What Is a Mass Extinction?

A mass extinction is defined as a period in which Earth loses more than 75% of its species within a geologically short timeframe. Earth has experienced five such events over the past 450 million years, each fundamentally reshaping the planet's biology:

• End-Ordovician (444 Ma): ~85% of species lost, caused by glaciation and sea level changes
• Late Devonian (372 Ma): ~75% of species lost over several million years
• End-Permian (252 Ma): ~96% of species lost—the "Great Dying," caused by massive volcanism
• End-Triassic (201 Ma): ~80% of species lost, linked to volcanic activity and climate change
• End-Cretaceous (66 Ma): ~76% of species lost, including all non-avian dinosaurs, caused by asteroid impact

Each previous extinction unfolded over thousands to millions of years. What makes the current crisis unprecedented is its speed—driven by a single species within centuries, not millennia.

The Evidence: Are We in a Sixth Mass Extinction?

The scientific case has strengthened dramatically over the past two decades:

Extinction Rates

In 2015, Ceballos, Ehrlich, and colleagues published a landmark paper in Science Advances demonstrating that modern species are disappearing at a rate tens to hundreds of times higher than the natural "background" extinction rate observed over the past 10 million years. Using conservative estimates, they concluded that even the lowest estimates of current extinction rates are far above normal—signaling that we have entered the sixth mass extinction.

Population Losses: "Biological Annihilation"

In 2017, the same researchers expanded their analysis beyond species extinction to population losses. Their findings, published in the Proceedings of the National Academy of Sciences, documented what they termed "biological annihilation":

• Of 27,600 vertebrate species examined, nearly one-third are declining in population size and range
• These are not rare species—they include common and widespread species experiencing severe population contractions
• The losses are so severe that they constitute a "frightening assault on the foundations of human civilization"

Vertebrates on the Brink

A 2020 follow-up identified 515 terrestrial vertebrate species with fewer than 1,000 individuals remaining—on the brink of extinction. Of these, 237 species had fewer than 250 individuals. The study found strong geographic clustering of these species, suggesting that targeted conservation of specific regions could prevent significant near-term extinctions.

Defaunation in the Anthropocene

Stanford researcher Rodolfo Dirzo coined the term "defaunation" to describe the parallel to deforestation: the systematic loss of animals from ecosystems. His 2014 paper in Science documented that beyond outright extinction, we are experiencing pervasive declines in animal abundance—a phenomenon he described as "the most under-recognized form of global environmental change."

Defaunation cascades through ecosystems. When large animals disappear, seed dispersal fails, vegetation changes, fire regimes shift, and carbon storage capacity decreases. The loss of megafauna has consequences that ripple through ecosystems for centuries.

The Insect Crisis

Perhaps the most alarming dimension of the sixth extinction involves insects—the foundation of terrestrial food webs:

• A 2017 German study found a 75% decline in flying insect biomass over 27 years in protected areas
• The decline is driven by a convergence of pesticide use, habitat loss, light pollution, and climate change—described as "death by a thousand cuts"
• Broad meta-analysis confirms terrestrial insect declines while freshwater insects show some increases in regions with improved water quality

Insects pollinate 87.5% of flowering plant species and are the primary food source for most birds, freshwater fish, and many mammals. Their decline threatens the entire web of terrestrial life.

What Makes This Different

The sixth mass extinction differs from all previous events in several critical ways:

• Speed: Previous extinctions unfolded over thousands to millions of years. The current one is occurring in centuries—too fast for evolution to respond.
• Cause: For the first time, a single species is the primary driver—through habitat destruction, pollution, climate change, overexploitation, and the spread of invasive species.
• Awareness: Also for the first time, the species causing the extinction has the knowledge and tools to stop it.
• Interconnection: Unlike previous mass extinctions, the current one operates through multiple simultaneous drivers that interact and amplify each other.

The Path Forward

As E.O. Wilson argued in Half-Earth (2016), preventing mass extinction requires dedicating approximately half of Earth's surface to nature conservation. OASA's model embodies this principle at the project level—mandating that at least 50% of all project land be dedicated to rewilding and biodiversity conservation.

The Kunming-Montreal Global Biodiversity Framework targets protecting 30% of land and ocean by 2030. But as the science makes clear, protection alone is insufficient without addressing the root causes—the economic systems that incentivize destruction.

OASA addresses this through perpetual commons—legally removing land from commodity markets permanently, while creating the governance and economic structures that align human prosperity with ecological regeneration. Indigenous peoples have demonstrated that humans can be keystone species—lands managed by Indigenous peoples typically exhibit higher levels of biodiversity than surrounding areas.

The sixth mass extinction is not inevitable. But avoiding it requires fundamental changes to how we relate to land, how we structure our economies, and how we govern shared resources—exactly the transformation that regenerative commons are designed to achieve.

Key References

• Ceballos, G., Ehrlich, P.R. & Barnosky, A.D. (2015). "Accelerated modern human-induced species losses: Entering the sixth mass extinction." Science Advances.
• Ceballos, G., Ehrlich, P.R. & Dirzo, R. (2017). "Biological annihilation via the ongoing sixth mass extinction." Proceedings of the National Academy of Sciences.
• Dirzo, R. et al. (2014). "Defaunation in the Anthropocene." Science.
• Barnosky, A.D. et al. (2011). "Has the Earth's sixth mass extinction already arrived?" Nature.
• Kolbert, E. (2014). The Sixth Extinction: An Unnatural History. Pulitzer Prize winner.
• Wilson, E.O. (2016). Half-Earth: Our Planet's Fight for Life.

Related Terms

• Biodiversity — The variety of all life on Earth and its current crisis
• Causes and Threats of Biodiversity Collapse — The five drivers and systemic forces
• Ecosystem Collapse and Tipping Points — Critical thresholds and cascading risks
• Keystone Species — Species whose loss triggers cascading ecosystem effects
• Rewilding — Restoring natural processes and species
• 50% Land Conservation — OASA's Half-Earth approach at project scale
• Humans as Keystone Species — How humans can support rather than degrade ecosystems