The variety of life on Earth, or biodiversity, is considered across three scales. Genetic diversity accounts for the variation in genes within a species, allowing populations to adapt or resist disease. Species diversity refers to the number and abundance of different species within an ecosystem. Ecosystem diversity encompasses the variety of habitats, communities, and ecological processes across the planet.
Anthropogenic forces, known as human disturbance, have accelerated the rate of species extinction far beyond the natural background rate. This loss is driven by mechanisms that fundamentally alter the planet’s biological and physical systems. Understanding how human activity compromises the natural world is the first step toward mitigating the current biodiversity crisis.
Habitat Loss and Fragmentation
The physical transformation of natural landscapes for human use is a primary driver of biodiversity decline globally. This disturbance occurs through two processes: the reduction of habitable area and the division of remaining habitat into smaller, isolated patches, known as fragmentation. The reduction in total area directly lowers the environment’s carrying capacity, meaning it supports fewer individuals of any given species.
When an ecosystem shrinks, the remaining area often becomes insufficient to sustain populations that require large territories, like apex predators or wide-ranging herbivores. This loss increases competition for resources and results in lower survival and reproductive success for specialized species. Fragmentation experiments demonstrated that this process can reduce local biodiversity by 13 to 75%, with effects magnifying over time.
Habitat fragmentation introduces the “edge effect,” which changes the ecological conditions of the remaining patches. Edges are the boundaries between natural habitat and human-altered landscape, such as a forest next to a cattle pasture. These perimeters experience altered microclimates, including increased light penetration, higher temperatures, and lower humidity compared to the forest interior.
The altered conditions along these edges make interior-dwelling species, sensitive to changes in moisture and shade, less likely to survive and reproduce. Edges also become conduits for invasive species, which thrive in disturbed environments, and for increased predation pressure from generalist species. As habitat patches become smaller, a larger proportion of the remnant is exposed to these edge effects.
The isolation of populations within small fragments hinders the natural flow of genes between groups. When a population is confined to a small patch, the risk of inbreeding rises, leading to the expression of harmful recessive traits and reduced genetic diversity. This loss of genetic variability makes the isolated population less resilient to future environmental changes or disease, increasing its vulnerability to local extinction.
Chemical Alteration and Pollution
Human industrial and agricultural activities introduce compounds that alter the chemical composition of ecosystems, often with toxic effects. One category involves direct toxins, including heavy metals, industrial compounds like polychlorinated biphenyls (PCBs), and modern pesticides. Many of these substances are persistent, meaning they do not easily break down, allowing them to accumulate and concentrate in animals higher up the food web.
Endocrine-disrupting chemicals (EDCs) include plasticizers, pharmaceuticals, and certain herbicides like atrazine. EDCs interfere with an organism’s hormone system by mimicking natural hormones or blocking their action. Exposure to these chemicals, even in minute concentrations, can impair the reproductive, nervous, and immune systems of wildlife.
The developmental effects of EDCs are severe, leading to reproductive failure in marine mammals and the feminization of male fish and amphibians, resulting in intersex individuals unable to reproduce. PCBs and DDT have been linked to impaired reproduction in seals. The presence of these compounds in aquatic habitats poses a chronic threat to many species.
A second chemical alteration is nutrient overloading, driven by the runoff of nitrogen and phosphorus from agricultural fertilizers and sewage. When these nutrients enter aquatic systems, they trigger rapid growth of algae, a process called eutrophication. This dense algal bloom blocks sunlight, killing submerged aquatic plants, and eventually the algae die.
The subsequent decomposition of dead organic matter consumes nearly all the dissolved oxygen in the water. This creates hypoxic or anoxic conditions, commonly referred to as “dead zones,” where oxygen levels are too low to support most aquatic life. Examples include the seasonal dead zone in the Gulf of Mexico and hypoxic areas in the Baltic Sea, which impact local fisheries.
Overexploitation of Species
Overexploitation is the unsustainable removal of organisms from their environment at a rate faster than their populations can naturally replenish. This extraction includes commercial activities such as industrial fishing, uncontrolled logging, poaching, and the harvest of wild plants. When target populations are subjected to this pressure, their numbers can rapidly decline, leading to stock collapse and a loss of genetic fitness.
Industrial fishing illustrates this pressure, with approximately one-third of the world’s fish populations considered overfished. Beyond collapsing target species, such as the Atlantic cod fishery off Newfoundland, fishing often results in significant bycatch—the unintended capture and death of non-target marine animals. This indiscriminate removal contributes to the mortality of protected species like endangered sea turtles and marine birds.
The removal of species through overexploitation can initiate a trophic cascade across the ecosystem. Trophic cascades occur when the removal of an apex predator or a keystone species destabilizes the food web structure. For example, the collapse of the Atlantic cod population led to an increase in their prey, such as shrimp, which heavily preyed upon zooplankton.
The decline of zooplankton then released phytoplankton from grazing pressure, causing an increase in their biomass. These shifts demonstrate how the removal of a single species affects the composition and function of the entire ecosystem. This destabilization reduces ecosystem resilience and can hinder the recovery of the original target species.
Systemic Change: Climate Disruption
The emission of greenhouse gases from human activities has initiated global alterations to the planet’s physical systems, posing a long-term threat to biodiversity. These systemic changes manifest as shifts in species ranges, changes to ocean chemistry, and altered biological timing, affecting nearly all species.
Warming temperatures force many terrestrial species to move their geographic distributions toward the poles or to higher altitudes to remain within their preferred thermal range. This phenomenon, known as range shift, is evidenced by plants altering their leafing and flowering times and insects adjusting their seasonal activity. However, a species’ ability to shift its range is constrained by physical barriers or migration speed, often leading to a mismatch with the rate of environmental change.
The change in the timing of seasonal events, or altered phenology, challenges species interactions. Rising temperatures cause events like the flowering of plants and the migration of animals to advance.
These shifts disrupt synchronized biological interactions, creating a phenological mismatch between interdependent species. If a plant flowers earlier, but its specific insect pollinator emerges later, the plant’s reproductive success will fail, and the pollinator loses a food source, causing cascading failure. This desynchronization also occurs between migratory birds and the peak abundance of their insect prey.
In the marine environment, the ocean absorbs excess carbon dioxide, fundamentally altering its chemistry. The absorbed CO2 reacts with seawater to form carbonic acid, which lowers the ocean’s pH in a process known as ocean acidification. This decrease in pH reduces the availability of carbonate ions, the building blocks many marine organisms use to construct their shells and skeletons.
Ocean acidification is detrimental to calcifying organisms, including reef-building corals, oysters, clams, and pteropods. The reduced availability of carbonate ions makes it harder for these organisms to grow and can cause the dissolution of existing shells. This threatens the entire marine food web, as the loss of corals compromises entire reef ecosystems and pteropods are consumed by numerous fish and whales.