Extinction�AP Topic 7.11

• Learning Objectives
• Describe factors that lead to the extinction of a population.
• Explain how the risk of extinction is affected by changes in the environment.
• Explain species diversity in an ecosystem as a function of speciation and extinction rates.
• Explain how extinction can make new environments available for adaptive radiation.

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Factors Leading to �Extinction of a Population��

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Humans Cause Biodiversity�Extinction of Species

hippo + c

habitat loss (fragmentation of land)

invasive species (introduction to nonnative species)

population (humans current population 8 billion)

pollution (environmental degradation)

overharvesting (damaging number of individuals of a population removed)

climate change (human pollution leading to atmospheric degradation

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Measuring Biodiversity

greater diversity contributes to ecosystem resilience

• species diversity
• the number of different species within a habitat
• genetic diversity
• the variation of the gene pool within a population
• ecological diversity
• missing organisms important for healthy environments
• keystone species

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Species Diversity

• species diversity in ecosystems is related to the rates of speciation and extinction
• equilibrium = speciation + extinction
• ↑ speciation ↓ extinction causes increase in diversity
• ↓ speciation ↑ extinction causes decrease in diversity

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Genetic Diversity

• Small Population Size
• Increased vulnerability to genetic drift
• inbreeding depression
• bottle-neck effect
• Loss of Genetic Diversity
• Reduced ability to adapt to changing conditions

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Ecological Diversity

• Trophic Cascade Effects
• Loss of key species disrupting food webs
• keystone species
• predators that keep herbivore populations in check
• e.g., wolves
• species that shape landscapes
• e.g., beavers

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Environmental Changes

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Risk of Extinction

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Extinction &� Adaptive Radiation

• extinction causes new niches open up
• extinction removes competitors, allowing surviving species to diversify
• example mammal radiation
• dinosaur extinction (65 mya) led to the rise of mammals
• example Darwin’s finches
• adaptive radiation on the Galápagos islands following colonization of a new environment

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Earth’s Mass �Extinction Events

• Ordovician Extinction (444 mya)
• Late Devonian Extinction (375 mya)
• Permian Extinction (252 mya)
• Triassic Extinction (201 mya)
• Cretaceous Extinction (66 mya)

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Ordovician Extinction �(444 MYA)

• Cause: Rapid glaciation, sea level drop, changes in ocean chemistry
• Impact: about 85% of marine species went extinct
• Adaptive Radiation: Diversification of jawed fish and early land plants in the Silurian period

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Late Devonian Extinction�(375 MYA)

• cause: global cooling, volcanic activity, ocean anoxia, asteroid impact
• impact: about 75% of species lost, particularly marine life like trilobites and reef-builders
• adaptive radiation: expansion of amphibians and first terrestrial vertebrates (early tetrapods).

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Permian Extinction�(252 MYA)

• The Great Dying
• cause: massive volcanic eruptions (Siberian Traps), global warming, ocean acidification
• impact: ~96% of marine species and 70% of terrestrial species went extinct
• adaptive radiation: rise of dinosaurs, early mammals, and modern coral reefs in the Triassic period

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Triassic Extinction�(201 MYA)

• cause: massive volcanic eruptions (central Atlantic Magmatic Province), climate change
• impact: about 80% of species lost, allowing dinosaurs to dominate
• adaptive radiation: rapid evolution of dinosaurs, early crocodiles, and the first true mammals

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Cretaceous Extinction�(65 MYA)

• cause: asteroid impact (Chicxulub crater), volcanic activity, climate change
• impact: ~75% of species lost, including non-avian dinosaurs
• adaptive radiation: mammals diversified rapidly, leading to primates, birds, and modern mammals

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Human-caused �Mass Extinction

• 6^th Mass Extinction Event
• Anthropocene Extinction
• currently happening
• accelerating loss of biodiversity loss
• rate of extinction 100-1000 times higher than natural levels
• large mammals, amphibians, and island/ endemic species at risk

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Summary�Topic 7.11

• Extinction is the permanent loss of a species, driven by environmental changes and human impact
• Species Diversity is shaped by the balance between speciation (new species forming) and extinction (species loss)

Speciation Rate > Extinction Rate → Increased Biodiversity

Extinction Rate > Speciation Rate → Decreased Biodiversity

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Summary continued �Topic 7.11

• Mass Extinctions have occurred five times in Earth’s history, leading to biodiversity loss followed by adaptive radiation
• Environmental Changes & Extinction
• Species with small populations, low genetic diversity, or specialized niches are more vulnerable
• Anthropocene Extinction: Human activities are accelerating extinction rates, causing long-term ecological consequences
• Extinction & Evolution: While extinction reduces biodiversity, it can create opportunities for surviving species to diversify and evolve

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