Extinctions: Videos & Practice Problems

Extinctions have significantly influenced the history of life on Earth, with background extinctions occurring sporadically due to factors like competition and environmental changes. In contrast, mass extinctions, such as the Permian and Cretaceous events, drastically reduced biodiversity, eliminating up to 96% of marine species and 75% of all species, respectively. The current extinction rate, driven by human activities like pollution and habitat loss, may indicate a potential sixth extinction, with rates 10 to 100 times higher than historical averages, threatening global biodiversity.

Extinctions

Extinctions have played a crucial role in shaping the history of life on Earth, acting as a natural pruning mechanism for the phylogenetic tree of life. This tree illustrates the diversification of species through speciation events, where lineages split and evolve. However, not all branches thrive; some species go extinct, highlighting the reality that not every lineage survives. This ongoing process of extinction is referred to as background extinction, which represents the average rate of extinction occurring due to factors such as competition, environmental changes, and the inability of species to adapt quickly enough to new conditions.

In contrast to background extinction, mass extinctions are significant events where a large number of species across various lineages go extinct simultaneously, often due to catastrophic environmental changes. These events can eliminate up to 75% or more of all species, drastically altering the course of evolution and biodiversity. Following a mass extinction, it can take millions of years for ecosystems to recover and reach similar levels of diversity, emphasizing the long-term impact of these events on life on Earth.

Throughout the Phanerozoic era, which spans from the Cambrian period to the present, scientists have identified five major extinction events, commonly referred to as the "big five." The Phanerozoic is characterized by significant diversification, particularly during the Cambrian explosion, which marks a period of rapid evolutionary development. The major eras within this timeframe include the Paleozoic, Mesozoic, and Cenozoic, each representing distinct phases in the history of life.

The big five extinction events include:

1. Ordovician extinction event
2. Late Devonian extinction event
3. Permian extinction event
- noted for the loss of approximately 95% of marine species.
4. End Triassic extinction event
5. Cretaceous extinction event
- famously known for the extinction of the dinosaurs, leading to the rise of mammals in the Cenozoic era.

Understanding these extinction events is essential for grasping the dynamics of evolution and the resilience of life on Earth. Each event not only reshapes the biodiversity of the planet but also sets the stage for new evolutionary pathways and the emergence of new species.

Extinctions Example 1

The Phanerozoic eon, which spans from approximately 541 million years ago to the present, is characterized by significant changes in marine biodiversity, as illustrated by the graph depicting the number of marine families over time. This graph highlights the fluctuations in marine family diversity across three major eras: the Paleozoic, Mesozoic, and Cenozoic, with various periods within each era. The total number of marine families is plotted against geological time, revealing both gradual increases and sharp declines, indicative of extinction events.

Extinction events are marked by noticeable dips in the graph, with five major events recognized as the "Big Five." These include the end-Ordovician, late Devonian, end-Permian, end-Triassic, and end-Cretaceous extinctions. Each of these events corresponds to significant losses in marine biodiversity, with the end-Permian extinction being particularly catastrophic, as it shows the most dramatic decline in the number of families.

In addition to the major extinction events, the graph also reveals smaller dips that suggest other extinction occurrences. These additional events, while not as severe as the Big Five, still indicate periods of significant loss in marine families. The overall trend, despite these extinctions, shows a general increase in the total number of marine families over time, suggesting resilience and recovery in marine ecosystems.

The focus on marine families in this data is largely due to the robust fossil record associated with marine organisms. Marine environments often provide excellent conditions for fossilization, allowing for a clearer understanding of historical biodiversity. This is crucial for studying extinction events and their impacts on marine life.

Interestingly, while the end-Cretaceous extinction is estimated to have eliminated 75% or more of all species on Earth, the graph does not reflect a corresponding 75% reduction in marine families. This discrepancy arises from the biological classification system, where a family encompasses multiple species. Therefore, even if a significant number of species within a family go extinct, the family itself may persist if at least one species survives. This highlights the complexity of biodiversity loss and the importance of understanding different levels of biological organization when interpreting extinction data.

How do mass extinctions differ from background extinctions?

• A. Mass extinctions affect larger numbers of diverse species simultaneously.
• B. Mass extinctions tend to affect larger organisms instead of smaller organisms.
• C. Mass extinctions can be caused by a change in the environment, while background extinctions cannot.
• D. Mass extinctions are typically caused by an increase in competition, while background extinctions are not.

Answer: (A)

What could be one reason why paleontologists mostly study extinctions in the Phanerozoic (since the Cambrian explosion)?

• A. Mass extinctions were rare before the Phanerozoic but have become more common since.
• B. Plants and animals did not live on land until the Phanerozoic.
• C. More hard-bodied organisms evolved in the Phanerozoic.
• D. The Phanerozoic was a time of more active volcanic activity, leading to more easily dateable fossils.

