When a predator disappears from a food web, the effects can spread far beyond the species it normally eats. Without predation, some prey populations may increase, which can put heavier pressure on the plants, animals, or other organisms below them. Those changes can then affect habitat, competition, nutrient cycling, and even the abundance of other species.
This chain of effects is known as a trophic cascade. It is one reason predators can have an influence on an ecosystem that is much larger than their own population might suggest.
Predators help regulate prey populations
A predator is an animal that hunts and eats other animals. Wolves, hawks, sharks, spiders, and many other species occupy predatory roles in food webs.
Predation does not necessarily keep prey populations low. Instead, it is one of several forces that helps regulate their numbers. Predators remove individuals from prey populations, while prey numbers are also shaped by food availability, disease, competition, weather, reproduction, and other factors.
When a predator disappears, that source of mortality is removed. If other factors do not compensate for the loss, the predator’s prey may become more abundant.
For example, if a predator normally eats herbivores, fewer predators can mean more herbivores. More herbivores can then consume more vegetation. The resulting decline in plants can affect insects, birds, and other organisms that depend on those plants.
The important point is that the predator’s disappearance can change relationships throughout the food web rather than producing only one simple predator-prey effect.
The food web can shift from the top down
Food webs show how organisms are connected through feeding relationships. Energy generally moves from organisms such as plants and algae to herbivores and then to predators.
A predator near the top of a food web can therefore influence organisms several feeding levels below it. When that predator is removed, changes can move downward through the web.
A simplified example might look like this:
Predator → herbivore → plant
If the predator disappears, the herbivore may increase. If the herbivore becomes more numerous or spends more time feeding, plant abundance can decline.
That is a top-down effect: a change higher in the food web influences organisms at lower levels.
The response does not always follow this neat sequence, however. Real ecosystems contain many species and overlapping feeding relationships. A predator may eat several kinds of prey, and those prey may compete with one another or feed on different resources. As a result, removing one species can produce a mixture of direct and indirect effects.
What is a trophic cascade?
A trophic cascade occurs when a change in one level of a food web produces effects that propagate through other levels.
The classic pattern involves the loss or reduction of a predator, followed by an increase in its prey and a decrease in organisms or resources consumed by that prey.
Suppose a predator eats a herbivore that feeds heavily on vegetation. Removing the predator can allow the herbivore population to grow. Greater herbivore pressure can reduce vegetation. That reduction may then affect species that use the vegetation for food, shelter, or nesting sites.
The cascade can therefore involve organisms that the missing predator never directly encountered.
Trophic cascades vary greatly in strength. In some ecosystems, the effect of losing a predator may be pronounced. In others, prey populations may not increase much because they are limited by food, disease, competition, other predators, or environmental conditions.
Other predators can sometimes fill the gap
The disappearance of one predator does not necessarily leave its prey without predators.
Many ecosystems contain multiple predators that eat some of the same prey. If one predator declines, another may continue to exert substantial pressure on the prey population.
Predators can also respond behaviorally or numerically to changes in prey abundance. A predator that previously relied on several food sources may consume more of a newly abundant prey species.
Because of these interactions, removing one predator can sometimes have a smaller effect than expected. In other cases, the missing predator may have occupied a role that other species cannot easily replace.
This is one reason scientists cannot determine the consequences of predator loss simply by looking at a single predator-prey relationship.
Prey behavior can change, too
Predators influence ecosystems not only by killing prey but also by changing how prey behave.
Animals that face predation may avoid certain areas, spend less time feeding in exposed locations, or become more cautious when predators are nearby. If predators disappear, prey may use habitats they previously avoided.
That behavioral change can alter an ecosystem even before the prey population becomes substantially larger.
For instance, herbivores may begin feeding more heavily in places where they once faced greater danger. The resulting increase in grazing or browsing can change plant communities and the organisms that depend on them.
This means the ecological effect of a predator can involve both numbers and behavior.
Plants can be affected even though predators do not eat them
One of the most striking consequences of predator loss is that plants can be affected by an animal that never feeds on them.
Consider a food web in which a predator controls a herbivore population. If the predator disappears, herbivores may increase or change where they feed. More intense herbivory can reduce certain plants.
Changes in vegetation can then affect insects that depend on particular plants, birds that eat those insects or use vegetation for nesting, and other animals that rely on the habitat.
In this way, a predator can indirectly influence plant communities through several feeding relationships.
The effects can spread sideways through the food web
Not every consequence of predator loss moves straight downward.
Species at the same level of a food web often compete for food, space, or other resources. If one prey species increases after a predator disappears, it may consume more of a shared food source, making that resource less available to another species.
The increase of one species can therefore contribute to the decline of another, even if the missing predator never interacted with the declining species.
Predator loss can also change interactions among predators. If one predator disappears, another may face less competition or encounter more prey. Its population or behavior may then change, producing additional effects elsewhere in the ecosystem.
This interconnectedness is why the term food web, rather than simply food chain, is so important. Most organisms participate in multiple feeding relationships.
Why predator loss does not always cause a dramatic cascade
It is tempting to assume that removing a predator will always cause prey populations to explode and ecosystems to unravel. Real ecosystems are more complicated.
A prey population may be limited by its food supply even when predators are absent. Disease can prevent a population from growing indefinitely. Competition can become stronger as population density rises. Other predators may compensate for the missing species.
Environmental conditions can also overwhelm the effects of predation. Drought, severe winters, habitat changes, or changes in food availability may strongly influence population sizes independently of predator abundance.
The strength of a predator’s ecological role also depends on the species involved, the structure of the food web, and the environment in which they live.
Some predators have unusually large ecological effects
The influence of a predator is not determined simply by how many individuals are present.
A predator can have an especially important role when it strongly controls an abundant prey species, affects prey behavior, or interacts with several parts of the food web. Such species are sometimes described as keystone species when their ecological influence is disproportionately large relative to their abundance.
But not every predator is a keystone species, and losing a predator does not automatically create a trophic cascade.
The useful lesson is that ecological importance and population size are not always the same thing. A species can be relatively uncommon yet still influence the structure of the community around it.
What happens over the long term?
The consequences of predator loss can develop over different timescales.
An immediate effect may be a change in prey behavior. Over longer periods, prey abundance may change as reproduction and mortality respond to the altered conditions. Vegetation or other resources may then change, followed by changes in species that depend on those resources.
Eventually, the ecosystem may settle into a new state rather than simply returning to its previous condition.
That new state can have different population sizes, species interactions, vegetation patterns, and resource availability. If the predator later returns, the ecosystem may not immediately return to its former condition because the organisms and habitats involved may have changed in the meantime.
Predator loss can change the balance of an entire food web
When a predator disappears, the most important consequence is not simply that one species is gone. The loss changes the pressures acting on the species that remain.
Prey may become more abundant or alter their behavior. Plants and other resources may experience heavier use. Competitors may gain or lose an advantage. Other predators may change what they eat or how they use the habitat. Those changes can propagate through the food web in several directions.
The result is an ecosystem whose relationships have been rearranged.
Predators are therefore more than consumers at the top of a food chain. Through their effects on prey abundance and behavior, they can help shape the structure and functioning of the broader community.
