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Chapter 14. Parasitism What’s a parasite? – hard to define – – – – Intimate contact (feed off host) Usually do not kill host (parasitoids do) Herbivores(?) Parasitic Plants Holoparasites (lack chlorophyll) – Rafflesia (biggest flower) Hemiparasites (photosynthesize) – Mistletoe – Microparasites – reproduce inside host Bacteria, viruses – Macroparasites – release juvenile outside E.g. trematodes – Ectoparasites vs. endoparasites “Weird” Parasites Nest Parasites – Brownheaded Cowbird – European Cuckoo Sexual Parasites – Gynogenetic fishes Amazon molly Resided/Finescale Dace hybrid Parasitism Common Possibly more parasites than anything else – 50% of insects parasitic – Potentially 4:1 parasites:free-living forms Often complex life cycles – E.g. lancet fluke, other trematodes Several intermediate hosts Modeling Parasitism Complex because of intermediate hosts, and infection rate – Not usually sensitive to “actual” r for parasite (this is gigantically high) – Important variables: Rp – number of infected hosts If Rp > 1 then parasite spreads – For microparasites Rp = NBL N – density of susceptible hosts B – transmission rate of parasite L – length of time host is infectious – Nt (host pop. size) = 1/BL (if Rp = 1) Critical host density (upshot is disease cycles as Nt reached by recruitment) Effects on natural populations Introduced parasites – large effect – Chestnut blight, Dutch elm etc. Natural systems – Dodder (Cuscuta) – plant parasite – may act to maintain diversity – Fuller and Blaustein – deer mice Found infected had lower overwinter survival – Hurtrez-Bousses – microwaved blue tit nests Found higher size at fledging and lower failure rate – Red Grouse Community Effects Brainworm – host is white-tailed deer – Not much effect – All other cervids and pronghorns susceptible – “apparent competition” – as white-tailed deer expand range, other species affected Other examples of effects – Flour beetles, Anolis lizards Biocontrol Some success (about 16%) E.g. myxoma and rabbits in Australia Evolution of reduced virulence – How much of the rest deleterious uncertain – Pesticides degrade in environment – Introduced parasites remain Switch hosts?? Cause other problems? Some advocate shotgun approach Some advocate “targeted” approach – I think – last-ditch effort (and maybe not even then) Mutualism Both species benefit Plant-pollinator – Often tightly coevolved relationships E.g. figs and fig wasps – 900 species of figs, each with its own pollinating wasp Yucca plants and yucca moths – Perhaps each trying to “cheat”? Reciprocal parasitism? Seed Dispersal Fruits attract dispersers – Color, smell, abundance etc. Hypotheses for seed dispersal – Reduced competition – Colonization hypothesis – Directed dispersal hypothesis (ants) – Predator escape hypothesis Variety of Mutualisms Resources – Leaf cutting ants/fungus – Nitrogen fixing bacteria / plants Protection – Cleaner fish and “customers” Some are mimics (cheaters) – Ants and aphids – Ants and acacia trees (herbivory) Obligate mutualisms – – – – – Lichens (algae and fungus) Ruminants/bacteria Deep sea fishes/luminescent bacteria Corals/zooxanthellae Endosymbiont theory Modeling Mutualism Similar to Lotka-Volterra comp. eqns. – Replace negative effect with positive – Change K to X (carrying capacity is raised) Can become weird (unstable) or can become stable when facultative Obligate mutualisms even more unstable (though obviously there are stable areas) Indirect effects on community Mycorrhizal fungi / plants – Reduce herbivory Increased vigor Increased antiherbivore defenses – Increased mycorrhizal diversity can be positive for community – Or…introduced mutualists can outcompete (endophytes in Indiana) Commensalisms Cattle egrets/cattle Clinging seeds and hosts Flower mites and hummingbird nostrils