Environmental Engineering Reference
In-Depth Information
competitively dominant species, which would normally physically usurp
space that could be occupied by other species, is suppressed by a carnivore.
This allows less competitive species to gain a foothold in the system.
A textbook example of such an interaction occurs in rocky intertidal
ecosystems where the starfish (
Pisaster ochraceus
) preferentially feeds on com-
petitively superior barnacles and mussels.This allows other marine species
including several species of algae, a sponge, and herbivores (limpets and chi-
tons) to persist in the ecosystem. Removal of the starfish leads to a cascade
of events including aggressive preemption of other species' space by barna-
cles. Barnacles are then overtaken by mussels.At the same time, many of the
algae species are lost owing to a lack of space and their associated herbivores
are lost due to a lack of food.A study of one such sequence of events found
that diversity collapsed from fifteen to eight species (Paine 1966).
In a multitrophic level variation on this theme (figure 2.2d), a top car-
nivore (C) species interacts with an herbivore (H) species that consumes
two or three plant (P) species leading to an indirect keystone effect. At the
heart of this system lies an asymmetrical plant competition interaction in
which the middle species, P
2
, is competitively dominant to the other two
species (depicted by a thick arrow of effect toward the other species).This
can lead to a host of indirect effects. In one such field system (Schmitz 2003)
a hunting spider carnivore (
Pisaurina mira
) interacts with a grasshopper her-
bivore (
Melanoplus femurrubru
), a grass species (
Poa pratensis
), and the com-
petitively dominant species of goldenrod (
Solidago rugosa
).The grasshopper
eats both the grass and the goldenrod. But, it prefers the grass in the absence
of predators owing to its high nutritional value and can inflict consider-
able damage to it. Mortality risk caused by predator spiders causes grasshop-
pers largely to forego feeding on grass and to seek refuge in and forage on
less nutritious but safer leafy goldenrod.This in turn causes high damage
levels to this species.Thus, spiders exert strong cascading effects by having
a positive indirect effect on grass abundance and a negative indirect effect
on goldenrod abundance. Because of the spider's indirect effect on the com-
petitively dominant plant, it releases other plant species from competition
thereby having a positive indirect effect on plants not consumed by the
grasshopper.The top predator here has an overall net diversity-enhancing
effect on plants.
Both systems are examples of what is now called “keystone predation”
(sensu Paine 1966): the former a direct keystone effect, the latter an indirect
keystone effect.This concept derives its name from the keystone in an arch.
