Threats to Biodiversity: Causes, Examples and Conservation

Cut timber stacked beside a hut in Madagascar — Cut timber in Madagascar, photographed in 2015. The photograph illustrates resource use; it does not establish whether a particular harvest was sustainable. Rod Waddington / Wikimedia Commons; CC BY-SA 2.0. Image displayed without cropping.

Ecology study notes • GCSE foundations, A-level and introductory university extension

Biodiversity declines when pressures reduce the variety of genes, species or ecosystems. The five major human-driven pressures are changes in land and sea use, direct exploitation, climate change, pollution and invasive alien species. To explain their effects, connect each pressure to a biological mechanism and then to evidence of population or ecosystem change.

The five major threats to biodiversity

A threat is a process that can damage a component of biodiversity. A driver is a cause of change. Direct drivers act on organisms or habitats; indirect drivers, such as consumption patterns and governance, influence the strength of those pressures. Their importance differs among ecosystems, so there is no single ranking that fits every location.

Direct pressure	Biological mechanism	Useful response
Land and sea-use change	Conversion or altered management removes habitat, changes conditions or separates populations.	Protect remaining habitat; restore appropriate native communities.
Direct exploitation	Harvest removes organisms faster than the population can replace them.	Set evidence-based limits and monitor recruitment and survival.
Climate change	Temperature, rainfall or seasonal changes alter suitable conditions and interactions.	Reduce greenhouse-gas emissions and maintain opportunities for movement.
Pollution	Toxic substances, excess nutrients, noise or artificial light disrupt survival and reproduction.	Reduce the source and monitor the relevant pollutant and biological response.
Invasive alien species	Introduced organisms can compete, prey, transmit disease or alter ecosystem processes.	Prevent introductions; detect and respond early.

The 2019 Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services global assessment identified these five pressures. Its global findings are a starting point for local investigation, not a substitute for measurements at a particular site.

Habitat loss, fragmentation and degradation

Habitat loss reduces the amount of suitable habitat. Draining a wetland to build houses is an example. Fragmentation divides habitat into separate patches. A road may divide woodland even when substantial tree cover remains. Degradation reduces habitat quality; a wetland may remain on a map while contaminated water makes it unsuitable for sensitive organisms.

These processes often occur together, but they are not interchangeable. Fragmentation can reduce dispersal and gene flow. Small isolated populations may experience stronger genetic drift and greater risk from chance fluctuations. Edge conditions can differ from the interior in temperature, light, wind and disturbance; effects depend on the organism and landscape.

Worked geometry example: a square reserve 1 kilometre on each side has area 1 square kilometre and perimeter 4 kilometres. Four separate square patches, each 0.5 kilometres on each side, have the same combined area but total perimeter 8 kilometres. This idealised example holds habitat area constant and shows increased boundary length. Real fragmentation can also remove habitat, and ecological effects require field evidence.

Corridors can help some species move between patches. Their value depends on corridor width, quality and the species concerned; poorly designed connections may also facilitate disease or invasive species. Protecting intact habitat usually remains essential.

How overexploitation changes populations

Harvest affects population size through direct removal, but the outcome depends on reproduction, natural mortality and movement. Slow-growing species that mature late can recover particularly slowly. Removing breeding adults may also reduce future recruitment. Overharvesting and sustainable yield explains this mechanism with a population-budget example.

Effects can extend beyond the harvested species. Removing a major predator can change prey abundance; removing seed-dispersing animals can affect forest regeneration. Such changes are context-dependent, so an exam answer should explain the particular food-web connection rather than assume every harvest causes a trophic cascade.

Pollution, eutrophication and climate change

Pollution can cause direct toxicity or change the physical and chemical environment. For example, excess nutrient inputs can stimulate algal growth. When organic matter decomposes, aerobic microorganisms consume dissolved oxygen. Oxygen depletion can then harm fish and other aerobic aquatic organisms. This is a common eutrophication pathway; nutrient enrichment does not always produce the same outcome because water flow, temperature and nutrient limitation differ.

Bioaccumulation is a substance building up within an organism. Biomagnification is increasing concentration across successive trophic levels. Persistent substances that are retained in tissues can biomagnify, but not every pollutant does so. Air pollution and biodiversity covers ozone injury, acid deposition and nitrogen enrichment.

