Natural Selection and Adaptation
2. Competition and Survival
Learning outcomes
- I can explain why organisms compete for resources.
- I can identify factors that limit population growth.
- I can describe the struggle for survival.
- I can explain how competition affects populations.
- I can relate competition to natural selection.
Why Do Organisms Compete?
Every organism needs resources to survive, grow, and reproduce. However, resources in an environment are limited.
Organisms may need:
- Food.
- Water.
- Light.
- Space.
- Nutrients.
- Shelter.
- Territory.
- Nesting sites.
- Mates.
When several organisms need the same limited resource, competition occurs.
Competition is the interaction between organisms when they require the same resource and there is not enough of that resource for all organisms to obtain as much as they need.
Competition is an important ecological interaction because it can affect survival, reproduction, population size, and natural selection.
Resources Are Limited
No environment contains unlimited resources.
A grassland may contain only a certain amount of:
- Grass.
- Water.
- Shelter.
- Nesting space.
A forest has limited:
- Light.
- Soil nutrients.
- Water.
- Space.
Even when a resource is abundant at one time, it may become scarce later.
For example, water may be plentiful during a rainy season but scarce during a drought.
Competition Between Animals
Animals commonly compete for:
- Food.
- Water.
- Territory.
- Shelter.
- Nesting sites.
- Mates.
For example, several predators may hunt the same prey species.
If prey becomes scarce, competition increases.
Individuals that obtain sufficient resources are generally more likely to survive and reproduce.
Competition Between Plants
Plants also compete.
Important resources for plants include:
- Light.
- Water.
- Mineral nutrients.
- Space.
In a dense forest, tall trees may capture much of the available sunlight.
Plants beneath them may receive less light and therefore carry out less photosynthesis.
Below ground, plant roots compete for water and mineral nutrients.
Competition therefore occurs even though plants cannot move from place to place like animals.
Intraspecific Competition
Intraspecific competition occurs between members of the same species.
For example:
Two male deer competing for mates.
Two oak trees competing for light.
Two wolves competing for food.
Members of the same species often experience strong competition because they usually require very similar resources.
Interspecific Competition
Interspecific competition occurs between members of different species.
For example:
Lions and hyenas may compete for prey.
Different plant species may compete for light and water.
Different bird species may compete for nesting sites.
Both forms of competition can influence population sizes and species distributions.
What Limits Population Growth?
Populations cannot normally increase forever.
Eventually, environmental conditions limit their growth.
A limiting factor is something that restricts the size or growth of a population.
Limiting factors include:
- Food availability.
- Water availability.
- Space.
- Light.
- Nutrients.
- Predation.
- Disease.
- Competition.
- Temperature.
- Drought.
- Flooding.
- Availability of mates.
Different populations are affected by different combinations of limiting factors.
Biotic Limiting Factors
Biotic factors are living components of an ecosystem.
Biotic limiting factors include:
- Competition.
- Predation.
- Disease.
- Parasites.
- Food availability.
For example, an increase in predators may reduce the population of a prey species.
An outbreak of disease may also reduce population size.
Abiotic Limiting Factors
Abiotic factors are non-living environmental conditions.
Examples include:
- Temperature.
- Water availability.
- Light intensity.
- Soil conditions.
- Salinity.
- Oxygen availability.
- Drought.
- Flooding.
A severe drought can reduce plant growth, which may then reduce food availability for herbivores.
Abiotic factors can therefore affect entire food webs.
Population Growth
Suppose a small population enters an environment containing abundant food and space.
Initially:
Resources are abundant
→ survival is high
→ reproduction is high
→ population increases.
As the population becomes larger:
More individuals
→ greater demand for resources
→ increased competition
→ fewer resources available per individual
→ population growth slows.
This is one reason populations do not normally increase indefinitely.
Carrying Capacity
The carrying capacity of an environment is the approximate population size that the environment can support over time under particular conditions.
Carrying capacity depends on factors such as:
- Food.
- Water.
- Space.
- Shelter.
- Predation.
- Disease.
- Climate.
If environmental conditions change, carrying capacity can also change.
For example, a drought may reduce the amount of vegetation available to herbivores.
Less food means the environment may support fewer herbivores.
A population approaching carrying capacity typically experiences increasing competition and slower population growth.
Carrying Capacity Is Not Fixed
Carrying capacity can change over time.
