What controls the size of populations, and how can ecosystems be used sustainably?
The factors that limit population size, predator-prey relationships and carrying capacity, the sampling of populations, and the principles of sustainable management of ecosystems.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on populations and sustainability, covering the factors limiting population size, predator-prey cycles and carrying capacity, the sampling of populations, and the sustainable management of ecosystems.
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What this dot point is asking
Edexcel wants you to explain the factors that limit population size, describe predator-prey relationships and carrying capacity, describe how populations are sampled, and explain the principles of sustainable management of ecosystems. Mark-release-recapture calculations and predator-prey graph interpretation are common exam tasks.
Factors limiting population size
Population size is controlled by:
- Abiotic factors such as temperature, light, water and oxygen.
- Biotic factors such as food supply, competition (for resources or mates), predation and disease.
As a population grows towards the carrying capacity, resources become limiting and growth slows.
Predator-prey relationships
Sampling populations
Because we cannot count every organism, we sample and estimate.
- Quadrats and transects estimate the abundance and distribution of plants and slow-moving organisms.
- Mark-release-recapture estimates the size of mobile animal populations. The estimate is (the Lincoln index). It assumes no births, deaths or migration between samples, random mixing, and that marking does not affect survival.
- Random sampling (using random coordinates to place quadrats) avoids bias, while a transect (a line across a habitat) is used to study how distribution changes along an environmental gradient, such as up a shore.
Sustainable management
Examples include controlled harvesting of fish or timber (taking no more than is replaced so the population stays near its carrying capacity), fishing quotas and net-size limits (so young fish can breed), rotational coppicing of woodland, and replanting. The aim is to balance human needs against protecting the ecosystem for the long term.
Examples in context
Example 1. North Sea cod quotas. Overfishing pushed North Sea cod below the level at which the population could replace itself, so catches collapsed. Sustainable management set catch quotas, larger net mesh sizes (letting young fish escape and breed) and closed breeding areas, allowing the population to recover towards its carrying capacity. This shows sustainability balancing a human food need against the survival of the stock.
Example 2. Snowshoe hare and lynx. Long-term Canadian records show the lynx (predator) population peaking a year or two after the snowshoe hare (prey) population, in a roughly ten-year cycle. When hares are abundant, lynx have more food and breed successfully; the rising lynx numbers then drive the hares down, after which the lynx decline. This is the classic data set used to illustrate the predator-prey time lag.
Try this
Q1. Explain what is meant by the carrying capacity of an environment. [2 marks]
- Cue. The maximum population size the environment can support over time, given its resources.
Q2. Describe how mark-release-recapture is used to estimate population size. [3 marks]
- Cue. Capture, mark and release a sample; recapture later; use the proportion marked in the second sample to estimate the total population.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20195 marksIn a mark-release-recapture study, woodlice were caught and marked, then released. In a second sample, woodlice were caught, of which were marked. Calculate the estimated total population size, and state two assumptions of this method.Show worked answer →
A worked Lincoln-index calculation plus assumptions.
Population woodlice. Assumptions (any two): there are no births, deaths, immigration or emigration between the two samples; marked individuals mix randomly back into the population; the marking does not affect survival or behaviour (for example does not make them easier for predators to see); and marks are not lost.
Markers reward: correct working giving ; and two valid assumptions.
Edexcel 20224 marksUsing a predator-prey relationship as an example, explain why the predator population peaks shortly after the prey population peaks.Show worked answer →
Markers want the lag explained by cause and effect.
When prey numbers are high there is plenty of food for predators, so more predators survive and reproduce, and the predator population rises. This rise lags behind the prey peak because it takes time for the predators to feed, reproduce and for their offspring to mature. The growing predator population then eats more prey, so the prey numbers fall; with less food the predator numbers then fall too, allowing the prey to recover, and the cycle repeats.
Award marks for: high prey means more food for predators; predators reproduce so numbers rise; time lag for reproduction and growth; predators then reduce prey, then predators fall.
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Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)