Climate change can shift suitable ranges and seasonal timing. A plant may flower earlier without its pollinator shifting by the same amount, potentially reducing successful pollination. Range movement is limited by dispersal, barriers, habitat availability and other species. Ocean warming and acidification are distinct processes, although both are linked to rising atmospheric carbon dioxide.

Invasive species: distinguish introduction from harm

An alien or non-native species occurs outside its natural range because of human introduction. An invasive alien species establishes and spreads in ways that cause harmful impacts. Therefore, non-native does not automatically mean invasive.

Introduced brown tree snakes severely affected native birds on Guam; the snakes were not introduced into the Pacific Ocean as an aquatic habitat. Predation can be especially damaging where prey have not evolved effective defences against the introduced predator. Zebra mussels can compete with native mussels and attach to their shells, interfering with normal functioning. It is incorrect to describe their overall invasive effect as simply providing native mussels with food and space.

Prevention can include checks on transported organisms and cleaning equipment between water bodies. A management proposal should identify the introduction pathway, the organism at risk, and a measurable outcome. Control methods also need evaluation for effects on non-target species.

How to answer an ecology exam question

Use the sequence pressure → mechanism → biological effect → evidence → response. For example: a road divides a habitat; movement between breeding groups declines; gene flow may decrease; compare movement or genetic data before and after construction; evaluate a wildlife crossing and protect surrounding habitat.

Original practice data: with the same survey method and effort, a wetland population estimate falls from 240 to 180 individuals. The reduction is (240 − 180) ÷ 240 × 100 = 25%. This demonstrates a decline in that population, not automatically the extinction of the species or a 25% decline in all biodiversity. A survey should report uncertainty and account for detectability.

To evaluate restoration, compare treated and untreated sites where feasible, repeat sampling over time, and measure outcomes such as native species abundance, recruitment or water quality. Counting planted seedlings alone does not show that an ecosystem has recovered. UK ecology revision often asks students to explain and evaluate human impacts; introductory United States ecology study also emphasises population interactions, biodiversity and ecosystem disruption. These are study connections, not claims of exam-board endorsement.

Frequently asked questions

What is the biggest threat to biodiversity?

It depends on the ecosystem and scale. Land-use change is especially important in terrestrial and freshwater systems, while direct exploitation is a major marine pressure. Use a dated assessment and local evidence for a specific claim.

Are habitat loss and fragmentation the same?

No. Loss reduces suitable habitat area; fragmentation separates it into patches. The two often occur together.

Does an invasive species have to be a predator?

No. Invasive organisms can cause harm through competition, disease, herbivory or changes to ecosystem processes as well as predation.

Can biodiversity be measured by counting species only?

Species richness is useful, but it misses relative abundance, genetic variation and ecosystem differences. Choose a measure suited to the question.

Will planting trees always restore biodiversity?

No. Species choice, habitat type, connectivity and management matter. Planting trees on a naturally open habitat can damage the biodiversity already present.

How can I make a conservation answer more precise?

Name the pressure, explain its mechanism, choose an appropriate intervention and state how success would be measured.

20 revision MCQs with explained answers

Choose one answer before opening the explanation. The interactive quiz below uses a different set of questions.

