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Lecture 15 Understanding the Structure and Function of Community Structure: Food Chains 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 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. o o 3 consumers 3 consumers 1) Differences across trophic levels o 2 consumers Help o 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 1 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 5 4 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 3 2 1 A B Possible Food Chains E 2 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 3 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 4 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 o 3 consumers o 2 consumers Grazers & Detritivores Detritus http://video.google.com/videoplay?docid=2564669773825384723&q=planet+earth+shark&ei=UD9ESIeOBIjS4QLe_PX_CA&hl=en 5 Biodiversity Lecture 16 • Biodiversity – # of species in an area • • • • • 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) 1. 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 1 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 4 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 2 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 3 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 4 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. 2. 3. 4. 5. 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 5 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) 6 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 7 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 9 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 10