Option G.5 – Population Ecology


Option G.5 – Population Ecology

G.5.1 – Distinguish between r-strategies and K-strategies

Population ecology focuses on the numbers and structures in populations. Part of this study involves reproduction strategies in relation to habitat.

Because K-strategists are larger, they consequently require a larger habitat. Their later maturity also means that they need more protection from predators. They will perform better in environments where competition is high and there are no vacant niches. Examples include elephants, whales and tortoise.

G.5.2 – Discuss the environmental conditions that favour either r-strategies or K-strategies.

In a more predictable environment, K-strategies will be used to maximise fitness, by investing resources in long-term development and longer life.

When the environment is unstable, it is more beneficial to produce a greater number of offspring as quickly as possible, thus employing r-strategies. Ecological disruption favours r-strategists, which include pathogens and pest species.

It is not always possible for biologists to agree on which strategies organisms display. For example, if we look at human populations, the strategies seem to depend on

For example, if we look at human populations, the strategies seem to depend on the circumstances. In developed parts of the world, most people will conform to K-strategies. In less developed parts, however, more r-strategies are used. Some organisms display

Some organisms display extreme r- or K- strategies, however most organisms have life histories that are on the intermediate of the continuum. Some species can even switch strategies depending on the environmental conditions.

G.5.3 – Describe one technique used to estimate the population size of an animal species based on a capture-mark-release-recapture method

In the study of populations, the collection of accurate information on the size of populations present in a habitat is very important. A total count of all the members of a population is called a census. This gives the

A total count of all the members of a population is called a census. This gives the most accurate data, however it is usually impractical. This is because the population may be located in a very large area, fast moving or only active for short periods of time.

A Lincoln index, or the capture, mark, release, recapture (MRR) technique, is a more practical method for estimating the size of a population when they are small and mobile. This can be done with rings, tags, or dabs of paint or nail varnish.

The first sample is caught and marked (n1). The method of marking should be resistant to moisture or the deliberate actions of the animal. It also should not harm the animal, such as increased visibility to predators. For more significant results, are larger sample should be caught.

After their release, the marked individuals should be free to distribute themselves randomly among the whole population. Once this has happened, the second sample is taken (n2). Any from sample 2 that were marked (therefore belonging also to sample 1) are classified into sample 3 (n3). Once this data has been collected, the following formula is used.

A number of assumptions are made when using this technique:


G.5.4 – Describe the methods used to estimate the size of commercial fish stocks

Since fish are an important source of food for many human populations, they tend to be exploited commercially, more often marine species. Populations are commonly shared among a number of harvesting countries. As a result, over-fishing is becoming an increasing problem. In this case, there is conflict between the economic and conservation interests. The demand for harvest and the need for economic returns for fishermen must be balanced with our need to sustain these resources. In recent years, the scale of commercial fishing in Western Europe has seriously depleted stocks.

If these populations are consistently over-fished, stocks will be rapidly depleted to point where they collapse and can no longer support a commercial fishery.

Fishery research services of national governments measure and assess the four parameters in order to gain information about the state of fish stocks. These include:

Fishing mortality – the proportion of fish stocks taken each year during commercial fishing

Spawning stock biomass – the total mass of mature fish in the population

Recruitment – the number of young fish produced each year which survive to enter the spawning stock

Landings – the total annual tonnage of fish landed by the fishing fleet.

Ecological computer models are used with this data to create a picture of the total biomass of living fish across the area studied, and also track changes in trends

G.5.5 – Outline the concept of maximum sustainable yield in the conservation of fish stocks

The maximum sustainable yield (or MSY) of a stock represents the maximum average catch that a stock can sustain over an indefinite period of time. This will correspond to the optimum balance between the reproductive rate and the growth rate of the stock, as well as the death due to harvesting and natural mortality. The MSY should be exactly half the carrying capacity of the species, since it is at this stage that the population growth is the highest. Harvesting at MSY will usually result in lower harvest rate at many fisheries. The maximum sustainable yield is usually higher than the optimum sustainable yield.


For the MSY to be calculated, a good knowledge of the relationship between the size of the stock and the number of juveniles produced each year is required. However, there is a natural variation in the production of juveniles, which makes establishing this relationship difficult. This may lead to setting the maximum sustainable yield too high or low. Data needs to be collected over at least 20 years. There are still reliable methods for obtaining an adequate approximation of the MSY

G.5.6 – Discuss international measures that would promote the conservation of fish

Fishing nations have had to implement various control measure to cope with their over exploiting of their fishing resources. These have included bans on the fishing of certain species. If these are respected, breeding stocks are allowed to recover.

The European Union is regulating fishing around the shores of the member states through various measures:

At the same time, funding is maintained for national marine biology station to carry out research to gain a clear picture of the ecology in that area, and of the key stages of the life cycles organisms in the relevant food chain. Other measures are:

  • Monitoring of stocks and reproduction rates
  • Quotas for catches
  • Minimum net sizes to stop immature fish being caught
  • Banning drift nets, which catch many different species indiscriminately
  • Moratoria (temporary suspension) on catching endangered species