What Is the Background Extinction Rate? A Comprehensive Guide

Key Takeaways

• The background extinction rate is the natural, long-term rate at which species disappear due to normal evolutionary and environmental processes, excluding catastrophic mass extinction events.
• It is typically measured in extinctions per million species-years (E/MSY), with most estimates ranging between 0.1 and 1 E/MSY for marine invertebrates and around 0.1 to 2 E/MSY for mammals.
• This rate serves as a critical baseline for assessing whether current extinction rates are elevated, helping scientists determine if Earth is entering a sixth mass extinction.
• Understanding the background rate is essential for conservation biology, as it provides context for human impacts on biodiversity and informs policy decisions.

What Is the Background Extinction Rate?

The background extinction rate is the standard, long-term rate of species loss that occurs in the absence of major environmental upheavals or human influence. It reflects the natural turnover of life on Earth, where species go extinct due to factors such as competition, predation, genetic drift, and gradual environmental changes. This rate is not constant across all groups of organisms or through time, but it provides a baseline against which elevated extinction rates—such as those observed during mass extinction events or the current biodiversity crisis—can be compared.

In paleontology and conservation biology, the background extinction rate is usually expressed as the number of extinctions per million species-years (E/MSY). This metric normalizes extinction counts by both the total number of species and the time interval, allowing comparisons across different taxa and time periods. For example, if 10 out of 1,000 species go extinct over 1 million years, the rate is 10 E/MSY. Most estimates for the fossil record suggest a background rate of 0.1 to 1 E/MSY for marine invertebrates, which are the most commonly studied group due to their abundant fossil record. For mammals, the estimated background rate is slightly higher, around 0.1 to 2 E/MSY, based on fossil and molecular data.

The concept is fundamental to understanding the history of life and the current extinction crisis. By establishing what is “normal,” scientists can quantify how much human activities have accelerated species loss. The background rate is not a single number but a range that varies by taxonomic group, geographic region, and geological era. It is also distinct from the “mass extinction rate,” which is many times higher and occurs during geologically brief intervals.

How It Works

The background extinction rate is estimated primarily from the fossil record, which provides a long-term archive of species appearances and disappearances. Paleontologists count the number of extinctions of well-preserved taxa (such as marine invertebrates) over specific time intervals and then standardize these counts to a per-million-species-years basis. This involves estimating the total number of species present at the time, which is challenging because the fossil record is incomplete. Statistical methods, such as rarefaction and capture-recapture models, are used to account for sampling biases.

For modern organisms, molecular phylogenetics can also be used to infer background extinction rates. By analyzing the branching patterns of evolutionary trees and the timing of speciation events, scientists can estimate how often lineages have gone extinct in the absence of major perturbations. These methods often yield rates that are consistent with fossil-based estimates, providing cross-validation. The background rate is not a fixed number but a statistical average over millions of years, and it can vary between different clades and ecosystems. For instance, some groups like insects may have higher background rates due to their rapid speciation and extinction dynamics, while others like sharks have lower rates.

To calculate the rate, researchers use the formula: (number of extinctions) / (estimated total species × time interval in millions of years). The result is then expressed as E/MSY. Because the fossil record is biased toward hard-bodied organisms, the background rate for soft-bodied creatures is less certain. Additionally, the rate can be influenced by the taxonomic level studied; species-level extinctions are more common than genus-level extinctions, so the rate depends on the resolution of the data.

What the Evidence Shows

Fossil evidence indicates that background extinction rates have fluctuated over geological time but generally remain low. For marine invertebrates, the average background rate is estimated at 0.1–1 extinctions per million species-years. For mammals, the fossil record suggests a rate of about 0.1–2 E/MSY. Birds, with a less complete fossil record, are estimated to have a similar background rate. These numbers mean that, under normal conditions, one would expect roughly 0.1 to 2 species extinctions per year among every 10,000 mammal species, for example.

In contrast, current extinction rates for many groups are estimated to be 100 to 1,000 times higher than the background rate, leading many scientists to conclude that a sixth mass extinction is underway. For instance, a 2015 study in Science Advances estimated that the current extinction rate for vertebrates is up to 100 times the background rate, and that species are going extinct at a pace not seen since the dinosaur extinction. However, these comparisons depend heavily on the assumed background rate and the accuracy of modern extinction counts. Some researchers argue that the background rate may be lower than commonly cited, which would make current rates appear even more alarming.

