Understanding Loss: Defining Mass Extinction Thresholds

Mass extinctions are periods with much higher extinction rates than normal. They are defined by both magnitude and rate. Magnitude is the percentage of species lost, and rate is how quickly this happens. While there is no precise percentage that constitutes a mass extinction, it is generally understood that a mass extinction event occurs when about 75% of the world's species are lost in a short period of geological time, typically less than 2 million years. In the last 500 million years, there have been at least five mass extinction events, with the current rate of species extinction estimated to be between 1,000 and 10,000 times higher than natural rates.

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The current mass extinction

The planet has experienced five previous mass extinction events, with estimates ranging from as few as five to more than twenty. The last mass extinction event occurred 65.5 million years ago and wiped out the dinosaurs. Experts now believe that we are in the midst of a sixth mass extinction, driven by human activity.

Unsustainable food production and consumption contribute to greenhouse gas emissions, causing global temperature rise and severe droughts and storms. This, in turn, creates challenges for food production and makes habitats inhospitable. The current species extinction rate is estimated to be between 1,000 and 10,000 times higher than natural extinction rates. While extinctions are a normal part of the evolutionary process, the current rates threaten important ecological functions that support human life, such as a stable climate, predictable precipitation patterns, and productive farmland and fisheries.

Scientists have identified five major mass extinctions in the Phanerozoic Eon through methods such as geological change, ecological impact, and diversity loss. The current mass extinction is unique in that it is caused exclusively by human activities. Human-induced biodiversity loss is unprecedented, and if the current trend continues, half of Earth's higher life forms could be extinct by 2100. Conservation biologists believe that human activities have already triggered a mass extinction or are on the cusp of doing so.

Urgent action is needed to curb human impacts on biodiversity and reverse the current mass extinction trend. This includes increasing conservation efforts, changing patterns of land use, and reducing greenhouse gas emissions. While it will require significant efforts, addressing the current mass extinction is crucial for preserving the planet's biodiversity and ensuring its ability to support current and future generations.

Human activity

The current mass extinction, the sixth in the history of the planet, is the first to be caused by a single organism—humans. Human activity is killing nature at an unprecedented rate. The current species extinction rate is estimated to be between 1,000 and 10,000 times higher than natural extinction rates. While extinctions are a normal and expected part of the evolutionary process, the current rates of species population decline and species extinction are high enough to threaten important ecological functions that support human life on Earth, such as a stable climate, predictable regional precipitation patterns, and productive farmland and fisheries.

The cumulative effects of deforestation, overfishing, ocean acidification, and wetland destruction have further destabilized ecosystems. The decline in amphibian populations, in particular, serves as an early indicator of broader ecological collapse. Introduced species compete with local species for resources and often diminish the quality of biodiversity in the area, sometimes causing extinction.

The current rate of extinction is between 100 and 1,000 times higher than the pre-human background rate of extinction. If we do not course-correct, we will continue to lose life-sustaining biodiversity at an alarming rate. These losses will take decades to reverse, resulting in a planet less able to support current and future generations. Urgent action is needed to curb human impacts on biodiversity.

The Big Five mass extinctions

The "Big Five" mass extinction events are defined by their impact on Earth's biodiversity, wiping out a large percentage of species in a short period of geological time. While the exact percentage of loss varies, these events stand out for their severity and their role in shaping life on our planet. Here is an overview of the "Big Five" mass extinctions:

• The Ordovician-Silurian Mass Extinction (443 million years ago): This extinction event eradicated approximately 85% of all species. It was likely caused by plummeting temperatures, the formation of huge glaciers, and a subsequent rapid warming period. This drastic shift in climate led to the extinction of many small marine species.
• The Devonian Mass Extinction (374 million years ago): This event killed about three-quarters of the world's species, primarily marine invertebrates that inhabited the ocean depths. Like the first mass extinction, this event was likely triggered by extreme temperature fluctuations and changes in sea levels.
• The Permian-Triassic Mass Extinction (250 million years ago): Known as the "Great Dying," this was the largest and most catastrophic of the "Big Five." It wiped out more than 95% of all species, including most of the vertebrates. Scientists attribute this extinction to either an asteroid impact or massive volcanic activity, both of which would have had profound global consequences.
• The Triassic Mass Extinction (200 million years ago): This extinction event eliminated about 80% of Earth's species, including many types of dinosaurs. While the exact causes are still debated, it is believed to be linked to the same environmental changes and catastrophes that characterised the other mass extinctions.
• The Cretaceous Mass Extinction (66 million years ago): This event, also known as the K-T extinction, killed 78% of all species, including the remaining non-avian dinosaurs. The primary cause is thought to be an asteroid impact, potentially compounded by volcanic activity.

