Threats to Biodiversity: Habitat Loss, Overexploitation, and Climate Change

The main threats to biodiversity are habitat loss and fragmentation, overexploitation, pollution, invasive species and climate change, which together drive species toward extinction. In brief: a threatened species is one at risk of becoming endangered, an endangered species is at high risk of extinction, and extinction is the permanent loss of a species. Scientists regard habitat loss as the largest single threat on land, and overexploitation (mainly fishing) as a major threat in the oceans.

Most species decline because of several pressures acting together. This guide explains the status categories, each of the main threats, how fragmentation affects populations, how climate change adds to the pressure, a worked example showing how threats combine and the mistakes to avoid.

What are threatened, endangered and extinct species?

The International Union for Conservation of Nature (IUCN) keeps the Red List of Threatened Species. It sorts species into categories according to risk, based on population size, rate of decline and range.

• Vulnerable, Endangered and Critically Endangered: these three categories together are called “threatened”. The risk of extinction in the wild rises from one to the next.
• Extinct in the Wild: only individuals in captivity or outside the natural range remain.
• Extinct: no living individuals remain.

In everyday biology classes, “threatened” is also used more loosely to mean a species whose numbers are falling. Extinction is a natural process, but the present rate is estimated to be much higher than the long-term background rate, and human activity is considered the main cause.

How does habitat loss and fragmentation threaten species?

Habitat loss occurs when natural areas are cleared or altered for farming, cities, mining, roads or dams. Deforestation in the tropics and the draining of wetlands are major examples. Animals and plants lose food, shelter and breeding sites.

Habitat fragmentation happens when a large habitat is broken into small, isolated patches by roads, fields or buildings. Fragmentation has several effects:

• Small patches support small populations, which are more likely to die out by chance.
• Animals cannot move between patches to find mates, so inbreeding increases and genetic diversity falls.
• Edges are exposed to wind, heat and invaders. A patch with a large proportion of edge has less true interior habitat.

Wildlife corridors, such as strips of habitat or bridges over roads, can reconnect patches.

What is overexploitation?

Overexploitation means harvesting a species faster than it can reproduce. It includes overfishing, excessive hunting, collecting plants and illegal wildlife trade. Examples include the extinction of the passenger pigeon in the early twentieth century after massive hunting, and the collapse of the Atlantic cod fishery off Newfoundland in the early 1990s.

Slow-reproducing species are especially vulnerable. Sharks, rhinoceroses and elephants have few young and mature late, so they recover slowly. Demand for products such as ivory, horn or exotic pets can drive illegal trade even when a species is protected.

How does pollution harm biodiversity?

Pollution changes the chemical conditions that organisms need. Fertilizer runoff causes eutrophication, in which algae grow excessively, die and use up oxygen in water, which kills fish. Pesticides can poison non-target species. Some insecticides, notably neonicotinoids, are linked to harm to bees and other pollinators. Plastic waste is eaten by marine animals or entangles them, and oil spills coat the feathers of birds.

The pesticide DDT is a classic case. In the mid-twentieth century it built up in food chains and caused thin eggshells in birds of prey such as the bald eagle and the peregrine falcon. After DDT was banned in many countries in the 1970s, populations recovered.

How does climate change affect biodiversity?

Rising temperatures shift the places where species can live. Many species move toward the poles or up mountains, but those already at the top of a mountain or in a small area have nowhere to go. Changes in timing can disrupt relationships, for example when insects emerge earlier than the birds that feed on them.

Warm water causes coral bleaching, in which corals expel their algae partners and may die. Ocean acidification, caused by the absorption of carbon dioxide, makes it harder for organisms such as corals and shellfish to build shells and skeletons. Climate change also increases the frequency of extreme events like drought and wildfire. It interacts with other threats and often makes them worse.

Major threats compared

Worked example: why does a forest bird decline?

Consider an imaginary forest bird that nests in old trees and eats insects. Its population has fallen by 60 percent in 30 years. Identify the likely threats by checking each one.

1. Habitat loss: If old trees are logged, nest sites vanish. This directly reduces the number of breeding pairs.
2. Fragmentation: If the remaining forest is split into small patches, pairs in isolated patches cannot find mates, and genetic diversity decreases.
3. Pollution: If insecticides reduce insects, chicks starve.
4. Invasive species: If introduced rats eat eggs, breeding success falls.
5. Climate change: If insects peak earlier in spring, chicks hatch after the peak and find less food.

The next step is to gather evidence. Surveys of nest sites, counts of insects and records of egg losses can show which threats are important. A reduction of 60 percent in 30 years means about 40 percent of the original population remains, and the average yearly decline is roughly 3 percent. Such numbers help conservationists decide which action is most effective.

What has history shown?

Past extinctions caused by humans include the dodo, the Steller’s sea cow and the passenger pigeon. The last known passenger pigeon, named Martha, died in a zoo in 1914. Paleontologists have also recorded five mass extinctions in Earth’s history. Many scientists argue that we may be entering a sixth, though this is debated and depends on how quickly losses continue.

There are also recoveries. The American bald eagle, the humpback whale and the Arabian oryx have all recovered after protection, which shows that action can work.

Which mistakes are common?

Mistake 1: Naming a single cause for a decline

Most declines involve several interacting threats. Good answers consider more than one.

Mistake 2: Confusing endangered with extinct

Endangered species still exist but are at high risk. Extinct species are gone.

Mistake 3: Saying that fragmentation is the same as habitat loss

Fragmentation can occur even when the total area of habitat stays similar. The effect comes from isolation and edges.

Mistake 4: Believing that climate change affects only polar species

Species on every continent and in every ocean are shifting their ranges or timing, from corals to alpine plants.