Speciation and Conservation
3. Extinction
Learning outcomes
- I can define extinction.
- I can identify causes of extinction.
- I can explain how extinction affects ecosystems.
- I can analyze examples of past extinctions.
- I can evaluate the impact of human activities on extinction rates.
What Is Extinction?
Extinction occurs when the last living individual of a species dies.
Once a species becomes extinct, it no longer exists anywhere on Earth.
Extinction is a natural part of evolutionary history. Species have appeared, changed, and disappeared throughout Earth's history.
However, extinction rates can increase dramatically when environmental conditions change rapidly or when human activities place intense pressure on populations.
Extinction and Evolution
Species do not exist forever.
Over geological time:
- New species form through speciation.
- Existing species evolve.
- Some species become extinct.
The diversity of life at any particular time therefore depends partly on the balance between:
Speciation
and
Extinction
If extinction occurs faster than new species form, biodiversity decreases.
Local Extinction and Global Extinction
It is useful to distinguish between local extinction and complete extinction.
A local extinction, sometimes called extirpation, occurs when a species disappears from one particular area but survives elsewhere.
For example, a species might disappear from one forest while populations survive in neighboring regions.
Global extinction occurs when the species disappears everywhere.
Once the final individual dies:
the species is extinct.
Why Small Populations Are Vulnerable
Species with very small populations are particularly vulnerable to extinction.
Small populations may experience:
- Difficulty finding mates.
- Reduced genetic diversity.
- Inbreeding.
- Greater effects of genetic drift.
- Greater vulnerability to disease.
- Greater vulnerability to environmental disasters.
A single event such as a fire, storm, or disease outbreak can have a much larger effect on a small population than on a large, widespread population.
Causes of Extinction
Extinction can have many causes.
Natural causes include:
- Climate change.
- Volcanic eruptions.
- Asteroid impacts.
- Changes in sea level.
- Disease.
- Competition.
- Predation.
- Environmental change.
Human activities can also contribute through:
- Habitat destruction.
- Habitat fragmentation.
- Overexploitation.
- Pollution.
- Introduction of invasive species.
- Human-driven climate change.
Often, several factors act together.
Environmental Change
Species are adapted to particular environmental conditions.
If conditions change, a population may:
- Move to another suitable habitat.
- Adapt through evolutionary change over generations.
- Decline.
- Become extinct.
The outcome depends partly on:
- Speed of environmental change.
- Available genetic variation.
- Population size.
- Ability to migrate.
- Availability of suitable habitat.
Rapid environmental change can be especially difficult for species to survive.
Climate Change and Extinction
Climate has changed many times throughout Earth's history.
Changes in:
- Temperature.
- Rainfall.
- Ocean conditions.
- Ice cover.
- Sea level.
can alter habitats and food webs.
Species unable to tolerate new conditions, migrate, or adapt may decline.
Today, human-driven climate change is adding substantial pressure to many ecosystems.
Habitat Destruction
Habitat destruction occurs when natural habitat is removed or changed so severely that species can no longer use it successfully.
Examples include:
- Deforestation.
- Wetland drainage.
- Urban development.
- Mining.
- Agricultural expansion.
Habitat destruction can remove:
- Food.
- Shelter.
- Nesting areas.
- Breeding sites.
- Hunting grounds.
It is one of the major pressures affecting biodiversity today.
Habitat Fragmentation
Habitat does not have to disappear completely to cause problems.
Habitat fragmentation occurs when a large continuous habitat is divided into smaller isolated patches.
Fragmentation can:
- Isolate populations.
- Reduce movement.
- Reduce gene flow.
- Make finding mates more difficult.
- Increase edge effects.
- Produce smaller populations.
Small isolated populations may have a greater risk of local extinction.
Overexploitation
Overexploitation occurs when organisms are removed from populations faster than those populations can replace themselves.
Examples include:
- Overfishing.
- Excessive hunting.
- Wildlife collection.
- Unsustainable logging.
If mortality consistently exceeds reproduction, the population declines.
If this continues far enough, extinction can result.
