figurski lecture- food web ecology

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Lecture 15Understanding the Structure and Function of

Communities

• Food chains• Primary Production and Energy Flux• HSS – Green World Hypothesis• Fretwell Hypothesis• Top Down vs. Bottom Up• Examples• Food Webs• Ecosystem-based Management

Community Structure: Food Chains

• 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.

Detritivores

Grazers&

2 consumers

3 consumerso

o

Detritus

Terrestrial Marine/ Aquatic

Litter

Detritivores

Grazers&

2 consumers

3 consumerso

o

Net Primary Production

General Patterns of Food Chains

Elton’s Pyramid (1940’s)

1) Differences across trophic levels

1) MORE animals LOWER on the food chain2) MORE biomass sequestered in LOWER food chain3) HIGHER turnover and Smaller animals LOWER in food chain

One of the founders of ecology.

Help

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General Patterns of Food ChainsLindeman (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

75-99% of energy is lost between trophic levels

Community structure: the number of species and their relative abundance in a community

• 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: Net Primary Production

HSS (Hairston, Smith, Slobodkin 1960)

Why is the world green?? Not productivity.

Top down Control (Trophic Cascade) 1) Nutrients are seldom limiting for plants.2) Herbivores are potentially strong

regulators of plant populations.BUT3) Predators keep herbivore densities

low and allow plants to proliferate

Problems with HSS1) Often not supported experimentally2) Many plants are defended chemically and structurally

(cactus spines, lignins, kelp secondary compounds. World is green because it green isn’t always edible….

Community Structure: Green World Hypothesis(Fretwell 1977) (Oksanen 1981)• Explains deviations from HSS by

considering number of trophic levels.1) Odd numbered trophic levels= Green2) Even number = Brown

Trop

hic

Leve

l

1

2

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5HSS

Community Structure: Fretwell-Oksanen

Possible Food ChainsA B E

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Top Down (Trophic Cascades): Predators controls herbivoresConsequently, herbivores have little effectPlants are abundantPrimary Production is not limiting (not regulating)

Predictions:Removal of predators should release herbivores, which will then decrease plants

Top Down vs. Bottom Up

Bottom-up model (Productivity) :Nutrients limit plant abundancePrimary Production is limitingPlants limit herbivores, herbivores limit carnivores, etc.

Predictions:Removal of top levels=little effect on lower levelsBut if you remove lower trophic levels, big effect on upper trophic levels

Keystone Species : A species that has a disproportionate effect on the community (Paine 1969)

•Keystone species often:1) Control dominant competitor (Mussels)2) Control dominant herbivore (Urchin)2) Modify habitat (Bears, Elephants in Serengeti)

•Removal of few results in massive change to the community structure•Important to ecosystem integrity

Top Down Control

1) Starfish and Mussel

2) Alaskan Sea Otter/ Urchin/ Kelp

3) Aleutian Islands Invasive Rat study

Examples

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 Ochraceous

Pisaster- Voracious predatorMytilus- Space competitor

Experiment: Remove Pisaster

Results: Loss of diversity: 25 to 1 species.

Criticisms: 1) Only counted primary space holders2) Was only done at one location

Top Down: Otter/ Urchin/ KelpKiller 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 IslandsIntroduced rats indirectly change marine rocky intertidal communities from algae- to invertebrate-dominated (Kurle, Croll, Tershy 2008)

Top Down: Invasive Rats on Aleutian Islands

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++

Birds

Rat InfestedRat Free

Limpets/ Snails

Sessile Inverts(not eaten by birds)

Algae

Bottom Up: Big Blue Big SurWhy is the ocean blue in Big Sur?

•Upwelling provides nutrients:• Phosphate, Nitrate, Silicic Acid

•Iron (Fe) is from continental shelf•Rivers have low sediment loads•Big Sur is Fe limitated

(Bruland et al. 1991)

Food webs• Food webs:

– More complex than food chains

– Show interactions between all species or functional groups in a community

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Food Webs: Coachella Valley

(Gary Polis 1991)

Get very complex- very fast!!

