Populations rise and fall
A population is all the members of one species in an area. Populations change size because of births, deaths and individuals moving in or out.
Population change = (births + immigration) − (deaths + emigration)
Limiting factors
A limiting factor is anything that keeps a population from growing forever.
| Type | Examples |
|---|---|
| Density-dependent (stronger in crowded populations) | Food supply, disease, competition, predation |
| Density-independent (affect populations regardless of size) | Floods, fires, droughts, extreme cold |
Carrying capacity
The carrying capacity is the largest population an environment can support over time. When a population has plenty of resources, it may grow quickly. As it nears carrying capacity, growth slows because of limiting factors. If it overshoots, it may crash.
Example: In 1944, 29 reindeer were released on St. Matthew Island in Alaska. With no predators, they grew to about 6000 by 1963. They ate all the lichen, and by 1966 only about 42 remained.
Food webs and energy
A food web shows feeding relationships. Each feeding level is a trophic level:
| Trophic level | Example |
|---|---|
| Producers | Grass, phytoplankton |
| Primary consumers (herbivores) | Rabbits, zooplankton |
| Secondary consumers | Foxes, small fish |
| Tertiary consumers | Eagles, seals |
| Decomposers (act at all levels) | Fungi, bacteria |
Only about 10% of energy passes to the next level. The rest is used for life processes or lost as heat. This is why energy pyramids are wide at the bottom and narrow at the top.
Predator–prey cycles
The famous records of the Hudson’s Bay Company showed that lynx and snowshoe hare populations rise and fall in cycles of about 10 years. When hares are plentiful, lynx increase. Lynx eat more hares, so hares decline. Then lynx starve and decline, allowing hares to recover.
Keystone species
A keystone species has a huge effect on its ecosystem compared with its numbers.
- Sea otters eat sea urchins. Without otters, urchins overgraze kelp forests, turning them into "urchin barrens."
- Beavers build dams that create wetlands for many other species.
- Wolves in Yellowstone changed elk behaviour, allowing streamside trees to recover.
Environmental change
Natural changes
Wildfires, floods, volcanic eruptions and droughts can reshape ecosystems. After a disturbance, ecosystems often recover through ecological succession, as pioneer species like mosses and grasses move in first, followed by shrubs and trees.
Human-caused changes
| Change | Effect |
|---|---|
| Habitat loss | The leading cause of species decline worldwide |
| Pollution | Fertilizer runoff causes algal blooms that use up oxygen in water |
| Climate change | Shifts ranges of species; warms oceans; affects timing of seasons |
| Overharvesting | The Atlantic cod fishery off Newfoundland collapsed in the early 1990s, leading to a moratorium in 1992 |
| Invasive species | Zebra mussels in the Great Lakes clog pipes and out-compete native species |
Biodiversity and resilience
Ecosystems with high biodiversity are usually more resilient: they can withstand and recover from disturbances better because many species fill similar roles.
Common mistakes
- Thinking populations always grow until they crash. Many stay near carrying capacity.
- Thinking removing a predator always helps prey. It can cause overpopulation and starvation.
Try it yourself
Use this data for a deer population:
| Year | Deer |
|---|---|
| 1 | 200 |
| 2 | 350 |
| 3 | 600 |
| 4 | 700 |
| 5 | 720 |
| 6 | 710 |
- Graph the data.
- Estimate the carrying capacity.
- Name two limiting factors that might be acting.
Quick recap
- Population size changes through births, deaths, immigration and emigration.
- Limiting factors keep populations near carrying capacity.
- Energy flows through trophic levels, with about 10% passed on.
- Keystone species have large effects on ecosystems.
- Human activities like habitat loss and invasive species change ecosystems.