Species go extinct when they can no longer survive or reproduce fast enough to maintain their population. The causes range from rapid environmental shifts to direct killing by humans, and they often overlap. Today’s extinction rate is roughly 1,000 times higher than the natural background rate, and projections suggest it could reach 10,000 times higher in coming decades. Understanding what drives these losses means looking at several interconnected forces.
Climate Change and Temperature Limits
Every species has a temperature range it can tolerate. Within that range, the body functions normally. Outside it, the animal or plant has to spend extra energy just to keep its basic systems running. Short bursts of heat stress are survivable, but prolonged periods of increased energy expenditure gradually reduce an organism’s ability to feed, grow, and reproduce.
Heat tolerance appears to be a hard physiological ceiling. Research shows that the upper temperature limit a species can handle is strongly conserved across evolutionary lineages, meaning it doesn’t shift easily even over long stretches of evolutionary time. Cold tolerance, by contrast, is more flexible. This matters enormously for climate change: species can adapt to cooling trends more readily than to warming ones.
Tropical species face the greatest risk. They already live close to their upper temperature limits, so even a small rise in ambient temperature can push them past the threshold. When you layer in declining rainfall, the picture worsens further, because many warm-blooded animals rely on water availability to cool themselves. A species that loses both tolerable temperatures and access to water simultaneously faces a survival crisis with very few escape routes.
Invasive Species and Competition
Biological invasions are the single most common factor in documented extinctions. Roughly 60% of all species extinctions involve invasive organisms, species transported by humans (intentionally or accidentally) into ecosystems where they didn’t evolve. Over 35,000 plant and animal species have been relocated this way, and about 3,500 of them cause serious ecological damage by outcompeting, preying on, or parasitizing native wildlife.
The damage is especially severe on islands, where native species evolved without exposure to mainland predators. When rats, cats, or snakes arrive on an island, local birds or reptiles that never developed defenses against them can vanish within a few generations. Invasive plants can similarly take over a habitat, crowding out the specific vegetation that native insects or herbivores depend on for food.
Overexploitation and Hunting
Humans have hunted and harvested species to extinction for centuries. The pattern continues today through commercial fishing, poaching, and the wildlife trade. Sturgeon species like the Atlantic sturgeon and beluga sturgeon have been wiped out across large parts of their range due to overfishing for caviar. The long-tailed chinchilla was hunted to local extinction across several areas of Chile for its fur. These are not isolated cases. Dozens of mammal species have suffered regional extinction linked directly to trade.
Overexploitation is particularly dangerous because it targets adults, the individuals most important for population stability. Remove enough breeding adults from a population and the birth rate drops below the death rate. Once that imbalance takes hold, the population spirals downward even if hunting eventually stops, because too few individuals remain to recover.
Disease and Fungal Epidemics
Infectious disease can devastate wildlife populations with a speed that leaves no time for adaptation. The most destructive example on record is a chytrid fungus that attacks amphibians. Around the world, roughly 90 frog and salamander species are thought to have gone extinct because of it, and at least 491 more have declined. Among those surviving species, 124 have lost 90% or more of their population. No other known disease has driven more species to extinction.
What makes wildlife diseases so dangerous is that they often arrive in a population with no prior exposure and therefore no immune defense. The chytrid fungus spread globally through the international trade of amphibians. White-nose syndrome, a fungal disease killing bats across North America, followed a similar pattern. These pathogens don’t need to kill every individual directly. They just need to reduce the population below the point where it can sustain itself.
Habitat Loss and Fragmentation
When forests are cleared, wetlands drained, or grasslands converted to farmland, the species living there lose the physical space they need to find food, shelter, and mates. Habitat destruction is the most widespread driver of population decline worldwide. But outright loss isn’t the only problem. Fragmentation, where a large continuous habitat gets broken into small disconnected patches, can be just as lethal over time.
Small, isolated patches support fewer individuals. Those individuals can’t travel between patches to find mates, which leads to inbreeding. Conservation biology uses a general framework called the 50/500 rule: a population needs at least 50 breeding individuals in the short term to avoid the damaging effects of inbreeding, and around 500 in the long term to maintain enough genetic diversity for the species to adapt to future challenges. When habitat fragmentation traps populations below these thresholds, extinction becomes a matter of time even if no other threat is present.
The Cascade Effect of Coextinction
Species don’t exist in isolation. Roughly half of all known species, and a larger share of unnamed ones, depend on a specific host or partner during at least one stage of their life. Parasites need their hosts. Pollinators need specific plants. Specialist predators need specific prey. When one species disappears, the species that depend on it can follow. This process, called coextinction, means the true toll of any single extinction is almost always larger than it first appears.
Coextinction is difficult to track because the dependent species are often small, poorly studied, or not yet described by science. A tree frog going extinct might take with it a parasitic worm, a symbiotic skin bacterium, and a specialist fly, none of which anyone had catalogued. This hidden multiplier effect means that headline extinction numbers likely undercount the real losses.
Why Multiple Threats Compound
In practice, species rarely go extinct from a single cause. A frog population already stressed by habitat loss becomes more vulnerable to disease. A bird species struggling with rising temperatures gets pushed over the edge when an invasive predator arrives. A fish population depleted by commercial harvesting loses the genetic diversity it would need to adapt to warming oceans. These threats interact, and their combined effect is greater than any one of them alone.
This compounding nature helps explain why the current extinction crisis is so severe. The background rate of extinction, the natural pace at which species disappeared before human influence, was low enough that new species evolved to replace them. At 1,000 times that rate, the losses are accumulating far faster than evolution can compensate.