Worked Example: Population Decline
Suppose a fish population contains:
20,000 fish
Each year:
3,000 new fish survive to join the population
but:
5,000 fish are removed by fishing
Ignoring other factors, the population experiences a net decrease of:
5,000 - 3,000 = 2,000 fish per year
If this pattern continues, the population will decline.
This demonstrates why harvesting rates must be considered relative to population reproduction and survival.
Invasive Species
An invasive species is a non-native species that spreads and causes significant harm.
Invasive species can threaten native populations by:
- Competing for resources.
- Eating native species.
- Spreading disease.
- Altering habitats.
- Disrupting food webs.
Island species can be particularly vulnerable because they may have evolved without certain predators or competitors.
Pollution
Pollution can reduce survival and reproduction.
Important forms include:
- Water pollution.
- Air pollution.
- Soil contamination.
- Pesticides.
- Plastics.
- Oil pollution.
- Heavy metals.
- Excess nutrients.
Some pollutants can accumulate in organisms and move through food webs.
Sensitive species may decline or disappear from polluted environments.
Disease
Disease can sometimes contribute to extinction, particularly when:
- Populations are already small.
- Species have little resistance.
- A new pathogen is introduced.
- Individuals are concentrated into small habitats.
Disease often interacts with other pressures such as habitat loss and climate change.
Competition
Species compete for limited resources such as:
- Food.
- Water.
- Space.
- Light.
- Nesting sites.
If environmental conditions change or a new competitor enters an ecosystem, a species may be placed at a disadvantage.
Competition alone does not automatically cause extinction, but it can contribute to population decline.
Predation
Changes in predation can also affect populations.
For example, introducing a new predator to an island can have severe consequences for native species that lack defenses against it.
Ground-nesting birds can be particularly vulnerable to introduced mammals such as rats or cats.
The Dodo
The dodo lived on the island of Mauritius.
It became extinct during the seventeenth century following human arrival.
Factors associated with its disappearance included:
- Hunting.
- Habitat changes.
- Introduced animals.
- Predation on eggs and competition associated with introduced species.
The dodo illustrates how island species can be particularly vulnerable to rapid environmental changes associated with human arrival.
The Passenger Pigeon
The passenger pigeon once occurred in enormous numbers in North America.
During the nineteenth century, its population collapsed.
Major pressures included:
- Large-scale hunting.
- Habitat loss.
The last known passenger pigeon, named Martha, died in captivity in 1914.
Its extinction demonstrates that even a species with an extremely large population can disappear if pressures become sufficiently intense.
The Thylacine
The thylacine, sometimes called the Tasmanian tiger, was a marsupial predator.
The last known captive individual died in Tasmania in 1936.
Factors associated with its decline included:
- Human persecution.
- Habitat changes.
- Reduced prey availability.
- Small remaining population size.
The thylacine is an example of a relatively recent human-associated extinction.
Extinction Before Humans
Extinction existed long before modern humans.
The fossil record contains countless organisms that no longer exist.
Examples include:
- Trilobites.
- Ammonites.
- Non-avian dinosaurs.
- Many prehistoric marine reptiles.
- Numerous ancient plant groups.
Scientists estimate that the overwhelming majority of species that have existed throughout Earth's history are now extinct.
Extinction is therefore a normal component of evolutionary history.
Background Extinction
Background extinction refers to the relatively continuous extinction of species that occurs during ordinary evolutionary history.
Possible causes include:
- Environmental change.
- Competition.
- Predation.
- Disease.
- Habitat changes.
These extinctions occur without a global catastrophic event.
Mass Extinction
A mass extinction is an interval during which an unusually large proportion of Earth's species disappears over a geologically short period.
Scientists commonly recognize five major mass-extinction events during the Phanerozoic Eon.
These events dramatically changed the course of evolution.
The End-Permian Mass Extinction
The largest known mass extinction of the Phanerozoic occurred about 252 million years ago, at the end of the Permian Period.
A very large proportion of marine species disappeared, along with major losses on land.
Evidence links the crisis to enormous volcanic eruptions in the Siberian Traps and resulting environmental changes.
