Transcript Community Structure

Community Structure:
Top-Down vs. Bottom-Up Control
Hairston, Smith and Slobodkin (1960)
Community Dynamics
Carnivores
Detritivores
control
Resource limited
Herbivores
Frees plants
from herbivore
control
Plants
Critiques
Too Simple
1) Species differences matter
2) Plant dominance could be explained by good defences
Other inferences of Hairston et al, 1960
1) Exceptions not important ??
2) All communities have 3 trophic levels
3) Omnivory not important
X
X
4) External abiotic factors - not controllers X
Robles et al, 1995
Mean
Density
(seastars/m2)
Recruitment index of Mytilus
Menge and Sutherland, 1976
Effects of predation by whelks.
Predation is weak
High wave energy - effects of predation -weak
Moderate wave energy
- effects of predation - strong
Menge
Sutherland
Bottom Up Control
Fretwell, 1977, 1987
- availability of plant material governs structure of food chains
- Low productivity - 1 link (plants)
- Higher productivity - add links
Ecological Relationships in Kelp Forests
Kelp
Orca
Sea Otter
Urchins
Transplant mussels and barnacles (filter feeders) to urchin-dominated and
kelp-dominated substrates
Expected (top down)
Urchin
Kelp
Transplant mussels and barnacles (filter feeders) to urchin-dominated and
kelp-dominated substrates
Expected (top down)
Urchin
Observed (bottom up)
Urchin
Kelp
Kelp
Clearly - can be a complex interaction
Increased nutrient
Increased algae
Increased benthic filter feeders
Increased consumers (predation)
control
Interaction of Systems
High flow
Leonard et al, 1998
Low flow
Interaction of Systems
• increased seaweed growth
• increased filter feeder growth
• increased larval settlement
• low consumer efficiency
• higher densities of
organisms with planktonic
•larvae
more spatial competition
• increased consumer pressure
• increased sedimentation
• lower densities of
organisms with planktonic
larvae
• increased consumer mortality
• less spatial competition
Leonard et al, 1998
Hydrodynamics
Flow rate
Time
Leonard et al, 1998
Community structure
High flow
Low flow
T
i
d
e
h
e
i
g
h
t
Percent cover
Percent cover
barnacles
Fucus
mussels
Bare space
Leonard et al, 1998
High flow
Recruitment rates
Barnacles
Density
(#/100 cm2)
Low flow
Mussels
Snails
Leonard et al, 1998
Crab predation
High flow
Low flow
Predation Intensity
(% mortality)
On Littorina, Nucella, Mytilus
Leonard et al, 1998
crabs
grazers
diatoms
Nutrients
mussels
Larvae
barnacles
Plankton
Leonard et al, 1998
crabs
crabs
grazers
diatoms
Nutrients
whelks
grazers
mussels
Larvae
barnacles
Plankton
diatoms
Nutrients
mussels barnacles
Larvae
Plankton
Interference competition,
exploitative competition
for resources other than
food
(-)
Predators
(-)
(-)
Induced morphological or
chemical defenses,
hiding, retreat to refuges
Depletion of more
nutritious,
palatable or
accessible prey
- (+)
Consumers
+ (-)
Cover from (for) predators
Stimulation of areaspecific primary
productivity
Plants
(+)
Powers. ‘92. Ecology 73: 733