Extinction debt threatens many species

Because of the destruction of appropriate habitats, many animal and plant species face the threat called extinction debt. It means that the species will not disappear immediately, but it is likely to vanish later. The period of decline may be rather long. A large European research group, coordinated by the Finnish Environment Institute, has collected the available information on extinction debt. The results have recently been published in the internationally respected series Trends in Ecology and Evolution.  

Extinction debt is the number of species that will disappear before the ecosystem has reached a new equilibrium after a change. In most cases the change is either a reduction of the area of habitats or a deterioration of their quality. Extinction debt seems to be a common phenomenon in many groups of organisms and in various kinds of habitats around the world. For nature conservation the phenomenon sets new challenges, especially in regions like rain forests and old boreal forests in which large areas of natural habitats have been lost in the last decades. In such regions, many species can go extinct long after the destruction of habitats has been stopped. On the other hand, as long as an endangered species survives in the deteriorated conditions, its perspectives for the future can be improved by active habitat management and restoration measures.

Long-lived species more threatened by extinction debt

Habitat changes are especially dangerous to species that have strict demands on their environment, e.g. beetles that live on rotted wood in old-growth forests. Extinction debt threatens especially long-lived species such as perennial plants, or lichens and fungi on tree trunks, or relatively long-lived animals like birds and primates. It may take decades before meadow plants disappear from a site where conditions are no longer favourable. Some meadow plants can survive for more than one hundred years. On the other hand, short-lived species, e.g. insects that each year produce a new generation, disappear rather quickly from a changed area because of their short generation cycle.

Highest extinction debt in large areas changed by humans

The time delay of extinctions is longer and the quantity of extinction debt is  higher if the examined area is large. A species disappears rather quickly from a patch of forest when conditions change and the environment becomes unfavourable. In a large network of similar forest patches the process takes a longer time. For example many beetle species that live on rotted wood in old-growth forests have disappeared from almost all forests in Finland but not yet from the entire country. According to an estimate, the number of species in forests threatened by extinction debt amounts to more than one thousand. 

Research on extinction debt and quantitative estimates are difficult when the examined area is large. Studies of extinction debt have usually focused on rather limited habitat patches. Studies on national level have been few. So far most studies have concentrated on the effects of destruction and fragmentation of habitats, but in many regions it can be expected that climate change and invasive species, in addition to changes in land use, will in the future be significant causes of extinction debt.

More information

• Publication: Kuussaari, M., Bommarco, R., Heikkinen, R.K., Helm, A., Krauss, J., Lindborg, R., Öckinger, E., Pärtel, M., Pino, J., Rodà, F., Stefanescu, C., Teder, T., Zobel, M. & Steffan-Dewenter, I. 2009: Extinction debt: a challenge for biodiversity conservation. Trends in Ecology and Evolution 24, 564-571.

Mr Mikko Kuussaari, research specialist, Finnish Environment Institute SYKE, phone +358 40 525 6249, firstname.lastname@ymparisto.fi

Mr Risto Heikkinen, research specialist, Finnish Environment Institute SYKE, phone  +358 40 740 3521, firstname.lastname@ymparisto.fi