A-Level Biology AQA Notes

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Ecosystems & Population Size

A community is all of the populations of different species living and interacting in a place at the same time.
An ecosystem is the dynamic interaction between all the living (biotic) and non-living (abiotic) factors in a given area.
Within an ecosystem, every organism occupies a specific ecological niche
A niche includes all the abiotic and biotic conditions of the environment which organisms are adapted to.
The carrying capacity is the maximum population size that can be maintained over a period in a particular habitat.
The limiting factors of the carrying capacity include abiotic factors:
- Temperature & pH- each species has its optimum levels, and deviations from this optimum reduces population growth
- Light- low light levels reduce the carrying capacity of producers, reducing the population size of consumers
- Water- low water availability reduces the population size
The limiting factors of the carrying capacity include biotic factors:
- Interspecific competition (between different species)
- Intraspecific competition (within the same species)
- Predation
The size of a population can be estimated by:
- Randomly placing quadrats, or quadrats along a belt transect, for slow-moving or non-motile organisms. Can count the number of individuals of each species in the quadrat or percentage cover.
- The mark-release-recapture method for motile organisms. It assumes there is no deaths, births, migration, marking has no effect and enough time for the animals to mix.

Succession

Succession is the variety of processes that occur over time in a species that occupy a certain area.
Primary succession is the progressive colonisation of bare rock or other barren terrain by living organisms.
- The area is first colonised by the pioneer species, changing the abiotic factors to be less hostile for other species to survive.
- Different species may be present at each stage, who change the environment so that it becomes more suitable for other species with different adaptations but less suitable for the previous species- changing biodiversity.
- The climax community is when a stable state is reached, where there is high biodiversity and a number of new species.
Secondary succession is the recolonization of an area after an earlier community has been removed or destroyed.

Conservation

Conversations is the maintenance of biodiversity, including diversity between species, genetic diversity within species and maintenance of a variety of habitats and ecosystems.

Conservation involves active human involvement and is often orientated around managing a community by halting succession, to preserve species that would be extinct by the climax community being established
The rate of growth of the human population creates an increasing demand for raw material and food. A balance between conversation and human needs is necessary in order to maintain the sustainability of natural resources

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