Ch 13 LO's- Parasitism

Parasitism

Define parasitism.

• Relationship where 1 organism (the parasite) lives on/in another organism (the host) and benefits at the host’s expense

Differentiate between micro- and macroparasites, ecto- and endoparasites. Give examples of each

• Microparasites: small (viruses, bacteria, protists)

• Macroparasites: larger, visible to eye (flatworms, flukes, roundworks, lice, fleas, ticks) 

• Ectoparasites: Live on outside of host 

  • fleas, ticks, lice

• Endoparasites: live inside host, under skin in blood or organs

  • tapeworks, roundworms

Describe the trade-offs faced by endoparasites vs. ectoparasites

• Endoparasites

  • protected from envt 

  • constant food supply from host tissues/fluids 

  • BUT must deal with host’s immune system, harder to spread to new hosts 

• Ectoparasites 

  • easier to disperse to new habitats (can jump/crawl)

  • Avoid hosts’ immune system 

  • BUT eposed to envt (temp changes) 

  • Risk of being removed by host

  • Must stay attached

Differentiate between the different ways that hosts can defend themselves from parasites. Explain briefly what mechanisms endo vs. ectoparasites overcome these defenses

• Physical defense→ skin 

• ectoparasites cut through it (mosquitoes/lice) 

• Immune system defenses 

  • Endoparasites can supress it/evade or disguise it 

• Self medication (animals consume plants that contain chems to treat their infection)

  • endoparasites can become tolerant 

Define two mechanisms of parasite transmission.

• direct transmission→ when a parasite moves directly from 1 host to another without intermediate organism

  • physical contact, or ingestion like lice 

• Vector transmission→ when a parasite is trasmitted bw hosts by an intermediate vector (often invertebrate) 

  • ex: malaria has plasmodium carried by mosquiotes 

Define virulence. What evolutionary trade-offs do parasites face between virulence and transmission. Why might pathogens become less virulent over time

• virulence is how strong an impact a parasite has on a host. 

  • Usually in reference to endoparasites. High virlence is deadly.  

• Trade-off b/w viruelnce and transmission

  • If a parasite is too virulent it can kill the host too quickly, decrease the chance to spread

  • If it has low virulence host may live longer but it slows parasite population growth

• Pathogens become less virulent over time because.. 

  • hosts evolve better defenses and parasites evolve to cause less harm so they can cause survive longer inside

  • Selection favors parasites that keep their hosts alive and infectious for longer

Explain how parasites can impact host populations? Why might this be density dependent and lead to population cycles

• drive local hosts extinct 

• decrease geographic ranges

  • ex: chestnut blight

• At higher densities there are more parasite outbreaks so can lead to cycles where host population rises and falls in response to parasite pressure. When low densities fewer infections, and pop rebounds

Describe the general structure of an SIR model of host-parasite dynamics. Explain generally under what conditions the density of infected (I) individuals will increase. Translate this to impacts on parasite evolution and strategies humans use to reduce infection rates.

• S→ I→ R or → death 

• S= susceptible, I= infected, R= recovered/immune

• B= transmission coefficient

• SI= probabilty of susceptibl and incected encountering each other

• m= death + recovery rate

  • when m low parasites more likely to spread

Conditions where the density of I increases

• When new infections > recoveries/deaths

  • BSI-mI > 0

Impacts on parasite evolution

• Parasites evolve to maximme transmission

• To decrease infection rates humans wash hands, wear masks to decrease B

• To decrease SxI→ decrease encounters bw S+ I (physical distancing)

• Decrease S→ vaccination 

Generally explain how parasites can modify communities by (a) changing species interactions, and(b) altering community composition.

a) changing species interactions 

• parasites can change competition, predation, or mutualism among species

• ex: infection weakens a dominant competitor allows weaker species to dominant

• Parasites reduce prey health→ make them easier for preds to catch

  • or opposite make predators less able to catch food 

b) alter community composition 

• Parasites can shift which species are present or abundant in a community

  • ex: parasite attacks a dominant species allows other species to increase

  • ex: fungal outbreaks decrease pops of dominant herbivore in streams, allows other species to increase. 

Describe the impacts that climate change is having on disease in wild systems and humans. Interpret examples showing how climate is changing parasite transmission and incidence. Link to what you’ve learned about evolution and population

• affects distribution of diseases

  • increases water temps→ increases disease in corals, shellfish and amphibians 

  • vectors like mosquitoes are more active and abundant 

• Habitat destruction 

  • biodiversity conservation can protect humans from EIDS (emerging infectious diseases)

If parasites reduce host pops can cause extinction vortezes causes by alee efects, inbreeding depression gen drift, demographic stochasticity, environmental stochasticity