Chapter 14 Parasitism n Whats a parasite hard

Chapter 14. Parasitism n What’s a parasite? – hard to define – – Intimate contact (feed off host) Usually do not kill host (parasitoids do) Herbivores(? ) Parasitic Plants n n Holoparasites (lack chlorophyll) – Rafflesia (biggest flower) Hemiparasites (photosynthesize) – Mistletoe – Microparasites – reproduce inside host n Bacteria, viruses – Macroparasites – release juvenile outside n E. g. trematodes – Ectoparasites vs. endoparasites

“Weird” Parasites n Nest Parasites – Brownheaded Cowbird – European Cuckoo n Sexual Parasites – Gynogenetic fishes Amazon molly n Resided/Finescale Dace hybrid n

Parasitism Common n Possibly more parasites than anything else – 50% of insects parasitic – Potentially 4: 1 parasites: free-living forms n Often complex life cycles – E. g. lancet fluke, other trematodes n Several intermediate hosts

Modeling Parasitism n Complex because of intermediate hosts, and infection rate – Not usually sensitive to “actual” r for parasite (this is gigantically high) – Important variables: n n Rp – number of infected hosts If Rp > 1 then parasite spreads – For microparasites n n 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) n Critical host density (upshot is disease cycles as Nt reached by recruitment)

Effects on natural populations n Introduced parasites – large effect – Chestnut blight, Dutch elm etc. n Natural systems – Dodder (Cuscuta) – plant parasite – may act to maintain diversity – Fuller and Blaustein – deer mice n Found infected had lower overwinter survival – Hurtrez-Bousses – microwaved blue tit nests n Found higher size at fledging and lower failure rate – Red Grouse

Community Effects n 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 n Other examples of effects – Flour beetles, Anolis lizards

Biocontrol n Some success (about 16%) n n 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 n 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 n Plant-pollinator n – Often tightly coevolved relationships E. g. figs and fig wasps – 900 species of figs, each with its own pollinating wasp n Yucca plants and yucca moths n – Perhaps each trying to “cheat”? n Reciprocal parasitism?

Seed Dispersal n Fruits attract dispersers – Color, smell, abundance etc. n Hypotheses for seed dispersal – Reduced competition – Colonization hypothesis – Directed dispersal hypothesis (ants) – Predator escape hypothesis

Variety of Mutualisms n Resources – Leaf cutting ants/fungus – Nitrogen fixing bacteria / plants n Protection – Cleaner fish and “customers” n Some are mimics (cheaters) – Ants and aphids – Ants and acacia trees (herbivory) n Obligate mutualisms – – – Lichens (algae and fungus) Ruminants/bacteria Deep sea fishes/luminescent bacteria Corals/zooxanthellae Endosymbiont theory

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

Indirect effects on community n Mycorrhizal fungi / plants – Reduce herbivory Increased vigor n Increased antiherbivore defenses n – Increased mycorrhizal diversity can be positive for community – Or…introduced mutualists can out-compete (endophytes in Indiana)

Commensalisms Cattle egrets/cattle n Clinging seeds and hosts n Flower mites and hummingbird nostrils n