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Bio205H5: lecture 11 mutualism & community structure mutualism: interactions between species that benefit both parties mutualism ≠ altruism mutualism = kind of “reciprocal parasitism”: each obtains what is needed from mutualist at lowest cost to itself related terms: types of benefits: • transportation: transport gametes or individuals • nutrition: exchange of nutrients • protection: attack or remove predators, competitors, parasites (may be a combination) commensalism commensalism: one species receives benefit; other not affected symbiosis: long-term, intimate association may include • parasitism (+ / -) • commensalism (+ / 0) • mutualism (+ / +) • one species benefits; other is unaffected • e.g. human bot fly (Dermatobia hominis) & mosquitos: myiasis: larval stages of flies inhabiting humans or animals • bot fly: large insect (10-15 mm); loud • unlikely to effectively deposit eggs on host 1. diffuse mutualism: interactions among many species e.g. most mycorrhizae; pollinators and seed dispersers 2. direct mutualism: • facultative mutualism: species can survive without mutualistic partner • obligate mutualism: species’ survival depends on mutualistic partner coevolution - reciprocal evolutionary responses between species • bot fly captures mosquito; transfers eggs • eggs hatch when mosquito alights • phoresis: one species uses another only as means of transportation 1 diffuse mutualism • lichen: mutualistic relationship between fungi and alga or cyanobacteria Rhizobium and legumes: • not always mutualistic! • in habitats where lichen occur, neither partner can exist alone • photosynthetic part provides carbohydrates; fungal hyphae protect from water loss • fungi may be parasitic (kill algal cells) mycorrhizae: • cause legumes to form nodules on roots • nodules harbor and protect bacteria; bacteria fix nitrogen arbuscular mycorrhizae: penetrate root cortex cells ectomycorrhizae: surround root cortex cells provide plant with nitrogen & phosphorous; receive carbon seed dispersers: • may eat fruit; seed passed unharmed • eat some, but not all seeds • heavy seeds must be transported to prevent competition with parent plant 2 direct mutualism: tightly-coupled relationship • e.g. bottle gentian • flower stays closed • nectary can only be accessed by large bumblebees • faithful pollinators; reliable source of nectar evolution of mutualism: 1. many evolve out of initially antagonistic pair-wise interactions e.g. yucca moths: evolution of mutualism: • moth deposits eggs in ovary, but does not fertilize plant • result: few seeds develop, little food for moth larvae • moth deposits eggs in ovary; delivers “pollen ball” • result: many seeds develop; ample food for moth larvae; plant successful • carrion beetles bring mites to source of fly eggs • mites consume competitor’s eggs 2. mutualism may involve 3 or more species, including an antagonistic pair: a) carrion beetles, phoretic mites & bluebottle flies b) oropendulas, cowbirds, bees & botflies c) ants, treehoppers & spiders • cowbirds are nest parasites • tolerated when nests far from bees & wasps • without bees, botfly infestation common • cowbird chick eats botfly larvae 3 • ants harvest honeydew from treehoppers • do ants protect treehoppers from jumping spiders? if mutualism is beneficial to both, shouldn’t it always evolve? • not always – it may be profitable to cheat! results depend on density of jumping spiders: when density low, treehoppers do not benefit vs. mutualism → parasitism interactions between species are fluid! evolution of mutualism from parasitism: 1. indirect transmission: parasite attacks host; leaves (success of host & parasite separate) nectar robbers deceptive flowers 2. direct transmission: parasite’s offspring stay on host (fate of host affects fate of parasite) 4 1. indirect transmission: if parasite “cooperates”, another parasite may attack and damage host • parasite not assured of benefit; better to be antagonistic 2. direct transmission: better for parasite to cooperate (fate of offspring depends on fate of host) • cooperation of parasite → cooperation of host • mutualism will evolve if mutualists have greater fitness than non-mutualists • under what conditions does it benefit the sunflower to produce extra-floral nectaries? modeling the costs and benefits of mutualism: • e.g. facultative mutualism between ants and sunflowers: extra-floral nectaries feed ants; ants protect from seed predators let: H = amount of plant tissue damage without any defenses (measure of threat of seed predation) D = amount of protection plant has without ants (e.g. chemical defenses) A = amount of protection offered by ants IA = investment in extrafloral nectaries ID = investment in other defenses 5 mutualistic plants will evolve if: p ⎡⎣ H (1 − D ) A⎤⎦ > I A help provided by ants risk of damage proportion of pop that attract enough ants to mount a defense risk of damage without ants cost of being a mutualist community structure • communities contain assemblages of interacting species • affected by abiotic properties • are communities highly structured, or are they loose assemblages of species? evolution of mutualism: • mutualism often evolves from other types of interactions • involves various types of benefits • may be facultative or obligate; may be direct or diffuse • may involve several species including an antagonist • evolution depends on relative costs & benefits (may vary in space and time) 2 views: Clements: communities are stable, integrated and orderly • communities pass though predictable seral stages • culminates in climax community • nature of climax community set by climate; does not change Gleason: communities are neither stable nor predictable • plant and animal communities are ephemeral • abiotic factors and history determine what species are found in a community (which colonists first arrive, etc.) 6 conclusion: • total of 61 species found in all ponds • each pond had only 31 – 39 species • some species are good dispersers and colonists; found in all ponds • arrival of certain predators or competitors early in process affects what can establish after • both hypotheses partially supported interactions in food webs strong and weak interactions: • feeding activity of a few species has a dominant influence on community structure 7 • at least 3 species of gallfly • at least 14 species of parasitoid wasp attack gallflies • how do they coexist? do consumers increase or decrease diversity of prey species? prefers Enteromorpha Littorina intertidal snail eats Chondrus if nothing else available keystone species: low density: Enteromorpha crowds out other algae medium density: prevents exclusion; increases diversity high density: intense grazing reduces diversity • species that has a great impact on the structuring of a community • e.g. bison: small fraction of total mass of grassland species • maintain diversity of forbs (herbaceous flowering plants) by grazing down grasses 8 next lecture: • trophic interactions, disturbance & succession • wrapping it all up! • review of midterm; Q&A after next week’s lecture 9