Biodiversity declines when species become less numerous, populations shrink, habitats disappear or deteriorate, or species go locally or globally extinct. The main causes are changes that make it harder for organisms to survive and reproduce, especially habitat loss, overexploitation, pollution, invasive species, and climate change. These pressures often act together, making their effects stronger than any one threat alone.
Biodiversity includes the variety of life at several levels: the number and variety of species, the genetic differences within species, and the different ecosystems in which organisms live. A decline can therefore happen even before a species becomes extinct. A species with a large range may become much less abundant, for example, while still remaining widespread.
Habitat loss is a major cause of biodiversity decline
Species need suitable places to feed, reproduce, find shelter, and complete their life cycles. When natural habitats are destroyed or converted for agriculture, cities, roads, mining, logging, or other human activities, organisms can lose the conditions they need to survive.
Habitat loss does not always mean that every part of an ecosystem disappears. A forest can be divided into smaller patches, wetlands can be drained or altered, and grasslands can be converted into farmland. The remaining habitat may no longer be large enough or connected enough to support the same populations.
This process is called habitat fragmentation. When a once-continuous habitat becomes separated into isolated patches, populations may become smaller and less able to move between areas. Individuals can have greater difficulty finding mates or recolonizing areas where populations have disappeared. Small populations are also more vulnerable to random events and genetic problems.
Habitat degradation can have a similar effect without completely removing the habitat. Changes in water quality, vegetation, food availability, soil conditions, or shelter can make an area less suitable for the species that depend on it.
Overexploitation removes organisms faster than populations can recover
Biodiversity also declines when people harvest wild organisms faster than their populations can replace themselves. This is known as overexploitation.
Fishing, hunting, logging, and collecting wild plants or animals can all contribute to population declines when extraction is excessive. The problem is not simply that organisms are being used; populations can often withstand some level of harvesting. The problem arises when mortality from harvesting remains greater than the population’s ability to recover.
Overexploitation can also affect species indirectly. Removing large numbers of one species can alter food webs and change relationships among predators, prey, competitors, and other organisms.
Pollution changes the conditions organisms depend on
Pollution can harm biodiversity by changing air, water, and soil or by introducing substances that organisms cannot tolerate.
Chemical pollutants can directly injure organisms, interfere with reproduction, or cause death. Nutrient pollution can also transform ecosystems. When excessive nutrients enter aquatic environments, they can stimulate large amounts of algal growth. As this material decomposes, oxygen can be depleted from the water, creating conditions that many aquatic organisms cannot survive.
Pollution does not have to kill organisms immediately to reduce biodiversity. A pollutant that lowers reproductive success, disrupts development, or reduces access to food can gradually shrink a population over generations.
Other forms of environmental pollution, including excessive artificial light, noise, and plastic waste, can also alter the conditions under which organisms live. Their effects vary greatly among species and ecosystems.
Invasive species can disrupt native communities
When a species is introduced to an ecosystem where it did not previously occur, it can sometimes spread rapidly and disrupt native organisms. Such a species is often called an invasive species when its introduction and spread cause ecological or other significant harm.
Invasive species can compete with native organisms for food, space, light, or other resources. They may prey on native species, introduce diseases, alter habitats, or change ecological processes.
Native species are not automatically harmed by every introduced species. Many nonnative organisms have little lasting effect on biodiversity. The greatest problems occur when an introduced organism establishes itself successfully and creates strong pressures on native populations.
Climate change is altering habitats and ecological relationships
Climate change can reduce biodiversity by changing the physical conditions that organisms have adapted to. Rising temperatures, changing precipitation patterns, shifts in seasonal timing, and changes in the frequency or intensity of some extreme events can affect survival and reproduction.
Some species can move toward areas with more suitable conditions, but movement is not always possible. Mountains, cities, farms, roads, fragmented habitats, and other barriers can restrict movement. Species that cannot shift their ranges or adapt quickly enough may experience population declines.
Climate change can also disrupt relationships between species. The timing of flowering, insect activity, migration, breeding, and food availability may change at different rates. If interacting species become out of sync, organisms may have difficulty obtaining food or completing important parts of their life cycles.
Climate change can therefore intensify other threats. A population already weakened by habitat loss, pollution, or overexploitation may have less ability to cope with additional environmental change.
These causes often reinforce one another
Biodiversity rarely declines because of a single isolated pressure. Multiple threats can operate simultaneously.
For example, habitat fragmentation can leave a species with a small population. Pollution may then reduce its reproductive success, while climate change alters the conditions within its remaining habitat. If the species is also heavily harvested, its population may have little capacity to recover.
This interaction matters because the effects of different pressures can accumulate. A species may tolerate each stress individually but struggle when several occur at the same time.
Why small populations are especially vulnerable
As populations become smaller, they generally have less ability to withstand environmental disturbances. A disease outbreak, severe weather event, shortage of food, or other unexpected change can eliminate a larger fraction of a small population than of a large one.
Small populations can also lose genetic diversity. Genetic variation gives populations differences that can help them respond to changing conditions. When populations become very small and isolated, chance events and mating among relatives can also become more important, potentially reducing their ability to persist.
This creates a potential cycle: environmental pressures reduce a population, the smaller population becomes more vulnerable, and further losses become easier to trigger.
Biodiversity can decline without immediate extinction
Extinction is the most severe form of biodiversity loss, but it is not the only one that matters.
A species can remain present while its population falls sharply. An ecosystem can retain many of its species while losing particular organisms that play important ecological roles. Genetic diversity within a species can also decline even when the species itself remains widespread.
These changes can affect how ecosystems function. The loss or reduction of organisms can alter predation, competition, pollination, decomposition, nutrient cycling, and other ecological processes.
For that reason, biodiversity decline is better understood as a gradual process of biological loss rather than simply a count of species that have gone extinct.
The main causes are connected
The major drivers of biodiversity decline—habitat loss and degradation, overexploitation, pollution, invasive species, and climate change—are not separate categories operating independently. Human activities can produce several pressures at once, and one pressure can make organisms more vulnerable to another.
The central issue is whether populations and ecosystems can continue to provide the conditions organisms need to survive, reproduce, and maintain viable populations. When those conditions deteriorate faster than species can adapt, move, or recover, biodiversity declines.