Suppose a lake normally supports a population of fish.
If pollution reduces oxygen levels:
Less dissolved oxygen
→ fewer fish can survive
→ carrying capacity decreases.
If habitat quality later improves, carrying capacity may increase again.
Carrying capacity is therefore determined by current environmental conditions rather than being a permanent number.
Density-Dependent Factors
Some limiting factors become stronger as population density increases.
These are called density-dependent factors.
Examples include:
- Competition.
- Disease.
- Parasitism.
- Some forms of predation.
Imagine 20 rabbits living in a large field.
There may be plenty of grass.
If the population increases to 2,000 rabbits, competition for grass becomes much greater.
Competition therefore becomes stronger as population density increases.
Density-Independent Factors
Other factors can affect populations regardless of their density.
These are called density-independent factors.
Examples can include:
- Drought.
- Floods.
- Fires.
- Severe storms.
- Extreme temperatures.
A wildfire may affect both a small population and a large population.
However, the effects of these events can still depend on habitat conditions and the characteristics of the population.
The Struggle for Survival
Darwin recognized that organisms often produce more offspring than can survive and reproduce.
For example:
- Fish can produce thousands of eggs.
- Plants can produce thousands of seeds.
- Insects can produce hundreds of offspring.
If every offspring survived and reproduced, populations would increase extremely rapidly.
This usually does not happen.
Many individuals:
- Are eaten.
- Die from disease.
- Fail to obtain enough food.
- Cannot find suitable habitat.
- Fail to reproduce.
Darwin described this general situation as a struggle for existence.
The Struggle for Survival Does Not Always Mean Fighting
The phrase "struggle for survival" can be misleading.
Organisms do not necessarily physically fight each other.
A plant growing in shade may struggle because it receives too little light.
A bird may fail to reproduce because it cannot find a nesting site.
A rabbit may die because insufficient grass is available.
Competition can therefore occur without direct contact between individuals.
Exploitative Competition
Sometimes organisms compete indirectly by consuming the same limited resource.
This is called exploitative competition.
For example:
Two plant species absorb water from the same soil.
Neither plant directly attacks the other.
However:
Plant A absorbs water
→ less water remains
→ Plant B receives less water.
The organisms affect each other indirectly through resource use.
Interference Competition
Competition can also involve direct interactions.
This is called interference competition.
Examples include:
- Animals defending territories.
- Birds chasing competitors away from nests.
- Animals fighting over mates.
- Plants releasing chemicals that affect nearby plants.
In these situations, organisms directly interfere with another organism's access to a resource.
Competition Can Affect Survival
Consider a population of deer.
Suppose food is plentiful.
Most deer obtain enough food.
Then a drought reduces vegetation.
Now:
Less vegetation
→ increased competition
→ some deer obtain insufficient food
→ survival decreases.
Individuals that are particularly effective at obtaining food may have an advantage.
Competition Can Affect Reproduction
Competition can affect reproduction even when organisms survive.
For example, male animals may compete for access to mates.
Individuals that are more successful may reproduce more often.
Their inherited characteristics can therefore become more common in later generations.
Natural selection depends on reproductive success, not simply survival.
Competition and Population Size
Competition can help regulate population size.
Consider a rabbit population.
When the population is small:
- Food is plentiful.
- Competition is relatively low.
- Reproduction may be high.
As population size increases:
- Food becomes scarcer.
- Competition increases.
- Reproduction may decrease.
- Mortality may increase.
Population growth therefore slows.
This creates a form of negative feedback.
Population Fluctuations
Real populations do not usually remain at exactly the same size.
They may fluctuate because of changes in:
- Food.
- Weather.
- Predators.
- Disease.
- Reproduction.
- Competition.
A population may temporarily exceed the environment's carrying capacity.
Resources then become scarce, potentially causing the population to decline.
Worked Example: Rabbits in a Grassland
Imagine 100 rabbits living in a grassland.
There is enough grass to support approximately 500 rabbits.
Initially:
Population = 100
→ abundant food
→ little competition
→ rapid population growth.
Eventually the population reaches approximately 500.
Now:
Population near carrying capacity
→ stronger competition for grass
→ reduced survival and reproduction
→ slower population growth.
The population may then fluctuate around the carrying capacity.
When Carrying Capacity Changes
Suppose a severe drought reduces grass production by half.
