The Organism in the EnvironmentEdexcel IGCSE Biology: Revision notes

These four terms describe organisms at different scales:

• Population — all the organisms of one species living in a particular habitat at a particular time
• Community — all the populations of different species living and interacting in a habitat
• Habitat — the place where an organism lives
• Ecosystem — a community of organisms together with the non-living (abiotic) parts of their environment, all interacting with each other

A quadrat is a square frame of known area used to sample organisms in a habitat.

1. Place quadrats randomly across the study area (e.g. using random coordinates) to avoid bias
2. Count the number of individuals of the chosen species in each quadrat
3. Calculate the mean number of individuals per quadrat
4. Multiply the mean by (total area of the habitat ÷ area of one quadrat) to estimate total population size

Using more quadrats, placed randomly, gives a more reliable estimate.

Biodiversity is the variety of different species living in an area (or on Earth as a whole). A habitat with many different species present in reasonably even numbers has high biodiversity; one dominated by a single species has low biodiversity.

Quadrats can also be used to measure biodiversity and species distribution: by recording every species present (not just one target species) across many randomly placed quadrats, and counting how many different species are found, an estimate of biodiversity can be made. Placing quadrats along a line (a transect) shows how the distribution of species changes across an area, such as moving away from a hedge or up a shoreline.

Abiotic factors are the non-living physical and chemical conditions in an environment. Examples and their effects:

• Light intensity — affects rate of photosynthesis, limiting plant growth and population size
• Temperature — affects rates of enzyme-controlled reactions such as photosynthesis and respiration
• Water/moisture availability — limits the distribution of organisms that need water to survive or reproduce
• pH of soil or water — some species can only tolerate a narrow pH range
• Oxygen/carbon dioxide concentration — affects respiration in animals and photosynthesis in plants, particularly in aquatic habitats

Biotic factors are the living, biological influences on a population. Examples:

• Predation — an increase in predator numbers reduces prey population size
• Competition — for food, light, space or mates, between members of the same species (intraspecific) or different species (interspecific), which limits population growth
• Disease — pathogens can reduce population size, especially where individuals live at high density
• Availability of food — determines how many consumers a habitat can support