Speciation and Conservation
4. Threats to Biodiversity
Learning outcomes
- I can identify major threats to biodiversity.
- I can explain how habitat loss affects species.
- I can describe the effects of pollution and climate change.
- I can analyze human impacts on ecosystems.
- I can evaluate strategies for reducing biodiversity loss.
Biodiversity Under Pressure
Biodiversity is the variety of life at the genetic, species, and ecosystem levels.
Earth's biodiversity has always changed through evolution, speciation, environmental change, and extinction. However, human activities can change ecosystems rapidly and place strong pressures on species.
Major threats to biodiversity include:
- Habitat loss and degradation.
- Habitat fragmentation.
- Overexploitation.
- Pollution.
- Invasive species.
- Climate change.
These threats often interact, making their combined effects greater than the effect of any one threat alone.
Habitat Loss
Habitat loss occurs when a natural habitat is destroyed or changed so extensively that organisms can no longer live there successfully.
Examples include:
- Forests cleared for agriculture.
- Wetlands drained for development.
- Grasslands converted to farmland.
- Coastal habitats replaced by buildings.
- Rivers altered by dams.
- Natural areas removed for roads or mining.
When habitat disappears, organisms may lose the resources they require for survival and reproduction.
Why Habitat Matters
A habitat provides much more than simply a place to live.
Organisms depend on their habitats for:
- Food.
- Water.
- Shelter.
- Nesting sites.
- Breeding areas.
- Protection from predators.
- Suitable temperature and moisture.
- Access to mates.
Removing or altering habitat can therefore affect many aspects of an organism's life.
Deforestation
Deforestation is the large-scale removal of forests.
Forests may be cleared for:
- Agriculture.
- Timber.
- Roads.
- Mining.
- Settlements.
- Infrastructure.
Forests contain complex communities of organisms.
Removing trees can affect:
- Plants.
- Mammals.
- Birds.
- Reptiles.
- Amphibians.
- Insects.
- Fungi.
- Soil microorganisms.
The effects can therefore spread throughout the ecosystem.
Habitat Degradation
A habitat does not need to disappear completely to become less suitable.
Habitat degradation occurs when habitat quality decreases.
Examples include:
- Pollution entering a river.
- Heavy grazing damaging grassland.
- Removal of vegetation.
- Soil erosion.
- Changes in water flow.
- Noise and light pollution.
- Introduction of invasive species.
A degraded habitat may support fewer individuals and fewer species.
Habitat Fragmentation
Habitat fragmentation occurs when a large continuous habitat is divided into smaller, isolated patches.
For example, a forest may be divided by:
- Roads.
- Farms.
- Cities.
- Railways.
- Industrial development.
Even if some habitat remains, fragmentation can create serious ecological problems.
Effects of Habitat Fragmentation
Fragmentation can:
- Reduce available habitat.
- Divide populations.
- Reduce gene flow.
- Make finding mates more difficult.
- Restrict migration.
- Increase exposure to predators.
- Increase contact with humans.
- Produce smaller populations.
Small isolated populations may also experience reduced genetic diversity and increased risk of local extinction.
Edge Effects
Fragmentation creates more edges between habitats.
Conditions near the edge of a forest can differ from conditions deep inside it.
Forest edges may experience:
- More sunlight.
- Higher temperatures.
- Lower humidity.
- Stronger winds.
- Greater human disturbance.
- Different predators.
These changes are called edge effects.
Species adapted to the interior of a forest may decline even if some forest remains.
Worked Example: A Fragmented Forest
Suppose a continuous forest supports a population of 2,000 animals.
A highway divides the forest into several isolated patches.
The animals now experience:
- Reduced movement between patches.
- Fewer available mates.
- Smaller local populations.
- Reduced gene flow.
- Increased road mortality.
The total amount of forest may not immediately fall to zero, but the population can still become much more vulnerable.