The environment may no longer support 500 rabbits.
The carrying capacity might fall substantially.
The rabbit population may then experience:
- Greater competition.
- Reduced reproduction.
- Increased mortality.
Eventually, population size may decline toward the new carrying capacity.
Competition Can Reduce Population Growth
Competition affects population growth because individuals that cannot obtain sufficient resources may:
- Grow more slowly.
- Produce fewer offspring.
- Become more vulnerable to disease.
- Have less energy for reproduction.
- Die.
Competition therefore influences both birth rates and death rates.
Competition Can Affect Distribution
Competition can also influence where organisms live.
Suppose two bird species require similar nesting sites.
If one species dominates the best sites, the other may be restricted to different habitats.
Competition can therefore influence:
- Population size.
- Habitat use.
- Geographic distribution.
Resource Partitioning
Species can sometimes reduce competition by using resources differently.
This is called resource partitioning.
For example, different bird species might feed:
- At different heights in a tree.
- On different insects.
- At different times of day.
By using different portions of the available resources, species may coexist more successfully.
Competition and Ecological Niches
An organism's ecological niche describes its role and requirements within an ecosystem.
This includes:
- What it eats.
- Where it lives.
- When it is active.
- What conditions it tolerates.
- How it interacts with other organisms.
Species with very similar niches may compete strongly.
Differences in resource use can reduce competition.
Competitive Exclusion
If two species depend on exactly the same limited resources in the same way, long-term coexistence may be difficult.
One species may eventually become more successful at obtaining the resource.
The other species may:
- Decline.
- Move elsewhere.
- Shift its resource use.
- Disappear locally.
This idea is known as the competitive exclusion principle.
Real ecosystems are complex, so environmental variation and differences in resource use can allow apparently similar species to coexist.
Competition and Natural Selection
Competition is closely connected to natural selection.
Individuals within a population vary.
Some of those differences may affect how successfully individuals compete for resources.
Suppose some birds have inherited beak shapes that allow them to open hard seeds more efficiently.
When food becomes scarce:
Variation in beak shape
→ competition for limited seeds
→ some birds obtain food more successfully
→ these birds survive and reproduce at higher rates
→ inherited advantageous characteristics are passed to offspring
→ those characteristics become more common.
Competition can therefore act as a selection pressure.
Worked Example: Giraffes Competing for Food
Imagine a population of ancestral giraffes containing inherited variation in neck length.
Suppose food near the ground becomes scarce while leaves remain available higher in trees.
Individuals able to reach more food may have an advantage.
If neck length has a heritable component:
Variation
→ some individuals can reach more food
→ competition favors these individuals
→ they have greater reproductive success
→ inherited characteristics associated with greater reach become more common over many generations.
This does not mean individual giraffes stretched their necks and passed longer necks to their offspring.
Natural selection acts on inherited variation already present in populations.
Worked Example: Plants Competing for Light
Imagine a population of plants varies genetically in height.
They grow in a crowded habitat where light is limited.
Taller plants may receive more sunlight.
More light may allow:
- More photosynthesis.
- Faster growth.
- Greater seed production.
If plant height is heritable:
Variation in height
→ competition for light
→ taller plants receive more light
→ greater reproductive success
→ genes associated with advantageous height may become more common.
However, being tall also requires resources, so the advantage depends on the environment.
Competition Does Not Always Favor the Biggest
It is incorrect to assume that competition always favors:
- The biggest.
- The strongest.
- The fastest.
The advantageous characteristic depends on the resource and environment.
For example:
During a drought, a smaller plant that requires less water might survive better than a larger plant.
In another environment, deep roots might provide an advantage.
Natural selection favors characteristics that increase reproductive success under the particular conditions present.
Competition Can Change When the Environment Changes
Suppose a population of birds normally eats small seeds.
After a drought:
- Small seeds become scarce.
- Large hard seeds remain.
Competition now occurs mainly for the remaining hard seeds.
Birds with stronger beaks may have an advantage.
If rainfall later returns and small seeds become abundant again, the selection pressure may change.
The environment therefore influences:
Which resources are limited
and
which characteristics provide an advantage.
Competition Between Predators
Predators can compete for the same prey.
Suppose two predator species eat rabbits.
If rabbit numbers decline:
Less prey available
→ stronger competition
→ predator survival or reproduction may decline.
