Biodiversity and Conservation
3. Threats to Biodiversity
Learning outcomes
- I can identify major threats to biodiversity.
- I can explain how habitat destruction affects species.
- I can describe the impacts of invasive species.
- I can explain how pollution and climate change threaten biodiversity.
- I can analyze human activities that reduce biodiversity.
What Is Biodiversity?
Biodiversity is the variety of life found in an area or across Earth as a whole.
Biodiversity can be considered at several levels:
- Genetic diversity – variation in genes within a species.
- Species diversity – variety of species in an ecosystem.
- Ecosystem diversity – variety of habitats and ecosystems.
A tropical rainforest, for example, may contain thousands of species interacting within a complex ecosystem.
High biodiversity contributes to functioning ecosystems and provides humans with resources including food, medicines, materials, and ecosystem services.
Why Biodiversity Matters
Species within ecosystems are connected through:
- Food webs.
- Competition.
- Predation.
- Pollination.
- Seed dispersal.
- Decomposition.
- Nutrient cycling.
Removing one species can therefore affect other organisms.
For example, a decline in an important pollinator could reduce reproduction in flowering plants. This could then affect animals that depend on those plants for food.
Biodiversity can also increase the range of responses available within an ecosystem when environmental conditions change.
The Major Threats to Biodiversity
Several major processes are responsible for biodiversity loss.
These include:
- Habitat destruction and fragmentation.
- Invasive species.
- Pollution.
- Climate change.
- Overexploitation of organisms.
- Disease.
- Interactions between several threats.
Most current threats are strongly influenced by human activities.
Habitat Destruction
Habitat destruction occurs when a natural habitat is removed or changed so extensively that its original organisms can no longer survive there.
It is one of the most important causes of biodiversity loss.
Habitats may be destroyed for:
- Agriculture.
- Housing.
- Roads.
- Mining.
- Logging.
- Industry.
- Dams and reservoirs.
- Commercial development.
When a forest is cleared, organisms do not simply "move somewhere else." Suitable habitat may already be occupied, too distant, or unavailable.
How Habitat Destruction Affects Species
A habitat provides organisms with resources necessary for survival.
These include:
- Food.
- Water.
- Shelter.
- Breeding sites.
- Nesting areas.
- Suitable temperatures.
- Protection from predators.
Removing the habitat can therefore reduce the carrying capacity of an environment.
A smaller habitat can support fewer individuals.
If the population becomes sufficiently small, it becomes increasingly vulnerable to extinction.
Habitat Fragmentation
Habitats do not always disappear completely.
Sometimes a large continuous habitat is divided into smaller isolated areas.
This is called habitat fragmentation.
For example, a road constructed through a forest can divide one population into two smaller populations.
Agricultural fields and cities can create even larger barriers.
Why Fragmentation Is a Problem
Fragmentation can:
- Reduce available habitat.
- Isolate populations.
- Make movement between populations difficult.
- Reduce access to mates.
- Reduce gene flow.
- Increase exposure to predators.
- Increase contact with humans.
- Create more habitat edges.
Small isolated populations can be especially vulnerable to environmental change.
Genetic Consequences of Fragmentation
Suppose a population of 1,000 animals becomes divided into several small populations.
Individuals may no longer move easily between them.
As a result, gene flow decreases.
Small populations are more vulnerable to:
- Loss of genetic diversity.
- Inbreeding.
- Random changes in allele frequencies.
- Reduced ability to respond to future environmental change.
Habitat fragmentation can therefore reduce both population size and genetic diversity.
Edge Effects
Fragmentation creates boundaries between habitats.
These boundaries are called edges.
Conditions near a forest edge may differ from conditions deeper inside the forest.
Edges may experience:
- More sunlight.
- Higher temperatures.
- Lower humidity.
- Stronger winds.
- Greater human disturbance.
- Different predators.
Species adapted to conditions deep inside a forest may therefore decline even when some forest remains.
Invasive Species
An invasive species is a non-native species that spreads and causes ecological, economic, or other significant harm in its introduced environment.
Not every non-native species becomes invasive.
A species becomes particularly problematic when it establishes, spreads, and disrupts the existing ecosystem.
How Invasive Species Arrive
Humans can move species far beyond their natural ranges.
Introductions may be intentional or accidental.
