Populations don't grow forever — they're shaped by births, deaths, migration, and limiting factors. Explore how a population grows toward its environment's carrying capacity.
A population's size changes due to four factors:
| Factor | Effect |
|---|---|
| Births | Increases population size |
| Deaths (mortality) | Decreases population size |
| Immigration | Individuals entering — increases population size |
| Emigration | Individuals leaving — decreases population size |
Population sizes naturally fluctuate over time as these four factors change.
Ecologists estimate population size using sampling methods such as the quadrat method (counting organisms in sample squares and scaling up) or mark-recapture (marking a sample of individuals, releasing them, then recapturing a second sample to estimate total numbers from the proportion re-caught).
Populations tend to follow one of two growth patterns:
Limiting factors — such as food, water, space, and disease — restrict how large a population can grow. The carrying capacity is the maximum population size an environment can sustainably support given these limiting factors.
| Term | Meaning |
|---|---|
| Intraspecific competition | Competition between individuals of the same species for an identical set of resources — usually the most intense form of competition, since their needs overlap completely |
| Interspecific competition | Competition between individuals of different species for a shared (but not identical) resource |
| Ecological niche | The specific role a species plays within its community — including its habitat, feeding relationships, and interactions with other organisms |
| Census | A method of directly counting every individual in a population — practical only when a population is small enough, and visible/stationary enough, to count completely |
| r-strategists | K-strategists | |
|---|---|---|
| Number of offspring | Very many, per reproductive event | Few, per reproductive event |
| Parental care | Little to none | Extensive |
| Offspring size | Small | Large |
| Typical environment | Unstable/unpredictable, where population stays well below carrying capacity | Stable, where population lives close to carrying capacity |
| Examples | Insects, many fish and plants | Elephants, whales, humans |
An ecological footprint is a measure of how much land, water, and resources are needed to support a population's consumption and absorb its waste — essentially, a measure of how dependent that population is on natural resources.
In some social animals (e.g. ants, bees, termites, naked mole rats), individuals within a colony are organised into castes — groups that perform specialised roles (such as workers, soldiers, or a single reproductive queen) rather than each individual doing everything itself.
Inbreeding is the mating/breeding between closely related individuals. Because related individuals already share many of the same alleles, inbreeding has serious genetic consequences:
Ecological succession is the gradual, predictable change in the species that make up a community over time, in a particular area.
| Primary succession | Secondary succession | |
|---|---|---|
| Starting point | Bare, lifeless substrate with no soil (e.g. bare rock left by a retreating glacier, or new volcanic lava) | An area that already HAS soil and some surviving organisms, following a disturbance (e.g. fire, flood, abandoned farmland) |
| Speed | Very slow — soil must form first | Much faster — soil and some organisms are already present |
| Typical pioneers | Lichens and mosses, which help break down rock into early soil | Fast-growing grasses and small plants that quickly recolonise the disturbed area |
Adjust the growth rate and carrying capacity, then press Play to watch the population grow along a logistic (S-shaped) curve.
| Year | Population size |
|---|---|
| 0 | 50 |
| 2 | 100 |
| 4 | 380 |
| 6 | 850 |
| 8 | 980 |