A-Level Biology OCR 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.

Biomass

Plants synthesise organic compounds from atmospheric, or aquatic, carbon dioxide.
Most of the sugars synthesised by plants are used as respiratory substrates. The rest are used to make other groups of biological molecules, forming the biomass of the plant.
Biomass is the total mass of living material in a specific area at a given time
Dry biomass shows the chemical energy store in an organism and can be measured by the process of calorimetry. A dry sample is weighed and burnt in pure oxygen within a sealed chamber, the temperature increase of the fixed volume of water is used to calculate the energy released.

Production & Productivity

Gross primary production (GPP) is the total quantity of chemical energy stored in plant biomass, in a given area or volume.
Net primary production (NPP) is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account
The NPP is available for plant growth and reproduction. It is also available for consumers in the food chain such as herbivores and decomposers.

Net production (N) is the total chemical energy consumers store after energy losses to faeces, urine and respiration have been taken away from the chemical energy store of the ingested plant food
Primary and secondary productivity is the rate of primary or secondary production, respectively. It is measured as biomass in a given area in a given time e.g. kJ ha^–1 year^–1
The percentage efficiency of energy transfer from one tropic level to another can be calculated as

Farming practices increase the efficiency of energy transfer to increase yields by:
- Reducing respiratory loses in a human food chain e.g. reduce movement of animals
- Simplifying food chains to reduce energy loss to non-human food chains e.g. killing weeds and pest using herbicides and insecticides

Nutrient Cycles

There is a finite supply of nutrients on Earth, which are recycled within natural ecosystems.
The Nitrogen cycle:

Microorganisms in Nutrient Cycles

Microorganism	Role
Mycorrhizae	Certain types of fungi associate with roots of plants to increase the surface area for absorption of water and mineral ions, including phosphate ions.
Free-Living Nitrogen-Fixing Bacteria	In the soil, they reduce nitrogen gas to ammonia.
Mutualistic Nitrogen-Fixing Bacteria	Use nitrogen gas to produce amino acids
Saprobiontic organisms	Break down dead organism to release phosphate, ammonia or ammonium compounds
Nitrifying bacteria	Free living in soil, oxidise ammonium ions into nitrites and nitrites into nitrates
Anaerobic denitrifying bacteria	Use nitrates in respiration to produce nitrogen gas

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