Extinction

Understanding Extinction: A Journey Through Time

Imagine a world where every species that ever lived is now gone. That’s the stark reality of extinction—where once vibrant life forms are no more, leaving behind only fossils and stories. How do we define this phenomenon? In essence, extinction is the termination of a taxon by the death of its last member. But what about when a species loses its reproductive capacity? Is it functionally extinct before its final demise?

The Extinction of Species

More than 99% of all species that have ever existed are estimated to be extinct, with around 8.7 million eukaryotic species currently existing globally. Notable examples include the non-avian dinosaurs, saber-toothed cats, dodos, mammoths, and passenger pigeons. These iconic creatures once roamed the Earth but now exist only in our imaginations and museums.

Causes of Extinction

The current high rate of extinctions is largely due to human actions such as habitat destruction, unsustainable fishing, hunting, and logging. According to the IPBES report, biomass of wild mammals has fallen by 82%, natural ecosystems have lost about half their area, and a million species are at risk of extinction.

Examples of extinct species include Steller’s sea cow, dodo, Chinese paddlefish, great auk, thylacine, and passenger pigeon. A species is declared extinct when the last existing member dies, and extinction requires a clear definition of the species in question.

The Extinction Vortex

In ecology, extinction refers to the local or global termination of a species’ existence. Local extinctions may be made good by reintroducing individuals from other locations, and species not globally extinct are termed extant yet threatened with extinction. Critically endangered species have been listed as ‘extinct in the wild’ (EW) due to lack of living specimens in the wild, and zoos or artificial environments maintain these species through breeding programs.

The extinction of one species’ population can have knock-on effects causing further extinctions, known as ‘chains of extinction,’ especially with keystone species. A 2018 study estimated that restoring mammal diversity could take up to 5-7 million years. Extinction of a parent species where daughter species or subspecies are still extant is called pseudoextinction or phyletic extinction.

Lazarus Taxa and Rediscovery

Pseudoextinction can be difficult to demonstrate without strong evidence linking living species to pre-existing species. Lazarus taxa refer to species thought to be extinct but rediscovered, such as the coelacanth and Attenborough’s long-beaked echidna. Some species currently considered extinct have been recently rediscovered, sparking speculation about their possible existence and potential for reintroduction.

Human-Driven Extinction

The human-driven extinction started over 60,000 years ago when humans migrated out of Africa. Humans can cause extinction through overharvesting, pollution, habitat destruction, introduction of invasive species, overhunting, and other influences. Several species have been listed as extinct since 2004. The International Union for Conservation of Nature (IUCN) has recorded 784 extinctions since the year 1500.

Genetic factors can affect extinction rates, including genetic pollution such as uncontrolled hybridization, introgression, and genetic swamping that lead to homogenization or out-competition from introduced species. Endemic populations face extinction due to new populations or selective breeding, interbreeding with more abundant species depleting the rarer gene pool.

Habitat Degradation

A large gene pool is associated with robust populations that can survive intense selection, while low genetic diversity increases the chance of extinction. Habitat degradation is a main cause of species extinctions, primarily due to agriculture and urban sprawl, leading to environmental changes that alter the fitness landscape and reduce survival rates.

Habitat destruction removes vegetation, enhances erosion, and diminishes nutrient availability in terrestrial ecosystems. Habitat degradation through toxicity can kill off species rapidly or over longer periods at lower levels of contamination. Diminished resources and introduction of new competitor species often accompany habitat degradation, allowing some species to expand their range and compete with other species for limited resources.

Natural Causes and Human Impact

Predation, competition, and disease are natural causes of extinction, but human activities have become a significant additional cause in recent geological time. Invasive alien species can cause catastrophic consequences for native species and ecosystems when they become established. Human populations can act as invasive predators. The ‘overkill hypothesis’ suggests that human presence led to the extinction of megafauna in areas such as Australia, North America, Madagascar, Hawaii, and New Zealand.

Coextinction refers to the loss of a species due to the extinction of another, often occurring when a keystone species goes extinct or when a predator loses its prey. Coextinction is especially common in mutualistic and parasitic relationships.

Climate Change

Climate change has caused extinctions throughout history, including during the Carboniferous Rainforest Collapse 305 million years ago. It is predicted that climate change will cause 15-37% of land species to be ‘committed to extinction’ by 2050.

Sexual Dimorphism and Extinction

Species with high sexual dimorphism are at a higher risk of extinction and die out faster than less sexually dimorphic species. The Permian-Triassic extinction event, which killed 90% of species 250 million years ago, was likely caused by a massive eruptive event that released tephra particles into the atmosphere. The Cretaceous-Paleogene extinction event occurred 66 million years ago, wiping out non-avian dinosaurs and many other species.

Historical Perspectives on Extinction

A large gene pool is associated with robust populations that can survive intense selection, while low genetic diversity increases the chance of extinction. Habitat degradation is a main cause of species extinctions, primarily due to agriculture and urban sprawl, leading to environmental changes that alter the fitness landscape and reduce survival rates.

Georges Cuvier: The Father of Extinction

The concept of extinction became central to Charles Darwin’s On the Origin of Species, where less fit lineages disappeared over time. However, it wasn’t until 1982 that David Raup and Jack Sepkoski published their paper on mass extinctions, vindicating Cuvier and accepting catastrophic extinction as a mechanism.

Georges Cuvier is credited with establishing the modern conception of extinction in 1796. The current understanding of extinction is a synthesis of cataclysmic and background extinction events, combining elements from Cuvier, Lyell, and Darwin’s theories.

Human Attitudes Towards Extinction

Human attitudes towards extinction are complex, ranging from concern to indifference, driven by factors such as commercial interests, governments’ policies, and people’s priorities. Organizations like the Worldwide Fund for Nature aim to preserve species, while governments enact laws to protect habitats and prevent pollution.

Scientific Interest in Biodiversity

The scientific community stresses the importance of maintaining biodiversity, with some technologies having devastating effects on wildlife despite minimal harm to humans. Governments sometimes prioritize ecotourism over conservation efforts, while advocacy groups like The Wildlands Project work to educate the public and pressure governments into action.

Philosophical Perspectives

People living near nature are often dependent on species’ survival, yet their priorities may conflict with conservation goals due to human overpopulation and economic pressures. Philosophers David Benatar and Jason Hickel have argued that humanity’s indifference to non-human species extinction is rooted in our perception of ourselves as separate from the natural world and the logic of capitalism.

Cloning Extinct Species

Theoretically, cloning extinct species using DNA from remains could be possible. However, attempts so far have been unsuccessful, including failed Pyrenean ibex cloning attempts. The scientific community is divided on whether such efforts are ethically justifiable.

As we stand on the brink of a potential sixth mass extinction, it is clear that our actions today will shape the future of countless species. The challenge lies not only in understanding but also in acting with urgency and compassion towards preserving the biodiversity that sustains us all.