These included:
- Global warming.
- Ocean acidification.
- Changes in ocean oxygen levels.
- Disruption of carbon cycles.
This event profoundly altered Earth's ecosystems.
The End-Cretaceous Mass Extinction
About 66 million years ago, a major mass extinction occurred at the end of the Cretaceous Period.
This event eliminated the non-avian dinosaurs along with many other organisms.
Strong evidence connects the extinction to the impact of a large asteroid near what is now the Yucatán Peninsula of Mexico.
The impact caused major environmental disruption, including material entering the atmosphere and reducing sunlight reaching Earth's surface.
Food webs were severely affected.
Dinosaurs Did Not Completely Disappear
It is common to say:
"The dinosaurs became extinct."
More precisely:
Non-avian dinosaurs became extinct.
Birds evolved within the dinosaur lineage and survived the end-Cretaceous extinction.
Modern birds are therefore living dinosaurs in the evolutionary sense.
Evidence for Past Extinctions
Scientists investigate past extinction events using several forms of evidence.
These include:
- Fossils.
- Rock layers.
- Isotopes.
- Sediments.
- Impact craters.
- Volcanic deposits.
- Changes in fossil abundance.
A sudden disappearance of many fossil species from younger rock layers can provide evidence of a major extinction event.
Fossil Evidence
Suppose Species A appears throughout many rock layers.
At a particular boundary, its fossils suddenly disappear.
If the pattern occurs consistently at many locations, scientists may infer that the species became extinct around that time.
If many unrelated species disappear around the same geological boundary, this can provide evidence of a larger extinction event.
Extinction Changes Ecosystems
Every species interacts with other parts of its ecosystem.
A species may act as:
- Predator.
- Prey.
- Pollinator.
- Seed disperser.
- Decomposer.
- Competitor.
- Habitat-forming organism.
Removing a species can therefore affect many others.
Food-Web Effects
Consider:
Plants → Rabbits → Foxes
If rabbits disappeared:
Foxes would lose an important food source.
Some plants might experience reduced grazing.
Other herbivores might face altered competition.
The effects could spread through the food web.
This type of indirect ecological effect is sometimes called a trophic cascade.
Keystone Species
A keystone species has an ecological effect that is disproportionately large relative to its abundance.
If a keystone species disappears, the structure of an ecosystem can change dramatically.
For example, losing an important predator could allow prey populations to increase greatly.
This might then reduce plant populations and affect many other species.
Pollinator Loss
Many flowering plants depend on animals for pollination.
If an important pollinator becomes extinct:
- Plant reproduction may decline.
- Plant populations may decrease.
- Animals depending on those plants may lose food.
- Other ecological relationships may change.
One extinction can therefore have consequences for many species.
Coextinction
Coextinction occurs when the extinction of one species contributes directly to the extinction of another species that depends strongly upon it.
This could involve:
- Parasites and their hosts.
- Specialized pollinators and plants.
- Organisms dependent on one particular food species.
Highly specialized ecological relationships can therefore create linked extinction risks.
Extinction and Ecological Niches
When a species disappears, its ecological role may become vacant.
Other species may eventually:
- Expand into the available niche.
- Increase in abundance.
- Evolve to exploit available resources.
However, ecological replacement is not necessarily immediate or complete.
The extinction of a species permanently removes its unique evolutionary history.
Human Activities and Modern Extinction
Humans now alter ecosystems on a global scale.
Major human-related pressures include:
- Habitat conversion.
- Overexploitation.
- Pollution.
- Invasive species.
- Climate change.
These pressures frequently interact.
For example:
A species may already have a small population because of habitat loss.
Climate change then reduces suitable habitat further.
An invasive predator may place additional pressure on the remaining population.
The combined effects can be much greater than any single pressure.
Extinction Risk
Not every species faces the same extinction risk.
Species can be especially vulnerable if they have:
- Small populations.
- Small geographic ranges.
- Highly specialized diets.
- Specific habitat requirements.
- Low reproductive rates.
