Cascade Effects

The loss of a species can have various effects on the remaining species in an ecosystem--what kind and how many depends upon the characteristics of the ecosystem and upon the species' role in its structure. Cascade effects occur when the local extinction of one species significantly changes the population sizes of other species, potentially leading to other extirpations. Such cascade effects are particularly likely when the lost species is a "keystone predator," a "keystone mutualist," or the prey of a "specialist predator."

Keystone Predators

How much the loss of a predator affects its prey population size depends on how much the predator limits the prey population. If the size of the prey population is determined by factors other than predation, it is said to be donor-controlled. For example, such bottom-dwelling invertebrates as mussels or barnacles consume only a small fraction of the ocean's plankton; consequently, removing these invertebrates would not affect the plankton's population size much. Similarly, although the passenger pigeon (Ectopistes migratorius), Carolina parakeet (Conuropsis carolinensis), and wild turkey (Meleagris gallopavo) were the dominant large seed-predators of eastern North America's forests, the extinction of the first two species and the near-extinction of the third apparently changed the forest little. The loss of the bird species increased seed survival, but this increase had no effect on the density of trees because what limited the tree populations was the availability of space, not seed predation.

In a predator-controlled system, the size of the prey population is determined by predation. In such systems, the impact of the loss of the predator can be substantial, especially if the predator is relatively high in the food chain. So-called keystone predators affect not only their prey's population size but also the community's species diversity. By limiting the population size of a species that would otherwise outcompete other species, keystone predators can help maintain high species diversity. When the keystone predator Pisaster ochraceus (a starfish) is removed from the intertidal zone in the Pacific northwest of the United States, the intertidal community changes from one with a high diversity of relatively large bottom-dwelling invertebrates to a virtual monoculture of the starfish's favorite prey--the mussel (Mytelus edulis).

Keystone predators represent a vexing problem for conservation. Many keystone predators, such as sea otters, face a high risk of extinction because they tend to be high in the food chain and relatively sparsely distributed. Yet, determining which species play keystone roles in a community may be extremely difficult. For example, the small moth Cactoblastis catorum, introduced into Queensland from South America, appears to be a minor component of the Queensland community since its population size and biomass aren't great. However, it is a keystone predator in Australia and probably also in South America. In the absence of the moth, Opuntia cactus (also introduced from South America) covered 62,000 square kilometers of Queensland but in its presence Opuntia has been reduced to small isolated patches. In general, determining whether a given species plays a keystone role requires considerable study and experimentation.

Although certain species have much more influence than others on an ecosystem's structure, not all ecosystems include a single species that exerts such a pervasive influence. In fact, most ecosystems are somewhat sensitive to the loss of any one of many species, though some losses have greater impact on the system than others. Nevertheless, determining which species function as keystone predators in an ecosystem can help managers find-tune conservation activities.

Mutualists and Keystone Mutualists

Species involved in mutualisms--mutually beneficial interactions such as pollination--are deeply affected if their partner mutualists are lost. For example, the loss of a fruit-eating species may severely affect the plants whose fruits it disperses. Similarly, rare tropical plants dependent on specific species for pollination would be threatened with extinction by the loss of the pollinator. Some orchids, for instance, rely on a single species of euglossine bee for pollination; without the bee, the plant would become extinct. By the same token, pollinators that specialize on a specific plant species could die out if the plant does. Most species of fig (Ficus spp.) are each pollinated by a different species of wasp, and each species of wasp pollinates only one fig, so the loss of a fig species could wipe out one of the specialist wasps.

Maintaining plant-pollinator interactions in tropical forests is of utmost importance since tropical forests are much more sensitive to extinctions of pollinators than are temperate forests. Whereas less than 30 percent of trees in the northern United States are pollinated by animals (as opposed to wind), more than 95 percent of the trees in one dry forest in Costa Rica are believed to rely on animal pollination.

Certain species--keystone mutualists--involved in mutualistic interactions may sometimes assume great importance in the community. For example, during the dry season in the tropical forest at the Cocha Cashu Biological Station in Manu National Park of southeastern Peru, only 12 of the approximately 2,000 plant species support the entire fruit-eating community of mammals and birds. During this period, fruit production drops to less than 5 percent of the peak production, so the food available to the fruit-eating species is severely restricted. The 12 plant species still producing fruit and nectar meet the food needs of as much as 80 percent of the entire mammal community and a major fraction of the avian species at the site. Clearly, the loss of one or more of these "keystone mutualists" could significantly harm frugivore populations.

While the identification of keystone predators may be extremely difficult, keystone mutualists can generally be identified through non-experimental observations of the resources that the species use. Once identified, keystone mutualists can be managed to ensure the survival of the species dependent upon them. By increasing the abundance of the keystone mutualists for instance, it might be possible to increase populations of dependent species.

Specialist Predators

In general, the loss of a species tends to have greater impacts on its own predators if the species is relatively high in the food chain. Populations of such species, including carnivores, tend to be limited by the amount of prey available, whereas species low in the food chain, such as herbivores, are more often limited by their predators or pathogens. Consequently, the loss of an herbivore tends to directly reduce the supply of the factor (food) limiting the carnivore population, while the removal of a plant species may have no effect on the herbivore population.

One exception to this pattern occurs when the plant species removed is the prey of a specialist predator--that is, a species with no alternative food source, For example, many insects eat just one species of plant because they have evolved mechanisms for detoxifying the plant's chemical defenses. If a such a plant were lost, the highly specialized insect would be too. Widespread and common prey species are more likely to have specialist predators that are rare or locally endemic species, so the loss of common species would tend to have a large impact on species higher in the food chain.