Extinction and Causes of extinction

Extinction occurs when the last individual of a species dies. Ultimate fate of all species. It is an irreversible process. Once a species is extinct it cannot reappear. Extinction has increased rapidly since the industrial revolution. Two types:   

Local extinction occurs when a species disappears from a part of its range but persists elsewhere. 

Global extinction means that a species becomes extinct everywhere.

Causes of extinction

1. Population risk
2. Environmental risk
3. Natural catastrophe
4. Genetic risk
5. Human actions

Population Risk

Random variations in population rates can cause a species in low abundance to become extinct. For example, if in one year most whales were unsuccessful in finding a mate, then birth could be dangerously low. It is a risk especially to species that consist of only a single population in one habitat, can occur without any change in the environment.

Environmental Risk

It involves variation in the physical or biological environment, including variations in predator, prey, symbiotic, or competitor species. In some cases, species are sufficiently rare and isolated that such normal variations (e.g. late snow & freeze) can lead to their extinction.

Natural Catastrophe

Fires, major storms, earthquakes, and floods are catastrophes on land. Changes in currents, upwellings, and tsunami are ocean catastrophes. These catastrophes can cause many species to become extinct.

Genetic Risk

Detrimental change in genetic characteristics is called genetic risk. Genetic changes can occur in small populations from reduced genetic variation, genetic drift, and mutation. In a small population, only some of the possible inherited characteristics will be found. The species is vulnerable to extinction because it lacks variety.

Human Actions

Humans have had a significant impact on the extinction of many kinds of species. Wherever humans have modified the environment for their purposes (farming, forestry, cities, hunting, introducing exotic sp), species are typically displaced from the area. If large areas are modified, entire species may be displaced. Other human actions that cause extinction include:

v  Intentional hunting or harvesting

v  Disruption or elimination of habitat

v  Introduction of new parasites, predators, or competitors of a species (Exotic Species)

v  Pollution of the environment.

Although extinction is a natural process humans have become a major force in the premature extinction of species. The IUCN estimates that 75% of the extinction of birds and mammals since 1600 were caused by human beings. Hunting caused 42% & 33% of the extinctions of birds and mammals, respectively. During this century human expansion may cause the premature extinction of up to a quarter of the Earth’s current species.

       

                                                                      

Fig: The Passenger Pigeon, one of hundreds                              Fig: The Bali Tiger was declared extinct in

       of sp. of extinct birds, was hunted to extinction                          1973 due to hunting & habitat loss.

       over the course of a few decades