What changes populations

How populations grow

A population changes in four ways: births and immigration (animals moving in) increase it, while deaths and emigration (moving out) decrease it.

change in population = (births + immigration) − (deaths + emigration)

When a few individuals arrive in a new area with plenty of resources, numbers grow slowly at first, because there are few parents. Then growth becomes rapid, because more individuals means more births. Eventually the population runs short of food, water, shelter or space, deaths rise and births fall, and numbers level off. The largest population an environment can support long-term is its carrying capacity. In real ecosystems, populations go up and down around it rather than sitting exactly on it.

Limiting factors

Anything that stops a population growing is a limiting factor.

• Biotic factors (from living things): food supply, predators, disease, parasites and competition.
• Abiotic factors (non-living): water, temperature, light, nesting sites, soil nutrients, and events such as drought, flood and fire.

Seasons also matter. Many Australian animals breed when food is most plentiful: the male brown antechinus is famous for mating so frantically in late winter that all the males die within weeks, leaving the females to raise young when insects are most abundant in spring.

Australian case studies

Disease: the Tasmanian devil

Devil facial tumour disease was first noticed in 1996. It is a rare cancer that spreads between devils when they bite each other's faces while feeding or mating. Devil numbers fell by about 80% in two decades. Scientists set up "insurance populations" on the mainland and on Maria Island, and some wild devils now show signs of evolving resistance.

Boom and bust: mouse plagues

House mice were introduced with European settlement and live in farmland across southern and eastern Australia. A female can breed from about six weeks old and can have a litter of six to eight young every three to four weeks. Usually their numbers are held down by limited food, cold, predators and disease. After a drought breaks, though, the carrying capacity suddenly jumps: a huge grain crop, with spilt grain and weeds, provides food far beyond what the small surviving population can eat. With few limits, the population grows almost unchecked for several breeding cycles.

The bust comes when the population overshoots. Once the harvest is in and winter arrives, food runs out and cold weather stops breeding. Crowded, hungry mice fight, eat each other and spread disease, and numbers crash. The 2021 plague did enormous damage to farms and towns before this happened. CSIRO scientists now predict likely plague years from rainfall, crop and mouse survey data, so farmers can bait early, before the boom.

(Fire is another big influence on Australian populations. You studied how plants and animals respond to fire, and cultural and hazard reduction burning, in Year 8, 1I.)

Drought and water: the Murray–Darling

The Murray–Darling Basin supplies much of Australia's irrigation water. Taking too much water, combined with drought, means wetlands no longer flood regularly, so fish, frogs and waterbirds can't breed. In the summers of 2018–19 and 2023, millions of fish, including large Murray cod, died near Menindee when hot weather and low flows caused oxygen levels to crash.

Introduced species

Introduced species often have few natural predators, diseases or competitors in their new home, so they can grow far beyond the numbers they reach in their native range. Feral cats alone kill well over a billion native animals a year. You studied rabbits, foxes, cane toads and biological control in Year 8 (1E); the simulation below lets you model how events push an introduced population up and down.