3.8.8Additional lessons
Interspecific Competition — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 17 of 101
Part 1 of 5Niche and Competition
Niche
- A niche is the role of a species in its habitat, including its food, space, behaviour and interactions with other species.
Fundamental niche
- This is the full range of conditions and resources a species could use if there were no competitors.
Realised niche
- This is the narrower range actually occupied once competition and predators are taken into account.
Interspecific competition
- When two species share a resource, each lowers the other's survival, growth or reproduction.
Part 2 of 5Competitive Exclusion
Controlled laboratory experiments can show the effect of competition.
1
Gause grew two species of Paramecium, P. aurelia and P. caudatum, separately in culture and each grew well.
2
When the two species were grown together and fed on the same bacteria, P. aurelia increased more quickly.
3
P. caudatum declined and eventually disappeared from the mixed culture because it was out-competed for food.
4
The result illustrates the competitive exclusion principle: two species with the same niche cannot coexist indefinitely in the same place.
Part 3 of 5Coexistence and Evidence
Coexistence and Evidence
Resource partitioning
- Species divide a resource, for example by feeding at different heights or times, so overlap is reduced and both can persist.
Barnacles
- Connell found that Chthamalus lived higher on rocky shores than Balanus because Balanus out-competed it lower down,
- so its realised niche was smaller.
Character displacement
- Over generations, competing species may evolve differences, such as beak size, that reduce overlap in resource use.
Fieldwork
- Removing one species and observing the other shows how competition affects its distribution.
| Concept | Meaning |
|---|---|
| Fundamental niche | Potential range without competitors |
| Realised niche | Actual range with competitors |
| Competitive exclusion | One species eliminates another with an identical niche |
| Resource partitioning | Sharing a resource in different ways |
Part 4 of 5Describing Competition Experiments
1
Choose two species that might use the same resource and grow each alone to measure how well it does without a competitor.
2
Grow the two together under the same conditions, keeping temperature, food supply and space constant.
3
Count the numbers of each species at regular intervals and plot population size against time.
4
Compare the mixed cultures with the single-species controls to see whether either population was reduced by the other.
5
State a conclusion carefully, noting that laboratory results may not exactly match what happens in nature.
Part 5 of 5Exam Checklist & Tips
3.8.8 – Key Points for the Exam
- Niche The role of a species in its ecosystem.
- Competitive exclusion Loss of a species from an area because a competitor uses the resource better.
- Resource partitioning Division of resources so species do not compete directly.
- Realised niche The niche actually occupied.
- Fundamental niche The niche that could be occupied without competitors.
Common Mistakes
- Using niche to mean only the place where an organism lives, which is its habitat.
- Saying the weaker competitor always dies at once, when the outcome depends on conditions.
- Forgetting that competitive exclusion applies to species with the same niche.
- Mixing up fundamental and realised niches.
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