This demonstrates why habitat fragmentation can reduce biodiversity even when habitat remains.
Overexploitation
Overexploitation occurs when organisms are removed from an ecosystem faster than their populations can replace themselves.
Examples include:
- Overfishing.
- Excessive hunting.
- Unsustainable logging.
- Wildlife collection.
- Harvesting plants faster than they regrow.
If removal continually exceeds reproduction, populations decline.
Overfishing
Fish populations can reproduce and replace individuals that die.
However, if fish are caught faster than the population can reproduce, numbers decline.
Overfishing can also alter marine food webs.
For example:
A major predatory fish declines.
Its prey may increase.
Those prey organisms may consume more organisms at lower trophic levels.
The effects can therefore spread through the ecosystem.
Unsustainable Harvesting
Harvesting natural resources is not automatically harmful to biodiversity.
The important question is whether removal is sustainable.
Sustainable harvesting attempts to ensure that:
rate of removal does not exceed the population's ability to recover over the long term.
This requires information about:
- Population size.
- Reproductive rate.
- Age structure.
- Habitat quality.
- Natural mortality.
Pollution
Pollution occurs when harmful substances or forms of energy enter the environment.
Major types include:
- Air pollution.
- Water pollution.
- Soil pollution.
- Plastic pollution.
- Chemical pollution.
- Nutrient pollution.
- Noise pollution.
- Light pollution.
Different pollutants affect organisms in different ways.
Water Pollution
Water pollution can result from:
- Sewage.
- Fertilizers.
- Pesticides.
- Industrial chemicals.
- Oil.
- Plastics.
- Heavy metals.
Aquatic organisms may be affected directly by toxic substances or indirectly through changes in oxygen, food availability, or habitat quality.
Eutrophication
Excess nutrients entering lakes and rivers can cause eutrophication.
Nutrients such as nitrates and phosphates may enter water from fertilizers or sewage.
A simplified sequence is:
Excess nutrients enter water
↓
Rapid algal growth
↓
Large algal bloom
↓
Algae and other organisms die
↓
Decomposers break down dead material
↓
Respiration by decomposers uses dissolved oxygen
↓
Oxygen levels fall
↓
Aquatic animals may die
This can greatly alter aquatic biodiversity.
Bioaccumulation
Some pollutants are difficult for organisms to break down or remove.
They may gradually accumulate inside an organism's tissues.
This is called bioaccumulation.
An organism exposed repeatedly to a pollutant can therefore develop increasing concentrations over time.
Biomagnification
The concentration of some persistent pollutants can increase at higher trophic levels in a food chain.
This is called biomagnification.
For example:
Water → Plankton → Small fish → Large fish → Bird of prey
A predator consumes many contaminated prey organisms.
As a result, organisms near the top of the food chain can receive particularly high doses.
Plastic Pollution
Plastic can remain in ecosystems for long periods.
Wildlife may:
- Become entangled.
- Swallow plastic.
- Mistake plastic for food.
- Be exposed to microplastics.
Marine ecosystems are particularly affected because plastics can be transported over enormous distances by currents.
Invasive Species
An invasive species is a non-native species that spreads and causes significant ecological, economic, or other harm.
Humans can move organisms between regions:
- Intentionally.
- Through shipping.
- Through agriculture.
- Through trade.
- Through travel.
- Through the pet trade.
Some introduced organisms establish populations and spread rapidly.
How Invasive Species Affect Biodiversity
Invasive species may:
- Compete with native organisms.
- Prey on native species.
- Introduce diseases.
- Alter habitats.
- Change nutrient cycles.
- Disrupt food webs.
Native species may be particularly vulnerable if they have not evolved defenses against the introduced organism.
Worked Example: Introduced Predator
Imagine an island containing ground-nesting birds.
The birds evolved without mammalian predators.
Humans accidentally introduce rats.
The rats eat:
- Eggs.
- Chicks.
- Food used by the birds.
