Competition (AQA A-Level Biology): Revision Notes
Competition
Competition occurs when two or more individuals share resources that are insufficient to meet all their needs. These resources may include food, light, space, oxygen, water, or breeding sites. When resources become limited, organisms must compete to survive and reproduce successfully.
Types of competition
Competition between organisms takes two main forms depending on the species involved.
- Intraspecific competition happens when individuals of the same species compete against each other for limited resources. This type of competition directly influences population size because resource availability determines how many individuals can survive in a given area.
- Interspecific competition occurs when individuals from different species compete for the same resources. This competition can lead to one species gaining an advantage over another, potentially affecting the distribution and abundance of both species.
Intraspecific competition
When members of the same species compete, the availability of resources becomes the key factor determining population size. Greater resource availability supports larger populations, while limited resources result in smaller populations.
The relationship between resource availability and population size works through birth and death rates. When resources are abundant, more individuals survive and reproduce successfully. When resources become scarce, individuals may die from starvation or fail to reproduce, reducing population size.
Examples of intraspecific competition
Examples of Intraspecific Competition in Nature
Limpets compete for algae, their primary food source. As algae becomes more available, limpet populations can grow larger.
Oak trees in dense forests compete for essential resources including light, water, and soil minerals. Some trees grow larger and monopolise these resources, causing smaller trees to die. Over time, this reduces the population to fewer, but larger, dominant trees.
Robins compete for breeding territories. Female robins typically choose males who have established suitable territories that can provide adequate food for raising young. Limited suitable territories mean fewer breeding pairs and smaller population sizes.
Interspecific competition
When different species compete for limited resources, one species typically gains a competitive advantage. The population of the more successful species gradually increases while the less competitive species declines.
The competitive exclusion principle
The Competitive Exclusion Principle
This principle explains that when two species compete for identical limited resources, the species that uses these resources most effectively will eventually eliminate the other from that habitat. Complete removal occurs because no two species can occupy exactly the same ecological niche indefinitely when resources are limiting.
Niche differentiation
Some species avoid direct competition by occupying different niches.
Niche Separation Example
Seabirds like shags and cormorants both feed on fish from coastal waters but have developed different feeding strategies. Analysis shows that shags feed primarily on sand eels and herring, while cormorants consume mainly flat fish, gobies, and shrimps. This niche separation allows both species to coexist in the same habitat.
Population size and competition
Understanding how competition affects population size requires examining birth and death rates rather than simply counting individuals. An increase in food supply doesn't automatically create more individuals - it may simply produce larger, healthier individuals. The effect on population size depends on whether the improved conditions lead to increased survival rates and reproductive success.
When food supply decreases, individuals may die from starvation, directly reducing population size. Conversely, increased food availability improves survival chances and reproductive success, leading to population growth over time.
Case study: Red and grey squirrel competition
Case Study: Red and Grey Squirrel Competition in Britain
The introduction of grey squirrels to Britain around 130 years ago provides a clear example of interspecific competition effects. Originally, red squirrels occupied woodland niches exclusively throughout the British Isles.
Population Changes (1970-1990):
- Grey squirrel populations: significantly increased
- Red squirrel populations: dramatically declined
Current Population Numbers:
- Grey squirrels: approximately 2.5 million
- Red squirrels: only 160,000
Current Distribution: Red squirrels now survive mainly in Wales, Scotland, and smaller populations in northern England and on islands like Anglesey and the Isle of Wight. This distribution pattern suggests that geographical isolation helps protect red squirrel populations from grey squirrel competition.
Competitive Advantage: The competitive advantage of grey squirrels may stem from behavioural differences. Grey squirrels spend more time foraging on forest floors compared to red squirrels, potentially allowing them to locate food more efficiently.
Experimental evidence: Paramecium competition
Experimental Evidence: Paramecium Competition Study
Laboratory experiments with Paramecium species demonstrate competitive exclusion under controlled conditions.
Experimental Setup:
- Species tested: P. aurelia and P. caudatum
- Food source: yeast
- Conditions: controlled laboratory environment
Results when grown separately: Both species reached stable population sizes
Results when grown together:
- P. aurelia: population increased to nearly 100%
- P. caudatum: population declined to near zero over a 20-day period
Conclusion: This experiment supports the competitive exclusion principle by showing that two similar species cannot coexist indefinitely when competing for identical limited resources.
Challenges in studying competition
Key Challenges in Competition Research
Proving that competition causes observed population changes presents several difficulties:
Many factors influence population size beyond competition, including disease, climate change, habitat destruction, and predation pressure. Establishing direct causal links between competition and population changes requires careful experimental design and long-term data collection.
Time delays often occur between competitive interactions and observable population changes, making it difficult to connect cause and effect clearly.
Natural population data can be unreliable or difficult to obtain, particularly for wild populations over extended time periods.
Key Points to Remember:
- Intraspecific competition occurs within the same species and directly controls population size through resource availability
- Interspecific competition happens between different species and can lead to competitive exclusion of the weaker competitor
- The competitive exclusion principle states that two species competing for identical resources cannot coexist indefinitely
- Competition effects on population size work through changes in birth and death rates, not just individual size
- Niche differentiation allows similar species to coexist by reducing direct competition for resources