Species can travel through:
- Ships.
- Ballast water.
- Cargo.
- Agriculture.
- Pet releases.
- Aquaculture.
- Horticulture.
- International trade and travel.
Once established, some introduced organisms can spread rapidly.
Why Can Invasive Species Be Successful?
In a new environment, an introduced species may encounter:
- Few natural predators.
- Few parasites or diseases.
- Abundant food.
- Suitable climate.
- Limited competition.
This can allow rapid population growth.
An invasive species may then compete with native species for:
- Food.
- Space.
- Light.
- Water.
- Nesting sites.
Competition with Native Species
Suppose a new herbivore enters an ecosystem and eats the same plants as a native herbivore.
If the invasive species obtains food more efficiently, the native species may receive less food.
Its population could decline.
Competition from invasive species can therefore change:
- Population sizes.
- Species distributions.
- Community structure.
Invasive Predators
Introduced predators can be especially damaging when native prey have not evolved effective defenses against them.
Island ecosystems can be particularly vulnerable because many island species evolved in environments with relatively few predators.
Introduced rats, cats, snakes, and other predators have contributed to severe declines in some island populations.
Invasive Species and Food Webs
The effects of an invasive species can extend beyond the species it directly interacts with.
Imagine an invasive predator reduces a native herbivore population.
Fewer herbivores could cause plant populations to increase.
This could affect:
- Other herbivores.
- Pollinators.
- Seed dispersers.
- Predators.
- Decomposers.
A change in one population can therefore spread through a food web.
Pollution
Pollution occurs when harmful substances or forms of energy enter the environment at levels that cause damage.
Important forms include:
- Air pollution.
- Water pollution.
- Soil pollution.
- Plastic pollution.
- Chemical contamination.
- Nutrient pollution.
- Oil pollution.
- Noise pollution.
- Light pollution.
Different pollutants affect organisms in different ways.
Chemical Pollution
Chemicals can enter ecosystems through:
- Industrial waste.
- Agricultural runoff.
- Mining.
- Sewage.
- Household waste.
Some chemicals are directly toxic.
They may:
- Damage tissues.
- Reduce reproduction.
- Interfere with development.
- Cause death.
Persistent pollutants can remain in ecosystems for long periods.
Bioaccumulation
Some pollutants are not easily broken down or removed from organisms.
They can gradually accumulate in an individual's tissues.
This is called bioaccumulation.
For example, an aquatic organism may absorb small amounts of a pollutant repeatedly from its environment and food.
Over time, the concentration in its tissues can increase.
Biomagnification
Pollutant concentrations can also increase at higher trophic levels.
This is called biomagnification.
A simplified example is:
Water → plankton → small fish → large fish → fish-eating bird
A predator consumes many contaminated prey organisms.
As a result, organisms near the top of the food chain can sometimes accumulate particularly high concentrations.
Nutrient Pollution and Eutrophication
Fertilizers contain nutrients such as nitrates and phosphates.
If large quantities enter lakes or rivers through runoff, they can stimulate rapid growth of algae.
This can produce an algal bloom.
When algae and other organisms die, decomposers break down the organic matter.
Decomposition uses oxygen.
This can reduce dissolved oxygen concentrations.
The process is called eutrophication.
Low oxygen levels can kill fish and other aquatic organisms, reducing biodiversity.
Plastic Pollution
Plastic can persist in ecosystems for long periods.
Animals can be harmed by:
- Becoming entangled.
- Swallowing plastic.
- Habitat contamination.
- Exposure to microplastics.
Large plastic objects can gradually break into smaller pieces called microplastics, which can enter food webs.
Climate Change
Earth's climate has always changed naturally, but current rapid warming is driven primarily by increased concentrations of greenhouse gases from human activities.
Major sources include:
- Burning fossil fuels.
- Deforestation.
- Agriculture.
- Industrial processes.
Climate change affects ecosystems through more than increasing average temperature.
It can alter temperature, rainfall, sea level, ocean conditions, seasonal timing, and extreme weather patterns.
Changing Species Distributions
Every species has environmental conditions within which it can survive and reproduce successfully.
As temperatures change, suitable habitats may shift.
Species may move:
- Toward higher latitudes.
- To higher elevations.
- Into deeper or cooler water.
But movement is not always possible.