Bird reproduction declines.
Populations become smaller.
Some bird species may eventually disappear from the island.
This demonstrates how one introduced species can affect several parts of an ecosystem.
Climate Change
Earth's climate naturally changes over long periods, but current global warming is primarily driven by human emissions of greenhouse gases.
Important greenhouse gases include:
- Carbon dioxide.
- Methane.
- Nitrous oxide.
Human activities increasing greenhouse gas concentrations include:
- Burning fossil fuels.
- Deforestation.
- Agriculture.
- Industrial processes.
The Greenhouse Effect
Earth naturally retains some thermal energy because greenhouse gases absorb and re-emit infrared radiation.
This natural greenhouse effect keeps Earth warm enough for life as we know it.
Increasing greenhouse gas concentrations strengthens this effect and changes Earth's climate.
The problem is therefore not the existence of the greenhouse effect itself.
It is the rapid human-driven enhancement of it.
Climate Change and Biodiversity
Climate change can affect:
- Temperature.
- Rainfall.
- Drought frequency.
- Wildfire conditions.
- Ocean temperatures.
- Sea level.
- Seasonal timing.
- Species distributions.
Species may respond by:
- Moving to new areas.
- Changing migration timing.
- Changing breeding times.
- Experiencing population decline.
- Adapting over generations.
Some species cannot move or adapt quickly enough.
Changing Species Distributions
As temperatures change, suitable habitats can shift.
Some species have moved:
- Toward higher latitudes.
- To higher elevations.
- Into deeper or cooler water.
However, movement is not always possible.
Species may encounter:
- Cities.
- Farmland.
- Mountains.
- Coastlines.
- Fragmented habitats.
Habitat loss and climate change can therefore interact.
Coral Reefs and Climate Change
Coral reefs support extremely high biodiversity.
When ocean temperatures become unusually high, corals can lose the photosynthetic algae living within their tissues.
This produces coral bleaching.
Bleached coral is not necessarily immediately dead, but prolonged or repeated heat stress can cause coral mortality.
Loss of reef structure can then affect many organisms that depend on reefs for food and shelter.
Ocean Acidification
Oceans absorb some atmospheric carbon dioxide.
When carbon dioxide dissolves in seawater, chemical reactions reduce ocean pH.
This process is called ocean acidification.
It can affect organisms that build calcium carbonate structures, including some:
- Corals.
- Mollusks.
- Plankton.
Climate change and ocean acidification therefore create multiple pressures on marine ecosystems.
Changes in Seasonal Timing
Many biological events depend on seasonal cues.
Examples include:
- Flowering.
- Migration.
- Breeding.
- Insect emergence.
Climate change can alter the timing of these events.
If interacting species respond differently, an ecological mismatch may occur.
For example, birds may arrive at breeding areas after the seasonal peak in their insect food supply.
Human Population and Resource Demand
Increasing human consumption can increase pressure on ecosystems through demand for:
- Food.
- Water.
- Energy.
- Timber.
- Minerals.
- Land.
- Transportation.
The environmental effect depends not only on population size but also on:
- Consumption patterns.
- Technology.
- Resource management.
- Waste production.
Human impacts therefore vary greatly between societies and activities.
Multiple Threats Can Interact
Species rarely experience only one environmental pressure.
Consider a frog population experiencing:
- Habitat loss.
- Pollution.
- Increasing temperatures.
- An introduced predator.
Each pressure could reduce the population.
Together, their effects may be particularly severe.
This is why conservation scientists often investigate multiple interacting threats rather than searching for only one cause.
Worked Example: A Wetland Ecosystem
A wetland is located near expanding farmland and a growing city.
Changes include:
- Part of the wetland is drained.
- Fertilizer enters the remaining water.
- A non-native fish is introduced.
- Summer temperatures increase.
Possible effects include:
Habitat loss
Less wetland is available.
