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Berkeley BIOLOGY 1B - Power 8 FoodWebsChains

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1Mutualism (+,+) leaf cutter ants (Atta spp)culture fungi (Ignacio Chapela, ESPM)Mark Moffett photos2MinorsdefendworkersfromparasiticphoridfliesHerbivory/fungivory: leaf cutter ants3Fungal garden inundergroundchamber of antcolonyYoung queen carries bitof fungus to start newcolony4Mutualisms (+,+)5Extrafloralnectarieson Acaciafeed antsthatprotect theAcaciatrees fromherbivoresand vines6Clown fish live in mutualisticsymbiosis with sea anemone• Clown fishmucusprotects itfromanemonesting, andclown fishfeedingimportscrumbs thatfeed anemone73 species interactions: Mutualists and cheaters…. Cleaner wrasses, host fish, cheaterblenniesMilkweed, bee, cheater orchidPollinator, moth, bumblebee nectarburglarSelection for refinement of cuesand mimicry in co-evolutionary raceHow much cheating can be toleratedbefore mutualism breaks down?8Taxon2 speciesLump allorganisms into‘trophic levels’Focus on particular species whosestrong interactions limitimportant populations9• Community: a group ofspecies that co-occur intime and space• Food (interaction) websdepict feeding (or othersignificant (e.g. populationlimiting or regulating))relationships amongmembers of a communityInteraction webs,food webs, food chainsSome of these interactionsare strong, others are weak.System: group of entitiesunited by interaction orinterdependence to formor act as an entire unitHow to simplify for study?10Sometimes strong effects are mediated through food chains.Hairston, Slobodkin and Smith (1960): “The world is greenbecause predators hold herbivores in check.” (assumes a 3level food chain)+++--11Fretwell (Kansas): world might havemore or fewer than 3 trophic levels:Odd numbers !Green Even numbers !BarrenLevels: 1 2 3 412• Trophic level: functional grouping of organisms according to their primary food source– Bottom up level: number of energy transfers from fixation of organic carbon to reachlevel– Top down level: number of lower levels that are alternatively released and suppressed when this level is removed (plus one).Odd number oflevels: green (plantsresource limited =“bottom up”limitation)Even number oflevels: barren(plants consumerlimited = “top down”limitation)Need to test which factoris limiting to understandfood web controls onpopulation abundance….13Indirect interactions:food webs, food chains,trophic cascadesMy enemy’senemy is myfriend--“-” x “-” = “+”++--141. Green stuff could be inedible(world is one trophic level)2. Consumers are co-limited bypredators and food (or otherfactors)3. Omnivory blurs trophic levels4. Factors other than consumers orfood limit populationsWhy food chain theory shouldn’t work:15Focus on Keystone Species whose stronginteractions limit potentially Dominant populations16Species traits and interaction strengthR.T. PaineNot all species are createdequal…Pisaster17Importance of top-down forcing18without Pisaster: mussel‘monoculture’With Pisaster: diverse intertidal19LE 53-16Without Pisaster (experimental)With Pisaster (control)1963’64 ’65’66’67’68 ’69’70’71’72’7320151050Number of speciespresent20Paine 1969 “Keystone Species”• Keystone species: aspecies thatconsumes and limitsthe population ofanother speciesthat wouldotherwise dominatethe system• Dominant species21Keystone specieshave effects thatare strong, andstronger thanwould beexpected fromtheir abundancesPower et al. 1996PisasterSea otterDistempervirusRhinovirusCorals, musselskelp, grass, treesKeystonesDominants22Sea Otter23Strong interactors that triggertrophic cascades: Sea otters (JimEstes et al.)• Can forage down to 60 m• Voracious-eat 20-25% body weight per day• Prey on sea urchins, voracious grazers that canextirpate kelp• Kelp is a potential dominant, structuring immediatesubtidal and adjacent intertidal ecosystems24Urchins as strong interactors• Can graze and detach kelp, boringthrough basal stipe, exporting hugeamounts• Collectively, urchins can wrestlekelp and weight it down• Can persist/subsist on uptake ofdissolved organic matter, orgrazing microalgae, so can persistwithout large kelp, graze all newsporelings, and maintain “urchinbarrens”25Kelp as strong interactor• Prodigious growthrates and huge biomass• Refuge for fin fish• Wave break, decreaseserosion in intertidal,causing sedimentdeposition• Supplies intertidal withdetritus and allowsmudflats to bedeposited (Duggins)262 vs 3-level food chains: Aleutian Islands, SE AlaskaUrchinBarrens(2trophiclevels)Otterstate(3 trophiclevels)27Sea otters present28Sea otters absent29Evidence (Estes and Palmisano 1974)• Rat Island (otters)- Complete cover of offshore kelp– Rare urchins– Lots of fin fish, harbor seals, bald eagles•Near Island (no otters)- Urchin barrens covered in mussels and barnacles–Dense large mussels–Few fish, harbor seals, or birds30Killer whale link (Estes et al. 1998, Campbell Fig. 53.15)• 27 years of monitoring the northeastern Atlantic revealedprecipitous sea otter decline in 1990s throughoutarchipelago off western Alaska, down to 10% of 1980spopulations• Hypotheses:– Toxins (PCBs)– Disease– Starvation– Climate stress– Predation• 1st killer whale attack on otter observed in 1991. Computedprobability that if rate of whale predation constant over27 y of census, probability of not observed before 1990 =0.006.• Ecosystem responses—urchins increasing 8x and kelpdecreasing 10x over 12 y where otters missing• Clam Lagoon (otter refuge too shallow for whales to enter)otters still present, no other food web or ecosystemchanges (urchins rare and small, kelp communities in tact).31Estes, Springer hypothesis forNorthern Pacific food web:birdsfishsealionskiller whalesottersurchinskelpHumanfishing, whalingGreat whales32CetaceansPinnipeds123HistoricalCurrentBiomass (tones X 10 6)Biomass estimates for great whales (blue) and seals (red) beforeand after their recent declines in the Aleutian Islands and BeringSea region. Springer et al. 2003.WhalesSealsBiomass (millions of tons)33Estes, Springer hypothesis forNorthern Pacific food web:birdsfishsealionskiller whalesottersurchinskelpHuman industrialfishing, whalingGreat whales34Geography of reported whale harvests (all species) in theNorth Pacific Ocean and Bering Sea from 1946 through1976. Springer et al. 2003.35Year1950 1960 1970 1980 1990 2000Proportion of maximum0.00.20.40.60.81.0HarborSealsSteller Sea


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Berkeley BIOLOGY 1B - Power 8 FoodWebsChains

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