Same species, same place, same time: A population is a group of organisms of the same species, living in the same area at the same time, that can interbreed.
The points to remember
- The same species: only one kind of organism is counted.
- The same area: one place with a clear edge, such as one forest or one lake.
- The same time: counted over one period, such as one year.
- Capable of interbreeding: the individuals can meet and mate with each other.
- So a population is an interbreeding unit.
Remember it as: Same species, same place, same time, can breed together.
Real example: the mountain gorillas of the Virunga Mountains, on the borders of Rwanda, Uganda and the Democratic Republic of Congo, are one population: one species, one area, counted in one census, and able to meet and breed.
Give all four parts: A definition that leaves out the area or the time is not complete. Write all four parts.
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A species is bigger than a population: it is every individual of that kind, wherever it lives.
The points to remember
- A species is every individual of that kind, wherever it lives.
- A population is the part of a species in one area at one time.
- One species may be one population or many.
- Two groups that are too far apart to meet and mate are two populations.
- Mixing two species (lions and zebras) gives a community, not a population.
Remember it as: Species = everywhere. Population = here and now.
Species
- Every individual, worldwide.
- Example: all the tigers on Earth.
Population
- One area, one time, can interbreed.
- Example: the tigers of one reserve in India.
Real example: mountain gorillas live as two populations about 30 km apart: about 604 in the Virunga Mountains (2016) and about 459 in Bwindi forest in Uganda (2018). They are one species, but the two groups cannot meet to breed.
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Papers often give a graph of a population's numbers and ask you to calculate how much it changed.
Calculating a percentage change
- Read the starting and final numbers from the graph.
- Change = final number minus starting number.
- Percentage change = change ÷ starting number × 100.
- Give the answer with a % sign; a fall gives a negative answer.
- Numbers rise with births and arrivals; they fall with deaths and departures.
Remember it as: Change, divide by the start, times 100.
Worked example: from 1981 to 2016 the Virunga gorillas rose from 242 to 604. The change is 604 − 242 = 362. Then 362 ÷ 242 × 100 = 150% (149.6%).
Divide by the starting number: Divide by the number you started with, not the final one, or the answer will be wrong.
You can investigate a population in a local ecosystem, such as your school grounds or a park.
Investigating a local population
- Choose one species that you can recognise.
- Mark out one area with a clear edge, such as a school field or a pond.
- Count at one time, and write down the date and how long you looked.
- If there are too many to count, estimate from samples (later pages show how).
- Record the conditions too, such as temperature, so results can be compared.
Remember it as: One species, one area, one time: then count.
Real example: in the RSPB Big Garden Birdwatch, more than half a million people in the UK each count the birds in one garden for one hour on one weekend in January: one area, one time, so the counts can be compared.
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How this comes up: Paper 1: a graph of a species' numbers; calculate the percentage increase between two years [1].
Figure 1 shows the number of mountain gorillas counted in the Virunga Mountains between 1981 and 2016.
Calculate the percentage increase in the number of gorillas between 2003 and 2016.
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