It is important to note that the background rate is not uniform across all taxa. Some groups, like amphibians, are currently experiencing extinction rates far above background, while others, like certain plants, may be less affected. The evidence also shows that background rates can be elevated during periods of rapid climate change or after major geological events, but these are still considered part of the natural variability unless they reach mass extinction thresholds.

Importance and Impact

The background extinction rate is a critical benchmark in conservation biology. It allows scientists and policymakers to gauge the severity of human impacts on biodiversity. If current extinction rates significantly exceed the background rate, it signals that ecosystems are under stress and that biodiversity loss may have cascading effects on ecosystem services, such as pollination, water purification, and climate regulation. The concept also underpins the definition of “mass extinction” events, which are periods when extinction rates spike far above background levels.

Moreover, understanding the background rate helps in setting conservation priorities. Species that are naturally rare or have high extinction rates even under normal conditions may require more urgent protection. The background rate also informs models of future biodiversity loss, helping to predict how many species might be lost if current trends continue. This, in turn, influences international agreements and conservation strategies, such as the Convention on Biological Diversity’s targets. For example, the Aichi Biodiversity Targets aimed to reduce the rate of biodiversity loss, implicitly using the background rate as a reference.

In addition, the background extinction rate provides a long-term perspective on evolution. It reminds us that extinction is a natural process that opens ecological niches for new species. However, when the rate is unnaturally high, the evolutionary process cannot keep pace, leading to a net loss of biodiversity that may take millions of years to recover. This has profound implications for the future of life on Earth and the services ecosystems provide to humanity.

Common Misconceptions

One common misconception is that the background extinction rate is a single, universally accepted number. In reality, it varies by taxonomic group, time period, and estimation method. Another misunderstanding is that all extinctions are bad; extinction is a natural part of evolution, and without it, new species could not flourish. The concern today is the abnormally high rate, not extinction itself. Additionally, some people confuse the background rate with the “normal” rate of extinction caused by human activities before the industrial era; however, even pre-industrial human impacts may have elevated extinction rates above the true geological background.

Another misconception is that the background rate can be directly observed in real time. Because it is measured over millions of years, it is an inferred average, not something we can witness annually. Short-term fluctuations in extinction rates are normal, and a few high-profile extinctions do not necessarily indicate a mass extinction unless the rate is sustained and global. Furthermore, the background rate is sometimes mistakenly thought to apply equally to all species, but some groups are naturally more extinction-prone than others.

Finally, there is a misconception that the background rate is irrelevant because we are already in a mass extinction. While current rates are alarming, the background rate remains essential for quantifying the magnitude of the crisis and for setting recovery targets. Without a baseline, it is impossible to measure progress in conservation.

Data Limitations and Uncertainties

Estimating the background extinction rate is fraught with uncertainties. The fossil record is incomplete, with only a tiny fraction of all species that ever lived being preserved and discovered. Soft-bodied organisms, for example, are severely underrepresented. Additionally, taxonomic practices can influence counts: what one paleontologist considers a single species might be split into multiple species by another, affecting extinction rate calculations. The time intervals used also matter; longer intervals tend to smooth out fluctuations and may underestimate short-term variability.

For modern comparisons, the background rate is often extrapolated from fossil data, but the fossil record is dominated by marine invertebrates, while current extinction concerns focus on terrestrial vertebrates and plants. This mismatch introduces uncertainty. Moreover, the background rate may have varied significantly over geological time, so using an average may not be appropriate for all contexts. Despite these limitations, the background extinction rate remains a valuable concept for contextualizing current biodiversity loss.

Another challenge is that the background rate is typically calculated at the species level, but many extinctions today are of subspecies or populations, which are not captured in the fossil record. This can lead to underestimation of the true extinction debt. Additionally, molecular methods for estimating background rates rely on assumptions about constant speciation and extinction rates, which may not hold over long periods. As a result, the background rate should be viewed as a rough guide rather than a precise number.