A species may encounter:
- Cities.
- Roads.
- Agricultural land.
- Mountains.
- Coastlines.
- Other unsuitable habitats.
Climate change and habitat fragmentation can therefore interact.
Timing Problems
Organisms often time important biological events according to seasonal conditions.
Examples include:
- Flowering.
- Migration.
- Breeding.
- Hatching.
- Insect emergence.
Climate change can alter the timing of these events.
If interacting species respond differently, a phenological mismatch can occur.
For example, chicks may hatch after the seasonal peak in the insects they depend upon for food.
Coral Reefs and Climate Change
Coral reefs contain extremely high biodiversity.
Corals live in association with microscopic algae that provide them with much of their energy.
When water becomes unusually warm, corals may lose these algae.
This produces coral bleaching.
Bleached coral is not necessarily immediately dead, but prolonged or repeated stress can cause coral mortality.
Loss of coral habitat can then affect many fish and invertebrates that depend on reefs.
Ocean Acidification
Oceans absorb some of the carbon dioxide released into the atmosphere.
Dissolved carbon dioxide reacts with seawater and changes its chemistry, reducing pH.
This process is called ocean acidification.
It can make it more difficult for some organisms to build calcium carbonate structures.
Potentially affected organisms include:
- Corals.
- Some mollusks.
- Some plankton.
Changes to these organisms can influence entire marine food webs.
Overexploitation
Overexploitation occurs when organisms are removed from a population faster than they can be replaced through reproduction.
Examples include:
- Overfishing.
- Overhunting.
- Unsustainable logging.
- Wildlife collection.
Harvesting a species is not necessarily unsustainable.
The problem occurs when the rate of removal exceeds the population's ability to recover.
Overfishing
Fish populations can reproduce and replace individuals that are caught — up to a point.
If too many fish are removed:
- Breeding populations decline.
- Fewer young may be produced.
- Population recovery becomes slower.
- Food webs may be disrupted.
Removing large predators can also change the populations of species at lower trophic levels.
Illegal Wildlife Trade
Animals and plants may be removed from the wild for:
- Pets.
- Traditional products.
- Decorative materials.
- Food.
- Collecting.
Rare species can become particularly valuable, which can increase harvesting pressure as their populations decline.
Small populations then become increasingly vulnerable to extinction.
Human Population and Resource Use
Biodiversity loss is influenced not simply by the number of humans but also by how resources are produced and consumed.
Human activities can increase demand for:
- Agricultural land.
- Timber.
- Fresh water.
- Energy.
- Minerals.
- Fish.
- Transportation infrastructure.
- Housing.
The environmental impact depends on population, consumption patterns, technology, management practices, and policy.
Agriculture and Biodiversity
Agriculture is essential for feeding human populations, but some agricultural practices can reduce biodiversity.
Potential effects include:
- Habitat conversion.
- Pesticide use.
- Fertilizer runoff.
- Soil degradation.
- Water extraction.
- Reduction of habitat diversity.
Large areas containing a single crop species are called monocultures.
These generally support a different and often smaller range of species than the natural ecosystems they replace.
Urbanization
As cities expand, natural habitats may be:
- Destroyed.
- Fragmented.
- Surrounded by roads.
- Exposed to artificial light.
- Exposed to noise.
- Altered by pollution.
Some species adapt successfully to urban environments.
Others decline because they cannot obtain the resources or conditions they require.
Roads and Biodiversity
Roads may appear to occupy relatively little land, but their ecological effects can extend much farther.
Roads can:
- Divide habitats.
- Prevent animal movement.
- Cause collisions.
- Increase noise.
- Increase pollution.
- Allow humans and invasive species to enter previously isolated habitats.
Wildlife crossings and habitat corridors can sometimes reduce these effects.
Threats Often Work Together
A species rarely experiences only one threat.
Consider a frog population.
It might simultaneously experience:
- Wetland destruction.
- Agricultural chemicals.
- An invasive predator.
- Changing rainfall patterns.
- Disease.
The combined effect can be greater than the effect of any one threat alone.
This makes biodiversity conservation particularly challenging.
Worked Example: Forest Fragmentation
Imagine a large forest containing 500 individuals of a species.
Agricultural development divides the forest into five isolated patches.
The animals may now experience:
- Smaller populations.
