The Holocene Extinction is the current, accelerated period of species loss spanning the last several millennia. This ongoing global biodiversity crisis is distinct because its primary driver is the activity of a single species: Homo sapiens. The speed and magnitude of this modern die-off have led scientists to label it the “Sixth Mass Extinction.” Unlike past events caused by asteroid impacts or supervolcanoes, the Holocene Extinction is a direct consequence of human population growth and the resulting alteration of planetary systems.
Placing the Extinction in Geological Time
The standard measure for species turnover is the background extinction rate, the natural, low-level pace at which species disappear. For most groups of organisms, this pre-human rate is estimated to be approximately 0.1 to 1 extinction per million species years (E/MSY). This rate is calculated from the fossil record.
Comparing the current crisis to this baseline reveals a biological anomaly. Earth has previously experienced five major mass extinctions, such as the event 66 million years ago that ended the reign of the dinosaurs, where at least 75% of all species vanished in a geologically brief time. These “Big Five” events were caused by sudden, planet-altering natural forces. The modern extinction is now occurring at a rate comparable to those ancient catastrophes, but the cause is purely biotic.
The current era is sometimes referred to as the Anthropocene Extinction, a proposed geological epoch that recognizes humanity’s dominant influence on the planet’s geology and ecology. However, the popular and established term remains the Holocene Extinction, referring to the geological epoch that began approximately 11,700 years ago. The defining feature is the unprecedented acceleration of species loss since the rise of human civilization and industrialization.
The Anthropogenic Mechanisms of Loss
The primary engine of the Holocene Extinction is habitat destruction and fragmentation, driven largely by the conversion of wildlands for agriculture, urbanization, and resource extraction. When forests are cleared or wetlands are drained, the immediate effect is the complete loss of the ecological niche for countless resident species. Remaining habitats are often isolated into small patches, which prevents gene flow and isolates populations, making them vulnerable to localized extinction events.
Direct overexploitation of species through unsustainable hunting, fishing, and harvesting represents another major mechanism of loss. For marine species, industrial fishing practices frequently remove biomass at a rate that exceeds the species’ reproductive capacity, leading to population collapse. Terrestrial species, particularly large mammals, have been historically and continue to be targeted by overhunting and poaching, which removes apex predators and large herbivores, destabilizing entire food webs.
Pollution acts as a pervasive stressor on biodiversity, introducing toxic substances into all major ecosystems. Chemical pollutants, including pesticides, industrial waste, and plastics, compromise the health and reproductive success of organisms in water and soil. Furthermore, the introduction of invasive species, often transported globally by human commerce and travel, disrupts native ecosystems by outcompeting local organisms for resources or preying upon them. The introduction of non-native diseases via invasive species has devastated amphibian populations globally.
These drivers are compounded by climate change, which acts as a multiplier of extinction risk by altering temperature and precipitation regimes faster than species can adapt. Rising global temperatures force species to migrate to maintain their climatic niche, but habitat fragmentation often makes this movement impossible. Species confined to small islands or mountain tops, like the Bramble Cay melomys, are particularly susceptible to extinction from sea-level rise and habitat transformation.
Assessing the Extinction Rate
Scientists quantify the severity of the current crisis using the metric Extinctions per Million Species Years, or E/MSY. This standardized measurement allows researchers to compare the modern extinction rate against the historical background rate. The current rate of species loss is conservatively estimated to be between 100 and 1,000 times the natural background rate.
This acceleration means species that would have naturally persisted for millions of years are now disappearing in decades or centuries. For vertebrates, the average background extinction rate is estimated to be less than two E/MSY, yet the current rate has surged, indicating that hundreds of species have been lost since the year 1900. The most vulnerable taxonomic groups today include amphibians, which face threats from habitat loss and disease, and many plant species.
The true scope of the crisis extends beyond species already declared extinct and includes a concept known as “extinction debt.” This term refers to species that have suffered such severe population decline and habitat loss that their eventual extinction is considered inevitable, even if the immediate cause of stress is removed. Scientists estimate that approximately one million plant and animal species are currently threatened with extinction, underscoring the scale of this impending debt.
Consequences for Ecosystem Stability
The loss of species does not merely reduce a biological count; it compromises the stability and functionality of entire ecosystems, which provide services upon which human civilization depends. These ecosystem services are the natural processes that support life, including the purification of water and air, the cycling of nutrients, and the regulation of local climates. As biodiversity declines, the resilience of these systems to withstand disturbances like drought, disease outbreaks, or extreme weather events is greatly diminished.
The disappearance of specific organisms can trigger cascading effects that reverberate throughout an ecological community. For instance, the loss of pollinators, such as bees and certain insect species, directly threatens the productivity of major food crops, impacting global food security. Similarly, the decline of natural pest controllers, including birds and bats, can lead to increased crop damage and a greater reliance on chemical pesticides.
Biodiversity loss also severely affects biogeochemical processes, like the cycling of carbon, nitrogen, and phosphorus. Wetlands and forests, for example, naturally filter water and sequester carbon, but their degradation compromises water quality and contributes to greenhouse gas emissions. Ultimately, the destabilization of these natural processes links species loss directly to human well-being, threatening economic stability and increasing the risk of zoonotic disease transfer as human and animal habitats overlap.