Losing one species can affect many others because species in an ecosystem are connected through food, competition, pollination, seed dispersal, shelter, disease, and other relationships. When one species disappears, those connections can change or break, altering the resources and conditions available to other organisms.
The effects are not always large. Some species have many ecological connections, while others have relatively few. The impact also depends on what the missing species did and whether another species can perform a similar role.
Species are connected through food webs
The clearest way one species can affect others is through feeding relationships. Most ecosystems contain food webs, not simple food chains. A single species may eat several kinds of organisms while also serving as food for several predators.
If a species disappears, the organisms that depended on it as food may lose an important resource. At the same time, organisms that were eaten by that species may become more abundant because one of their predators is gone.
For example, suppose a predator feeds heavily on a particular herbivore. Removing the predator can allow the herbivore population to increase. More herbivores may then consume more vegetation, reducing the abundance of plants that other animals also use for food or shelter.
The original species does not have to interact directly with every affected organism. Changes can move through the food web from one relationship to another.
Some species have unusually important ecological roles
Not every species has the same influence on its ecosystem. Some have effects that are much larger than their abundance might suggest.
A keystone species is a species whose ecological effects are disproportionately large compared with its abundance. Removing such a species can cause substantial changes in the structure or functioning of an ecosystem.
A predator can be a keystone species when its presence keeps particular prey populations from becoming dominant. A plant can also play a critical role if many organisms depend on it for food, shelter, or other resources.
This does not mean every species is a keystone species. In many ecosystems, losing a species produces relatively limited changes because other organisms can partly replace its ecological role.
Losing a species can change competition
Species also affect one another by competing for resources such as food, water, light, nesting sites, or space.
When one species disappears, competitors may gain access to resources that were previously being used. Their populations can increase, potentially changing the balance among several species.
The opposite can also happen. If the lost species was helping another organism indirectly—for example, by modifying habitat or providing a resource—its disappearance can make competition more difficult for that organism.
Because species often compete with multiple organisms at once, a change in one population can alter several relationships simultaneously.
Pollinators and plants can depend on each other
Some species have especially close relationships. Many flowering plants rely on animals such as bees, butterflies, moths, birds, and bats to transfer pollen between flowers.
If an important pollinator disappears, plants that depend heavily on it may produce fewer seeds. That can eventually reduce the number of those plants available as food or habitat for other organisms.
The effects can work in the other direction as well. If a plant species disappears, a pollinator that relies heavily on its flowers may lose an important source of nectar or pollen.
These relationships can therefore connect the survival of one species to the reproduction or population size of another.
Seed dispersers can influence plant communities
Animals can also affect plants by moving their seeds. Birds, mammals, and other animals may carry seeds away from the parent plant, sometimes after eating fruit.
If an important seed disperser disappears, fewer seeds may reach suitable places to grow. Some plants may therefore become less successful at reproducing or spreading.
Over time, changes in plant abundance can affect other organisms that depend on those plants for food, nesting sites, shade, or shelter. A change that begins with an animal that disperses seeds can therefore eventually alter parts of the plant community.
Species can provide habitat and other resources
An organism can affect other species without being food for them. Some species create or modify physical environments that other organisms use.
Trees, for example, can provide nesting sites, cavities, shade, and surfaces for other organisms to live on. Burrowing animals can alter soil and create spaces used by other species. Large organisms can also change vegetation structure, water conditions, or the availability of shelter.
When such a species disappears, the physical habitat can change along with the loss of the organism itself.
Ecosystems can respond through indirect effects
The most important effects of species loss are sometimes indirect.
Imagine three species: A, B, and C. A may eat B, while B competes with C. If A disappears, B may increase. The increase in B can then reduce C. In this case, A never needed to interact directly with C for its disappearance to affect C.
This kind of chain of effects is one reason ecological changes can spread beyond the species that is initially lost.
The strength and direction of these effects vary among ecosystems. A species may be extremely important in one environment but have a much smaller influence in another because the surrounding species and relationships are different.
Losing one species does not always cause a cascade
Species loss does not automatically trigger a major ecological collapse. Ecosystems often contain functional redundancy, meaning that different species can perform similar ecological roles.
If one plant disappears but several other plants provide similar food or habitat, other organisms may experience little change. Likewise, if a predator disappears and another predator consumes much of the same prey, the missing predator’s role may be partly replaced.
The more specialized a species’ ecological role is, however, the harder it may be for other species to compensate for its loss.
Why the effects can grow over time
The consequences of species loss may not appear immediately. A species can disappear today while some of the ecological changes become noticeable much later.
For example, losing a seed-dispersing animal may initially leave an ecosystem looking largely unchanged. If fewer plants successfully reproduce over many years, however, the plant community can gradually shift. Organisms that depend on those plants may then be affected as well.
This means biodiversity loss can involve changes that unfold across different timescales rather than appearing as a single, immediate event.
The broader lesson
A species is part of a network of relationships rather than an isolated component of nature. Its role may involve eating other organisms, being eaten, competing for resources, pollinating plants, dispersing seeds, creating habitat, or changing physical conditions.
When that species disappears, some of those relationships may weaken or vanish. Other species may increase, decline, or change their behavior in response. In ecosystems with highly interconnected or specialized relationships, the effects can spread through multiple levels of the ecological community.
That is why the loss of one species can matter far beyond the species itself.