One predator may switch to another prey species.
This can reduce competition.
Competition can therefore influence food-web relationships.
Competition and Food Webs
Competition does not affect only the organisms directly competing.
Changes can spread through food webs.
For example:
Drought
→ fewer plants
→ stronger competition among herbivores
→ herbivore population decreases
→ less prey available for predators
→ predator population may decline.
One environmental change can therefore affect several trophic levels.
Competition and Disease
High population density can increase both competition and disease transmission.
Imagine many animals crowded into a small habitat.
They may experience:
- Less food per individual.
- Greater competition.
- More frequent contact.
- Faster disease transmission.
Multiple limiting factors can therefore act simultaneously.
Competition and Invasive Species
An introduced species can sometimes compete with native species for:
- Food.
- Water.
- Space.
- Nesting sites.
If the introduced species obtains resources more effectively, native populations may decline.
Competition is therefore one mechanism through which invasive species can affect biodiversity.
Applying the Idea to Natural Selection
When asked to explain how competition could lead to natural selection, use this sequence:
1. Variation
Individuals have different inherited characteristics.
2. Limited resources
Not every organism can obtain everything it needs.
3. Competition
Individuals compete for those resources.
4. Advantage
Some inherited characteristics improve competitive success.
5. Reproductive success
Individuals possessing those characteristics produce more surviving offspring.
6. Inheritance
Offspring inherit the advantageous characteristics.
7. Population change
The characteristics become more common over generations.
This connects ecology directly to evolution.
Scenario: Birds During a Drought
A population contains birds with different beak sizes.
A drought causes soft seeds to become scarce.
Hard seeds remain available.
Birds with larger, stronger beaks can open these seeds more effectively.
The sequence is:
Drought
→ fewer available food resources
→ increased competition
→ strong-beaked birds obtain more food
→ greater survival and reproductive success
→ genes associated with stronger beaks become more common.
Over generations, the population may change.
Scenario: Plants in a Dry Environment
A plant population contains variation in root depth.
A long dry period reduces water near the soil surface.
Plants with deeper roots can access underground water.
If root depth is heritable:
Water becomes limited
→ competition increases
→ deep-rooted plants obtain more water
→ they survive and reproduce more successfully
→ inherited characteristics associated with deeper roots become more common.
This is natural selection caused partly by competition for water.
Competition Is Not the Only Selection Pressure
Competition is an important selection pressure, but it is not the only one.
Natural selection can also result from:
- Predation.
- Disease.
- Temperature.
- Drought.
- Salinity.
- Chemicals.
- Mate choice.
Often several selection pressures act at the same time.
For example, an animal may simultaneously face:
- Competition for food.
- Predation.
- Disease.
- Competition for mates.
Natural selection therefore occurs within complex ecological systems.
Survival Alone Is Not Enough
Suppose an animal survives for many years but never reproduces.
Its genes make little or no direct contribution to future generations.
Another animal survives for a shorter time but produces many surviving offspring.
From an evolutionary perspective, the second animal may have greater fitness.
Natural selection therefore depends on:
Survival + successful reproduction
with reproductive success being the key evolutionary outcome.
Competition and Adaptation
Over many generations, competition can contribute to adaptations.
Examples might include:
- Beaks suited to particular foods.
- Roots reaching particular soil depths.
- Behaviors used to defend territories.
- Structures used to obtain food.
- Characteristics involved in attracting mates.
Adaptations are not created because organisms decide they need them.
Instead, inherited variations that improve reproductive success become more common through natural selection.
A Changing Environment Changes Competition
Environmental change can alter competition.
For example:
Climate becomes drier
→ water becomes scarcer
→ competition for water increases.
New predator arrives
→ prey behavior changes
→ competition for safe habitats may increase.
New species arrives
→ competition for food may increase.
Food becomes abundant
→ competition for food may decrease.
Competition is therefore dynamic rather than constant.
Why Competition Matters
Competition is one of the major processes connecting ecology and evolution.
At the ecological level, competition can affect:
- Population size.
- Population growth.
- Species distribution.
- Community structure.
At the evolutionary level, competition can affect:
- Reproductive success.
- Selection pressures.
- Adaptations.
- Changes in populations over generations.
Understanding competition therefore helps explain both how ecosystems function and how populations evolve.
