OxfordAQA International A Level Biology 3.3.1 The Effect of Biotic and Abiotic Factors on Populations Exam Questions

About OxfordAQA International A Level Biology 3.3.1 The Effect of Biotic and Abiotic Factors on Populations

This topic opens Unit 3, Populations and genes. You need to define population, community, ecosystem and niche, and explain how abiotic factors, intraspecific and interspecific competition and predation change population size. You must be able to analyse population data and tell the difference between correlation and causation. The second part covers primary succession from pioneer species to a climax community, including how each stage changes the environment for the next, and how conservation often involves managing succession. Required practical 9, on the effect of competition on seedling growth, links here. The topic connects to energy transfer, nutrient cycles and natural selection.

What you need to know

• Key terms A population is all the organisms of one species in a habitat. A community is all the populations of different species in a habitat. An ecosystem is the community plus the abiotic factors. A niche is the role of a species, shaped by its adaptations to biotic and abiotic conditions.
• Abiotic factors Non-living factors such as temperature, light, pH, water and soil nutrients affect population size. When conditions are close to a species' optimum, more individuals survive and reproduce.
• Competition Intraspecific competition is between members of the same species and limits population size to the carrying capacity. Interspecific competition is between different species and can reduce the population of one or exclude it from the habitat.
• Predation Predator and prey populations often fluctuate in cycles. As prey numbers rise predators have more food and increase; more predation then reduces prey numbers, and predator numbers fall.
• Succession Primary succession starts on bare ground with pioneer species, which change the abiotic conditions (for example forming soil) so other species can colonise. Biodiversity usually increases until a stable climax community is reached.
• Conservation Conserving some habitats means managing succession, for example by grazing, mowing or controlled burning to stop the community progressing to the climax.

Common mistakes in 3.3.1 The Effect of Biotic and Abiotic Factors on Populations

• Saying pioneer species make conditions better for themselves. They change conditions so they become less suitable for themselves and more suitable for other species.
• Confusing intraspecific and interspecific competition. Intra means within one species; inter means between species.
• Assuming two population curves that rise and fall together show cause and effect. Explain the mechanism and note that other factors may be involved.
• Defining niche as where an organism lives. That is its habitat; the niche is its role in the ecosystem.

Try three 3.3.1 The Effect of Biotic and Abiotic Factors on Populations exam-style questions