Energy, Feeding Relationships, Nutrient Cycles, and Populations

Key terms and definitions covering energy transfer, food webs, ecological pyramids, nutrient cycles, and population dynamics for Cambridge IGCSE Biology.

Producers

Organisms that produce their own organic nutrients, usually using energy from sunlight, such as plants that carry out photosynthesis to make glucose.

Consumers

Organisms that obtain energy by feeding on other organisms, categorized as primary, secondary, tertiary, or quaternary according to their position in a food chain.

Herbivore

An animal that gets its energy by eating plants.

Carnivore

An animal that gets its energy by eating other animals.

Decomposers

Organisms that get their energy from dead or waste organic material.

Food Chain

A diagram showing the transfer of energy from one organism to the next through ingestion, starting with a producer.

Food Web

A network of interconnected food chains that illustrates the interdependence of organisms within an ecosystem.

Pyramid of Numbers

A diagram showing the relative number of individual organisms present at each trophic level of a food chain.

Biomass

A measure of the dry mass of an organism once water has been removed from its tissues.

Pyramid of Biomass

A pyramid-shaped diagram showing the amount of living matter (dry mass) present at each trophic level of a food chain.

Pyramid of Energy

A diagram representing the amount of stored energy at each trophic level over time, highlighting energy lost between levels.

Carbon Cycle

The biogeochemical cycle by which carbon is continuously transferred between atmospheric carbon dioxide, living organisms, decomposers, and fossil fuels.

Combustion

The burning of fossil fuels that combines carbon with oxygen, releasing carbon dioxide into the atmosphere.

Nitrogen Fixation

The process of converting inert atmospheric nitrogen gas ($N_2$) into ammonium compounds or nitrates, carried out by free-living or root nodule bacteria, or lightning.

Nitrification

The conversion of ammonium compounds into nitrites and subsequently into nitrates by soil nitrifying bacteria.

Denitrification

The process by which anaerobic denitrifying bacteria convert nitrates in the soil back into atmospheric nitrogen gas ($N_2$).

Nitrogen Cycle

The ecosystem cycle involving nitrogen fixation, feeding, decomposition, nitrification, and denitrification to recycle nitrogen.

Population

A group of organisms of one species living in the same area at the same time.

Community

All of the populations of different species living and interacting within an ecosystem.

Ecosystem

A unit containing the community of organisms and their environment, interacting together.

Sigmoid Growth Curve

An S-shaped population growth curve for organisms growing in an environment with limited resources, consisting of lag, log, stationary, and death phases.

Lag Phase

The initial period in a population growth curve where organisms adapt to their environment before active reproduction takes place.

Log Phase (Exponential Phase)

The phase of rapid population growth where resources are abundant, birth rate is high, and death rate is low.

Stationary Phase

The phase where population size levels out because a limiting environmental factor causes birth rate and death rate to become equal.

Death Phase

The phase of population decline where death rate exceeds birth rate due to nutrient depletion or buildup of toxic metabolic waste.