Download Lecture 15

Lecture 15
Understanding the Structure and Function of
Community Structure: Food Chains
Communities
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Food chains
Primary Production and Energy Flux
HSS – Green World Hypothesis
Fretwell Hypothesis
Top Down vs. Bottom Up
Examples
Food Webs
Ecosystem-based Management
Marine/ Aquatic
Terrestrial
• Food Chains
– A diagrammatic representation of the flow of
energy through trophic levels in an ecological
community.
• Trophic Levels
– functional classification of organisms in an
ecosystem according to feeding relationships
– Essentially groupings of species based on
what they eat and what eats them.
General Patterns of Food Chains
Elton’s Pyramid (1940’s)
One of the founders of ecology.
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3 consumers
3 consumers
1) Differences across trophic levels
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2 consumers
Help
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Net Primary Production
Grazers
&
Detritivores
2 consumers
Grazers
&
Detritivores
Litter
1) MORE animals LOWER on the food chain
2) MORE biomass sequestered in LOWER food chain
3) HIGHER turnover and Smaller animals LOWER in food chain
Detritus
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General Patterns of Food Chains
Lindeman (1942)- pioneer of thinking about ecosystems in terms of energy
flux
Thermo-dynamics Explains:
1) Energy transfer between trophic levels (1-25%)
2) Food Chain lengths 3-5
Community structure: the number of species and their relative abundance in a
community
75-99% of energy is lost
between trophic levels
Community Structure: Net Primary Production
• NPP is important to
food chain and
community structure
• NPP is extremely
variable across
ecosystems (0-3000
gC/m2 per year)
• So food chains are
driven by productivity
right?!
Community Structure: Green World Hypothesis
Community Structure: Fretwell-Oksanen
HSS (Hairston, Smith, Slobodkin 1960)
(Fretwell 1977) (Oksanen 1981)
Why is the world green?? Not productivity.
• Explains deviations from HSS by
considering number of trophic levels.
Problems with HSS
1) Often not supported experimentally
2) Many plants are defended chemically and structurally
(cactus spines, lignins, kelp secondary compounds.
World is green because it green isn’t always edible….
1) Odd numbered trophic levels= Green
2) Even number = Brown
HSS
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Trophic Level
Top down Control (Trophic Cascade)
1) Nutrients are seldom limiting for plants.
2) Herbivores are potentially strong
regulators of plant populations.
BUT
3) Predators keep herbivore densities
low and allow plants to proliferate
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Possible Food Chains
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Top Down vs. Bottom Up
Top Down (Trophic Cascades):
Top Down Control
Keystone Species : A species that has a disproportionate
effect on the community (Paine 1969)
Predators controls herbivores
Consequently, herbivores have little effect
Plants are abundant
Primary Production is not limiting (not
regulating)
•Keystone species often:
1) Control dominant competitor (Mussels)
2) Control dominant herbivore (Urchin)
2) Modify habitat (Bears, Elephants in Serengeti)
Predictions:
Removal of predators should release
•Removal of few results in massive change to the community structure
•Important to ecosystem integrity
herbivores, which will then decrease plants
Bottom-up model (Productivity) :
Nutrients limit plant abundance
Primary Production is limiting
Plants limit herbivores, herbivores limit
carnivores, etc.
Examples
1) Starfish and Mussel
2) Alaskan Sea Otter/ Urchin/ Kelp
Predictions:
3) Aleutian Islands Invasive Rat study
Removal of top levels=little effect on lower levels
But if you remove lower trophic levels, big effect
on upper trophic levels
Top Down: Starfish and Mussel
Food Web Complexity and Species Diversity (Paine 1966 American Naturalist)
Hypothesis: Diversity will decrease in areas where
Pisaster is removed compared to controls
Pisaster- Voracious predator
Mytilus- Space competitor
Experiment: Remove
Pisaster
Results: Loss of diversity: 25
to 1 species.
Criticisms:
1) Only counted primary space holders
2) Was only done at one location
Pisaster Ochraceous
Top Down: Otter/ Urchin/ Kelp
Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems
(Estes, Tinker, Williams, Doak 1998)
(Estes and Duggins 1995)
•Otters almost hunted to extinction
•During recovery kelp forests recovered
•Due to relaxation of urchin herbivory
•Orcas switch diets and eat otters
•Suddenly otter populations crashed
•Urchin grazing increased
•Kelp forests were denuded and were lost
Only takes a few pods to switch diet
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Top Down: Invasive Rats on Aleutian Islands
Top Down: Invasive Rats on Aleutian Islands
Introduced rats indirectly change marine rocky intertidal communities from algae- to
invertebrate-dominated (Kurle, Croll, Tershy 2008)
Rat Infested
Rat Free
Birds
Limpets/ Snails
Algae
+ +
Bottom Up: Big Blue Big Sur
Why is the ocean blue in Big Sur?