Which process directly reduces the amount of suitable habitat?
1. Habitat loss
2. Gene flow
3. Ecological succession in every case
4. Recombination
Answer and explanation
A. Habitat loss — Conversion to an unsuitable land use removes habitat area.
What does habitat fragmentation describe?
1. The total removal of every organism
2. Division into separated patches
3. An increase in all native species
4. A change in allele sequence
Answer and explanation
B. Division into separated patches — Fragmentation changes the spatial arrangement of habitat.
Which is an indirect driver of biodiversity change?
1. Habitat conversion
2. Harvest mortality
3. Consumer demand
4. Toxic metal exposure
Answer and explanation
C. Consumer demand — Demand influences activities that apply direct pressure.
Which set contains the five major human-driven pressures?
1. Mutation, meiosis, mitosis, fertilisation and dispersal
2. Rainfall, latitude, longitude, altitude and slope
3. Predation, competition, mutualism, commensalism and parasitism
4. Land-use change, exploitation, climate change, pollution and invasive species
Answer and explanation
D. Land-use change, exploitation, climate change, pollution and invasive species — The first set groups the major anthropogenic direct drivers.
What is habitat degradation?
1. A reduction in habitat quality
2. Any change in scenery
3. An increase in habitat area
4. A new species appearing anywhere
Answer and explanation
A. A reduction in habitat quality — Quality is judged by ecological conditions and functions, not beauty alone.
What is a possible consequence of isolation?
1. Equal allele frequencies everywhere
2. Reduced gene flow between populations
3. Guaranteed reproductive isolation in one generation
4. Elimination of all mutation
Answer and explanation
B. Reduced gene flow between populations — Barriers can reduce movement and mating between populations.
Why can edge effects matter?
1. All species prefer identical light levels
2. Edges prevent every form of dispersal
3. Conditions near boundaries differ from the interior
4. Edges always contain no organisms
Answer and explanation
C. Conditions near boundaries differ from the interior — Boundary conditions can favour some species and disadvantage others.
Which organism was introduced to Guam and harmed birds through predation?
1. Indus river dolphin
2. Snow leopard
3. Markhor
4. Brown tree snake
Answer and explanation
D. Brown tree snake — Brown tree snake predation severely affected Guam's native bird fauna.
Why can zebra mussels harm native mussels?
1. Competition and attachment can impair native mussels
2. They remove all oxygen by photosynthesis
3. They turn freshwater into seawater
4. They always increase native mussel reproduction
Answer and explanation
A. Competition and attachment can impair native mussels — Competition and fouling are recognised mechanisms of harm.
Which statement about non-native species is accurate?
1. Only mammals can be non-native
2. Not all non-native species become invasive
3. Every non-native species is harmless
4. Every non-native species immediately causes extinction
Answer and explanation
B. Not all non-native species become invasive — Introduction and harmful invasion are different concepts.
What commonly consumes oxygen after an algal bloom dies?
1. Evaporation alone
2. Sediment photosynthesis at night
3. Aerobic decomposition
4. Nitrogen fixation in every organism
Answer and explanation
C. Aerobic decomposition — Aerobic decomposers use dissolved oxygen while breaking down organic matter.
What is biomagnification?
1. Increasing body size with age
2. A substance building up only within one organism
3. A rise in total species richness
4. Increasing concentration across trophic levels
Answer and explanation
D. Increasing concentration across trophic levels — Biomagnification concerns concentration differences along a food chain.
Which trait can slow recovery after heavy harvest?
1. Late reproductive maturity
2. High reproductive output in all conditions
3. Immediate breeding after birth
4. Very rapid generation turnover
Answer and explanation
A. Late reproductive maturity — Late maturity delays replacement of removed breeders.
Which is a source-control response to nutrient pollution?
1. Counting buildings near the lake
2. Reducing nutrient runoff into a lake
3. Adding fish before measuring oxygen
4. Removing every aquatic plant
Answer and explanation
B. Reducing nutrient runoff into a lake — Preventing excess nutrient entry addresses the initiating pressure.
What can climate-driven timing changes disrupt?
1. All gravity-dependent processes
2. The definition of a species count
3. Interactions between flowering plants and pollinators
4. The existence of DNA in plants
Answer and explanation
C. Interactions between flowering plants and pollinators — Different shifts in seasonal timing can create a mismatch.
What does species richness measure?
1. Total number of organisms
2. Number of habitats only
3. Proportion of one allele
4. Number of species
Answer and explanation
D. Number of species — Richness counts species without describing their relative abundance.
Why should a wildlife corridor be evaluated for each species?
1. Movement requirements and risks differ
2. Every corridor restores all biodiversity
3. Corridors always prevent disease
4. Width never affects corridor function
Answer and explanation
A. Movement requirements and risks differ — Design and surrounding habitat determine ecological usefulness.
A population falls from 200 to 150. What is the reduction?
1. 75%
2. 25%
3. 50%
4. 33.3%
Answer and explanation
B. 25% — The reduction is 50 divided by the original 200, multiplied by 100.
What is extirpation?
1. An increase in birth rate
2. Successful colonisation
3. Local disappearance of a species
4. Global disappearance in every case
Answer and explanation
C. Local disappearance of a species — A species can disappear locally while surviving elsewhere.
Which provides the strongest evidence that restoration is working?
1. The number of publicity posters
2. A single attractive photograph
3. The name of the conservation project
4. Repeated measures of ecological outcomes
Answer and explanation
D. Repeated measures of ecological outcomes — Outcome measurements test recovery rather than activity alone.