600 |
| 100 | 3,200 |
After just 100 minutes the population has grown 32-fold. This doubling pattern is why bacterial contamination of food must be taken seriously — a single cell can become millions within hours at body temperature.
Real-world J-curves:
- Introduced species in a new habitat with no predators (e.g. grey squirrels introduced to the UK in the 1870s)
- Bacteria growing on fresh culture medium in a laboratory
- Human population growth from 1800 onwards (though slowing in recent decades)
What is logistic (S-shaped) growth?
Logistic growth occurs when limiting factors kick in as the population grows. The S-shaped (sigmoid) curve has three distinct phases:
- Lag phase — population is small; growth is slow. Individuals establish, find mates, and reproduce for the first time.
- Exponential phase — resources are still relatively plentiful; the population grows rapidly (the steep, J-shaped middle section).
- Stationary phase — the population reaches the carrying capacity (K); births approximately equal deaths; growth levels off.
In some populations a decline phase follows if the environment is damaged by overpopulation (overgrazing, pollution).
What is carrying capacity?
Carrying capacity (K) is the maximum population size that a given environment can sustainably support, set by the availability of limiting resources.
The population oscillates around K: if it overshoots K, competition intensifies and death rates rise until the population falls back; if it drops below K, competition eases and birth rates rise again.
Factors that set carrying capacity include:
- Food (energy) supply
- Water availability
- Nesting sites or shelter
- Mineral nutrient levels (for plants)
- Predation and disease pressure
What are the limiting factors in population growth?
Limiting factors prevent continued exponential growth:
| Factor type | Examples |
|---|---|
| Biotic (living) | Food availability, predation, disease, competition for mates |
| Abiotic (non-living) | Temperature, water, light, pH, mineral ions |
Density-dependent factors become more severe as population size increases. For example, food per individual decreases as more individuals compete for the same resource; disease spreads more easily in a crowded population.
Density-independent factors affect the population regardless of its size — a severe frost kills plants whether there are 10 or 10,000 of them.
How do human activities affect population curves?
Humans have altered the carrying capacity of many ecosystems:
- Agriculture raises the carrying capacity for humans by increasing food production, driving the human population rise from ~1 billion in 1800 to over 8 billion today.
- Habitat destruction lowers the carrying capacity for other species, sometimes below minimum viable population size, leading to local extinction.
- Pest control and culling aim to artificially reduce populations of species humans consider harmful, pushing them below their natural K.
- Conservation (protected areas, captive breeding) attempts to raise K for endangered species by restoring habitat and removing limiting factors.
Frequently asked questions
Why can the human population grow past the natural carrying capacity?
Humans bypass natural limiting factors through technology: agriculture produces more food per unit of land than natural ecosystems; medicine and sanitation reduce death from disease; heating and shelter reduce the impact of cold. Each technological advance effectively raises the carrying capacity for humans. However, this cannot continue indefinitely — soil degradation, freshwater scarcity, and climate change are already acting as new limiting factors at a global scale.
What causes a population crash after overshooting carrying capacity?
When a population exceeds K it consumes resources faster than they can be replenished. Food per individual falls, starvation increases, weakened individuals are more susceptible to disease, and reproductive success drops. The result is a rapid die-off. In small isolated populations this can lead to extinction. Classic examples include reindeer introduced to St Matthew Island in Alaska, whose numbers rose from 29 in 1944 to about 6,000 by 1963 before crashing to near-zero in 1966 after overgrazing destroyed the lichen that supported them.
What is the difference between intraspecific and interspecific competition?
Intraspecific competition occurs between individuals of the same species competing for the same resources (food, mates, territory). This is the main force driving logistic growth towards carrying capacity. Interspecific competition occurs between individuals of different species. When both species need the same resource, one typically outcompetes the other over time — Gause's competitive exclusion principle states that two species competing for identical resources cannot coexist indefinitely; one will displace the other or the species will evolve to use resources differently.
How do ecologists measure population size in practice?
Small, visible animals can be counted directly. For mobile animals, the mark-release-recapture method is used: capture a sample, mark them, release them, recapture a second sample, and use the formula: N = (n₁ × n₂) / m, where N is estimated population size, n₁ is the first sample, n₂ is the second sample, and m is the number of marked individuals in the second sample. For plants, quadrat sampling estimates population density per unit area. Both methods rely on the assumption that the population is closed (no births, deaths, or migration during the study period).
For Socratic GCSE biology with Professor Darwin — from individual organisms to ecosystem-level patterns, one question at a time — visit aitutors.me.