The organism in the environment — Edexcel International GCSE Science (Double Award)

Revision notes

4 short notes, in the order of the specification. Each one in short:

1. Population, community, habitat and ecosystem

   A population is all the organisms of one species living in the same area, and a community is all the populations of different species living there. A habitat is the place where an organism lives. An ecosystem is a community together with the non-living, or abiotic, parts of its environment, such as soil, water, light and temperature.

2. Practical: population size with quadrats

   Practical

   A quadrat is a square frame used to sample the organisms in a small, known area. To compare population size in two areas, place at least 10 quadrats at random coordinates in each area, count the chosen species in each quadrat and calculate the mean number per quadrat. Random placement avoids bias, and more quadrats give a more reliable mean.

3. Estimating population size

   Population size is estimated by counting organisms in randomly placed quadrats and scaling up: estimated population = mean number per quadrat × (total area ÷ area of one quadrat). For example, a mean of 4 daisies in 0.25 m² quadrats across a 100 m² field gives 1600 daisies. Plants hard to count individually are measured by percentage cover.

4. Abiotic and biotic factors

   Abiotic factors are non-living factors, such as temperature, light and water, and biotic factors are living factors, such as competition and predation. Both affect population size and distribution. Organisms compete for limited resources like food, water, light and space, and predator and prey numbers rise and fall in a cycle, with the predator peak coming after the prey peak.