Cascade effect (ecology) - AbsoluteAstronomy.com

An ecological cascade effect is a series of secondary extinctions that is triggered by the primary extinction of a key species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

in an ecosystem

Ecosystem

An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

. Secondary extinctions are likely to occur when the threatened species are: dependent on a few specific food sources, mutualistic(dependent on the key species in some way), or forced to coexist with an invasive species

Invasive species

"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....

that is introduced to the ecosystem. Species introductions to a foreign ecosystem can often devastate entire communities, and even entire ecosystems. These exotic species monopolize the ecosystem’s resources, and since they have no natural predators to decrease their growth, they are able to increase indefinitely. Olsen, T.M. et al. showed that exotic species have caused lake and estuary

Estuary

An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....

ecosystems to go through cascade effects due to loss of algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, crayfish

Crayfish

Crayfish, crawfish, or crawdads – members of the superfamilies Astacoidea and Parastacoidea – are freshwater crustaceans resembling small lobsters, to which they are related...

, mollusks, fish, amphibians, and birds. However, the principal cause of cascade effects is the loss of top predators as the key species. As a result of this loss, a dramatic increase (ecological release

Ecological release

Ecological release occurs when a species is introduced to an environment other than its native habitat, the foreign species will either establish a local population, or die out from the region...

) of prey species occurs. The prey is then able to overexploit its own food resources, until the population numbers decrease in abundance, which can lead to extinction. When the prey’s food resources disappear, they starve and may go extinct as well. If the prey species is herbivorous, then their initial release and exploitation of the plants may result in a loss of plant biodiversity

Biodiversity

Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

in the area. If other organisms in the ecosystem also depend upon these plants as food resources, then these species may go extinct as well. An example of the cascade effect caused by the loss of a top predator is apparent in tropical forests. When hunters cause local extinctions of top predators, the predators’ prey's population numbers increase, causing an overexploitation of a food resource and a cascade effect of species loss.

Current example of cascade effect

One example of the cascade effect caused by the loss of a top predator has to do with sea otters (Enhydra lutris). Starting before the 17th century and not phased out until 1911 when an international treaty was signed to prevent their further exploitation, sea otters were hunted aggressively for their pelts, which caused a cascade effect through the kelp forest

Kelp forest

Kelp forests are underwater areas with a high density of kelp. They are recognized as one of the most productive and dynamic ecosystems on Earth. Smaller areas of anchored kelp are called kelp beds....

ecosystems along the Pacific Coast of North America. One of the sea otters’ primary food sources is the sea urchin

Sea urchin

Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

(Class: Echinoidea). When hunters caused sea otter populations to decline, an ecological release of sea urchin populations occurred. The sea urchins then overexploited their main food source, kelp

Kelp

Kelps are large seaweeds belonging to the brown algae in the order Laminariales. There are about 30 different genera....

, creating urchin barrens where no life exists. No longer having food to eat, the sea urchins populations became locally extinct as well. Also, since kelp forest ecosystems are homes to many other species, the loss of the kelp ultimately caused their extinction as well. In conclusion, the loss of sea otters in local areas along the Pacific coast seems to have caused a cascade effect of secondary extinctions, continuing into the present day.

Current example of cascade effect

See also