Eutrophication
Nutrient runoff causes algal growth and oxygen depletion.
Invasive species
Native aquatic organisms face new competition or predation.
Climate change
Higher temperatures alter water conditions.
The combined effect may cause a substantial decline in biodiversity.
Measuring Human Impact
Scientists use many types of evidence to investigate human effects on ecosystems.
They may measure:
- Species richness.
- Species abundance.
- Population size.
- Water quality.
- Air quality.
- Forest cover.
- Habitat area.
- Genetic diversity.
- Temperature.
- Pollution levels.
Long-term monitoring is particularly valuable because it reveals changes over time.
Indicator Species
Some organisms are especially sensitive to environmental conditions.
These can sometimes be used as indicator species.
For example, certain aquatic invertebrates are sensitive to pollution.
If sensitive species disappear while pollution-tolerant species remain, this may provide evidence of declining water quality.
Scientists normally combine biological indicators with physical and chemical measurements.
Reducing Biodiversity Loss
There is no single solution to biodiversity loss.
Different threats require different strategies.
Major approaches include:
- Protecting habitats.
- Restoring damaged ecosystems.
- Connecting fragmented habitats.
- Controlling invasive species.
- Reducing pollution.
- Managing harvesting sustainably.
- Reducing greenhouse gas emissions.
- Protecting threatened species.
- Monitoring populations.
Protected Areas
Protected areas can conserve important habitats and species.
Examples include:
- National parks.
- Nature reserves.
- Marine protected areas.
- Wildlife sanctuaries.
Protected areas can reduce pressures such as:
- Habitat destruction.
- Hunting.
- Logging.
- Development.
However, protection must be effectively managed to produce conservation benefits.
Wildlife Corridors
Wildlife corridors connect separated habitat patches.
Corridors can allow organisms to:
- Move between populations.
- Find mates.
- Migrate.
- Access resources.
- Maintain gene flow.
For example, forest corridors can connect forest fragments separated by farmland.
Wildlife crossings can also reduce the effects of roads.
Habitat Restoration
Habitat restoration attempts to repair damaged ecosystems.
Examples include:
- Replanting native vegetation.
- Restoring wetlands.
- Removing invasive species.
- Restoring natural river channels.
- Rebuilding coral or oyster habitats.
- Reintroducing native species.
Restoration can improve ecosystem function and increase suitable habitat.
However, preventing severe damage is often easier than completely recreating a complex ecosystem afterward.
Reducing Pollution
Strategies include:
- Treating sewage.
- Reducing fertilizer runoff.
- Limiting harmful pesticides.
- Improving industrial waste treatment.
- Reducing plastic waste.
- Preventing oil spills.
- Recycling materials.
The best strategy depends on the type and source of pollution.
Sustainable Resource Use
Resources can sometimes be harvested while maintaining populations.
Strategies may include:
- Fishing quotas.
- Seasonal restrictions.
- Protected breeding areas.
- Minimum catch sizes.
- Sustainable forestry.
- Monitoring population sizes.
A sustainable system must consider the rate at which populations can recover.
Managing Invasive Species
Invasive species management can include:
- Preventing introduction.
- Early detection.
- Rapid removal.
- Biological control.
- Physical removal.
- Habitat management.
Prevention is particularly important because removing a widespread invasive species can be extremely difficult.
Reducing Climate-Related Threats
Long-term reduction of climate-related biodiversity pressures requires limiting greenhouse gas emissions.
Strategies include:
- Increasing energy efficiency.
- Expanding low-carbon energy.
- Protecting carbon-storing ecosystems.
- Reducing deforestation.
- Improving transportation systems.
Conservation can also help species adapt by protecting connected habitats through which populations may move as environmental conditions change.
In-Situ Conservation
In-situ conservation protects species in their natural habitats.
Examples include:
- National parks.
- Nature reserves.
- Marine protected areas.
- Habitat restoration.
