Keystone and Foundation Species

Types of Keystone Species
There are many types of keystone species, and some of them have been thoroughly studied. Predators are typically defined as keystone species, because it takes only a few to regulate populations of other species in lower trophic levels. Many species that create or modify habitats, called ecosystem engineers, are also keystone species.

Ecosystem Engineer Species that create or modify habitats, such as beavers ( Castor canadensis), can strongly affect ecosystem nutrient cycling. Shifts in available nutrients can directly and indirectly affect animal or plant species that use the same habitat.

Examples of Keystone Species 
Even prior to Paine’s seminal work and terminology, biologists had studied and defi ned many species as unique and necessary components of a given ecosystem, despite their rarity or low numbers. Many species have been widely studied in their role as keystone species.

1. Sea Stars
This is the quintessential example of a keystone species since Paine’s experiment in 1966. Sea stars are a key predator of mussels. Th e absence of sea stars can drastically impact ecosystems, including changes in diversity and abundance of other species in the habitat, affecting diff erent trophic levels. For example, in the absence of sea stars, diversity was reduced from fifteen species to only eight.

2. Bears
Brown bears ( Ursus arctos) as a predator constitute a keystone species by regulating the population of their prey species, but they also have a keystone species role regarding the cycling of nutrients, primarily nitrogen, by incorporating nutrients from rivers into riparian ecosystems. Th ese bears capture Pacifi c salmon when the fi sh are spawning in upstream rivers. Th e bears feed and deposit salmon carcasses further inland, where they decompose and fertilize the riparian areas with nutrients that otherwise may not be incorporated into the local terrestrial ecosystem. Brown bears thus act as nutrient vectors that aff ect an entire ecosystem.
3. Beavers 
Beavers ( Castor canadensis) are the classic example of an ecosystem engineer because they create dams in rivers. These dams significantly alter the flux of nutrients and therefore the growth and abundance of local plants and animals. Their tremendous eff ect can be observed in Tierra del Fuego, an area of South America (in Chile and Argentina) where they have been introduced. Beaver are not native to South America, and no other native species has the ability to create dams in rivers, so beavers are altering the local ecosystems, replacing the slow-growing Nothofagus trees for meadows. This change in the structure of the ecosystem provides evidence on the fundamental role that this species has in its native and exotic range.
4. Corals
The compact ivory bush coral ( Oculina arbuscula) is considered a keystone species because it creates new habitat. This coral species is endemic to the inshore and nearshore bottomland habitats of North and South Carolina in the United States. It is the only coral species found in this region. It forms complex branching colonies that provide shelter to over three hundred species of invertebrates that are known to live and complete much of their life cycle around the coral’s branches.

How Keystone Species Affect Ecosystems 

Many ecosystem eff ects are attributed to keystone species. For example, in Paine’s original work, he reported that diversity of the tidal pool community decreased dramatically when the keystone species (the sea star) was removed. Th e predator preferred to forage on the most abundant mussel (Mytilus californianus), and when the predator was removed, the population of this mussel exploded in numbers that prevented many other species from existing in the tidal pools. So this keystone predator increases community diversity by foraging on the most abundant species, which benefi ts less abundant prey species. Other studies on predator species have found similar results. Many dominant species depend on mutualism for their survival. Th erefore, such mutualistic species can play a fundamental role in ecosystem function, and their removal can change drastically its dynamics, such as in food webs containing hummingbirds. Similarly, keystone species that are ecosystem engineers can, by creating or altering habitats, directly aff ect other species that need these areas to obtain food or shelter. 

Foundation 

Dominant primary producer that provides food and shelter for many other species

For example Black rush (Juncus geraldi )  that increase species richness in a number of New England salt marsh zones, where Juncus helps prevent salt buildup in the soil by shielding the soil surface thus reducing evaporation and preventing oxygen depletion