Sessile Inverts
(not eaten by birds)
Food webs
• Food webs:
– More complex than
food chains
– Show interactions
between all species or
functional groups in a
community
(Bruland et al. 1991)
•Upwelling provides nutrients:
• Phosphate, Nitrate, Silicic Acid
•Iron (Fe) is from continental shelf
•Rivers have low sediment loads
•Big Sur is Fe limitated
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Food Webs: Coachella Valley
Food Webs: Who Cares?
(Gary Polis 1991)
Great Sharks
Characterize and Quantify All interactions
Get very complex- very fast!!
Useful for prediction:
Identify Keystone Species
Identify Strong Interactions/ Pathways
Identify Potential Indirect Interactions
Understand Top-Down/ Bottom-Up effects
Understand how communities interact
Smaller sharks and rays
Marine Subsidies of Islands
Marine
(Myers et.al. 2007)
Terrestrial
Food Webs: Ecosystem
Ecosystem--Based Management
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3 consumers
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2 consumers
Grazers
&
Detritivores
Detritus
http://video.google.com/videoplay?docid=2564669773825384723&q=planet+earth+shark&ei=UD9ESIeOBIjS4QLe_PX_CA&hl=en
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Biodiversity
Lecture 16
• Biodiversity – # of species in an area
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Biodiversity
Species-Area relationships
Island Biogeography
SLOSS debate
Latitudinal gradients in biodiversity
• Two different ways to measure biodiversity:
1. Species richness- number of species
2. Species richness and evenness
Richness = 3
Evenness Low
Diversity Low
Biodiversity
• Three different ways to measure species richness:
α (Alpha)
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2.
Diversity- within an ecosystem
Species Richness
Shannon Index- accounts for both richness and evenness.
pi= proportion of individuals for each species
n= number of species
K= constant
β
Richness = 3
Evenness High
Diversity High
Importance of Biodiversity
Importance of Biodiversity
1. Good Measure of Ecosystem Health
-metric for change
2. Ecosystem Stability (species redundancy)
3. Conservation Tool
-hotspots (bang for the buck)
(Beta) Diversity- between ecosystems or along a gradient (proportion
of diversity compared to the average)
γ (Gamma)
Diversity- over an entire region
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Distance will affect dispersal rate of species:
-difficult to survive
-island becomes a small target
-more forces need to act together to make a
successful colonization
Distance lowers local colonization rates
Mainland
P = # species
in source pool
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Extinction Rate
Why do farther islands have less species?
Equilibrium Model of Biogeography
DISTANCE EFFECT
Colonization Rate
Equilibrium Model of Island Biogeography
Species Richness
Species Richness
Equilibrium Model of Biogeography
DISTANCE EFFECT
Mainland
Mainland
Close
Far
Species Richness
P = # species
in source pool
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Close
Rate
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Extinction Rate
P = # species
in source pool
Colonization Rate
Equilibrium Model of Biogeography
DISTANCE EFFECT
Far
Species Richness
Sfar
Sclose
Species Richness
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Patterns of Biodiversity
• Species Area Relationship (One of the few laws in ecology)
A= Island Area
S= Species Richness
Log S= Log c + z log A
S=cAz
c and z= coefficients
Big islands have more species than small islands
Island Biogeography
Two observations about islands:
1) LARGER islands have MORE species
2) CLOSER islands to mainland have MORE species
than distant islands
Why do we see these patterns on islands?
1) Increased habitat heterogeneity
1) Many examples where this is not the case
#
Species
Log
Species
Area
2) Equilibrium Model of Island Biogeography
Log Area
-Include more habitats
-Equilibrium processes (Island Biogeography)
Equilibrium Model of Island Biogeography
Hypotheses: (MacArthur and Wilson 1967):
1) Island diversity represents balance between the
local rates of colonization and extinction
2) Therefore: Island diversity is at equilibrium
deaths
Why do larger islands have more species?
Non-equilibrium Hypotheses:
More habitats – explains some
Can support higher trophic levels – maybe
Equilibrium Hypothesis:
Sustain larger populations therefore:
lowering local extinction rates
Rate
births
Equilibrium Model of Island Biogeography
Stable Equilibrium
Population
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Equilibrium Model of Biogeography
Equilibrium Model of Biogeography
ISLAND SIZE EFFECT
Mainland
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Assumptions:
Permanent mainland source pool of species (P)
All P species have same dispersal capacity
All P species have same chance of going extinct on the island
P = # species
in source pool
Island
Species Richness
Species Richness
Equilibrium Model of Biogeography
ISLAND SIZE EFFECT
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Extinction Rate
P = # species
in source pool
Colonization Rate
Mainland
Equilibrium Model of Biogeography
ISLAND SIZE EFFECT
Mainland
Mainland
Extinction Rate
Colonization Rate
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Species Richness
Small
P = # species
in source pool
Island
Rate
P = # species
in source pool
Large
Species Richness
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Small
Large
Ssmall Slarge
Species Richness
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Distance will affect dispersal rate of species:
-difficult to survive
-island becomes a small target
-more forces need to act together to make a
successful colonization
Distance lowers local colonization rates
Mainland
P = # species
in source pool
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Extinction Rate
Why do farther islands have less species?
