Definitions (Ecosystems): Genetics, Evolution & Ecosystems: Biology OCR A Level

Abiotic factors

The non-living aspects of an ecosystem e.g. light, temperature, water availability, oxygen availability and soil pH.

Abundance

The number of individuals per species in a specific area at any given time.

Ammonification

The production of ammonium compounds when decomposers feed on organic nitrogen-containing molecules.

Azotobacter

A type of nitrogen-fixing bacteria that lives freely in the soil.

Belt transect

A line along a sampled area upon which quadrats are placed at intervals to determine the abundance and distribution of organisms in an ecosystem.

Biomass

The total mass of organic material, measured in a specific area over a set time period. This can be calculated in terms of dry mass.

Biotic factors

The living components of an ecosystem e.g. food availability, pathogens, predators and other species.

Carbon cycle

The cycle through which carbon (in the form of carbon dioxide) moves between living organisms and the environment, involving respiration, photosynthesis and combustion.

Carrying capacity

The maximum population size that can be indefinitely supported by an environment.

Climax community

The stable community of organisms that exists at the final stage of ecological succession.

Conservation

The maintenance of ecosystems and biodiversity by humans in order to preserve the Earth’s resources.

Consumers

Organisms that feed on other organisms to obtain energy.

Decomposers

Organisms that release enzymes which catalyse the breakdown of dead plant and animal material into simpler organic matter.

Deflected succession

Changes to the natural flow of succession due to human activity that result in a stable community known as a plagioclimax.

Denitrification

The conversion of nitrate ions to nitrogen gas by denitrifying bacteria.

Denitrifying bacteria

Anaerobic microorganisms, found in waterlogged soils, responsible for the reduction of nitrate ions to nitrogen gas.

Distribution

The spread of living organisms in an ecosystem.

Ecological efficiency

The efficiency of energy or biomass transfer between trophic levels is calculated using:
ecological efficiency = (energy or biomass available after transfer / energy or biomass available before transfer) x 100

Ecosystem

The community of organisms (biotic) and non-living (abiotic) components of an area and their interactions.

Food chain

Describes the feeding relationships between organisms and the resulting stages of biomass transfer. It takes the form:
producer ⟶ primary consumer ⟶ secondary consumer ⟶ tertiary consumer

Interspecific competition

A type of competition that takes place between members of different species.

Intraspecific competition

A type of competition that takes place between members of the same species.

Limiting factor

A variable that limits the rate of a particular process.

Line transect

A line along a sampled area. The species touching the transect at regular intervals are recorded to determine the abundance and distribution of organisms in an ecosystem.

Nitrification

The conversion of ammonium ions to nitrate ions by nitrifying bacteria. This takes place in two stages: ammonium ions are oxidised to nitrite ions; nitrite ions are oxidised to nitrate ions.

Nitrifying bacteria

Aerobic microorganisms found in the soil responsible for the oxidation of ammonium ions to nitrate ions.

Nitrobacter

A genus of nitrifying bacteria that oxidises nitrites into nitrates.

Nitrogen cycle

The cycle through which nitrogen moves between living organisms and the environment, involving ammonification, nitrification, nitrogen fixation and denitrification.

Nitrogen fixation

The conversion of atmospheric nitrogen gas into ammonia by nitrogen-fixing bacteria in the soil or root nodules of legumes.

Nitrogen-fixing bacteria

Microorganisms responsible for the conversion of atmospheric nitrogen gas into nitrogen-containing compounds. They can be free-living or mutualistic.

Nitrosomonas

A genus of nitrifying bacteria that oxidises ammonium compounds into nitrites.

Peat bogs

Areas of peat soil in wetland habitats formed by the accumulation of partially decayed organic matter. They store carbon and are commonly referred to as ‘carbon sinks’.

Pioneer species

Species that can survive in hostile environments and colonise bare rock or sand e.g. lichens.

Plagioclimax

A stage in succession in which artificial factors prevent the formation of a natural climax community.

Population

All organisms of the same species living with one another in a habitat at the same time.

Predator

An organism that eats other organisms.

Preservation

The restriction of human interference in an area in order to protect and maintain the ecosystem.

Prey

An organism that is eaten by predators.

Primary succession

A type of succession in which pioneer species colonise a newly formed or exposed area of land.

Producers

Photosynthetic organisms at the start of the food chain that manufacture biomass for all living things.

Quadrat

A square grid of known area used in sampling to determine the abundance of organisms in a habitat.

Rhizobium

A type of nitrogen-fixing bacteria that lives inside the root nodules of leguminous plants.

Succession

Describes changes in the community of organisms occupying a certain area over time.

Sustainable

The ability to maintain something for future generations.

Sustainable resource

A resource that can be continuously renewed; it will not diminish or run out.

Trophic level

The position of an organism in a food chain.