Key Area 3.8: Threats to Biodiversity

Extinction

A natural process, in which a group of organisms fail to adapt to their environment and therefore cannot survive.

How is the rate of extinction calculated?

The presence and absence of higher birds and animals, where normal extinction rates are compared to current extinction rates.

What happens after a mass extinction?

Biodiversity slowly increases, as remaining organisms adapt to their new environments.

Overexploitation

Where resources (eg. food, raw material, space) are used up quicker than they can be replaced.

Give examples of overexploitation:

- Overfishing; animals sold in the pet trade; over-harvesting.

- Overfishing of cod caused a depletion in stock, but government quotas aid a possible recovery.

Describe the effects of overexploitation:

- Populations can be reduced to a low level, but may still recover.

- Some species have a naturally low genetic diversity in their populations and still remain viable, eg. cheetahs.

Bottleneck effect

The loss of genetic variation necessary to enable evolutionary responses to environmental change, caused by the loss of many individuals from a small species, or when a population is almost wiped out by a natural disaster.

Describe causes of the bottleneck effect:

- Natural means, eg. forest fires, ice age, natural disasters

- Human activity, eg. overhunting

Explain how the bottleneck effect can lead to local extinctions of certain species:

- When a surviving population is very small, it loses genetic information.

- A loss of genetic diversity can be critical for many species, as inbreeding results in poor reproductive rates.

- Reduced populations can therefore be prevented from evolving or adapting in response to future environmental change.

Give an example of a species the bottleneck effect has occurred with:

Northern elephant seals, whose populations reduced severely because of human hunting.

What are the two possible outcomes of a bottleneck effect?

- Extinction, where a population fails to adapt to the new environment.

- Recovery, where a population successfully adapts to the new environment and reproduce.

Habitat fragmentation

The breakage of habitats into smaller fragments or sections, usually caused by the clearing of areas for human use.

Explain the effects of habitat fragmentation on species:

- Degradation of the edges of habitat fragments results in increased competition between species as fragments shrink in size.

- As habitat is lost and competition increases, biodiversity may also decrease.

- Each smaller fragment can only support a smaller biodiversity compared to the original habitat, as fragments are more isolated, and so exhibit a lower species diversity.

Describe benefits of linking fragmented habitats with habitat corridors:

- Allows movement between isolated fragments of animals, increasing access to food and choice of mate.

- Allows gene flow between fragments, increasing species diversity as it allows for the recolonisation of small fragments after local extinctions.

Introduced species

A non-native species which has been moved, intentionally or unintentionally, by humans, into a new geographical location they are not naturally found in.

Naturalised species

An introduced species which has become established in a new area or wild community.

Invasive species

A naturalised species which spreads rapidly and eliminates native species, eg. grey squirrels and Japanese knotweed in the UK.

Describe the impacts of invasive species:

Can outcompete or prey on native species and eliminate them, reducing species diversity.

In a new environment, invasive species may be free from:

- Predators

- Pathogens (disease)

- Parasites

- Competition occurring in their native habitats

Invasive species may:

- Prey on native species

- Outcompete native species for resources

- Hybridise with native species