The Importance of Extinction (College Board AP® Biology)

Extinction & Species Diversity

• Extinction is when a species comes to an end or dies out

• Extinction is a natural biological process that happens on planet Earth and studies of fossils and ancient DNA have shown that millions of species have gone extinct in the past

• Mass extinction events have also occurred in the past where a very large number of species went extinct at one time

  • The rate of extinction during these periods was very high

• Past mass extinctions were likely caused by major and sudden shifts in the environment such as an Ice Age or an asteroid hitting the Earth

• Scientists have been studying the current rates of extinction in recent years and many believe that the Earth is undergoing a current mass extinction with humans being the main cause

• Possible reasons for extinction include:

  • Climate change

  • Competition

  • Introduction of species

  • Hunting by humans

  • Degradation and loss of habitats

• Extinction (which reduces the number of species) can be thought of an an opposite force to speciation (which increases the number of species)

• The level of biodiversity in an ecosystem can be monitored by the rates of speciation and extinction

• Extinction is essential because it liberates niches that can then be occupied by different species, some of which are newly speciated

Climate Change

• The large scale burning of fossil fuels by humans in recent years has led to a large increase in the levels of carbon dioxide in the atmosphere, creating the greenhouse effect

• The increased carbon dioxide concentration in the atmosphere has had several consequent effects on ecosystems around the world

  • There has been an increase in the mean global temperature

  • Sea levels are rising

  • Ocean temperatures and acidity are rising

  • Ice caps are melting

• These consequences have massively changed the habitats of some species, so much so that some are no longer able to survive in the new environmental conditions

  • For example, polar bears are struggling to survive as more of their habitat melts away earlier each year

  • The earlier melting of the ice caps means they have to swim further to reach seal populations that they rely on for food

Competition

• When there is a limited supply of resources within an area competition between individuals for the same resource can occur

• The resources could be food, water, habitat and reproductive mates

• Competition can exist within species and between species

• Competition reduces the population size of a species

  • For example, millions of years ago there were many different species of wild dog that lived in North America

  • When several cat species spread from Asia into North America there were high levels of competition between these carnivores

  • As a result, there are only nine species of wild dog that exist in North America today

• Competition with humans has become a major problem for some species in the last 100 years as humans have outcompeted them for food, water and habitat

Introduction of species

• When humans colonized new land they would often exchange animal and plant species between their home country and the new land

  • These introduced species are non native

  • Non native species can be highly problematic as they often have no natural competitors, predators or pathogens that help limit population growth

  • Without these natural population checks, non native species can massively increase in number

  • The large numbers of non native species can negatively affect the native species through factors such as competition and disease

• Gray squirrels have led to the reduction of the red squirrel population in the UK

  • Gray squirrels were introduced to the UK in 1876

  • They quickly grew in numbers

  • The larger gray squirrels compete with red squirrels for food

  • They also carry and transmit a disease known as squirrelpox which is fatal to red squirrels

  

Red and gray squirrels as an example of interspecific competition threatening extinction

Hunting by humans

• In the past humans would have hunted, killed and ate wild animal species in order to survive

• Nowadays most of the meat that humans consume comes from domesticated livestock like cattle, sheep and chicken

• The evolution of livestock has negated the need for many humans to hunt wild animals

  • Some humans in underdeveloped countries still have to hunt animals for survival

• The hunting of wild animals is still common and has become a sport for some individuals

  • The rarer and more vulnerable species are often more desirable for a sport hunter

• If too many individuals within a species are killed then the population can become so small that it is no longer able to survive

Degradation & loss of habitats

• This is the main cause of species extinction

• Over recent years humans have aggressively destroyed animals habitats by cutting down forests, draining wetlands and polluting the water, soil and air

• This is highly problematic as species are adapted to survive within their specific habitat that has particular environmental conditions

• Without their habitat organisms will not get the resources they need to survive

• As their habitat area gets reduced a species will:

  • Search for other suitable habitats

  • Compete with others for the remaining habitat

• Eventually the range of habitat can become so small or nonexistent that a species is not able to survive and goes extinct