Trophic cascades are strong, indirect effects that can influence entire ecosystems. They happen when one level in a food chain is reduced. For example, a top-down cascade occurs when predators are effective at hunting, which reduces the number of their prey or changes their behavior. This allows the next level in the food chain to grow, especially if the prey is a plant-eater.

The idea of trophic cascades has helped scientists study ecosystems better. It helps explain the effects of removing top predators, like when humans hunt or fish too much.

A top-down cascade happens when the top predator controls the population of the next level. This allows plants or other producers to grow more. If the top predator is removed, the next level might grow too much and harm the producers. This can lead to fewer producers over time. Stability in food chains depends on competition and predation among higher-level animals. Invasive species can also change these patterns by becoming top predators or removing them. Sometimes, these changes can help ecosystems recover.

For example, if more large fish that eat other fish are added to a lake, the smaller fish that eat zooplankton may decrease. This could lead to more zooplankton, which might reduce the amount of phytoplankton (tiny water plants).

In a bottom-up cascade, the number of plants or phytoplankton controls how much energy is available for higher levels in the food chain. These plants need sunlight and nutrients to grow. If nutrients are plentiful, all populations in the food chain may grow.

A subsidy cascade happens when animals at one level get extra food from outside their habitat. For example, some predators might eat animals from another area, like livestock. This can increase their numbers and affect other species. A study found that animals in a rainforest in Malaysia used food from nearby oil palm plantations. This led to more wild boar, which built many nests and damaged young trees, reducing tree sapling numbers by 62% over 24 years. These effects can happen in many ecosystems and are a challenge for conservation.

Trophic interactions influence biodiversity worldwide. Human activities and climate change have greatly affected these patterns. For example, sea otters were removed from the Pacific coast of the United States due to human actions. Without otters, their main prey, sea urchins, grew too much and overate giant kelp, causing kelp forests to decline. This shows why protecting ecosystems is important.

Predators can also affect how much carbon is stored in the atmosphere. A study found that sea otters help kelp store carbon, with potential values between $205 million and $408 million in 2012.

Origins and theory

Aldo Leopold is often recognized for first explaining the ecological effect known as a trophic cascade. He observed that when wolves were removed from an area, deer overgrazed mountain slopes. Later, Nelson Hairston, Frederick E. Smith, and Lawrence B. Slobodkin introduced the concept into scientific discussions, though they did not use the term "trophic cascade." They proposed that predators help control the number of herbivores, which allows plants to grow more abundantly. This idea is called the green world hypothesis. It highlighted the importance of top-down forces, such as predation, and indirect effects in shaping ecosystems. Before this, scientists primarily used trophodynamics, which focused only on bottom-up forces, like limited resources, to explain community structures. Smith may have been influenced by experiments conducted by Czech ecologist Hrbáček, whom he met during a cultural exchange program. Hrbáček demonstrated that fish in artificial ponds reduced zooplankton numbers, which increased phytoplankton abundance.

Hairston, Smith, and Slobodkin suggested that ecological communities function as food chains with three trophic levels. Later models expanded this idea to include food chains with more or fewer than three levels. Lauri Oksanen argued that in food chains with an odd number of trophic levels (such as three), the top level increases the abundance of producers. However, in food chains with an even number of levels, the top level decreases producer abundance. He also noted that as an ecosystem becomes more productive, the number of trophic levels in its food chains tends to increase.

Examples

Although Hairston, Smith, and Slobodkin developed their theory using land-based food chains, the first real examples of trophic cascades were found in marine and, especially, freshwater ecosystems. Some well-known examples include:

• In North American lakes, fish that eat other fish (piscivorous fish) can greatly reduce the numbers of fish that eat zooplankton (zooplanktivorous fish). These fish can then change the types and numbers of zooplankton in the lake. Zooplankton, in turn, eat phytoplankton, which affects the clarity of the lake water. When piscivorous fish are removed, phytoplankton can grow more, making the water appear green instead of clear.
• In the Eel River in Northern California, fish like steelhead and roach eat fish larvae and predatory insects. These smaller predators also eat midge larvae, which feed on algae. When larger fish are removed, algae populations increase.
• In Pacific kelp forests, sea otters eat sea urchins. In areas where sea otters were hunted to extinction, sea urchins grow in number, and kelp populations decrease.
• The reintroduction of gray wolves (Canis lupus) to Yellowstone National Park is often seen as an example of a trophic cascade, with wolves changing the landscape. However, the Greater Yellowstone Ecosystem is too complex to be a simple example. It is influenced by many factors, including human activity, natural conditions, and other species like bison and elk, which are preyed on by multiple predators, including wolves, bears, cougars, and humans.
• Isle Royale Island has a simpler ecosystem. Moose arrived on the island about 50 years before wolves, which began hunting moose in the late 1940s. In 1979, the wolf population dropped by 80% due to a disease, and moose numbers increased. The wolf population did not fully recover, and moose numbers were later affected by factors like extreme weather and tick infestations. Wolves were important for keeping moose populations stable. In 2019, a project began to relocate wolves to the island to help restore balance. Studies on the moose-wolf relationship continue.
• In Banff National Park, a natural event occurred in 1986 when a wolf pack returned to the Bow Valley. The area has regions with high and low human activity, which corresponded to areas with high and low wolf presence. These areas showed differences in elk numbers, wolf predation, and environmental changes. High wolf activity led to more aspen and willow growth, more beaver lodges, and more songbirds. This example shows a simplified version of a trophic cascade, unlike the more complex system in Yellowstone.

