Food web Pisaster feeds on acorn barnacles and mussels in rocky intertidal
Exclosure experiment by R. Paine (Am. Nat. 1966) Pisaster removed by hand from sections of shore for 3 yrs. Observed change in bivalve community. Pisaster maintains diversity through predation of Mytilus californiana. Otherwise competitive exclusion results No Pisaster -> 8 species bivalve Pisaster present -> 15 species bivalve
Original keystone concept Two facets to being a Keystone species: –A predator preferentially eats and controls the abundance of a primary consumer –The primary consumer was capable of excluding (through competition or predation) other members of the community.
Contrasting kelp and urchin densities in Pacific NW Estes and Palmisano (Science 1974) Vegetation cover Urchin density Depth (m) Amchitka Shemya Veg Urchins
Diameter of urchins Shaded area = urchin biomass Open area = number of urchins Otters presentOtters absent
Sea otter impact on urchins Otters substantially reduce populations of large urchins. Estes and Palmisano (Science 1974)
Kelp density and otters 0 3 7 11 15 19 100 60 20 100 60 20
Otter effects on urchin biomass Estes (Ec. Monogr. 1995)
Other Keystones? Both the examples given are of keystone predators. Can you think of examples other than predation where one species has a large effect on a system?
Paine (1966) first usage Pisaster (top predator) was identified as the species that maintained diversity in a rocky intertidal community (exclosure experiments). Demonstrated that removal of Pisaster (top predator) led to reduction from 15 to 8 spp. in foodweb. Keystone predator may increase spp. diversity of prey by preventing competitive exclusion.
1) Keystone predator: sensu Paine (1966). Sea Otters:- limit sea- urchin density and hence allow development of kelp beds…with otters present there is a very different community (Estes and Palmisano; Science 1974). This meets the original definition in that removal of primary consumer changes environment.
More Keystone predators? Other authors have interpreted any predator with a major impact as a keystone...Risch and Carroll 1982...fire ants reduce number of arthropods harmful to agriculture. Ants are generalists preying on herbivores that are not specially competitive. Note the removal of fire ants does not change the environment and so this is a different meaning for “keystone”.
2) Keystone Prey (Holt 1974) Models demonstrated that if a prey species had sufficiently high reproductive output it could maintain predator popn that reduced density of other prey. i.e. rabbits support fox popn at a high level, foxes also prey on other mammals perhaps eliminating some rare ones. If this prey were removed species diversity would rise
3) Keystone mutualists (Gilbert 1980) Mobile links....pollinators and dedicated seed dispersers. Hummingbird pollinators and mammalian dispersers of mycorrhizal fungae. If these mutualists are absent the plant composition would change radically.
4) Keystone hosts: Hosts of the mobile links must also be keystones. Terborgh suggested palm nuts, figs and nectar sources are critical to the survival of 50-75% of tropical forest mammal and bird biomass.
5) Keystone modifiers: Beavers alter the hydrology of large areas, create wetlands and may be the cause of the majority of high elevation wetlands in the Rockies. Pleistocene megafauna may have been modifiers and as they started to be lost there was a snowball effect that led to the elimination of about half the mammalian genera with bodymass between 5 and 1000 kg at end of Pleistocene. Pocket gophers slow aspen invasion in mountain meadows.
Keystones and importance Classic food web theory assumes either that species-by-species interactions are uniform, or that their interaction strengths are drawn from symmetrical distributions. Keystone concepts demand that we quantify another measure….Community importance.
Keystone redefined Keystone is a species that is disproportionately important relative to its BIOMASS Proportional biomass of species Individual impact of species Keystone sp. Dominant sp. Ho hum
Test with exclosure experiments CI i = [(t N -t D )/t N ](1/p i ) –where t N is a quantitative measure of the trait in the intact community. –t D is the trait after species i has been deleted. –p i is proportional abundance of i prior to deletion. –If species has effect directly proportional to abundance CI =1. Values much greater than 1 needed to indicate keystone species.
Once a Keystone, not always a keystone Keystones can be habitat specific. Pisaster is only a keystone on exposed intertidal habitats. Does not have the same role in sheltered communities on same headland. (Menge, Ec. Monogr. 1994).
Keystone concept shows: Loss of some species of low abundance may have surprisingly dramatic effects: Preservation of a species of concern may depend upon the distribution and abundance of other species with which the target has no recognized interaction; and conversely, Loss of a species, such as a top carnivore, may reverberate to affect members of the entire community in hard to predict ways.