Extinction is a natural process that has shaped life on Earth since its beginnings. While the complete disappearance of a species might seem like a dramatic event, it occurs continuously through various mechanisms. Not all extinction events are the same, however, and understanding these differences is important for grasping the dynamics of life on our planet.
Defining Background Extinction
Background extinction refers to the standard, ongoing rate at which species disappear over Earth’s geological history, excluding major, rapid events. This process is a normal component of evolution and ecology, where individual species naturally fade away over long periods. It represents the baseline loss of species due to typical environmental and ecological pressures. This continuous, low-level species loss occurs gradually as species face challenges like environmental changes, resource competition, or the emergence of new species. The rate of background extinction has not remained constant throughout Earth’s history, though changes are measured over millions of years.
Background Versus Mass Extinction
The primary distinction between background and mass extinction lies in their scale, speed, and underlying causes. Background extinction is a slow, continuous process affecting a limited number of species at any given time, and is a gradual disappearance of species due to typical environmental or ecological factors.
Conversely, a mass extinction is a rapid, widespread, and significant reduction in Earth’s biodiversity. These events involve the disappearance of a large number of species across many different groups within a geologically short period. Mass extinctions are often triggered by catastrophic global events or widespread environmental shifts that occur too quickly for most species to adapt.
For instance, the Permian extinction, occurring about 252 million years ago, was Earth’s largest known mass extinction, causing approximately 90% of marine species to disappear. The Cretaceous extinction 66 million years ago led to the extinction of around 75% of species, including most dinosaurs, possibly due to an asteroid impact. These events stand in contrast to the typical, ongoing species loss seen in background extinction.
Natural Factors Driving Background Extinction
Natural ecological and evolutionary processes contribute to background extinction. Competition for limited resources, both between and within species, is a significant factor, as species less effective at acquiring them may decline and disappear.
Predator-prey dynamics also play a role. New predators or increased predator populations can drive prey species to extinction if they cannot adapt. Disease outbreaks from natural pathogens can decimate populations, especially if a species has low genetic diversity.
Natural environmental fluctuations are another contributor. Gradual climate shifts, habitat changes, or geological events like volcanic activity can alter ecosystems. Species unable to adapt quickly to these changing conditions may face extinction.
Assessing and Interpreting Background Extinction Rates
Scientists primarily estimate background extinction rates by analyzing the fossil record. They examine periods of Earth’s history between mass extinction events to determine how many species typically went extinct over time. This method helps establish a baseline rate of species loss.
Another approach involves studying species longevity, estimating how long species typically exist from their origination to their disappearance. For example, the average lifespan of a mammal species in the fossil record is estimated to be around 1 to 2 million years, though some can persist longer. For invertebrates, this average can be between 5 to 10 million years.
The background extinction rate is often expressed in “extinctions per million species-years” (E/MSY). While estimates vary, a commonly cited natural background rate is approximately one species extinction per million species per year. This means that if there were a million species on the planet, one would be expected to go extinct each year under natural conditions. This natural rate provides a reference point for evaluating current extinction trends.