12/9/25 Mass extinction Notes

​Answer the following questions while watching the video on your warm up page:
1. What causes mass extinction events?
2. How did the extinction of the dinosaurs help mammals thrive?
3. What is causing possibly the sixth mass extinction event?

​Warm Up

What are mass extinctions?

Mass extinctions occur when global extinction rates rise significantly above background levels in a geologically short period of time. Mass extinctions wipe out whole groups of organisms. On a graph, this looks like big spikes in the number of extinctions. The graph shows extinction rates of ocean animal families over the last 600 million years. Normally, only a few groups go extinct every million years, but during a mass extinction, that number suddenly becomes much higher.

Graphing diversity over time

Mass extinctions can also be observed by looking at diversity levels over time. The graph below shows number of marine genera (groups) alive at different points in life’s history. While diversity levels generally increase over time, mass extinctions cause sudden drop-offs in diversity. The largest mass extinctions in Earth’s history are marked on this graph.
*Genetic variation and environmental changes are the contributing factors to biodiversity within a species.

Multiple Choice

How do mass extinctions reduce biodiversity?

• By creating more habitats for species

• By wiping out entire groups of organisms

• By increasing competition between species

• By helping species evolve faster

Mass extinctions were first discovered by studying fossils in rock layers. There a large decrease in the number and variety of fossils in a rock layer. In older rock layers, scientists found many different kinds of fossils. But in the layers just above them, there were far fewer fossils and fewer types of organisms. Sometimes the rock layers above and below the extinction look very different. These changes in the rocks show that the environment was changing in a big way. They can also give clues about what caused the mass extinction.

Detecting mass extinctions in the fossil record

Multiple Choice

How can scientists detect that a mass extinction has occurred in the fossil record?

• By finding a sudden increase in the number and types of fossils

• By seeing that fossil layers contain only dinosaur bones

• By noticing a large decrease in the number and variety of fossils in a rock layer

• By measuring the size of individual fossils

Multiple Choice

How many mass extinction events are recognized by scientists to have occurred throughout Earth's history?

• 3

• 5

• 7

• 9

The next 5 slides will show each of the 5 mass extinctions. Write the name of the extinction and the % estimated for extinction.

Multiple Choice

Which period is known for the most significant mass extinction event, where it is estimated that about 96% of all species went extinct?

• End Cretaceous

• End Permian

• End Devonian

• End Ordovician

Mass Extinctions are complex and may have multiple causes.

For example, the end-Cretaceous mass extinction does seem to have been triggered by an asteroid impact, but continental-scale volcanic activity, climate change, and changes in atmospheric and oceanic chemistry likely also played a part.

Multiple Choice

Mass extinctions are complex events. Which statement best explains why they often happen?

• They are usually caused by only one simple event, such as a storm.

• They always happen because all plants stop growing at the same time, which reduces photosynthesis and kills animals.

• They only occur when new species evolve too quickly, which leads to the rapid decrease in biodiversity.

• They often have multiple causes, such as climate change, volcanic eruptions, and asteroid impacts happening together or close in time.

Some organisms may be more vulnerable during mass extinctions than others.

For example, during the end-Permian mass extinction, marine invertebrates with shells made of calcium carbonate — like clams — were more likely to go extinct than animals that don’t depend on this compound — such as certain kinds of sponges. That’s because the end-Permian mass extinction likely involved ocean acidification, which makes less calcium carbonate available for organisms that need it to build their shells.

The Role of Mass Extinctions in Evolution

Mass extinctions cause a widespread decrease in biodiversity that can lead to evolutionary changes in surviving organisms. Mass extinctions lower the number of different living things by wiping out entire groups of plants and animals. This is like cutting big branches off the tree of life. But mass extinctions can also help new species grow and change. When many organisms disappear, they leave behind space and resources. The species that survive can use those resources and slowly adapt to new environments. Over many generations, they may take on new roles in ecosystems. Because there is less competition, surviving species can spread out and become more common.

The Role of Mass Extinctions in Evolution Continued

​For example, mammals were around when dinosaurs ruled the Earth, but they were small and not very diverse. When the non-avian dinosaurs went extinct about 65 million years ago, mammals were able to grow in number and variety and fill many of the roles that dinosaurs once had.

Summarize this example:

Multiple Choice

Why did mammals become more diverse after dinosaurs went extinct?

• They moved to the ocean

• They faced more competition

• They stopped evolving

• They were able to fill empty ecological roles

Variety within a species' genetic heritage (DNA/genes)

Genetic Diversity

Variety of habitats, species, and ecological processes

Ecological Diversity

Evolution increases diversity on Earth

Species Diversity

Variety of different species in an area

Match

Match the following

• Ecosystem Diversity

• Species Diversity

• Genetic Diversity

Variety of habitats and species

Variety of organisms in area

Variety of genes/DNA present in species

• Variety of habitats and species

• Variety of organisms in area

• Variety of genes/DNA present in species

Why is genetic variation important?

Genetic variation contributes to biodiversity of a species. This means the organisms in a species are different from one another. This increases survival chances if there are changes to the environment and decreases the risk of extinction.

Multiple Select

Why are genetic variations important in a population? (Choose 2)

• It contributes to biodiversity of a species.

• This increases the risk of disease and makes the species less likely to survive if there is an outbreak.

• If the environment changes, there is an increased chance of survival.

• It makes all the species look different.