Humans are reducing global biodiversity at a pace roughly 100 times faster than the natural background extinction rate. Monitored wildlife populations have declined by an average of 73% between 1970 and 2020, according to the Living Planet Index. This loss is driven by a handful of interconnected pressures: habitat destruction, pollution, overexploitation, invasive species, and climate change. But humans also have the capacity to reverse some of the damage, and restoration efforts are showing measurable results.
Habitat Loss From Agriculture and Urbanization
The single greatest cause of species endangerment is habitat loss, and agriculture is the primary driver. Farming already covers roughly 40% of the world’s ice-free land, and that footprint is projected to grow as global population and wealth increase. When forests, wetlands, or grasslands are converted to cropland or pasture, the species that depend on those ecosystems lose the space and resources they need to survive. Many can’t simply relocate.
Urbanization compounds this pressure by fragmenting what habitat remains. Studies on bird populations show that higher levels of urbanization consistently reduce the number of breeding species, favoring a small subset of common, adaptable birds while pushing out specialists. Continuous urban fabric (dense buildings with little vegetation) supports the fewest species. Interestingly, low and intermediate levels of development can sometimes support diverse bird communities, because the mix of original and new habitats creates varied niches. Green spaces within cities help: research in Italy found that expanding urban parks and gardens by even a few hectares per neighborhood meaningfully increased bird species richness, with gains accelerating once green areas exceeded about 8 hectares per sampling zone.
How Humans Have Reshaped Mammal Life on Earth
One striking way to visualize human impact is through biomass. Domesticated mammals now outweigh all wild mammals by a factor of ten. The combined biomass of humans and their livestock has reached approximately 1,100 megatons. Wild mammals make up a small fraction of what remains. Over the last century, vertebrate species have disappeared at a rate that would normally take 800 to 10,000 years under natural conditions, depending on the group.
Overfishing and Marine Overexploitation
On land, hunting and poaching threaten thousands of species. In the ocean, the pressure is industrial. The UN Food and Agriculture Organization monitors over 2,000 fish stocks globally, and its 2025 report estimates that 35.5% are fished at unsustainable levels. When populations of key predator or prey species collapse, the effects cascade through marine food webs, altering the structure of entire ecosystems.
Nutrient Pollution and Dead Zones
Humans have fundamentally altered the nitrogen cycle. Synthetic fertilizers, sewage, and fossil fuel combustion flood ecosystems with reactive nitrogen that would not naturally be there. When this nitrogen washes into rivers, lakes, and coastal waters, it triggers eutrophication: explosive algal blooms that block sunlight, consume dissolved oxygen, and create dead zones where most aquatic life cannot survive.
The process follows a predictable pattern. Excess nutrients favor fast-growing algae and cyanobacteria over the diverse mix of organisms that normally inhabit a water body. As these blooms die and decompose, oxygen in deeper water plummets. Bottom-dwelling communities shift toward fewer, more tolerant species. Fish, invertebrates, coral reefs, and seagrass beds are all particularly sensitive to this oxygen depletion. Roughly 60% of freshwater systems across most continents now exceed baseline thresholds for nitrate concentration. Coastal dead zones have been growing steadily in both size and persistence, accompanied by increasing toxic algal blooms like red tides.
On land, nitrogen deposition changes which plants can thrive. Wildflowers, mosses, lichens, and nutrient-poor shrubs get outcompeted by grasses adapted to high-nutrient conditions. Over time, plant diversity drops, and the animals that depended on those plants follow.
Ocean Acidification and Coral Decline
The ocean absorbs about 25% of human carbon dioxide emissions. That sounds helpful for the atmosphere, but it’s slowly poisoning marine chemistry. CO₂ reacts with seawater to form a weak acid, and surface ocean pH is falling by about 0.002 units per year. As acidity rises, the concentration of carbonate ions drops, making it harder for corals, shellfish, and other calcifying organisms to build and maintain their shells and skeletons.
Warm-water coral reefs need a certain level of carbonate saturation in the water to form. Even under moderate emission scenarios, tropical waters are expected to fall below that threshold. In the Arctic, waters that are already corrosive to shell-building minerals are spreading rapidly. Research at natural CO₂ seep sites, where conditions mimic future ocean chemistry, shows around a 30% decline in animal biodiversity as pH drops from 8.1 to 7.8. Oysters and mussels decline sharply along these gradients. Many shellfish are especially vulnerable during their larval stages, when their tiny, newly forming shells dissolve faster than they can grow.
Invasive Species and Ecosystem Collapse
Humans move species around the planet, intentionally and accidentally, at a pace no natural process could match. When a non-native plant or animal establishes itself in a new ecosystem, it can outcompete native species for food, water, light, and space. Invasive plants reduce native biodiversity through several mechanisms: direct competition, changes to soil chemistry through the release of toxic compounds, and by making ecosystems more prone to fire.
Perhaps the most concerning dynamic is what ecologists call “invasion meltdown.” Once one invasive species gains a foothold, it can make conditions more favorable for additional invaders, creating a cascading cycle of displacement. Native species that co-evolved over millennia get replaced by a smaller set of aggressive generalists, simplifying ecosystems and making them less resilient to future shocks.
Microplastic Contamination in Aquatic Systems
Plastic pollution has become a pervasive threat to aquatic biodiversity. Microplastics, tiny fragments under 5 millimeters, are now found in virtually every marine and freshwater environment. About 54.5% of ocean microplastics are polyethylene (the plastic in bags and bottles), with another 16.5% being polypropylene (used in food containers and packaging). Their small size makes them easy for aquatic organisms to ingest, from zooplankton to fish to seabirds.
Once consumed, microplastics cause reduced food intake, developmental disorders, and behavioral changes. They also act as carriers for other toxic chemicals that accumulate on their surfaces. These effects move up the food chain as smaller contaminated organisms are eaten by larger ones.
Rewilding and Restoration Efforts
The picture isn’t entirely bleak. A global analysis of 42 rewilding case studies found that restoration efforts produce positive outcomes about 70% of the time, with 10% neutral and 20% negative. The most effective interventions include reintroducing herbivores (both wild and domestic) and removing invasive plants, which tend to boost ecosystem resilience against future biological disturbances like new invasions.
Results vary depending on the type of challenge. Rewilding projects show stronger success against biological disturbances than against abiotic ones like drought or wildfire. Projects that restore natural grazing patterns or rebalance food webs tend to improve biodiversity metrics, population stability, and the physical structure of habitats. Urban greening efforts, even modest ones, consistently benefit local wildlife. The evidence suggests that while human activity has driven biodiversity loss at an extraordinary scale, targeted restoration can meaningfully bend the curve back, particularly when it addresses the root pressures of habitat loss and species displacement.