What's the difference between a keystone species and a foundation species?

A keystone species has a disproportionately large effect on its environment relative to its abundance. These species are described as playing a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem. They help to determine the types and numbers of various other species in the community. Without a keystone species, the ecosystem would be dramatically different or cease to exist altogether.

For example, the jaguar acts as a keystone predator by its widely varied diet, helping to balance the mammalian jungle ecosystem with its consumption of 87 different types of species.

A foundation species is a species that has a strong role in structuring a community. The activities of foundation species physically modify the environment and produce and maintain habitats that benefit other organisms using that benefit. The presence of foundation species has the ability to either reduce or increase species diversity depending on its particular role in a specific ecosystem. This term is often applied to a range of terrestrial and aquatic ecosystems.

For example, the existence of kelp provides shelter and suitable habitat for numerous other organisms in marine ecosystems.