Marine Terrestrial

Characterize and Quantify All interactions

Useful for prediction:Identify Keystone SpeciesIdentify Strong Interactions/ PathwaysIdentify Potential Indirect InteractionsUnderstand Top-Down/ Bottom-Up effectsUnderstand how communities interact

Marine Subsidies of Islands

(Myers et.al. 2007)

Food Webs: Who Cares?

Great Sharks

Smaller sharks and rays

Food Webs: EcosystemFood Webs: EcosystemFood Webs: Ecosystem---Based ManagementBased ManagementBased Management

Detritivores

Grazers&

2 consumers

3 consumerso

o

Detritus

http://video.google.com/videoplay?docid=2564669773825384723&q=planet+earth+shark&ei=UD9ESIeOBIjS4QLe_PX_CA&hl=en

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Lecture 16• Biodiversity• Species-Area relationships• Island Biogeography• SLOSS debate• Latitudinal gradients in biodiversity

• Biodiversity – # of species in an area

• Two different ways to measure biodiversity:

1. Species richness- number of species

2. Species richness and evenness

Biodiversity

Richness = 3Evenness LowDiversity Low

Richness = 3Evenness HighDiversity High

• Three different ways to measure species richness:

α (Alpha) Diversity- within an ecosystem1. Species Richness2. Shannon Index- accounts for both richness and evenness.

β (Beta) Diversity- between ecosystems or along a gradient (proportion of diversity compared to the average)

γ (Gamma) Diversity- over an entire region

Biodiversity

pi= proportion of individuals for each speciesn= number of speciesK= constant

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)

Importance of Biodiversity

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Why do farther islands have less species?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

Equilibrium Model of Island Biogeography

Distance lowers local colonization rates

Mainland

P = # species in source pool

Island

Col

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n R

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Ext

inct

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Rat

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P = # species in source poolS = # species on islandI = maximum immigration rate

(of all P species)

Species Richness Species Richness

Equilibrium Model of BiogeographyDISTANCE EFFECT

Mainland


Island

Col

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n R

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Ext

inct

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Rat

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(of all P species)



Far

Close

Mainland


Island

Rat

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(of all P species)

Species Richness


Far

Close

Sfar Sclose

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• Species Area Relationship (One of the few laws in ecology)

A= Island AreaS= Species Richnessc and z= coefficients

Log Species

Log Area

# Species

Area Log Area

# Species

Area

Patterns of Biodiversity

S=cAz

Big islands have more species than small islands

-Include more habitats-Equilibrium processes (Island Biogeography)

Log S= Log c + z log A

Two observations about islands:1) LARGER islands have MORE species2) 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

2) Equilibrium Model of Island Biogeography

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


Rat

e

Population

births deaths

Stable Equilibrium

Why do larger islands have more species?Non-equilibrium Hypotheses:

More habitats – explains someCan support higher trophic levels – maybe

Equilibrium Hypothesis:Sustain larger populations therefore:

lowering local extinction rates


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Mainland


IslandP = # species in source poolS = # species on islandI = 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

Equilibrium Model of Biogeography

Mainland


Island

Col

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Ext

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Rat

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(of all P species)


Equilibrium Model of BiogeographyISLAND SIZE EFFECT

Mainland


Island

Col

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n R

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Ext

inct

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Rat

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(of all P species)


Large

Small


Mainland


Island

Rat

e


(of all P species)

Species Richness

Large

Small

Ssmall Slarge


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Why do farther islands have less species?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


Island

Col

oniz

atio

n R

ate

Ext

inct

ion

Rat

e


(of all P species)



Mainland


Island

Col

oniz

atio

n R

ate

Ext

inct

ion

Rat

e


(of all P species)



Far

Close

Mainland


Island

Rat

e


(of all P species)

