Parasite regulation of host populations

Question 1

what is wrong with the “delicate balance idea”

Answer 1

• natural selection has no foresight - it acts blindly by favouring genotypes that lead to the highest reproductivee success under the present conditions
• virulence is a complex trait and not the only determinant of parasites lifetime reporoductive success (trade offs with survival, transmission and reproduction)

Question 2

What happens if you have fewer parasites than hosts

Answer 2

if you have fewer parasites than hosts then the host population will be regulated by other regulatory processes independent of the parasites
host death rate due to parasitism must be greater than intrinsic growth of the host

Question 3

What happens if the parasite populations greater than the natural growth of the host

Answer 3

if the parasite population is greater than the natural growthh of the host then you will start ti see the effects of parasite regulation

Question 4

What is the Evidence for parasite regulation of host populations

Answer 4

• perpetuate the system away from equilibrium and measuring changes in host populations
• many of these cycles take years and so a single experiment can take a decade
• have to have detailed data and ability to track natural animal populations

TRACKING ALL OF THIS IS REALLY DIFFICULT

Question 5

Reinderpest outbreak late 19th century

Answer 5

• The virus is believed to have been brought in by Italian imports of infected zebu cattle from Aden or Bombay in 1887. In 10 years it had spread throughout the continent. It is recorded that 80-90% of susceptible domestic and wild animals died.
• This outbreak also effect large number of endemic ruminants like buffalo and wildebeest.
• In Kenya, rinderpest led to starvation. A Masai man was recorded as saying, in regard to the ground being littered with the corpses of animals “so many and so close that the vultures had forgotten to fly.“
• evidence that the regulation was conducted by the pathogen was starting to be seen when the vaccination was implemented and they saw an increase in the population of wild and domesticated caddle

Question 6

Soay sheep population cycles

Answer 6

• in early 1989 two thirds of the sheep population on St Kilda died over 12 weeks
• there has been a hypothesis that maybe the population of parasitic worms has contributed to the decline
• treatment with anti-helminitics increases daily survival - the treated sheep had a higher survival rate and that supported the hypothesis

Question 7

Lab experiments with tape worms and mice

Answer 7

Rinderpest is an example of a observational evidence indicating that parasites might be able to regulate entire host populations. What about actual controlled experimental evidence for this kind of regulation?

Scott 1986 conducted an experiment by infected free living laboratory mouse colonies with tapeworm.

As you can see a normal mouse colony increases in numbers over time.

The researcher inoculated the colony with tapeworm

• Add parasites (decline in population)
• High transmission- population stays low
• Treat to clear parasites- population increases

Now in this case you might say well this was in the lab and were these infections and population dynamics of the parasites here even realistic?

conclusion : infection with parasites is able to surpress the population

Question 8

Field experiments - snowshoe hares

Answer 8

• trap and tag in plots
• in each plot treat half with ivermectin
• radio tagged a subset (10 from each treatment per plot)
• determined COD
• carcasses - determine parasite burden and measure bone fat marrow
• 80% of known study area residents died and were necropsied
• 95% of hares died of predation and they were not as fat as the ones that were not killed - they look like they’ve experienced some level of starvation
• parasite burden was much higher in hares that were killed