Why do species become extinct?

The termination of a species, marked by the final death of its last individual or the irreversible loss of its ability to reproduce, is a persistent feature of life on Earth. ^[2] By nearly all reckonings, more than $99.9%$ of every species that has ever existed is now gone. ^[8] While this process is a natural component of evolution, the rate at which it is currently occurring alarms scientists, signaling that we are likely immersed in the sixth mass extinction event in the planet's history. ^[2][5] Understanding why species vanish requires looking both at the slow mechanisms of deep time and the accelerated pressures of the present day.

# Natural Pace

For billions of years, extinction has occurred at a "background rate," a slow, almost constant turnover where species appear and disappear over timescales often measured in millions of years. ^[2][5][7] Geological evidence suggests that the typical lifespan for a species might be somewhere between one and ten million years. ^[2] Paleontologists have cataloged at least five major mass extinction events over the last 600 million years, periods when biodiversity plummeted rapidly on a geological scale. ^[5] For instance, the Permian-Triassic event wiped out the majority of marine species, and the Cretaceous-Paleogene event famously ended the reign of the non-avian dinosaurs, likely due to an asteroid impact. ^[5][8] In these catastrophic ancient eras, the scale of loss—with up to $95%$ of the planet’s species vanishing—was far greater than what is observed between these large events. ^[1]

When comparing these historical markers to today, the difference is stark. Current extinction rates are estimated to be hundreds, or even thousands, of times higher than the natural baseline rate. ^[7] While documented losses recorded by organizations like the IUCN are small—only about 800 in the past 400 years—these figures are recognized as only the tip of the iceberg, as most extinctions happen without scientific documentation. ^[1][2] Some prominent scientists estimate that up to 150 species could be lost every single day, although these high figures rely heavily on complex computer modeling rather than direct observation. ^[1]

# Evolutionary Lag

A common question arises when facing this loss: if evolution is the mechanism that drives life forward, why don't species simply adapt to new conditions instead of dying out? The answer lies in the speed of change. ^[4]

Evolution is not a conscious process where organisms decide to acquire a needed trait. ^[4] Instead, it relies on random mutations appearing within a population, which are then selected for if they provide a reproductive advantage in the current environment. ^[4] This process, even under the best circumstances, requires significant time—often spanning many generations. ^[4][3]

When environmental shifts are gradual, evolution can keep pace, meaning the species that survive are those that adapted slowly, bit by bit. ^[3][5] However, the changes imposed by modern humanity are incredibly swift. The sheer speed of change—driven by industrialization, land conversion, and global temperature shifts over just a century or two—outpaces the generational capacity for most complex organisms to evolve solutions. ^[3][4]

For example, larger, more complex organisms like whales tend to be outpaced by faster-reproducing entities like viruses because complexity often demands stringent error correction during reproduction, which limits the rate of beneficial, random mutation. ^[4] Furthermore, species adapted to very narrow, specific ecological niches—such as an insect reliant on a single host plant—lack the inherent flexibility to pivot when their specialized environment is destroyed or altered. If the required trait for survival in the new environment is not already present in the gene pool, the population simply cannot conjure it in time. ^[4] It is a race where modern environmental shifts are running a marathon pace while many species are stuck in a slow, steady evolutionary trot. ^[4]

How does habitat fragmentation affect species?

# Human Drivers

While natural processes like disease, competition, or climate shifts have always driven extinctions, current biodiversity loss is overwhelmingly attributed to human activity. ^[2][7] These contemporary drivers often interact synergistically, creating an "extinction vortex". ^[2]

# Habitat Loss

The most critical anthropogenic cause of species decline today is habitat destruction and degradation. ^[7] This includes the clearing of forests for agriculture, the relentless expansion of urban and industrial centers, mining operations, and the degradation of aquatic environments. ^[3][7] When a species' specific niche is eliminated—like the dense shade provided by old-growth forests for certain ferns—the species loses the necessary infrastructure for survival. ^[3]

What we are witnessing today is not just the outright removal of habitat, but its fragmentation. When large tracts are broken into smaller, disconnected parcels, species that require large home ranges or cannot easily cross disturbed land struggle to find mates or sufficient resources. ^[7] This fragmentation is a crucial modifier of how species respond to other changes, such as climate change, because intact habitat is essential for a population to shift its range to track shifting climate envelopes. ^[7] A truly insightful way to view this is through the lens of extinction debt: the destruction of habitat today means that many species are effectively doomed, even if they are still technically alive. They are living on borrowed time in remnant areas, and their final disappearance is a delayed consequence of past, irreversible habitat removal. ^[2]