- Wildlife corridors.
An important advantage is that species remain within their ecological communities and continue interacting with other organisms.
Ex-Situ Conservation
Ex-situ conservation protects organisms outside their natural habitats.
Examples include:
- Zoos.
- Aquariums.
- Botanical gardens.
- Seed banks.
- Captive breeding programs.
- Frozen genetic material.
Ex-situ conservation can be particularly useful when wild populations become extremely small.
However, it does not replace the need to protect functioning natural ecosystems.
Evaluating Conservation Strategies
A conservation strategy should not simply be described as "good" or "bad."
Scientists can evaluate it using evidence.
Questions include:
- Does it address the main threat?
- Does the target population increase?
- Does habitat quality improve?
- Does genetic diversity remain adequate?
- Are other species negatively affected?
- Is the strategy sustainable over time?
- Can it operate over a sufficiently large area?
- Can results be monitored?
Different ecosystems may require different combinations of strategies.
Worked Evaluation: Wildlife Corridor
Suppose two forest populations have become isolated by farmland.
A wildlife corridor is created between the forests.
After several years, scientists observe:
- Animals using the corridor.
- Increased movement between populations.
- Greater gene flow.
- Reduced genetic differences between populations.
These observations provide evidence that the corridor is improving habitat connectivity.
However, scientists should continue monitoring the populations to determine whether these changes improve long-term survival.
Worked Evaluation: Marine Protected Area
Suppose fishing is restricted within part of a marine ecosystem.
Scientists monitor fish populations before and after protection.
They find:
- Increased abundance of several fish species.
- More large breeding adults.
- Increased species richness.
These results would provide evidence that the protected area is contributing to recovery.
However, scientists should also compare protected and unprotected locations and consider other environmental changes.
Prevention Versus Restoration
Preventing biodiversity loss can often be more effective than attempting to reverse severe damage later.
For example:
Preventing introduction of an invasive species
may be easier than removing it after it spreads.
Protecting an intact forest
may preserve ecological relationships that are difficult to recreate through reforestation.
Preventing extinction
preserves genetic information that cannot be recovered once the species disappears.
This is why conservation often combines prevention with restoration.
Biodiversity and Ecosystem Services
Protecting biodiversity can also protect ecosystem services.
These are benefits humans receive from functioning ecosystems.
Examples include:
- Pollination.
- Water purification.
- Soil formation.
- Nutrient cycling.
- Carbon storage.
- Food production.
- Flood regulation.
Protecting biodiversity therefore has ecological and human benefits.
Common Mistakes
Thinking Habitat Loss Means Only Complete Habitat Destruction
Habitat degradation and fragmentation can also reduce biodiversity.
Thinking Pollution Only Kills Organisms Directly
Pollution can alter oxygen levels, reproduction, food webs, and habitat quality.
Confusing Bioaccumulation and Biomagnification
Bioaccumulation occurs within an organism over time.
Biomagnification involves increasing concentrations through trophic levels.
Thinking Every Introduced Species Is Invasive
An introduced species is considered invasive when it spreads and causes significant harm.
Thinking the Greenhouse Effect Is Entirely Harmful
The natural greenhouse effect is essential for maintaining Earth's temperature. Human-driven increases in greenhouse gases are intensifying the effect.
Thinking Climate Change Only Means Higher Temperatures
It also affects rainfall, oceans, sea level, extreme conditions, seasons, and species distributions.
Thinking One Conservation Strategy Can Solve Every Problem
Different threats require different approaches.
Thinking Protected Areas Automatically Solve Biodiversity Loss
Protected areas require appropriate location, size, connectivity, management, and enforcement.
Ignoring Interactions Between Threats
Habitat loss, pollution, invasive species, overexploitation, and climate change can act simultaneously.
