Population Size
Senior 3 · Biology
Population Size
Lesson 3 of 5
25 min
A pair of rats can produce dozens of young in a year, and each of those can breed within a few months. If every rat survived and bred, a grain store would be buried in rats within a few years. It never happens. Something always stops a population from growing for ever: food runs short, predators eat more, disease spreads, or there is no more space. These are the limiting factors.
Definition
Limiting factor: a condition that stops a population from growing any larger, such as a shortage of food, predation or disease.
Environmental resistance: all the limiting factors acting together on a population.
Density-dependent factor: a limiting factor whose effect grows stronger as the population becomes more crowded. Examples: competition for food, predation, disease.
Density-independent factor: a factor that affects a population by the same proportion whatever its density. Examples: a flood, a fire, a very cold night, a landslide.
Immigration: individuals moving into a population; emigration: individuals moving out.
Only four things change the size of a population (Figure 1). Limiting factors act on them: they raise the death rate, lower the birth rate, or make animals leave.
Figure 1: The four processes that change a population, and where limiting factors act.
Worked example 1: a year in an impala population
At the start of a year a park had 1,200 impalas. During the year 300 were born, 180 died, 20 arrived from outside the park and 40 left. (Illustrative data.) Calculate (a) the population at the end of the year and (b) the percentage growth.
1. Food supply. When food is plentiful, animals are well fed, breed more and survive better. When food runs short, for example grass in a long dry season, the weakest animals starve, fewer young are born and more die. Competition for food grows stronger as the population becomes more crowded, so food supply is density-dependent. Plants are limited in the same way by light, water and mineral salts.
2. Predation. Predators remove prey. The more prey there are, the easier they are to find, and the more predators they can feed, so predation also increases with prey density. You saw the result in Unit 1: predator and prey numbers rise and fall in linked cycles.
3. Disease. Pathogens (bacteria, viruses, fungi and protists) and parasites spread from one individual to the next. In a crowded population contacts are more frequent, so an infection spreads faster and further. Disease is therefore strongly density-dependent. The IUCN lists diseases caught from people, from respiratory infections to Ebola, among the threats to mountain gorillas: a small population in close contact with humans is at risk.
Worked example 2: disease and crowding
A vet recorded the percentage of cattle infected with a contagious disease in herds kept at different densities. (Illustrative data.)
| Density / cattle per hectare | 0.5 | 1 | 2 | 4 |
|---|---|---|---|---|
| Cattle infected / % | 5 | 12 | 26 | 48 |
Figure 2: Infection rises with herd density (illustrative data).
(a) Describe the pattern. (b) Is the disease a density-dependent or density-independent factor? Explain. (c) In a herd of 60 cattle kept at 2 cattle per hectare, how many would be expected to be infected?
Worked example 3: sorting limiting factors
Classify each as density-dependent (DD) or density-independent (DI): (a) a hailstorm that flattens a bean crop; (b) weevils spreading through a store of maize; (c) competition among fish for food in a crowded pond; (d) a landslide that buries part of a hillside.
Warning
Common mistakes. (1) Forgetting immigration and emigration when calculating a population change. (2) Calling weather "density-dependent": a storm or drought affects a population whatever its density, although a crowded population may then suffer more from the food shortage that follows. (3) Writing "the population dies out": limiting factors usually stop growth or cause a fall, not extinction.
Limiting Factors in an Ecosystem (MooMooMath and Science). Watch for the difference between density-dependent and density-independent factors.
Tip
Memory hook: "crowding makes it worse". If crowding makes a factor stronger (food shortage, predation, disease, competition), it is density-dependent. If it hits a population the same way whether it is crowded or not (flood, fire, frost), it is density-independent.
Practice: limiting factors
Key points
Sources and credits
Written by Ganzaa: Figures 1 and 2, the worked examples (illustrative data) and the practice questions.
