What controls the size of a population in an ecosystem, and how do ecologists estimate it?
An ecosystem includes all the living organisms and all the abiotic conditions in a particular area. Within an ecosystem, populations of different species form a community. The population size of any species is limited by the effect of abiotic factors and biotic factors, such as interspecific and intraspecific competition and predation. Population size may vary as a result of the effect of abiotic factors and interactions between organisms; the carrying capacity is the maximum stable population size that an ecosystem can support over a long period. Students should be able to use given data to describe and interpret predator-prey relationships and to investigate populations and estimate the size of a population using randomly placed quadrats, transects and the mark-release-recapture method, including the assumptions made when using this method.
A focused answer to the AQA 3.7 dot point on populations in ecosystems. Defines ecosystem, community, population size and carrying capacity, explains abiotic and biotic limiting factors and predator-prey cycles, and details estimating populations with quadrats, transects and mark-release-recapture (with its assumptions).
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What this dot point is asking
AQA wants you to define ecosystem, community, population size and carrying capacity, explain how abiotic and biotic factors (competition and predation) limit population size, interpret predator-prey data, and estimate population size using quadrats, transects and mark-release-recapture, including its assumptions.
Carrying capacity and limiting factors
The population size of a species is limited by abiotic and biotic factors. The carrying capacity is the maximum stable population size an ecosystem can support over a long period.
Abiotic limiting factors are non-living: temperature, light intensity, water availability, pH, oxygen or carbon dioxide concentration and mineral availability. As a population grows, these become limiting and slow further growth.
Biotic limiting factors are interactions between organisms:
- Intraspecific competition: competition between members of the same species for the same resources (food, mates, territory). As population rises toward carrying capacity, intraspecific competition intensifies, reducing the rate of reproduction and survival, so the population stabilises around the carrying capacity.
- Interspecific competition: competition between members of different species for a shared resource. It can reduce the population sizes of both species, and if one is a much better competitor the other may be excluded from the habitat.
- Predation: predators reduce prey numbers, and prey availability limits predator numbers.
Predator-prey relationships
Predator and prey populations cycle, with the predator peak lagging behind the prey peak:
- When prey are abundant, predators have plenty of food, so predator numbers rise.
- More predators eat more prey, so prey numbers fall.
- With less prey, predators starve, so predator numbers fall.
- With fewer predators, prey numbers recover, and the cycle repeats.
The classic data set is the Canadian lynx and snowshoe hare, whose numbers oscillate out of phase. When interpreting predator-prey graphs, point out that the predator peak comes after the prey peak, and that other factors (disease, food supply for the prey, abiotic conditions) also influence the cycle.
Estimating population size
You cannot count every organism, so ecologists sample and scale up.
Random quadrats estimate the abundance of slow-moving or non-motile organisms (plants, limpets). Place quadrats at random coordinates (use a numbered grid and random numbers to avoid bias), count or estimate percentage cover in each, then calculate a mean per quadrat and multiply up to the total area. Random placement avoids sampling bias.
Transects measure distribution along an environmental gradient (for example, from the shore inland). Lay a tape (the transect line) and record species at intervals (an interrupted belt transect), showing how a community changes with an abiotic factor such as light or salinity.
Mark-release-recapture estimates the size of a mobile animal population using the Lincoln index.
The method relies on assumptions: marked individuals mix randomly back into the population before the second sample; the mark does not affect survival or behaviour and is not lost; and there is no significant birth, death, immigration or emigration between the two samples. If these fail, the estimate is unreliable.
Try this
Q1. Define carrying capacity and explain how intraspecific competition keeps a population near it. [3 marks]
- Cue. Carrying capacity is the maximum stable population an ecosystem can support; as numbers rise, intraspecific competition for resources increases, lowering reproduction and survival until the population stabilises.
Q2. Using a predator-prey graph, explain why the predator population peaks after the prey population. [3 marks]
- Cue. Abundant prey lets predators feed and reproduce, so predators rise after prey; more predators then reduce prey, predators then starve and fall, prey recover.
Q3. A student marked 30 snails, released them, then later caught 25 snails of which 5 were marked. Estimate the population and state two assumptions of the method. [3 marks]
- Cue. snails. Assumptions: marked snails mix randomly; the mark does not affect survival or get lost; no major births, deaths or migration between samples.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA3 marksScientists used the mark-release-recapture method to estimate the number of woodlice in a habitat. They captured and marked 60 woodlice. In a second sample of 80 woodlice, 12 were marked. Calculate the estimated population size. Give one assumption of this method.Show worked answer →
A full-mark answer applies the Lincoln index and states a valid assumption.
Estimate. woodlice.
One assumption (any one): marked individuals mix randomly back into the population; the mark does not affect survival or behaviour; no significant births, deaths, immigration or emigration between sampling; the mark is not lost.
Markers reward the correct calculation (400) and one valid assumption.
Related dot points
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