Mass Extinction Events

Question

Dinosaurs roamed Earth for 135 million years. Then, after an asteroid struck Earth, dinosaurs and many other organisms became extinct. An event, such as a large asteroid striking Earth, can cause widespread extinction in plants and animals. Scientists call these mass extinction events. Throughout the history of life on Earth, there have been five mass extinction events.

The diagram shows the periods and the impact of 5 mass extinction events. From left to right, the periods in Millions of Years Ago are Cambrian from 540 to 490, Ordovician from 490 to 445, Silurian from 445 to 415, Devonian from 415 to 360, Carboniferous from 360 to 300, Permian from 300 to 250, Triassic from 250 to 200, Jurassic from 200 to 145, Cretaceous from 145 to 65, Tertiary from 65 to 1.8, and Quaternary from 1.8 to 0. The number of living families is shown as trapezoids next to each other, and the broader end marks a mass extinction event. The first mass extinction event was at the end of the Ordovician Period, 445 million years ago, and 50% of animal families, including many trilobites, became extinct. The second mass extinction event was at the end of the Devonian Period, 360 million years ago, and 30% of animal families, including many marine species and insects, became extinct. The third mass extinction event was at the end of the Permian Period, 250 million years ago, and 60% of animal families, including many marine species, insects, amphibians, and all remaining trilobites, became extinct. The fourth mass extinction was at the end of the Triassic Period, 200 million years ago, and 35% of animal families, including many reptiles, became extinct. The fifth mass extinction event was at the end of the Cretaceous Period, 65 million years ago, and 50% of animal families, including the last dinosaurs and many marine species, became extinct.

Constructed Response Question
Use the diagram shown to respond to the following prompts.

A. Describe what happens to the number of animal families at and after each mass extinction event.

B. Some scientists believe that Earth is on the verge of its sixth mass extinction because so many animal species have become extinct in the relatively recent past. If this is the sixth mass extinction event, describe the most likely changes in the animal families on Earth over time.

Answer 1

Constructed Response

A. Describe what happens to the number of animal families at and after each mass extinction event.

At each mass extinction event, there is a significant decline in the number of animal families, indicated by the trapezoidal shapes in the diagram. The events cause sharp drops in biodiversity, followed by a gradual recovery over extended periods of time.

1. End of the Ordovician Period (445 million years ago): The first mass extinction event resulted in the extinction of approximately 50% of animal families. After this event, the number of animal families decreased dramatically, with a subsequent slow recovery leading to a gradual increase in biodiversity as new families evolved.

2. End of the Devonian Period (360 million years ago): The second mass extinction led to a loss of about 30% of animal families. Similar to the first event, the number of families dropped sharply but recovered over time as ecosystems adjusted and new species emerged.

3. End of the Permian Period (250 million years ago): This event was the most severe, with around 60% of animal families going extinct. The impact was profound, leading to a steep decline in biodiversity. Recovery was prolonged as ecosystems took millions of years to stabilize and diversify again.

4. End of the Triassic Period (200 million years ago): In this event, approximately 35% of animal families became extinct. While there was a notable reduction in the number of families afterwards, the recovery was again gradual, allowing for the emergence of new species in the subsequent Jurassic Period.

5. End of the Cretaceous Period (65 million years ago): The fifth mass extinction also led to a decline of about 50% of animal families, marking the end of the dinosaurs. Following this extinction, there was a significant but slow recovery, leading to the diversification of mammals and birds in the new ecosystems of the Tertiary Period.

Overall, each mass extinction was followed by a clear drop in the number of animal families, with a gradual recovery phase in which new families rose as ecosystems adjusted over millions of years.

B. If this is the sixth mass extinction event, describe the most likely changes in the animal families on Earth over time.

If the current extinction crisis is recognized as the sixth mass extinction event, we can anticipate a substantial decline in the number of animal families, similar to those seen in past events. Over time, the consequences may manifest as follows:

1. Immediate Decline: The ongoing loss of species due to habitat destruction, climate change, pollution, and other anthropogenic factors will likely lead to a sharp decline in biodiversity. Many animal families could face extinction, resulting in the loss of a significant portion of the planet's biological diversity.

2. Ecosystem Disruption: The extinction of various species will cause imbalances in ecosystems, affecting predator-prey relationships, pollination systems, and nutrient cycles. The extinction of keystone species could have cascading effects, leading to further declines in other species and, consequently, more extinction events.

3. Gradual Recovery: Eventually, after the immediate impacts of the sixth mass extinction subside, ecosystems may start to recover. This recovery, however, will likely take millions of years. New species and families may emerge to fill ecological niches left vacant by extinct species, leading to a different composition of biodiversity than previously existed.

4. Adaptation and Evolution: The species that survive may undergo rapid evolution and adaptation in response to changed environmental conditions. The emergence of new animal families could lead to increased biodiversity in the long term, but it may not match the diversity lost during the extinction event.

5. Human Influence: Unlike previous mass extinctions, the current crisis is driven by human activities. This ongoing influence may continue to shape the evolutionary paths of surviving species, potentially leading to either new forms of biodiversity or a heavily altered ecosystem dominated by human-facilitated species.

In summary, if the current situation is a sixth mass extinction, we can expect a significant drop in the number of animal families, ecosystem disruptions, a prolonged recovery phase, and a reshaping of biodiversity influenced by both natural processes and human actions.