Answer: (C)

Permian Extinction

The Permian extinction, often referred to as the "Great Dying," occurred approximately 252 million years ago, marking the end of the Permian period and the Paleozoic Era. This event is notable for its catastrophic impact on life on Earth, as it resulted in the extinction of about 96% of marine species and 70% of terrestrial species. At this time, the Earth was dominated by a supercontinent known as Pangaea, where various forms of life thrived, including early reptiles, fish, ferns, conifers, and a diverse array of insects.

The leading hypothesis for the cause of the Permian extinction is linked to extensive volcanic activity in what is now Siberia. This volcanic activity released vast amounts of gases, significantly increasing carbon dioxide (CO₂) levels in the atmosphere. The rise in CO₂ led to a dramatic increase in global temperatures, estimated at around 6 degrees Celsius (approximately 11 degrees Fahrenheit). Such rapid climate change posed severe challenges for many organisms, making adaptation nearly impossible.

Additionally, the volcanic eruptions contributed to acid rain, which devastated plant life. The decline of plants had a cascading effect on herbivores and, subsequently, on the entire food web. The oceans also faced dire consequences, as increased CO₂ levels led to ocean acidification and a decrease in oxygen levels, further threatening marine life. The combination of these factors created an inhospitable environment, resulting in the mass extinction event that reshaped the course of evolution on Earth.

The aftermath of the Permian extinction set the stage for the Mesozoic Era, a time that would eventually see the rise of dinosaurs and other new forms of life. Understanding this pivotal event is crucial for grasping the dynamics of Earth's biological history and the resilience of life in the face of catastrophic changes.

What is thought to have been the major cause of the end-Permian extinction event?

• A. An asteroid impact that caused global winter.
• B. The diversification of the dinosaurs forced other species into extinction.
• C. Major volcanic activity disrupted the global atmosphere and ocean chemistry.
• D. Continental drift shifted environments suddenly and drastically.

Answer: (C)

Cretaceous Extinction

The Cretaceous extinction event, also known as the KT extinction event, occurred approximately 66 million years ago and is recognized as the last of the five major extinction events in Earth's history. This catastrophic event is most famously associated with the extinction of the non-avian dinosaurs, which comprised about 75% of all species at the time. The term "KT" derives from the German spelling of Cretaceous (K) and Tertiary (T), the geological period that followed.

During the Cretaceous period, diverse life forms thrived, including various species of dinosaurs such as Tyrannosaurus rex and Velociraptors, alongside a rich variety of flowering plants and small mammals. Notably, social insects like bees and ants also evolved during this time. However, the extinction event drastically altered this biodiversity.

The leading explanation for the Cretaceous extinction is the impact hypothesis, which posits that a massive meteorite, approximately 10 kilometers wide, struck Earth near the Yucatan Peninsula. This impact released energy equivalent to about one billion atomic bombs, resulting in immediate and catastrophic consequences. The collision generated mega tsunamis that inundated coastal areas and triggered worldwide firestorms, potentially igniting forests across the globe.

In the aftermath of the impact, debris was propelled into the atmosphere, leading to a prolonged period of darkness and a global winter lasting around three years. This drastic climate shift severely hindered photosynthesis, disrupting food chains and ecosystems. Following this period, the Earth experienced acid rain and ocean acidification, further exacerbating the survival challenges for many species. The long-term effects included about 100,000 years of climate change, characterized by global warming.

Despite the widespread extinction, a small number of mammal species survived, likely due to their small size and generalist feeding habits, allowing them to adapt to the rapidly changing environment. While the meteorite impact is the most widely accepted cause of the extinction, some researchers suggest that volcanic activity may have also played a role, contributing to a gradual decline in biodiversity that the impact event punctuated.

Understanding the Cretaceous extinction event provides insight into the fragility of ecosystems and the complex interplay of catastrophic events and gradual changes in shaping the history of life on Earth.

The extinction at the end of the Cretaceous period is most closely associated with what?

• A. The extinction of many organisms that evolved in the Cambrian, such as trilobites.
• B. The extinction of the dinosaurs.
• C. Massive worldwide volcanic activity.
• D. Opening of terrestrial environments for colonization by vertebrates.

Answer: (B)

Which piece of evidence is consistent with the leading hypothesis for the cause of end-Cretaceous extinction?

• A. Massive lava flows have been found in Siberia, large enough to disrupt global atmospheric chemistry.
• B. Plant colonization of land led to a major increase in the nutrients available in the oceans and a worldwide decrease in oxygen levels.
• C. The super-continent Pangea broke up, forming many of the current continents during the Cretaceous.
• D. Worldwide, a layer of sediment dated to 66 mya contains iridium, an element common in asteroids.

Answer: (D)

Sixth Extinction

The concept of the "6th extinction" refers to a potential ongoing mass extinction event primarily driven by human activities, contrasting with the historical "big five" extinction events caused by natural disasters such as volcanic eruptions or asteroid impacts. Current scientific consensus suggests that the extinction rate today is alarmingly high, estimated to be 10 to 100 times greater than the background extinction rate observed in the fossil record. Some researchers argue that this figure may even underestimate the true scale of extinctions occurring.