Check Your Understanding
1. Name five major threats to biodiversity.
2. Define habitat loss.
3. Explain how habitat fragmentation differs from complete habitat destruction.
4. Why can fragmented populations experience reduced genetic diversity?
5. What are edge effects?
6. Define overexploitation.
7. Explain how overfishing can affect a food web.
8. Describe the process of eutrophication.
9. Explain the difference between bioaccumulation and biomagnification.
10. How can invasive species reduce native biodiversity?
11. Explain how increasing greenhouse gas concentrations affect climate.
12. How can climate change alter species distributions?
13. What is coral bleaching?
14. Explain why multiple environmental threats can be more serious than one threat acting alone.
15. What is a wildlife corridor?
16. Compare in-situ and ex-situ conservation.
17. Give three strategies for reducing pollution.
18. Explain how sustainable harvesting can help conserve biodiversity.
19. Why is preventing habitat destruction often preferable to restoring an ecosystem afterward?
20. Describe how scientists could determine whether a conservation program is successfully protecting biodiversity.
Key Terms
- Biodiversity – variety of life at genetic, species, and ecosystem levels.
- Habitat loss – reduction or destruction of habitat available to organisms.
- Habitat degradation – decline in habitat quality.
- Habitat fragmentation – division of continuous habitat into smaller isolated patches.
- Edge effect – environmental and ecological changes occurring near habitat boundaries.
- Overexploitation – removal of organisms faster than populations can replace themselves.
- Pollution – introduction of harmful substances or forms of energy into the environment.
- Eutrophication – nutrient enrichment of water that can lead to excessive algal growth and oxygen depletion.
- Bioaccumulation – accumulation of a substance within an organism over time.
- Biomagnification – increase in concentration of certain substances at higher trophic levels.
- Invasive species – non-native species that spreads and causes significant harm.
- Climate change – long-term change in climate conditions.
- Coral bleaching – loss of symbiotic algae from coral tissues, commonly associated with environmental stress such as unusually high water temperatures.
- Ocean acidification – reduction in ocean pH caused mainly by absorption of atmospheric carbon dioxide.
- Protected area – region managed to conserve biodiversity and natural resources.
- Wildlife corridor – habitat connection allowing organisms to move between separated populations.
- Habitat restoration – process of repairing damaged ecosystems.
- In-situ conservation – conservation of species within their natural habitats.
- Ex-situ conservation – conservation outside natural habitats.
- Ecosystem services – benefits humans receive from functioning ecosystems.
Key Takeaways
- Biodiversity is threatened by several interacting environmental pressures.
- Major threats include habitat loss, overexploitation, pollution, invasive species, and climate change.
- Habitat destruction removes resources required for survival and reproduction.
- Habitat fragmentation separates populations and can reduce gene flow.
- Edge effects can change environmental conditions within remaining habitat.
- Overexploitation occurs when organisms are removed faster than populations can recover.
- Pollution can affect organisms directly and indirectly.
- Nutrient pollution can cause eutrophication and oxygen depletion.
- Persistent pollutants can bioaccumulate and biomagnify.
- Invasive species can compete with, prey upon, or spread disease to native species.
- Human-driven greenhouse gas emissions are changing Earth's climate.
- Climate change can alter habitats, species distributions, seasonal timing, and ecological interactions.
- Ocean warming can contribute to coral bleaching.
- Ocean acidification can affect marine organisms that build calcium carbonate structures.
- Environmental threats often interact.
- Protecting habitats is a major strategy for conserving biodiversity.
- Wildlife corridors can reconnect fragmented populations.
- Habitat restoration can improve damaged ecosystems.
- Sustainable harvesting can reduce pressure on wild populations.
- Invasive-species prevention and management can protect native ecosystems.
- In-situ and ex-situ conservation provide complementary approaches.
- Conservation strategies should be evaluated using measurable evidence.
- Protecting biodiversity also helps maintain ecosystem services.
- Reducing biodiversity loss requires addressing the causes of decline as well as protecting and restoring species and ecosystems.