These "Big Five" mass extinctions serve as stark reminders of the fragility of our planet's biodiversity. While the current consensus identifies five major mass extinctions, some sources suggest that there have been more, with the differences arising from the definition of a "major" extinction event and the data used to measure past diversity.

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Geological time

Mass extinctions are periods with much higher extinction rates than normal. They are defined by both magnitude and rate. Magnitude is the percentage of species lost, and rate is how quickly this happens. These metrics are inevitably linked, but both are needed to qualify as a mass extinction.

In a mass extinction, at least 75% of species go extinct within a relatively short period of time—less than 2 million years. In geological time, a "short" period can span thousands or even millions of years.

There have been five mass extinction events in Earth's history, at least since 500 million years ago. The first known mass extinction occurred during the Ordovician period, from 485 to 444 million years ago. This extinction event struck at the end of a 30-million-year stretch of blossoming species diversity. As the period ended, massive glaciation locked up huge amounts of water in an ice cap that covered parts of a large south polar landmass. The Ordovician-Silurian mass extinction wiped out approximately 85% of all species.

The second worst mass extinction, the Late Devonian extinction, started 383 million years ago and eliminated about 75% of all species on Earth over a span of roughly 20 million years. The worst pulse of this extinction, called the Kellwasser event, came about 372 million years ago. As ocean oxygen levels dropped precipitously, many reef-building creatures died out, including a major group of sea sponges.

The Permian mass extinction, which happened 250 million years ago, was the largest and most devastating of the five mass extinctions. This event eradicated more than 95% of all species, including most of the vertebrates which had evolved by this time. The Triassic mass extinction event occurred 200 million years ago, eliminating about 80% of Earth's species, including many types of dinosaurs.

The Cretaceous mass extinction event, which marked the boundary between the Cretaceous and Paleogene periods, occurred 66 million years ago, killing 78% of all species, including the remaining non-avian dinosaurs. This extinction was most likely caused by an asteroid hitting the Earth in what is now Mexico, potentially compounded by ongoing flood volcanism in what is now India.

Extinction rates are currently estimated to be between 1,000 and 10,000 times higher than natural extinction rates. Experts believe that we are in the midst of a sixth mass extinction, driven by human activity, including unsustainable land and water use, climate change, and unsustainable food production.

Biodiversity loss

Mass Extinction Criteria

To understand the magnitude of biodiversity loss, it is essential to define mass extinctions. Mass extinctions are characterised by both magnitude and rate. Magnitude refers to the percentage of species lost, while rate refers to the speed at which this loss occurs. To qualify as a mass extinction, at least 75% of species must go extinct within a relatively short period of time, typically less than 2 million years, by geological standards.

Historical Mass Extinctions

Throughout Earth's history, there have been several mass extinctions, often referred to as the "Big Five." These events have been caused by a combination of factors, including rapid climate change, environmental composition changes, ocean acidification, and volcanic activity. The most recent mass extinction, the Cretaceous-Palaeogene extinction, occurred approximately 66 million years ago, wiping out 78% of all species, including the non-avian dinosaurs.

Current Biodiversity Loss

Currently, we are facing the consequences of a potential sixth mass extinction, primarily driven by human activity. The species extinction rate is estimated to be between 1,000 and 10,000 times higher than natural extinction rates. Human activities, such as unsustainable land use, water usage, and energy consumption, and climate change, are major contributors to this crisis. Agriculture, in particular, is responsible for 90% of global deforestation and 70% of freshwater use, severely impacting the species that depend on these habitats.

Impacts and Efforts

The ongoing biodiversity loss threatens important ecological functions that support human life on Earth, such as stable climates and productive farmland. Urgent action is needed to curb human impacts and mitigate the loss of life-sustaining biodiversity. Organisations like the World Wildlife Fund (WWF) are actively working towards conservation efforts and seeking donations to protect endangered species from extinction.

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