Common Mistakes
Thinking Competition Always Means Fighting
Competition can occur indirectly when organisms use the same limited resource.
Thinking Only Animals Compete
Plants compete strongly for light, water, nutrients, and space.
Thinking Organisms Compete Only With Their Own Species
Competition can be both intraspecific and interspecific.
Assuming Population Growth Continues Forever
Limiting factors prevent populations from increasing indefinitely.
Thinking Carrying Capacity Is Always Fixed
Carrying capacity changes when environmental conditions and resource availability change.
Assuming the Strongest Organism Always Wins
Competitive success depends on the particular environment and resource.
Confusing Survival With Evolutionary Fitness
Fitness is primarily about successful reproduction and contribution to future generations.
Saying Competition Causes Individuals to Develop Useful Traits
Competition acts as a selection pressure on existing heritable variation. Individuals do not develop inherited adaptations simply because they need them.
Forgetting That Several Limiting Factors Can Act Together
Food shortage, disease, predation, and environmental conditions may affect a population simultaneously.
Check Your Understanding
1. Define competition.
2. Why do organisms compete for resources?
3. Give four resources that animals may compete for.
4. Give four resources that plants may compete for.
5. Distinguish between intraspecific and interspecific competition.
6. Define a limiting factor.
7. Give three biotic limiting factors.
8. Give three abiotic limiting factors.
9. Explain what is meant by carrying capacity.
10. Why can carrying capacity change?
11. Explain why competition often increases as population density increases.
12. What is meant by the struggle for survival?
13. Why does the struggle for survival not necessarily involve physical fighting?
14. Explain how competition can affect population size.
15. Describe how resource partitioning can reduce competition.
16. Explain how competition can act as a selection pressure.
17. A drought causes food to become scarce in a rabbit population. Explain how this could affect competition and population size.
18. A population of plants varies in root depth. Water becomes scarce. Explain how competition could contribute to natural selection.
19. Two bird species use the same nesting sites. Predict what might happen if the number of available nesting sites decreases.
20. Explain the connection between variation → competition → reproductive success → natural selection.
Key Terms
- Competition – interaction occurring when organisms require the same limited resource.
- Resource – something an organism needs to survive, grow, or reproduce.
- Intraspecific competition – competition between members of the same species.
- Interspecific competition – competition between members of different species.
- Limiting factor – factor that restricts population growth or population size.
- Biotic factor – living component of an ecosystem.
- Abiotic factor – non-living environmental condition.
- Carrying capacity – approximate population size an environment can support under particular conditions.
- Density-dependent factor – limiting factor whose effect generally becomes stronger as population density increases.
- Density-independent factor – factor that can affect populations regardless of population density.
- Struggle for existence – competition and other environmental challenges affecting survival and reproduction.
- Exploitative competition – indirect competition through use of a shared resource.
- Interference competition – direct interaction preventing another organism from obtaining a resource.
- Resource partitioning – use of resources in different ways that can reduce competition between species.
- Ecological niche – role, environmental requirements, and interactions of an organism within an ecosystem.
- Competitive exclusion – principle that species occupying extremely similar niches and depending on the same limiting resources may be unable to coexist indefinitely.
- Selection pressure – environmental factor affecting survival or reproductive success.
- Fitness – reproductive success in a particular environment.
- Natural selection – process through which inherited characteristics affecting reproductive success become more or less common over generations.
Key Takeaways
- Organisms compete because environmental resources are limited.
- Animals may compete for food, water, territory, shelter, nesting sites, and mates.
- Plants compete for light, water, mineral nutrients, and space.
- Intraspecific competition occurs within a species, while interspecific competition occurs between species.
- Population growth is restricted by limiting factors.
- Limiting factors can be biotic or abiotic.
- As populations grow, competition for limited resources often becomes stronger.
- Carrying capacity describes the approximate population size an environment can support under particular conditions.
- Carrying capacity can change when environmental conditions change.
- Competition is an important density-dependent limiting factor.
- The struggle for survival does not necessarily involve direct fighting.
- Competition can affect survival, reproduction, population size, and species distribution.
- Species can sometimes reduce competition through resource partitioning.
- Competition can act as a selection pressure.
- Individuals with inherited characteristics that improve competitive success may produce more surviving offspring.
- Those inherited characteristics can become more common over generations.
- Natural selection therefore connects variation, limited resources, competition, reproductive success, and population change.