- Low genetic diversity.
- Limited ability to disperse.
Species found only in one small geographic region are called endemic species and may be particularly vulnerable if that habitat is damaged.
Worked Example: Island Bird
Imagine a bird species that:
- Lives on one small island.
- Nests on the ground.
- Produces one chick each year.
- Has never encountered mammalian predators.
Humans introduce rats to the island.
The rats eat eggs and chicks.
This species may have high extinction risk because:
- Its entire range is small.
- It reproduces slowly.
- Its nests are vulnerable.
- It has little evolutionary history with the introduced predator.
Extinction and Genetic Diversity
As a population becomes smaller, genetic diversity may decline.
This can cause problems because low genetic diversity may reduce a population's ability to respond to:
- Disease.
- Environmental change.
- New selection pressures.
Small populations can also experience increased inbreeding.
This can increase the chance that harmful recessive alleles are expressed.
The Extinction Vortex
A declining population can sometimes enter a cycle known as an extinction vortex.
A simplified sequence is:
Small population
↓
Reduced genetic diversity
↓
Inbreeding and genetic drift
↓
Reduced survival or reproduction
↓
Even smaller population
↓
Greater vulnerability
This feedback can make recovery increasingly difficult.
Humans Can Also Prevent Extinction
Human activities can increase extinction risk, but conservation actions can also help populations recover.
Strategies include:
- Habitat protection.
- Habitat restoration.
- Wildlife corridors.
- Legal protection.
- Sustainable harvesting.
- Control of invasive species.
- Captive breeding.
- Seed banks.
- Reintroduction programs.
Successful conservation requires identifying the specific pressures affecting a species.
Extinction and Natural Selection
Natural selection does not guarantee that a species will survive.
Natural selection can only act on existing heritable variation.
If environmental change is:
- Too rapid.
- Too severe.
- Outside the range of available variation.
a population may decline faster than evolutionary adaptation can occur.
Therefore:
"Adapt or go extinct" is an oversimplification.
Migration, ecological interactions, population size, genetic variation, and the rate of environmental change also matter.
Extinction Is Permanent
Individual organisms die naturally.
Populations may disappear locally and later return.
But global extinction is different.
When the last individual of a species dies, the species' unique evolutionary lineage ends.
Its particular combination of genetic information is lost from living biodiversity.
This is why preventing extinction is an important goal of conservation biology.
Worked Example: Interpreting Population Data
Scientists monitor a species:
2000: 12,000 individuals
2005: 9,500
2010: 6,200
2015: 3,800
2020: 2,100
2025: 1,200
The data show a strong population decline.
However, the species is not yet extinct.
Scientists would need to investigate:
- Cause of decline.
- Reproductive rate.
- Geographic distribution.
- Genetic diversity.
- Habitat condition.
- Whether other populations exist.
Declining population size indicates increased concern, but extinction occurs only when no living individuals remain.
Worked Example: Ecosystem Effects
Suppose an important predator disappears.
Before extinction:
Predators control herbivore numbers.
After extinction:
Herbivore populations increase.
Greater herbivory reduces vegetation.
Reduced vegetation affects insects, birds, and soil conditions.
This demonstrates how extinction can produce indirect effects throughout an ecosystem.
Comparing Past and Present Extinctions
Past extinction events resulted from causes including:
- Asteroid impacts.
- Massive volcanism.
- Climate change.
- Ocean chemistry changes.
Modern extinctions can also involve environmental change, but human activities are now major drivers of biodiversity loss.
This includes direct pressures such as habitat destruction and overexploitation, as well as indirect pressures such as climate change.
Extinction and Biodiversity
Extinction reduces species diversity.
It can also reduce:
Genetic diversity
because the extinct species' genetic information disappears.
Functional diversity
because the ecological roles performed by that species may be lost.
Evolutionary diversity
because an entire evolutionary lineage may disappear.
Extinction therefore represents more than simply removing one name from a species list.
Common Mistakes
Thinking Extinction Means a Species Is Rare
A rare species still exists.
Extinction means no living members remain anywhere.
