Extinction

The International Union for the Conservation of Nature (IUCN) ranks a species as extinct when there is no reasonable doubt that the last individual has died. This might sound like a simple definition, but it isn’t always easy to pinpoint exactly when a species has become extinct.

Occasionally, a species that has been presumed extinct reappears, for example, in New Zealand, the takahē was presumed extinct in the 1930s but was rediscovered in a remote valley in 1948. Despite a few examples like this, when a species becomes extinct, it is almost always gone forever.

The IUCN also has a separate rank ‘extinct in the wild’ when the species is only known to survive in captivity. Species are also sometimes referred to as ‘functionally extinct’ when only a very small number of individuals survive and factors such as poor health or age mean that they can not successfully reproduce.

Scientists have documented a large number of extinctions in New Zealand. These include at least 3 native frogs and a number of skinks and geckos. It is thought that there may have also been another species of tuatara that is now extinct, but this is difficult to prove without further DNA analysis of fossilised remains.

History of extinctions

Extinction is a natural process. Scientists estimate that up to 98% of all the species that have ever lived are now extinct. Most of these became extinct before the arrival of humans over a period of hundreds of millions of years.

Evidence from the fossil record suggests that there have been at least 5 mass extinctions during the Earth’s history. Mass extinctions are where a large number of species disappear within a relatively short period of time. The most well known example is the Cretaceous–Tertiary mass extinction. This resulted in the disappearance of the dinosaurs 65 million years ago.

So if extinction is a natural event, why should we be concerned about modern extinctions?

There are 2 main arguments:

• The extinction rate today is estimated to be significantly higher than the expected natural rate. This increased rate does not allow for ecosystems to recover or other species to occupy vacant niches.
• The primary cause of modern extinctions is human impact as opposed to natural phenomena.

Causes of extinction

There is no single factor responsible for extinction. Some species may gradually reduce in number over thousands of years as they are out-competed by another species. Others may become extinct very suddenly due to an unprecedented event, such as a large-scale fire.

Internationally, habitat loss and degradation is considered the most significant factor. In New Zealand, the biggest threat is introduced species. Other factors include over-exploitation, pollution, disease and climate change. Some species may be affected by a combination of factors. Species that have a very limited habitat or small population size are particularly vulnerable to extinction.

The impacts of extinction

The impact of isolated extinctions can be very difficult to predict and measure. In some ecosystems, the loss of one species may have an obvious negative effect, but the loss of another may have little or no immediate effect.

If the species that becomes extinct is a keystone species, the results can be dramatic. The removal of a keystone species characteristically results in a major change, in the same way that removing a keystone from an arch or bridge could cause the structure to collapse. A good example of this in New Zealand is the kererū. Kererū are the only birds that are able to swallow the largest fruits – and disperse the seeds – of many of our native trees, so the extinction of the kererū could result in the loss of many of our native tree species. This would have significant flow-on effects for the whole ecosystem.

From a human-centred perspective, many scientists are concerned about the impact of extinctions on our food supply, control of insect populations and potential medical treatments.

What are scientists doing to prevent extinction?

A large number of scientists around the world are involved in research to try and prevent extinction. An important aspect of this work is the collaboration between scientists, zoos and government bodies as well as the general public.

In New Zealand, conservation is an important research field. Key conservation tools include habitat restoration, pest eradication, captive breeding, translocation and treatment of disease. Have a look at the New Zealand Research articles in this context to find out more about how scientists at the University of Otago are working together to save our native reptiles and amphibians.