# Overshoot

Direct overexploitation, often termed overharvesting, remains a potent threat. ^[3] This goes beyond localized subsistence hunting; it encompasses industrial-scale pressures like modern fishing technology, which efficiently removes marine populations faster than they can recover. ^[3] Similarly, unsustainable logging or the extraction of other natural resources can decimate populations for food, medicine, or construction materials, such as the historical example of passenger pigeons being hunted to near oblivion. ^[3][5][8]

# Alien Threats

The introduction of non-native species into new ecosystems accelerates extinction risk dramatically, especially on islands. ^[7] Invasive species—whether they are predators (like cats or snakes introduced to islands), competitors, or disease vectors—often thrive because native species have not evolved defenses against them. ^[3][7] The impact can be profound, as seen in Hawaii where introduced diseases like avian malaria devastated native bird populations that lacked immunity. ^[7] In aquatic systems, the introduction of a predator like the Nile perch into Lake Victoria has resulted in the estimated loss or threat to 200 species of endemic cichlid fish. ^[7] This process can also manifest as genetic pollution, where hybridization with a more abundant introduced species swarms and dilutes the rare native gene pool until the original lineage is genetically swamped. ^[2]

# Pollution and Climate

Pollution adds chemical stress to biological systems. From oil spills killing coastal life to plastic debris being mistaken for food by sea turtles, contaminants weaken organisms and increase susceptibility to other threats. ^[3][8] Climate change, driven by greenhouse gases from fossil fuel use and methane from agriculture, also shifts temperature and precipitation regimes, destroying established habitats and forcing ranges to move. ^[1][3][7] While climate has always changed, the current rate of warming is rapid, and unlike ancient ice ages where species could migrate gradually across continuous landscapes, today's fragmented habitats prevent this necessary movement. ^[7]

# Genetic Vulnerability

Beyond external pressures, a species’ internal state dictates its ability to survive a shock. Genetic health is paramount. Populations that undergo a drastic reduction in size—a bottleneck—suffer a corresponding loss of genetic diversity. ^[2][8] A diverse gene pool provides the raw material for adaptation. When diversity shrinks, the population becomes susceptible to inbreeding and cannot generate the beneficial new traits needed to handle novel environmental challenges. In the worst cases, this leads to a positive feedback loop where small size causes low fitness, which causes smaller size, known as mutational meltdown. ^[2]

It is interesting to note that even traits evolved for reproductive success can increase extinction risk. Fossil studies suggest that species exhibiting high sexual dimorphism, often meaning elaborate male displays or ornaments, tend to die out faster. ^[2] If the genes responsible for these attractive but energetically costly traits are not neutral or beneficial under new selection pressures—or if they actively impede survival traits—the entire lineage may vanish more quickly than less ornamented counterparts. ^[2] Humans, in contrast, have evolved a massive prefrontal cortex that allows for cultural adaptation and expertise development, letting us change the environment to suit us, rather than waiting for genetic shifts. ^[4]

When examining the factors that cause extinction, whether natural or human-induced, we see that few events act in isolation. For instance, habitat loss can precede and exacerbate the impact of an introduced predator, or starvation resulting from environmental change can weaken a population enough to succumb to a new pathogen. ^[8] The extinction of one species can trigger a cascade via coextinction, where dependent species—like a parasite losing its host or a predator losing its prey—disappear right along with it. ^[2]

The critical difference between the slow turnover of the fossil record and the present crisis is a matter of tempo. Geological time operates in millennia, allowing speciation to potentially balance extinction. ^[7] Human industrial and agricultural activity compresses centuries of environmental transformation into mere decades. ^[3][4] In a world where a species might require thousands of generations to accumulate a necessary adaptive shift, facing a complete ecosystem collapse within one hundred years means the evolutionary "answer" simply never arrives in time. ^[3] This mismatch between evolutionary pace and anthropogenic pace is why we see so many species now listed as threatened, standing on the brink of becoming a footnote in Earth's next chapter.