Biodiversity represents the entirety of life’s variety on Earth, encompassing three interwoven levels: genetic, species, and ecosystem diversity. Genetic diversity is the variation within a species, providing the raw material for adaptation to environmental changes. Species diversity is the number and relative abundance of different life forms in a particular area. Ecosystem diversity refers to the variety of habitats, communities, and ecological processes across the landscape. This biological variation underpins the stability of natural systems, providing essential ecosystem services such as clean air and water, nutrient cycling, and climate regulation. The rate of biodiversity loss globally is accelerating, driven primarily by human activities through five major, interconnected factors.
Habitat Degradation and Loss
The largest threat to terrestrial biodiversity is the destruction of natural habitats for human use, a process that includes outright loss, fragmentation, and degradation. Habitat loss involves the complete removal of an ecosystem, such as draining wetlands for agriculture or clear-cutting rainforests for timber. This destruction eliminates the niche for specialist species, leading to sharp population declines and local extinctions.
Habitat fragmentation occurs when large, continuous tracts of nature are broken into smaller, isolated patches separated by infrastructure like roads and farms. This isolation prevents organisms from moving freely to find mates or resources. Small, isolated populations suffer from reduced gene flow, leading to increased inbreeding and a phenomenon known as genetic drift. This loss of genetic diversity compromises the long-term health and resilience of the population, making it less able to adapt to new diseases or environmental shifts. Furthermore, the edges of fragmented habitats experience “edge effects,” which are altered conditions like increased light, wind, and exposure to predators that degrade the quality of the remaining patches.
of Non-Native Species
The movement of non-native species, often called invasive species, to new geographic areas is a significant factor in biodiversity loss. An invasive species is an organism introduced outside its natural range that causes ecological or economic harm because its new environment lacks the natural predators or competitors that kept its population in check. These invaders displace native species through:
- Competition
- Predation
- Disease transmission
- Habitat alteration
Invasive species aggressively outcompete native organisms for limited resources like food, light, and space. For example, the zebra mussel filters massive amounts of plankton in North American freshwater systems, effectively starving native aquatic species. Predatory invaders, such as the brown tree snake on Guam, have caused the extinction of multiple native bird species in isolated ecosystems. Invasive organisms also introduce novel pathogens to native populations that have no evolutionary resistance, such as the chytrid fungus linked to global amphibian declines.
Climate Shifts
Long-term, global changes in climate patterns are fundamentally altering the conditions under which species must survive. Rising global temperatures force many species to shift their geographic ranges, typically moving toward cooler poles or higher altitudes, a phenomenon known as range shift. If species cannot relocate fast enough, or if their habitat is fragmented, they are unable to track suitable climate conditions, leading to population decline.
Altered precipitation patterns disrupt delicate ecological balances. Increased aridity shifts the composition of plant communities, favoring species that tolerate dry conditions while reducing overall species richness. In marine environments, rising sea surface temperatures cause coral bleaching, where corals expel the symbiotic microalgae living in their tissues, often leading to the death of the coral if the stress is prolonged.
A related mechanism is ocean acidification, which occurs as the ocean absorbs excess atmospheric carbon dioxide, lowering the water’s pH. This chemical change reduces the availability of carbonate ions, making it difficult for marine calcifiers, such as corals, oysters, and mussels, to form and maintain their calcium carbonate shells and skeletons. This weakens the physical structure of ecosystems like coral reefs, which provide habitat for a quarter of all marine life.
Pollution of Ecosystems
The introduction of contaminants that harm the environment represents a pervasive threat to the health of all ecosystems. Chemical pollution includes the widespread use of agricultural chemicals like neonicotinoid pesticides, which target insects’ nervous systems. Even at low concentrations, these systemic chemicals impair the navigation and reproduction of non-target species, such as wild bees and butterflies, leading to population crashes for essential pollinators.
Industrial waste contributes heavy metals, such as mercury and cadmium, which are non-biodegradable. These toxins bioaccumulate and biomagnify, meaning their concentration increases as they move up the food chain, causing severe toxicity and physiological damage to top predators. Another major form of contamination is nutrient loading, where excess nitrogen and phosphorus from agricultural runoff and sewage enter aquatic systems, triggering eutrophication.
This nutrient overload causes rapid growth of algae, which then die and decompose, consuming dissolved oxygen and creating vast hypoxic “dead zones” in coastal areas. Marine life cannot survive in these oxygen-depleted areas, resulting in mass die-offs and severe loss of species diversity. Physical pollution, primarily plastics, poses a mechanical and chemical threat as it degrades into microplastics that are mistaken for food by organisms from plankton to whales.
Overuse of Resources
The unsustainable harvesting of biological resources, termed overexploitation, directly reduces population sizes to levels from which recovery is difficult or impossible. In marine environments, this is visible in overfishing, where more than one-third of global fish populations are harvested at unsustainable levels. Destructive methods like bottom trawling scrape the seafloor, destroying complex habitats and resulting in high levels of bycatch, the incidental capture of non-target species.
On land, the illegal wildlife trade targets rare species for their parts, such as ivory or horns. This trade often selects for the largest and fittest individuals, reducing the genetic quality of the remaining breeding stock and causing severe sex-ratio imbalances. Excessive logging, particularly clear-cutting of old-growth forests, removes large, mature trees and simplifies the forest structure, eliminating the dead wood and multi-layered canopy that specialist species rely upon. This reduction in population numbers, especially for keystone species, can trigger a cascade of negative effects throughout the entire food web.