VMBO Year 3 · Ecology · Section 6.4
Ecological balance
Influences on organisms
An organism is affected by environmental factors. Biotic factors arise from living organisms, such as food organisms, predators, competitors and members of the same species. Abiotic factors are physical or chemical conditions, such as temperature, light and water availability.
Together, these factors influence survival and reproduction. Suitable conditions may allow a population to grow. Unfavourable changes can cause it to decline or disappear locally.
Levels in ecology
- An individual is one organism.
- A population consists of individuals of one species living in an area, potentially reproducing with one another.
- A community includes all populations in an area.
- An ecosystem includes the community, the abiotic environment and their interactions.
Processes at these levels continually influence one another.
Ecological balance and population size
Population size rarely stays exactly constant. It may fluctuate around a typical level under fairly stable conditions: this is a dynamic ecological balance. Population size is influenced by food availability, predators, disease and abiotic factors such as weather and temperature. Births and immigration increase numbers; deaths and emigration decrease them. Ecosystems do not always return to a fixed balance, especially after major changes.
Optimum and tolerance curves
Each species has a range of conditions it can tolerate. A tolerance curve can show how survival, growth or reproduction varies with an abiotic factor.
- Below the minimum, the organism cannot survive those conditions.
- At the optimum, performance is greatest for the process being measured.
- Above the maximum, survival is no longer possible.
The interval between minimum and maximum is the tolerance range. Near its limits, organisms may survive but grow or reproduce poorly. Other environmental factors can change the shape and limits of the curve.
Glossary
- Abiotic factors
- Non-living physical or chemical influences, such as temperature and rainfall.
- Ecological balance
- A dynamic situation in which populations fluctuate within a range under relatively stable conditions.
- Biotic factors
- Influences involving living organisms, such as food, predators and competition.
- Ecosystem
- A community and its abiotic environment, including the interactions between them.
- Individual
- One organism.
- Community
- All populations living in a particular area.
- Tolerance curve
- A diagram showing an organism's response to an environmental factor, including a minimum, optimum and maximum.
- Population
- Individuals of one species living in an area and potentially interbreeding.