Equilibrium Model of Biogeography
DISTANCE EFFECT
Colonization Rate
Equilibrium Model of Island Biogeography
Species Richness
Species Richness
Equilibrium Model of Biogeography
DISTANCE EFFECT
Mainland
Mainland
Close
Far
Species Richness
P = # species
in source pool
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Close
Rate
Island
P = # species in source pool
S = # species on island
I = maximum immigration rate
(of all P species)
Extinction Rate
P = # species
in source pool
Colonization Rate
Equilibrium Model of Biogeography
DISTANCE EFFECT
Far
Species Richness
Sfar
Sclose
Species Richness
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Mainland
Equilibrium Model of Biogeography
DISTANCE and AREA EFFECT
P = # species
in source pool
Patterns of Biodiversity
Island Effects vs. Mainland Area Effects
Mainland shows species/ area relationship, but weaker than islands
Suggests that processes specific to islands are important
colonization
extinction
Mainland dispersal is so high that area doesn’t affect colonization much
Island
Small
Close
HOWEVER- Habitat Fragmentation
makes mainland sites behave like
Islands!!
Gain or
Loss of
Species
Far
Large
Clearcut picture
n
ro
St
Sfar/small
n
tio
ela
gR
ip
sh
Sclose/large
Species Richness
Protecting Diversity: SLOSS?
Protecting Diversity: SLOSS?
SLOSS – single large or several small
SLOSS – single large or several small reserves?
reserves?
Why might large reserves be better?
1.
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Less edge effects
Larger contiguous populations
Protects species with large home-ranges
Supports higher trophic levels
Increased habitat diversity within one reserve
Why might small reserves be better?
MPA
MPA
1. More habitat diversity across reserves
2. Spreads risk of extinction
3. Easier politically
4. Easier enforcement?
MPA
VS.
MPA
MPA
MPA
MPA
MPA
MPA
VS.
MPA
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Latitudinal gradients of biodiversity
Latitudinal gradients of biodiversity
Predatory Birds: Falcons
Observation: Overall species richness declines at higher
latitudes
e.g. Tropics more diverse than tundra
(Rangel et.al. 2004)
Plants
Latitudinal gradients of biodiversity
Predatory Birds: Falcons
Latitudinal gradients of biodiversity: WHY?
Deep Sea Invertebrates
Case study: Bird Diversity
(John Terborgh 80’s)
Comparative study of Bird Diversity:
Manu, Peru vs. North Carolina
(Rangel et.al. 2004)
Plants
Goal: Identify the mechanisms that allow
greater diversity in the tropics
(Rex et.al. 2000)
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Comparing species richness: Peru vs
Carolina (50 ha plots)
Rio Llullapichis, Peru
North Carolina
Results from Manu
1. Extremely high alpha diversity in tropics:
319 bird species on 1 km2 plot
2. Big territories, low densities:
median densities: 2 pairs/km2
many rare species: 84 spp denisities < 1 pair/km2
(compare: lark buntings Colorado — 200 pairs/km2)
3. Comparison to North Carolina (50 ha plots):
207 species versus 40 species (167 more) 5X
USED A GUILD APPROACH- Species using same resources
Carolina
Peru
same guild, more finely
divided (more
specialization)
Results of Comparison using Guild
Approach
Guild Approach:
Diversity is higher in the tropics because:
same guild, broader
resource base
1.
New guilds 33%
2.
More species per guild 50%
Increased diet specialization
Increased habitat partitioning
new guild
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Tropical flycatchers show
extreme morphological & diet
variation
More species/guild
due to diet
specialization
Laughing Falcon —
snake specialist
extreme diet specialization
• only stingless bees
More species/guild due to vertical habitat
partitioning
Hypotheses for Biodiversity Gradients
Difficult question to answer:
Speciation (ecological time)
Maintenance (ecological time)
Examples of Gradients:
Latitudinal Gradient
Topographic Relief
East-West Gradients
Penninsular Lows
Wave exposure Gradients
Depth Gradients
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Hypotheses for Biodiversity Gradient
1) Climate stability over evolutionary/geologic time
Hypotheses for Biodiversity Gradient
4) Competition higher in tropics
Physical factors not as important/limiting as in temperate areas
Competition leads to species diversification
Glaciers had less impact in tropics
Greater speciation rate
Lower extinction rate
5) Predation higher in the tropics
2) Climate stability over ecological time
Low variation in resources allows for greater speciation
Intermediate levels of disturbance promote early and late succession spp.
Predators keep #’s down
Competition less important
No competitive exclusion occurs
6) Patterns of Productivity
3)More habitat heterogeneity
Explains ↑ in β diversity but does not explain ↑ in α diversity
Stable productivity allows for greater specialozation
Pulsed productivity results in lower species richness.
Deep-Sea: local conditions don’t vary much
Surface production is much more variable in north
Summer Volunteers Needed for Lab Work
[email protected]
o
3 consumers
o
2 consumers
Grazers
&
Detritivores
Detritus
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