Extinction (College Board AP® Biology): Study Guide

Extinction: causes & effects

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

• Extinction can be 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

Causes of extinction

• Climate change

  • Burning fossil fuels increases CO₂ levels, causing the greenhouse effect

  • This leads to rising temperatures, sea levels, ocean acidity, and melting ice caps

  • Habitat loss forces species like polar bears to travel further for food, threatening survival

• Competition

  • Limited resources (food, water, habitat, mates) drive competition within and between species

  • Species unable to compete effectively face population decline

  • E.g., wild dogs in North America declined due to competition with Asian cats

• Introduction of non-native species

  • Human introduction of species disrupts ecosystems

  • Non-native species often outcompete natives due to lack of predators

  • E.g. gray squirrels introduced to the UK displaced red squirrels through competition and disease (squirrel pox)

• Hunting by humans

  • Historically essential but now largely recreational or commercial

  • Rare species are often targeted, accelerating population decline

  • Overhunting can reduce numbers beyond recovery, leading to extinction

• Habitat destruction

  • The leading cause of extinction due to deforestation, pollution, and land conversion

  • Loss of habitat forces species to relocate or compete for limited space

  • If no viable habitat remains, species cannot survive and go extinct

Effects of extinction

• Extinction, which reduces the number of species, can be thought of as an opposing force to speciation

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

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

Genetic diversity & extinction

Impact of genetic diversity on populations

• Genetic diversity is the variety of alleles within a species or population

• Maintaining genetic diversity is crucial for the long-term survival and adaptability of populations in changing environments

• If environmental changes occur, different selection pressures act on the population

  • Under new selection pressures, alleles that were beneficial in one environment may become harmful

  • This will alter the fitness of individuals, influencing survival and reproduction rates

  • Populations with high genetic diversity are more likely to include individuals with traits that help them adapt and withstand new selective pressures

  • Populations with low genetic diversity are more vulnerable to changes

    • There is an increase in the risk of decline or extinction, as fewer individuals possess adaptive traits

Examples of genetic variety

California condors

• The California condor is the largest North American landbird

• Declined in the 20th century due to DDT use, poaching, and lead poisoning

• Low population numbers caused low genetic diversity, reducing resilience to environmental challenges

CC BY-SA 2.0 Don Graham, via Wikimedia Commons

Black-footed ferrets

• The black-footed ferret (Mustela nigripes) is a critically endangered predator

• Its diet primarily consists of prairie dogs, which have been exterminated as agricultural pests in parts of the USA and Canada

• Loss of prairie dog habitats has reduced the ferret's main food source

• The species' low genetic diversity makes it vulnerable to environmental changes, further threatening its survival

USFWS Mountain-Prairie, Public domain, via Wikimedia Commons

Prairie chickens

• The prairie chicken (Tympanuchus cupido), also called a boomer for its mating call, is highly endangered in North America

• Its habitat, tall grass prairie, has been largely lost to agriculture in the 20th and 21st centuries

CC BY 2.0 Ron Knight, via Wikimedia Commons

Potato blight

• Potato blight is a disease caused by the protoctist Phytophthora infestans, which shares some fungal characteristics

• It spreads via spores, initially causing dark brown marks on leaves that quickly spread

• The pathogen destroys potato and tomato crops, rendering them inedible

• The Irish Potato Famine in the mid-19th century, caused by potato blight, killed ~1 million people and spurred emigration to North America

• This highlights the importance of maintaining genetic diversity in crop plants to prevent similar agricultural disasters

I.Sáček, CC0, via Wikimedia Commons