Explain the factors that regulate population size and describe the main interactions between species, including competition, predation, mutualism and succession.

What this dot point is asking

This dot point asks you to explain why populations grow, shrink or stay steady, and to describe the ways species interact within a community. You should understand carrying capacity, the difference between density-dependent and density-independent factors, the main types of species interaction, and how communities change over time through succession, using Tasmanian examples where you can.

Population growth and carrying capacity

A population is all the individuals of one species living in an area. Its size changes through four processes: births and immigration add individuals, while deaths and emigration remove them. When resources are unlimited, a population can grow exponentially, producing a J-shaped curve. In the real world resources are finite, so growth slows as the population rises, producing an S-shaped (logistic) curve that levels off at the carrying capacity.

Carrying capacity is the maximum population size an environment can sustain given available food, water, shelter and space. A population may overshoot its carrying capacity and then crash, or fluctuate around it. Introduced species often grow rapidly at first because Tasmania lacks their natural predators, as seen with European rabbits and feral cats.

Limiting factors

Factors that slow or stop population growth fall into two groups. Density-dependent factors have a stronger effect as the population gets denser. Competition for food, predation, parasitism and the spread of disease all increase when individuals are crowded together. Devil Facial Tumour Disease spreads more easily where Tasmanian devils are dense, because the cancer is passed on through biting.

Density-independent factors affect the same proportion of a population regardless of its size. Bushfire, drought, flood and severe frost are typical examples; a Tasmanian summer bushfire can kill a large fraction of a population whether it is large or small.

Interactions between species

Species in a community interact in several characteristic ways.

• Competition occurs when two species need the same limited resource. Introduced rabbits compete with native herbivores for grazing.
• Predation is one species (the predator) eating another (the prey). Wedge-tailed eagles preying on small mammals help regulate prey numbers.
• Mutualism benefits both species, such as native bees pollinating flowering plants while gaining nectar.
• Parasitism benefits one species at the expense of another, as with ticks feeding on a host.

These interactions create feedback that helps regulate population sizes and keeps the community in a shifting balance.

Ecological succession

Communities are not fixed; they change over time through succession. Primary succession begins on bare ground with no soil, such as a newly exposed surface, where hardy pioneer species like lichens and mosses arrive first and slowly build soil. Secondary succession begins where a disturbance such as fire has removed vegetation but the soil remains, so recovery is faster.

In Tasmania, fire-driven succession is especially important. After a bushfire, fast-growing species colonise first, then shrubs, and eventually slower-growing eucalypts return. In the absence of fire over very long periods, wet eucalypt forest can give way to rainforest dominated by species such as myrtle beech, because rainforest seedlings tolerate shade while eucalypt seedlings need open, sunlit ground.

Modelling growth with the logistic curve

The logistic model captures these ideas in a single curve. Early on, when numbers are far below the carrying capacity ($K$), resources are plentiful and the population grows almost exponentially, so the curve rises steeply. As numbers climb toward $K$, the per-individual growth rate falls because there is less food, space and shelter to go around, so the curve bends over and flattens. The fastest absolute growth happens around the middle of the curve, near half the carrying capacity, where there are both many breeding individuals and still-abundant resources. Reading a population graph well means identifying which phase the population is in, because that tells you which factors are dominating.

Bringing it together

To answer this dot point well, describe how births, deaths and migration change population size, explain carrying capacity and the logistic curve, classify limiting factors as density-dependent or density-independent, and outline the main species interactions. Finish with a Tasmanian example of succession, such as fire-driven recovery of eucalypt forest, to show how communities develop over time.