Transcript Keystones,umbrellas and focal species

Keystone species
Rocky intertidal Bodega bay
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.
• Estes and Palmisano
(Science 1974)
Veg
Urchin density
Depth (m)
Contrasting kelp
and urchin
densities in
Pacific NW
Urchins
Shemya
Amchitka
Vegetation cover
Otters present
Diameter of urchins
Shaded area = urchin biomass
Open area = number of urchins
Otters absent
Diameter of urchins
Sea otter impact on urchins
• Otters substantially reduce
populations of large
urchins. Estes and
Palmisano (Science 1974)
Kelp density and otters
100
100
60
60
20
20
0 3 7 11 15 19
0 3 7 11 15 19
0 3 7 11 15 19
0 3 7 11 15 19
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 seaurchin 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 is a
species that is
disproportionately
important relative
to its BIOMASS
Individual impact of species
Keystone redefined
Keystone sp.
Dominant
sp.
Ho hum
Proportional biomass of species
Test with exclosure experiments
• CIi = [(tN-tD)/tN](1/pi)
– where tN is a quantitative measure of the trait in
the intact community.
– tD is the trait after species i has been deleted.
– pi 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.