Terrestrial trophic cascades

Scientists noticed that the earliest known examples of trophic cascades happened in lakes and streams. This led them to think that differences between water and land food webs might explain why trophic cascades are more common in aquatic environments. These cascades were often found in communities with few species, where a small number of species had a large impact, and food chains were simple. At the time, most well-documented trophic cascades involved algae as the main producer of energy. Strong suggested that trophic cascades may only occur in communities where producers grow quickly and have no defenses against being eaten by herbivores.

Later research showed that trophic cascades also happen in land ecosystems, such as:

• In Northern California’s coastal prairie, yellow bush lupines are eaten by root-boring caterpillars of the lupine ghost moth. These caterpillars are killed by nematodes, which help lupines survive and produce seeds.
• In Costa Rica’s rainforest, a clerid beetle eats ants that live on Piper plants. These ants protect the plants by eating insect herbivores. When the beetle reduces the number of ants, more herbivores damage the Piper plants.

Critics noted that most studies on land focused on only a few species, unlike aquatic studies that looked at many species. In water ecosystems, removing predators often reduced the total amount of plant life. On land, removing predators usually increased plant damage from herbivores, but it was unclear if this damage reduced plant numbers. A 2002 study found that trophic cascades on land are generally weaker, meaning changes in predator numbers cause smaller changes in plant life. However, a 2009 study showed that many tree species are affected by the loss of top predators.

In Yellowstone National Park, gray wolves were removed by 1930, which led to changes in the ecosystem. Without wolves, elk herds grew and damaged aspen, cottonwood, and willow trees, harming beaver colonies and other species. Wolves were reintroduced in 1995 and 1996, which led to some recovery of plants and animals, but the ecosystem did not fully return to its earlier state. This lack of full recovery is called hysteresis. A key reason for this is creek and river erosion caused by the loss of beavers and their dams. Experiments showed that fencing out herbivores and building artificial beaver dams helped willows grow again, but fencing alone was not enough.

Changes in elk behavior, such as avoiding areas with high predator density, also affected trophic cascades. Studies showed that these behavioral changes, called "trait-mediated indirect effects," are important to understanding how wolves influence the ecosystem.

The return of wolves in Yellowstone also led to more grizzly bears and cougars. Elk migrate between Yellowstone and Montana, where they are hunted by humans. Five predators—humans, wolves, cougars, grizzly bears, and black bears—now help control elk populations.

Trophic cascades affect biodiversity. In Yellowstone, scavengers like ravens and eagles benefit from wolf kills. Restored willow habitats helped songbirds, and bison numbers increased as elk declined.

Other examples of trophic cascades involving large mammals include:

• In Zion and Yosemite National Parks, increased human visits in the early 20th century led to fewer cougars. This caused mule deer populations to grow, damaging trees and reducing biodiversity. In areas where cougars remained, ecosystems were healthier.
• In sub-Saharan Africa, the decline of lions and leopards led to more olive baboons, which harmed other animals and caused conflicts with humans.
• In Australia, the absence of dingoes led to more red foxes, which reduced numbers of an endangered mouse species.

Marine trophic cascades

In addition to the classic examples listed above, more recent examples of trophic cascades in marine ecosystems have been identified:

• A trophic cascade in a complex, open-ocean ecosystem happened in the northwest Atlantic during the 1980s and 1990s. Long-term overfishing removed Atlantic cod (Gadus morhua) and other ground fishes. This caused the number of prey species for these ground fishes to increase, especially smaller forage fishes and invertebrates like the northern snow crab (Chionoecetes opilio) and northern shrimp (Pandalus borealis). The increase in these prey species changed the community of zooplankton, which are food for smaller fishes and invertebrates.

• A similar trophic cascade involving Atlantic cod occurred in the Baltic Sea at the end of the 1980s. After Atlantic cod numbers dropped, the number of its main prey, sprat (Sprattus sprattus), increased. This caused the Baltic Sea ecosystem to shift from being dominated by cod to being dominated by sprat. The next level of the cascade was a decrease in the number of Pseudocalanus acuspes, a copepod that sprat eat.

• On Caribbean coral reefs, several species of angelfishes and parrotfishes eat sponges that lack chemical defenses. Removing these sponge-eating fish species from reefs through fishing activities caused the sponge community to change. Fast-growing sponge species that lack chemical defenses became more common. These sponges are better at competing for space and overgrow and smother reef-building corals more on overfished reefs.

Criticisms

Although scientists agree that trophic cascades exist, they have long debated how common these effects are in nature. Hairston, Smith, and Slobodkin suggested that land ecosystems usually follow a simple pattern with three levels in the food chain, which led to disagreements among scientists. Some criticisms of their model and later models, such as Oksanen’s, include the following points:

• Plants have many ways to protect themselves from being eaten, which helps reduce the harm caused by herbivores.
• Herbivore populations may be limited by factors other than food or predators, such as the availability of nesting areas or space.
• For trophic cascades to be common, ecosystems would need to act like simple food chains with clear levels. However, most ecosystems have complicated food webs. In these webs, animals often eat at multiple levels (omnivory), change their diets as they grow, eat members of their own species (cannibalism), and receive resources from outside their local area, which makes it harder to define clear levels in the food chain.

This idea is sometimes called the "trophic trickle."