The primary drivers of this potential extinction crisis include pollution, climate change, and habitat loss, all of which stem from human actions that disrupt ecosystems and threaten biodiversity. Data illustrates a concerning trend: from 1500 to 2010, the rate of species extinction remained relatively stable until the last century, when a significant increase in extinction rates was observed. Notably, mammals exhibit the highest extinction rates, with approximately 2% of species already lost. While this percentage may seem small, the steepness of the extinction curve raises serious concerns about future biodiversity loss.

Historically, the fossil record indicates that it can take between 5 to 100 million years for ecosystems to recover to similar levels of diversity following mass extinctions. The irreversible nature of species loss emphasizes the urgency of addressing the factors contributing to this crisis, as the implications for the planet's ecological balance are profound and long-lasting.

During the late Pleistocene, between 50,000 and 10,000 years ago, about 65% of megafauna species (animals with mass over 44 kg, or 100 lb) went extinct around the world. Australia and the Americas had the highest megafauna extinction rates with 100% of mammals over 1000 kg (~2,200 lbs) going extinct. Many scientists consider these extinctions as the leading edge of the world’s 6th major extinction event. If the causes of the late Pleistocene extinctions and the causes of the 6th major extinction event are linked, which of the following statements about the Pleistocene extinctions is most likely true?

• A. Volcanic activity in South America led to widespread ocean acidification.
• B. Modern humans moved into new areas at the end of the Pleistocene, notably into Australia and the Americas.
• C. A large asteroid impact in what is today Northern Canada occurred during the late Pleistocene.
• D. An adaptive radiation of new species forced these resident species into extinction.

Answer: (B)

Here’s what students ask on this topic:

What are the main differences between background extinction and mass extinction?

Background extinction refers to the natural, low-level rate of species loss over time due to factors like competition, environmental changes, or inability to adapt. It occurs sporadically and impacts individual species or small groups. Mass extinction, on the other hand, is a catastrophic event that leads to the simultaneous loss of a large number of species across multiple ecosystems. Examples include the Permian extinction, which wiped out 96% of marine species, and the Cretaceous extinction, which ended the reign of dinosaurs. Mass extinctions drastically alter the trajectory of life on Earth and can take millions of years for biodiversity to recover.

What caused the Permian extinction, and why is it called the 'great dying'?

The Permian extinction, occurring about 252 million years ago, is called the 'great dying' due to its severity, wiping out 96% of marine species and 70% of terrestrial species. It was primarily caused by massive volcanic activity in what is now Siberia. This led to increased CO₂ levels, causing a 6°C rise in global temperatures, acid rain, ocean acidification, and reduced oxygen levels. These environmental changes made survival extremely difficult for most organisms. The extinction reshaped Earth's biodiversity, paving the way for new evolutionary paths.

What is the impact hypothesis for the Cretaceous extinction event?

The impact hypothesis suggests that the Cretaceous extinction, which occurred 66 million years ago, was caused by a meteorite striking Earth near the Yucatan Peninsula. The meteorite, about 10 km wide, released energy equivalent to a billion atomic bombs, causing mega tsunamis, global firestorms, and a three-year global winter due to debris blocking sunlight. This was followed by acid rain, ocean acidification, and 100,000 years of global warming. These events wiped out 75% of species, including non-avian dinosaurs, while small, adaptable mammals survived, leading to the age of mammals.

What evidence suggests that a sixth extinction is currently underway?

Evidence for a sixth extinction includes extinction rates that are 10 to 100 times higher than the background extinction rate, largely due to human activities like pollution, climate change, and habitat loss. Data shows a steep increase in species loss over the last century, with mammals experiencing a 2% extinction rate. If this trend continues, biodiversity could decline significantly, taking millions of years to recover. This potential extinction event highlights the urgent need for conservation efforts to mitigate human impact on ecosystems.

How long does it take for biodiversity to recover after a mass extinction?

After a mass extinction, it typically takes 5 to 10 million years for biodiversity to recover to similar levels. This recovery period depends on the severity of the extinction and the environmental conditions that follow. For example, after the Permian extinction, it took millions of years for ecosystems to stabilize and for new species to evolve. The recovery process involves the gradual diversification of surviving species and the emergence of new evolutionary paths, reshaping Earth's biodiversity.

Why did mammals survive the Cretaceous extinction event?

Mammals survived the Cretaceous extinction due to their small size and generalist diets, allowing them to adapt to the harsh post-impact conditions. Unlike dinosaurs, which relied on specific food sources, mammals could eat seeds, scavenge, and find alternative food sources. Their ability to burrow and avoid immediate environmental hazards also contributed to their survival. While most mammals went extinct, the few that survived evolved into the diverse mammalian species we see today.