Species Richness


Far

Close

Sfar Sclose

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Mainland

P = # species in source pool Island

Gain or Loss of Species

Far

Close

Species Richness

Large

Small

Equilibrium Model of BiogeographyDISTANCE and AREA EFFECT

Sfar/small Sclose/large

Island Effects vs. Mainland Area Effects

Strong Relationship

Patterns of Biodiversity

Mainland shows species/ area relationship, but weaker than islandsSuggests that processes specific to islands are important

colonizationextinction

Mainland dispersal is so high that area doesn’t affect colonization much

HOWEVER- Habitat Fragmentationmakes mainland sites behave likeIslands!!

Clearcut picture

SLOSS – single large or several small reserves?

Why might large reserves be better?1. Less edge effects2. Larger contiguous populations3. Protects species with large home-ranges4. Supports higher trophic levels5. Increased habitat diversity within one reserve

Protecting Diversity: SLOSS?

VS.MPA

MPA

MPA

MPA

MPA

Protecting Diversity: SLOSS?

VS.MPA

MPA

MPA

MPA

MPA

SLOSS – single large or several small reserves?

Why might small reserves be better?1. More habitat diversity across reserves2. Spreads risk of extinction3. Easier politically4. Easier enforcement?

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Latitudinal gradients of biodiversity

Observation: Overall species richness declines at higher latitudese.g. Tropics more diverse than tundra

(Rangel et.al. 2004)

Latitudinal gradients of biodiversityPredatory Birds: Falcons

Plants

(Rangel et.al. 2004)

Latitudinal gradients of biodiversityPredatory Birds: Falcons

Plants

(Rex et.al. 2000)

Deep Sea InvertebratesCase study: Bird Diversity(John Terborgh 80’s)

Latitudinal gradients of biodiversity: WHY?

Comparative study of Bird Diversity:Manu, Peru vs. North Carolina

Goal: Identify the mechanisms that allow greater diversity in the tropics

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Comparing species richness: Peru vsCarolina (50 ha plots)

Rio Llullapichis, Peru North Carolina

Results from Manu1. 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

new guild

same guild, more finely divided (more specialization)

same guild, broader resource base

CarolinaPeru

Guild Approach:Diversity is higher in the tropics because:

1. New guilds 33%

2. More species per guild 50%Increased diet specializationIncreased habitat partitioning

Results of Comparison using Guild Approach

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extreme diet specialization• only stingless bees

Tropical flycatchers showextreme morphological & diet

variation More species/guild due to diet specialization

Laughing Falcon —snake specialist

More species/guild due to vertical habitat partitioning Hypotheses for Biodiversity Gradients

Examples of Gradients:Latitudinal GradientTopographic ReliefEast-West GradientsPenninsular Lows Wave exposure GradientsDepth Gradients

Difficult question to answer:Speciation (ecological time)

Maintenance (ecological time)

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1) Climate stability over evolutionary/geologic time Glaciers had less impact in tropicsGreater speciation rate Lower extinction rate

2) Climate stability over ecological time Low variation in resources allows for greater speciation Intermediate levels of disturbance promote early and late succession spp.

3)More habitat heterogeneityExplains ↑ in β diversity but does not explain ↑ in α diversity

Hypotheses for Biodiversity Gradient Hypotheses for Biodiversity Gradient

4) Competition higher in tropics Physical factors not as important/limiting as in temperate areasCompetition leads to species diversification

5) Predation higher in the tropicsPredators keep #’s down

Competition less importantNo competitive exclusion occurs

6) Patterns of ProductivityStable productivity allows for greater specialozationPulsed productivity results in lower species richness.

Deep-Sea: local conditions don’t vary muchSurface production is much more variable in north

Detritivores

Grazers&

2 consumers

3 consumerso

o

Detritus

Summer Volunteers Needed for Lab WorkSummer Volunteers Needed for Lab WorkSummer Volunteers Needed for Lab [email protected]

figurski lecture- food web ecology

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