Biodiversity is harmed by five major forces: habitat destruction from agriculture and development, overexploitation of wildlife, climate change, invasive species, and pollution. These threats don’t operate in isolation. They overlap and amplify each other, and their combined effect has driven a 73% decline in the average size of global wildlife populations between 1970 and 2020, based on tracking of nearly 35,000 vertebrate populations across 5,495 species.
Agricultural Expansion and Habitat Loss
Converting natural land to farmland is the single largest driver of biodiversity loss on Earth. Forests, grasslands, and wetlands are cleared or fragmented to grow crops and raise livestock, eliminating the habitats that species depend on. A 2024 study in Nature Sustainability calculated that land-use change driven by agricultural expansion has caused a cumulative global potential species loss of 1.4% since 1995. That figure may sound small, but it exceeds the planetary boundary for biodiversity loss by roughly fifty times, meaning we have already blown past the level scientists consider safe for Earth’s ecosystems.
The damage concentrates in tropical regions, where biodiversity is richest and agricultural frontiers are expanding fastest. Much of this expansion is fueled by global demand for commodities like soy, palm oil, beef, and coffee. The species that disappear in Brazil or Indonesia are often lost to feed supply chains ending in Europe or North America. This makes habitat loss not just a local issue but a globally distributed one, where consumption patterns in wealthy countries drive extinction risk thousands of miles away.
Urban Sprawl and Fragmentation
Cities and suburbs don’t just erase habitat. They fracture what remains into smaller, disconnected patches. Roads, buildings, and infrastructure create barriers that prevent animals from migrating, finding mates, or accessing food across a landscape. Research modeling the effects of urban sprawl across European cities found that sprawl indirectly reduces genetic diversity by disrupting habitat connectivity. When populations become isolated in small fragments, inbreeding increases and their ability to adapt to environmental changes drops.
Connectivity between forest patches turned out to be a stronger predictor of genetic diversity than connectivity between urban green spaces alone. In other words, parks and gardens within a city help, but they can’t fully substitute for the continuous natural habitat that sprawl replaces.
Overfishing and Direct Exploitation
Harvesting wild species faster than they can reproduce pushes populations toward collapse. In marine ecosystems, roughly 25% of assessed fish stocks have collapsed at some point, meaning their numbers fell below 20% of what’s needed for a healthy, sustainable population. When looking at species-level data from landing records rather than formal stock assessments, that figure rises to about 35% of species experiencing at least one stock collapse.
The consequences ripple outward. Many of the species most vulnerable to collapse are small, short-lived fish near the bottom of the food web. These species act as a critical link, transferring energy from plankton up to larger fish, seabirds, and marine mammals. Even a temporary collapse in one of these populations can starve the animals that depend on it, destabilizing the broader ecosystem. Overhunting and poaching play a similar role on land, particularly for large mammals and birds targeted for meat, traditional medicine, or the wildlife trade.
Climate Change and Shifting Habitats
Rising temperatures force species to move. On average, species are shifting toward higher latitudes at about 11.8 kilometers per decade and toward higher elevations at about 9 meters per decade. Marine species tend to move even faster than terrestrial ones, partly because water offers fewer physical barriers and temperature gradients travel more directly through ocean currents.
The problem is that not all species can keep pace. Trees migrate slowly. Amphibians tied to specific streams can’t easily relocate. Species living on mountaintops have nowhere higher to go. And when species arrive in new areas, they encounter different competitors, predators, and food sources, with no guarantee of survival. Climate change also intensifies other threats: droughts dry out wetlands, coral bleaching destroys reef habitat, and shifting seasons disrupt pollination timing. The interaction between warming and habitat fragmentation is especially dangerous, because fragmented landscapes make it harder for species to move to newly suitable areas.
Invasive Species
Species introduced to ecosystems where they didn’t evolve can devastate native biodiversity. Historically, invasive species have been the leading cause of documented extinctions, particularly on islands where native animals evolved without exposure to predators like rats, cats, and snakes. Most recorded extinctions are mollusks and vertebrates on islands driven to extinction by introduced species.
But the threat is far from historical. Research estimates that 11% of the world’s phylogenetic diversity (a measure of evolutionary uniqueness) is represented by species currently threatened by invasive species. For birds specifically, invasive species threaten species hosting 40% of the world’s trait diversity, meaning unique ecological roles like specialized feeding strategies and body plans. The Neotropical and Oceanian regions face the highest risk. When these species go extinct, they don’t just reduce a count on a list. They take with them irreplaceable evolutionary history and ecological functions that took millions of years to develop, with the greatest losses concentrated among large-bodied herbivorous mammals and birds that feed near the ground.
Nutrient Pollution and Dead Zones
Excess nitrogen and phosphorus from agricultural runoff, sewage, and industrial discharge flow into rivers, lakes, and coastal waters, triggering a process called eutrophication. The chain reaction works like this: nutrients fuel explosive algae growth, which blocks sunlight from reaching underwater plants. Those plants die. Then the algae dies too. Bacteria decompose all that dead organic matter, consuming the dissolved oxygen in the water. The result is a hypoxic “dead zone” where fish, shellfish, and other aquatic life suffocate or flee.
The decomposition process also releases large amounts of carbon dioxide, which lowers the water’s pH. This acidification slows the growth of fish and shellfish and can prevent clams, mussels, and oysters from forming their shells at all. Freshwater ecosystems are especially vulnerable. Freshwater vertebrate populations have declined by 85% since 1970, the steepest drop of any ecosystem type, far exceeding the 69% decline in terrestrial populations and 56% in marine populations. Pollution is one of several forces behind that staggering number, alongside dam construction, water extraction, and habitat degradation along riverbanks.
Pesticides and Insect Decline
Chemical pollution harms biodiversity in ways that are harder to see than a cleared forest but no less damaging. Neonicotinoid pesticides, widely used as seed treatments on crops like oilseed rape (canola), illustrate the problem clearly. A large-scale study tracking wild bee populations across England found that neonicotinoid exposure increased local extinction rates. Wild bee species that foraged on treated crops were three times more negatively affected than species that didn’t visit those crops.
For five specific bee species, neonicotinoid exposure alone was responsible for their disappearance from more than 20% of the areas where they previously lived since 2002. Across a broader set, 24 species lost more than 10% of their range due to these pesticides. Because bees pollinate both wild plants and food crops, their decline cascades through ecosystems and agricultural systems alike. Insect losses are not limited to bees. Pesticides, light pollution, and habitat loss are driving declines across many insect groups, with consequences for the birds, bats, and fish that feed on them.
Why the Threats Compound
What makes biodiversity loss so difficult to reverse is that these threats reinforce each other. A forest fragment surrounded by farmland is more vulnerable to invasive species at its edges. A fish population weakened by pollution is less able to withstand warming waters. A bee colony stressed by pesticides has fewer resources to adapt when its habitat shrinks. The pattern that emerges from current threat assessments is notably different from past extinction waves. Most historically documented extinctions were island species killed by invasive predators. Most species threatened today are mainland species facing habitat destruction, a shift that reflects how dramatically human land use has reshaped the planet.
Freshwater ecosystems sit at the intersection of nearly every threat: they receive agricultural runoff, get fragmented by dams, lose water to irrigation, warm with the climate, and host some of the most successful invasive species on Earth. That convergence of pressures explains why freshwater biodiversity is declining faster than any other category.