- Reduced movement.
- Fewer potential mates.
- Reduced gene flow.
- Increased edge effects.
- Greater human contact.
Even though some forest remains, the species may be considerably more vulnerable than before.
Worked Example: An Invasive Predator
Imagine an island bird that nests on the ground.
For thousands of generations, there were no land predators on the island.
Humans accidentally introduce rats.
The birds may have few effective defenses against them.
Rats eat:
- Eggs.
- Chicks.
The bird population begins to decline.
This illustrates why isolated ecosystems can be especially vulnerable to invasive species.
Worked Example: Fertilizer Runoff
A farmer applies fertilizer to a field.
Heavy rainfall washes some fertilizer into a lake.
The sequence might be:
Fertilizer runoff → increased nutrients → algal bloom → algae die → decomposition increases → oxygen decreases → aquatic organisms die
This is an example of how an activity far from an organism can eventually affect its survival.
Worked Example: Climate Change and a Mountain Species
Imagine an animal adapted to cool mountain conditions.
As average temperatures increase, suitable conditions move farther uphill.
The population follows the cooler habitat.
Eventually, however, it approaches the mountain summit.
There is nowhere higher to move.
The available habitat becomes progressively smaller.
This is sometimes described as an escalator to extinction scenario for mountain species.
Population Decline and Extinction Risk
As populations become smaller, additional problems can develop.
Small populations may experience:
- Reduced genetic diversity.
- Inbreeding.
- Difficulty finding mates.
- Greater effects from random events.
- Greater vulnerability to disease.
- Reduced ability to recover after disasters.
This can create a downward cycle in which declining populations become increasingly vulnerable.
Local Extinction and Global Extinction
Local extinction, or extirpation, occurs when a species disappears from one particular area but survives elsewhere.
Global extinction occurs when the final individual of a species dies.
Once a species becomes globally extinct, its unique genetic information is permanently lost.
Extinction Is Natural — But Rate Matters
Extinction is a natural part of evolution.
Species have appeared and disappeared throughout Earth's history.
However, environmental changes caused by human activities can increase extinction risk by changing habitats more rapidly or extensively than some populations can tolerate or adapt to.
The important issue is therefore not simply whether extinction occurs, but also its rate, causes, and ecological consequences.
Biodiversity Loss Can Affect Ecosystems
Declining biodiversity can alter ecosystem processes.
Possible consequences include changes in:
- Pollination.
- Decomposition.
- Nutrient cycling.
- Food webs.
- Soil formation.
- Water quality.
- Population regulation.
Humans depend on many of these processes.
They are often called ecosystem services.
Ecosystem Services
Ecosystems provide benefits including:
- Pollination of crops.
- Purification of water.
- Soil formation.
- Nutrient cycling.
- Carbon storage.
- Flood regulation.
- Food.
- Medicines and potential medical compounds.
- Raw materials.
Protecting biodiversity is therefore not only about protecting individual species. It also helps maintain functioning ecosystems.
Analyzing Human Activities
When analyzing whether a human activity threatens biodiversity, ask:
What habitat changes?
Which species are affected?
Does population size decrease?
Does the activity reduce reproduction or survival?
Does it isolate populations?
Does it introduce pollutants or non-native species?
Are effects temporary or long-term?
Can the ecosystem recover?
This approach is more useful than simply labeling an activity as "good" or "bad."
A Cause-and-Effect Approach
Biodiversity problems can often be analyzed as chains of cause and effect.
For example:
Forest clearing
→ less habitat
→ smaller populations
→ reduced gene flow
→ reduced genetic diversity
→ greater extinction risk
Or:
Fossil fuel combustion
→ increased atmospheric greenhouse gases
→ climate change
→ changing habitat conditions
→ altered species distributions
→ changes in communities and biodiversity
Understanding these chains helps explain why particular activities threaten biodiversity.
Common Mistakes
Thinking Biodiversity Means Only the Number of Species
Biodiversity also includes genetic and ecosystem diversity.
Assuming All Non-Native Species Are Invasive
A non-native species is considered invasive when it establishes and spreads in ways that cause significant harm.
Thinking Habitat Destruction Only Matters When All Habitat Is Removed
Fragmentation and degradation can threaten populations even when some habitat remains.
