Chapter 14. Parasitism What’s a parasite? – hard to defineWhat’s a parasite? – hard to define

– Intimate contact (feed off host)Intimate contact (feed off host)– Usually do not kill host (Usually do not kill host (parasitoidsparasitoids do) do)– Herbivores(?)Herbivores(?)– Parasitic PlantsParasitic Plants

HoloparasitesHoloparasites (lack chlorophyll) – (lack chlorophyll) – RafflesiaRafflesia (biggest flower) (biggest flower) HemiparasitesHemiparasites (photosynthesize) – Mistletoe (photosynthesize) – Mistletoe

– MicroparasitesMicroparasites – reproduce inside host – reproduce inside host Bacteria, virusesBacteria, viruses

– MacroparasitesMacroparasites – release juvenile outside – release juvenile outside E.g. trematodesE.g. trematodes

– EctoparasitesEctoparasites vs. vs. endoparasitesendoparasites

“Weird” Parasites

NestNest Parasites Parasites– Brownheaded CowbirdBrownheaded Cowbird– European CuckooEuropean Cuckoo

SexualSexual Parasites Parasites– Gynogenetic fishesGynogenetic fishes

Amazon mollyAmazon molly Resided/Finescale Dace hybridResided/Finescale Dace hybrid

Parasitism Common

Possibly more parasites than anything Possibly more parasites than anything elseelse– 50% of insects parasitic50% of insects parasitic– Potentially 4:1 parasites:free-living formsPotentially 4:1 parasites:free-living forms

Often complex life cyclesOften complex life cycles– E.g. lancet fluke, other trematodesE.g. lancet fluke, other trematodes

Several intermediate hostsSeveral intermediate hosts

Modeling Parasitism Complex because of intermediate hosts, and infection Complex because of intermediate hosts, and infection

raterate– Not usually sensitive to “actual” r for parasite (this is gigantically Not usually sensitive to “actual” r for parasite (this is gigantically

high)high)– Important variables:Important variables:

RRpp – number of infected hosts – number of infected hosts If RIf Rp p > 1 then parasite spreads> 1 then parasite spreads

– For microparasitesFor microparasites RRpp = NBL = NBL N – density of susceptible hostsN – density of susceptible hosts B – transmission rate of parasiteB – transmission rate of parasite L – length of time host is infectiousL – length of time host is infectious

– NNt t (host pop. size) = 1/BL (if R(host pop. size) = 1/BL (if Rpp = 1) = 1) Critical host density (upshot is disease Critical host density (upshot is disease cyclescycles as N as Ntt reached by reached by

recruitment)recruitment)

Effects on natural populations Introduced parasites – large effectIntroduced parasites – large effect

– Chestnut blight, Dutch elm etc.Chestnut blight, Dutch elm etc. Natural systems Natural systems

– Dodder (Dodder (Cuscuta)Cuscuta) – plant parasite – may act to – plant parasite – may act to maintain diversitymaintain diversity

– Fuller and Blaustein – deer miceFuller and Blaustein – deer mice Found infected had lower overwinter survivalFound infected had lower overwinter survival

– Hurtrez-Bousses – microwaved blue tit nestsHurtrez-Bousses – microwaved blue tit nests Found higher size at fledging and lower failure rateFound higher size at fledging and lower failure rate

– Red GrouseRed Grouse

Community Effects

Brainworm – host is white-tailed deerBrainworm – host is white-tailed deer– Not much effectNot much effect– All other cervids and pronghorns All other cervids and pronghorns

susceptiblesusceptible– ““apparent competitionapparent competition” – as white-tailed ” – as white-tailed

deer expand range, other species affecteddeer expand range, other species affected Other examples of effectsOther examples of effects

– Flour beetles, Flour beetles, AnolisAnolis lizards lizards

Biocontrol Some success (about 16%)Some success (about 16%)

E.g. myxoma and rabbits in AustraliaE.g. myxoma and rabbits in Australia Evolution of reduced virulenceEvolution of reduced virulence

– How much of the rest deleterious uncertainHow much of the rest deleterious uncertain– Pesticides degrade in environmentPesticides degrade in environment– Introduced parasites remainIntroduced parasites remain

Switch hosts?? Cause other problems?Switch hosts?? Cause other problems? Some advocate shotgun approachSome advocate shotgun approach Some advocate “targeted” approachSome advocate “targeted” approach

– I think – last-ditch effort (and maybe not even then)I think – last-ditch effort (and maybe not even then)

Mutualism

Both species benefit Both species benefit Plant-pollinatorPlant-pollinator

– Often tightly Often tightly coevolvedcoevolved relationships relationships E.g. figs and fig wasps – 900 species of figs, E.g. figs and fig wasps – 900 species of figs,

each with its own pollinating waspeach with its own pollinating wasp Yucca plants and yucca mothsYucca plants and yucca moths

– Perhaps each trying to “cheat”?Perhaps each trying to “cheat”? Reciprocal parasitism? Reciprocal parasitism?

Seed Dispersal

Fruits attract dispersersFruits attract dispersers– Color, smell, abundance etc.Color, smell, abundance etc.

Hypotheses for seed dispersalHypotheses for seed dispersal– Reduced competitionReduced competition– Colonization hypothesis Colonization hypothesis – Directed dispersal hypothesis (ants)Directed dispersal hypothesis (ants)– Predator escape hypothesisPredator escape hypothesis

Variety of Mutualisms ResourcesResources

– Leaf cutting ants/fungusLeaf cutting ants/fungus– Nitrogen fixing bacteria / plantsNitrogen fixing bacteria / plants

ProtectionProtection – Cleaner fish and “customers”Cleaner fish and “customers”

Some are mimics (cheaters)Some are mimics (cheaters)– Ants and aphidsAnts and aphids– Ants and acacia trees (herbivory)Ants and acacia trees (herbivory)

Obligate mutualismsObligate mutualisms– Lichens (algae and fungus)Lichens (algae and fungus)– Ruminants/bacteriaRuminants/bacteria– Deep sea fishes/luminescent bacteriaDeep sea fishes/luminescent bacteria– Corals/zooxanthellaeCorals/zooxanthellae– Endosymbiont theoryEndosymbiont theory

Modeling Mutualism Similar to Lotka-Volterra comp. eqns.Similar to Lotka-Volterra comp. eqns.

– Replace negative effect with positiveReplace negative effect with positive– Change K to X (carrying capacity is raised)Change K to X (carrying capacity is raised)

Can become weird (unstable) or can Can become weird (unstable) or can become stable when facultativebecome stable when facultative

Obligate mutualisms even more Obligate mutualisms even more unstable (though obviously there are unstable (though obviously there are stable areas)stable areas)

Indirect effects on community

Mycorrhizal fungi / plantsMycorrhizal fungi / plants– Reduce herbivory Reduce herbivory

Increased vigorIncreased vigor Increased antiherbivore defensesIncreased antiherbivore defenses

– Increased mycorrhizal diversity can be Increased mycorrhizal diversity can be positive for communitypositive for community

– Or…introduced mutualists can out-Or…introduced mutualists can out-compete (endophytes in Indiana)compete (endophytes in Indiana)

Commensalisms

Cattle egrets/cattleCattle egrets/cattle Clinging seeds and hostsClinging seeds and hosts Flower mites and hummingbird nostrilsFlower mites and hummingbird nostrils