Extinction refers to the complete and permanent disappearance of a species from Earth. This phenomenon is a fundamental part of the planet’s evolutionary story, a natural process that has shaped life since its beginning. While species loss is an inherent feature of life, the current rate and cause of species disappearance are fundamentally different from those that governed the natural world. The modern crisis is something new entirely.
The Baseline: Natural Extinction Rates
The normal rate of species loss, known as the background extinction rate, provides a constant against which to measure modern changes. This baseline accounts for the continuous turnover of species due to standard ecological pressures over vast stretches of time. Species disappear naturally when they fail to adapt to localized environmental shifts, lose out in competition, or reach the end of their typical evolutionary lifespan.
Scientists estimate this background rate by analyzing the fossil record during periods of geological stability. A commonly accepted estimate is approximately 0.1 to 1 extinction per million species years (E/MSY). For example, one would expect only one mammalian species to go extinct naturally every few centuries.
These losses are generally balanced by the emergence of new species through speciation, maintaining a long-term equilibrium in biodiversity. The drivers of background extinction are slow-acting and biological, ensuring that only poorly adapted species are gradually filtered out.
Catastrophic Shifts in Geological History
In contrast to the slow background hum, Earth’s history is punctuated by rare, devastating mass extinction events. These events are defined as rapid, global occurrences that result in the loss of at least 75% of all species within a geologically short time frame. These events demonstrate that catastrophic species loss can be driven by natural, Earth-system processes.
The most severe of these, the Permian-Triassic extinction event about 252 million years ago, eliminated up to 96% of marine species and 70% of terrestrial vertebrate species. This cataclysm was likely triggered by massive volcanic activity in Siberia, resulting in rapid global warming, widespread ocean acidification, and oxygen-depleted ocean water.
Another notable example is the Cretaceous-Paleogene extinction event 66 million years ago, which famously ended the reign of the non-avian dinosaurs. This was caused by the impact of a massive asteroid near the Yucatán Peninsula in Mexico. The impact generated a global firestorm, tsunamis, and a thick dust cloud that blocked sunlight for months, collapsing food chains worldwide.
These historical mass extinctions were driven by non-biological, geological forces like super-volcanism and asteroid strikes. They were ultimately the result of planetary physics and geochemistry, not the sustained influence of a single biological species.
The Anthropocene Extinction Crisis
The current period of species loss, often termed the Anthropocene extinction, is widely considered by scientists to be the Earth’s Sixth Mass Extinction event. This crisis is distinct because its primary driver is the activity of a single species: Homo sapiens. While past mass extinctions were caused by external forces, this one is entirely anthropogenic, or human-caused.
The speed of the current species loss is the most dramatic difference when compared to the natural background rate. Current extinction rates are estimated to be between 100 and 1,000 times higher than the natural rate of 0.1 E/MSY. For vertebrates, the number of species that have gone extinct in the last century would have taken many millennia to disappear under normal conditions.
The mechanisms driving this rapid loss are directly linked to human industrial and population growth. The combination of these human-induced pressures has put approximately one million species at risk of extinction, confirming that the current biological crisis is non-natural in both its origin and its scale.
Primary Drivers of Anthropocene Extinction
The primary drivers include:
- Habitat fragmentation and destruction, mainly through the conversion of forests and wetlands for agriculture and urbanization. Agricultural expansion is the identified threat for over 85% of species currently at risk.
- Overexploitation and the introduction of invasive species. Unsustainable fishing, hunting, and logging directly reduce populations faster than they can recover.
- Invasive alien species are a major factor in an estimated 60% of all recorded extinctions, as they outcompete or prey on native organisms.
- Human-driven climate change, caused by greenhouse gas release, which rapidly destabilizes ecosystems. This forces species to adapt or migrate at speeds that exceed their evolutionary capacity.