Download Predation and Community Structure • Predator influence on

Predation and Community Structure
• Predator influence on community
structure/diversity
– Predation decreasing diversity
• Predators select for competitive
inferiors, allowing competitive
superiors to dominate.
• Eg. lizards preferred rarer spiders
as prey.
Predation and Community Structure
• When productivity is high
– Frequency dependent predation increases diversity.
– Generalist predators – less or no change in diversity (they
will not specialize in the competitively dominant prey).
– Competitive dominants put more resources into
production, less into avoiding predation.
Predation and Community Structure
• As with disturbance,
intermediate levels of
predation maximize
diversity.
Expected and unexpected effects
• The fact that we
observe unexpected
effects hints at
additional complexity in
communities.
• Intermediate predation pressure more likely to increases
diversity
– Too high – preferred prey driven to extinction
– Too low – superior competitors dominate
• Predation = biotic factor. Role or predators most pronounced
in deterministic communities.
1
Direct/Indirect Effects
Overview
Food Webs
Trophic Cascade
•
Increase in predator abundance
→ decreased consumer
abundance → increased producer
abundance
2
Trophic Cascades
• Best documented in aquatic systems
• Criticized as being
– “all wet”
– Species cascades and not trophic cascades
– Assume top-down control
– Less applicable to >3 level communities
• Community level cascades most common when:
– Habitat is homogeneous
– Prey are uniformly edible
– Producer and Prey populations both capable of rapid
increase
– Tropic levels are discrete, omnivores or multi-level
feeders rare
Top down vs. Bottom up control
• Hairston, Smith and Slobodkin (1960)
– The world is green, top-down forces more important
Top-Down vs. Bottom-Up
• Top-down control - consumers
control abundance of their prey.
Predator
Prey
Producer
• Bottom-Up (donor) control –
consumer abundance controlled by
resource availability.
Predator
• Trophic cascade – alternating topdown and bottom-up control.
• Are communities dominated by topdown or bottom-up forces?
Prey
Producer
Power (1990)
• Manipulated fish presence in
in-stream mesocosms that
allowed insect movement.
chironomids
• Oksanen (1981)
– Much of the world is white
– Productivity shifts the balance from top-down to
bottom-up control
Pred. insects
Stickleback fry
3
Power (1990)
Food Webs
• Second experiment excluded
fish and included only one (or
none) of the second level
consumers.
• Strong vs. weak interactors
• Historically, most important
species (keystone) viewed as top
predators, controlling top-down
forces
• Modern view - does not have to
be predator (eg. figs).
• Pollinating insects = Keystone
mutualists
Change in Ecosystem Function if Removed
Keystone Species
Keystone
species – rare,
but vital to
ecosystem
Abundant
species And vital role in
ecosystem
Moderate
abundance moderate role
in ecosystem
Rare species not a vital role
in ecosystem
Species Biomass
4