Download Lecture 25: Trophic Cascades The world is GREEN (Hairston et al

Lecture 25: Trophic Cascades
Top-down Control:
• Abundance/biomass/diversity of lower trophic
levels depends on effect of consumers from
higher trophic levels
• Trophic levels alternate between consumer &
resource limitation
• In terrestrial systems: top-down control is
limited to low diversity systems
The world is GREEN
(Hairston et al. 1960)
• e.g. Predators keep
herbivores in check,
primary producers
compete, carnivores
compete
• Removal of herbivores
= no effect on plants
Bottom-up
Control
• Structure of community
depends on factors from
lower trophic levels
(nutrient concentrations
& prey availability)
• Organisms limited by
trophic level below
them.
• Plants have primacy of
control but…
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The world is prickly & tastes bad
(Murdoch 1966)
• Plant defenses make herbivory hard.
• Herbivores may compete, predators
compete
• Removal of predators or herbivores does
not affect plants
The world is white,
white yellow & green
(Fretwell 1977, Oksanen et al. 1981)
Predator effects vary along a gradient of 1° productivity
Predictions:
• Unproductive : herbivores rare (not enough food)
• High productivity: few herbivores (pred. limited), green
• Intermediate: plants are pred. limited
(prey can’t support predators)
• High & Low productivity: plant competition
• Intermediate: herbivore competition
• High productivity: predator competition
Evidence for WYG
• Biomass: productivity relationship is
confirmed
• Herbivore removals = increase plants
• Plants only compete in low & high prod.
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Top-down vs. Bottom-up
Fath (2004)
• Control of community structure & change is
more complicated than a dichotomy
• Control is distributed among ecosystem
elements (all have input/output effects)
• Thus, must examine all elements to
determine control:
• After initial E input what affects energy
flow & storage?
Network Analysis
• Maximum donor control: sequential food
chain
• Maximum recipient control: one node is
final recipient for all energy flow
• Unrealistic & Idealized, but can use as a
comparison for relative strength of
donor/recipient control in real systems
Trophic Cascades
Paine (1980):
• Top-down effects of predators influenced
abundance of species at lower trophic
levels.
• e.g. Lake productivity : 50% of the
variability can be attributed to top-down
effects of top predators
But, trophic effects can cascade up & down
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Types of Trophic Cascades
Species-level:
• Changes in predator abundance affect the
success of a subset of plant species
• Generally do not explain habitat-wide
changes in biomass distribution.
Community-level:
• Substantially alter the distribution of plant
biomass throughout an entire community
Consensus (Polis et al. 2000)
• Top-down influence on structure
• Change in plant biomass must be significant
or plant community composition (amount of
change?)
• Community-level cascades are relevant,
species-level cascades are not
Terrestrial vs. Aquatic Systems
Strength of Cascades (Shurin et al. 2002):
• Strongest in lentic & marine benthos
• Weakest in marine plankton & terrestrial
• Effects on herbivores stronger than on plants
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More…
• Stronger effects in aquatic systems but, among
the aquatic systems, differences were as
significant as between wet & dry systems
• Stronger attenuation in terrestrial environments
• Weak representation of terrestrial experiments
Aquatic Systems
a) Discrete & homogeneous
habitats
b) Prey population dynamics
fast rel. to pred.
c) Common prey “uniformly
edible” (unlikely)
d) Simple & trophically
stratified communities,
strong interactions
(but, omnivory common
& diverse –scale issue)
Terrestrial:
Antiherbivore defenses
(weakens effect of carnivore removal)
a) Heterogeneous & poorly separated
habitats
b) Variable prey dynamics rel. to
predator (effect appears weak)
c) Prey rarely uniformly edible
d) Reticulate & complex systems, weak
& diffuse interactions
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Other differences
a) Body size ratios b/w prey & predator
larger in aquatic systems
b) Aquatic primary producers are more
nutritious
c) Aquatic: higher mass-specific biomass
production
d) Herbivores stronger effect on primary
producers in aquatic systems
But…
• Differences among systems account for
35% or less of variation in herbivore/plant
response
Within community differences are more
important:
• e.g. diversity, behaviour, productivity,
structural complexity…
Empirical Evidence: Aquatic
• Lots of evidence
• Primarily from lentic systems (marine &
streams as well)
• Community-level results rather than
species-level
• But, much more highly studied…
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Empirical Evidence: Terrestrial
North American forests (& European tundra):
• Wolves structure populations of moose & deer,
cascades to plant abundance
Canadian Boreal Forest:
• Strong predator-herbivore & herbivore-plant
relationships
• Weak predator-plant effects
Chilean Scrubland:
• Predator removal: herbivore increase, plant response
Empirical Results:
Schmitz et al. (2000):
• 41 studies of carnivore removals
• Quantified direct & indirect effects
• Species-level cascades not community-level
cascades
Schmitz et al. Results
• Species-level cascades are common (45/60)
• Similar magnitude of indirect & direct effects
as in aquatic systems
• Indirect effects are obvious quickly (< 6 mos.)
• Trophic cascades are weaker in food webs
with high intermediate species diversity
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Plant Biomass vs. Plant Damage
• When biomass was used, carnivore effects
tended to be attenuated
• Biomass considered better measure of
community effects, damage tends to be a
short-term response
Mechanism
• Important to understand pathways, especially
because presence/absence studies do not always
demonstrate control
e.g. Spiller & Schoener (1998):
• Lizard removal: no effect on plants, but direct effect
on herbivore abundance, therefore attenuation
• But, lizard removal: increase in spiders, intraguild
replacement
Behavioural Changes
• Predator removal/addition may not change
herbivore density but may affect herbivore
behaviour
• e.g. Herbivores may adjust prey preference,
habitat preference in response to predation
risk
• May affect plant species composition (must
examine species!)
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Problems
• Studies that show NO trophic cascade
generally not published (bias)
• Inadequate information about species
composition, modes of interaction, degree
of interconnectedness
• More emphasis on mechanism (behav.,
physiol., morpho.)
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