Confusing Local Extinction With Global Extinction
A species may disappear from one area while surviving elsewhere.
Thinking Extinction Is Always Caused by Humans
Extinction occurred for billions of years before humans existed.
Thinking Extinction Is Always Caused by One Factor
Multiple interacting pressures often contribute.
Thinking Only Small or Weak Species Become Extinct
Even extremely abundant species can become extinct.
Thinking Natural Selection Always Allows Species to Adapt
Environmental change may occur faster than populations can adapt.
Thinking All Dinosaurs Became Extinct
Non-avian dinosaurs disappeared, but birds survived as part of the dinosaur lineage.
Assuming Extinction Affects Only the Species That Disappears
Species interact within food webs, so extinction can affect many other organisms.
Check Your Understanding
1. Define extinction.
2. What is the difference between local extinction and global extinction?
3. Why are small populations particularly vulnerable to extinction?
4. Give four natural causes of extinction.
5. Give five human activities that can increase extinction risk.
6. Explain how habitat destruction can lead to extinction.
7. What is habitat fragmentation?
8. Explain how invasive species can threaten native species.
9. What is overexploitation?
10. Why can rapid environmental change increase extinction risk?
11. What is background extinction?
12. What is a mass extinction?
13. What evidence connects the end-Cretaceous extinction with an asteroid impact?
14. Why is it more accurate to say that non-avian dinosaurs became extinct?
15. Describe two factors associated with the extinction of the dodo.
16. How can the extinction of one species affect a food web?
17. What is coextinction?
18. Why can low genetic diversity increase extinction risk?
19. Explain how human activities can combine to increase extinction risk.
20. Explain how extinction and speciation together influence biodiversity.
Key Terms
- Extinction – permanent disappearance of a species when its last individual dies.
- Local extinction – disappearance of a species from a particular area while it survives elsewhere.
- Extirpation – another term for local extinction.
- Background extinction – ongoing extinction occurring during normal evolutionary history.
- Mass extinction – unusually large loss of species over a geologically short interval.
- Habitat destruction – severe removal or alteration of natural habitat.
- Habitat fragmentation – division of continuous habitat into smaller isolated areas.
- Overexploitation – removal of organisms faster than populations can replace themselves.
- Invasive species – non-native species that spreads and causes significant harm.
- Endemic species – species naturally restricted to a particular geographic area.
- Keystone species – species with a disproportionately large ecological effect relative to its abundance.
- Coextinction – extinction linked to the loss of another species upon which a species depends.
- Extinction vortex – feedback process in which declining population size increases factors that cause further decline.
- Genetic diversity – genetic variation within a population or species.
- Speciation – evolutionary formation of new species.
Key Takeaways
- Extinction occurs when the last member of a species dies.
- Local extinction means a species disappears from one area but survives elsewhere.
- Extinction has occurred naturally throughout Earth's evolutionary history.
- Background extinction occurs continuously at relatively low rates.
- Mass extinctions involve unusually widespread species loss.
- The fossil record provides evidence of past extinctions.
- The end-Permian extinction was the largest major mass extinction of the Phanerozoic.
- The end-Cretaceous extinction occurred about 66 million years ago and is strongly associated with the Chicxulub asteroid impact.
- Non-avian dinosaurs became extinct, while birds survived.
- Habitat destruction, fragmentation, overexploitation, pollution, invasive species, and climate change can increase extinction risk.
- Human pressures often interact rather than acting independently.
- Small and geographically restricted populations are particularly vulnerable.
- Low genetic diversity can make populations less able to respond to environmental change.
- Extinction can alter food webs and ecosystem processes.
- Loss of keystone species can have particularly large ecological consequences.
- One extinction can sometimes contribute to other extinctions.
- Natural selection does not guarantee survival when environmental change is rapid or severe.
- Conservation can reduce extinction risk through habitat protection, restoration, invasive-species management, sustainable resource use, and species recovery programs.
- Speciation adds new evolutionary lineages while extinction removes them.
- The balance between speciation and extinction is one of the major processes shaping biodiversity through time.