Confusing Bioaccumulation and Biomagnification
Bioaccumulation occurs within an individual organism over time.
Biomagnification describes increasing concentrations through trophic levels.
Thinking Pollution Only Means Chemicals
Noise, artificial light, heat, plastics, and excess nutrients can also act as pollutants.
Assuming Climate Change Only Means Higher Temperatures
Climate change can alter rainfall, sea level, ocean chemistry, seasonal timing, extreme events, and species distributions.
Thinking All Harvesting Is Overexploitation
Harvesting becomes overexploitation when organisms are removed faster than populations can replace them.
Assuming Species Can Simply Move When Habitats Change
Movement may be prevented by geographic barriers, fragmented habitats, unsuitable climate, competition, or limited dispersal ability.
Check Your Understanding
1. Define biodiversity and identify its three major levels.
2. Explain why habitat destruction can reduce population size.
3. What is habitat fragmentation?
4. Explain how fragmentation can reduce genetic diversity.
5. Distinguish between a non-native species and an invasive species.
6. Explain two ways an invasive species could cause a native population to decline.
7. Distinguish between bioaccumulation and biomagnification.
8. Explain how fertilizer runoff can eventually cause fish deaths.
9. Describe two ways plastic pollution can affect animals.
10. Explain how climate change can alter species distributions.
11. What is a phenological mismatch?
12. Explain how coral bleaching can affect biodiversity beyond the corals themselves.
13. Define overexploitation.
14. Explain why removing a top predator can affect several trophic levels.
15. Why can small isolated populations be especially vulnerable to extinction?
16. Explain how two different threats to biodiversity could interact.
17. A road is constructed through a previously continuous forest. Predict three possible effects on biodiversity and explain the mechanism behind each.
Key Terms
- Biodiversity – variety of life at genetic, species, and ecosystem levels.
- Genetic diversity – genetic variation among individuals within a population or species.
- Species diversity – variety of species within an area.
- Ecosystem diversity – variety of ecosystems and habitats.
- Habitat destruction – removal or severe alteration of natural habitat.
- Habitat fragmentation – division of a continuous habitat into smaller isolated areas.
- Gene flow – movement of genetic information between populations through reproduction.
- Edge effect – ecological changes occurring near habitat boundaries.
- Invasive species – non-native species that spreads and causes significant harm.
- Pollution – introduction of harmful substances or forms of energy into the environment.
- Bioaccumulation – accumulation of a substance within an organism over time.
- Biomagnification – increasing concentration of some substances at higher trophic levels.
- Eutrophication – nutrient enrichment of water that can lead to algal growth and oxygen depletion.
- Climate change – long-term changes in climate patterns.
- Coral bleaching – loss of symbiotic algae from stressed corals.
- Ocean acidification – decrease in ocean pH caused primarily by absorption of atmospheric carbon dioxide.
- Overexploitation – removal of organisms faster than populations can replace them.
- Extirpation – disappearance of a species from a particular area.
- Extinction – permanent global disappearance of a species.
- Ecosystem services – benefits humans obtain from functioning ecosystems.
Key Takeaways
- Biodiversity includes genetic, species, and ecosystem diversity.
- Major threats include habitat loss, invasive species, pollution, climate change, and overexploitation.
- Habitat destruction removes resources that organisms need to survive and reproduce.
- Habitat fragmentation can isolate populations and reduce gene flow.
- Small isolated populations can lose genetic diversity and become more vulnerable to extinction.
- Invasive species can threaten native organisms through competition, predation, disease, and changes to habitats or food webs.
- Pollution can directly harm organisms or alter entire ecosystems.
- Bioaccumulation occurs within organisms, while biomagnification occurs through food chains.
- Nutrient pollution can cause eutrophication and oxygen depletion.
- Climate change can alter species distributions, seasonal timing, marine ecosystems, and habitat availability.
- Coral reefs are vulnerable to warming and changing ocean chemistry.
- Overexploitation occurs when populations are harvested faster than they can recover.
- Agriculture, urbanization, transportation, resource extraction, and other human activities can affect biodiversity in multiple ways.
- Threats often interact, making their combined effects more severe.
- Biodiversity loss can disrupt food webs and important ecosystem processes.
- Analyzing biodiversity threats requires following cause-and-effect relationships from human or environmental change to its effects on populations and ecosystems.