4a The Organism in the Environment¶
Part of 4 Ecology and the Environment.
Ecology begins by naming the levels at which organisms interact with their surroundings. It then moves into ways of measuring populations and biodiversity in real habitats.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
4a-lo-1 |
4.1 understand the terms population, community, habitat and ecosystem | Ecological Levels |
4a-lo-2 |
4.2 practical: investigate the population size of an organism in two different areas using quadrats 4.3B understand the term biodiversity 4.4B practical: investigate the distribution of organisms in their habitats and measure biodiversity using quadrats |
Quadrats, Distribution and Biodiversity |
4a-lo-3 |
4.5 understand how abiotic and biotic factors affect the population size and distribution of organisms | Abiotic and Biotic Factors |
Ecological Levels¶
- An individual is a single organism of one species.
- A population is all the organisms of one species living together in a particular habitat.
- A community is all the populations of different species living together in the same habitat. Populations within a community are often interdependent.
- A habitat is the specific place where an organism lives.
- An ecosystem is the community of organisms (biotic components) and the non-living (abiotic) parts of the environment and their interactions.
Organisms are adapted to live in the conditions of their particular environment.
Using these terms precisely makes later ecology questions much easier because they refer to different scales of interaction.
Quadrats, Distribution and Biodiversity¶
Quadrats are square frames of known area. They are used to estimate the distribution and abundance of organisms within a large area by sampling small, representative sections of it.
Using quadrats with transects: - A transect is a defined line or strip across the habitat. - Quadrats are placed at regular intervals along the transect, and the number or percentage cover of each species is recorded. - This method is useful for looking at how species distribution changes along an environmental gradient (e.g. light level from a woodland edge into open ground).
Random placement: - Quadrats can also be placed at random coordinates across an area (e.g. using a random number generator). - Random placement reduces sampling bias and improves the reliability of population estimates. - Percentage cover is a quick method but is subjective; counting gives a more precise, quantitative value.
Improving reliability: - Take more samples or repeat sampling. - Use larger transects. - Ensure consistent quadrat size.
Biodiversity is the variety of species in an ecosystem. To measure biodiversity, the number of different species in each quadrat is counted (rather than just the number of individuals of one species). Greater numbers of samples and random placement improve the reliability of biodiversity estimates.
Abiotic and Biotic Factors¶
Abiotic and biotic factors affect the size, distribution and survival of populations.
Abiotic (non-living) factors:
| Factor | How it affects organisms |
|---|---|
| Light intensity | Required for photosynthesis; affects plant growth and therefore food and shelter for other species |
| Temperature | Affects enzyme activity and metabolic rate; determines which species can survive |
| Moisture levels | Plants and animals both need water to survive |
| Soil pH and mineral content | Soil pH affects decomposition rate; mineral levels determine which plants can grow, affecting the entire food web |
Biotic (living) factors:
| Factor | How it affects organisms |
|---|---|
| Food availability | More food allows more successful breeding; population size increases |
| New predators | A new predator introduced to an ecosystem can rapidly reduce prey populations |
| New pathogens | A new disease to which existing populations have no resistance can cause rapid population decline |
| Competition | When two species compete for the same resource, the better-adapted species tends to outcompete the other, reducing the weaker species' population |
Key Terms¶
- Population: all the organisms of one species in a particular area.
- Community: all the populations of different species living together in an area.
- Habitat: the place where an organism lives.
- Ecosystem: the community of organisms and the non-living environment interacting together.
- Biodiversity: the variety of species living in an ecosystem.
- Quadrat: a square frame of known area used for ecological sampling.
- Transect: a defined line across a habitat along which samples are taken at regular intervals.
- Abiotic factor: a non-living environmental factor that affects the distribution and abundance of organisms.
- Biotic factor: a living factor in the environment that affects organisms.
- Distribution: the spread or pattern of where organisms occur in a habitat.