Parasites and host populations dynamics Notes
By: Victory Obiefuna
January 22, 2014: Leading the Discussion
1. “Prevention of Population Cycles by Parasite Removal” (1998)
i. Math models have shown that densitydependent response acting
with a time delay can generate population cycles
ii. Aim of the study: To investigate the capacity of parasites to cause
iii. Information on the organisms in the study:
1. Lagopus lagopus scoticus, aka red grouse.
a. Gamebird native to Great Britain and Ireland.
i. Game bird = bird that is hunted for sport or
b. Known to have cycles in its numbers
2. Trichostrongylus tenuis , aka strongyle worm. Direct life
a. Gut nematode (nematodes are round worms) found
in the United Kingdom.
b. Known to be responsible for crashes in the
population of grouse birds
c. Transmission: through the feces of the red grouse.
Eggs are present in them.
d. Eggs hatch and L1 larvae come out. They moult to
the L3 stage (infective stage). Then, another grouse
ingests it. L3 stage moults to the adult in the
caecum of the grouse. Makes lots of eggs and
continues the cycle.
e. Worms feed on the contents of the intestines of the
grouse. Makes it harder for them to survive.
Especially in the early spring when the food quality
iv. Many grouse populations tend to have lots of population
v. High worm load l ▯ ow population growth rate. Breeding production
is affected negatively. This is how the parasite could cause
vi. High worm load ▯decrease host fecundity ( c is pronounced as a k)▯
decreased host survival. b. Materials and Methods
i. Longterm data was taken from 6 grouse moors between 1989 and
1. Do you see any issues with the materials and methods
section as it is described? They didn’t mention the size of
the moors. Didn’t mention the number of grouse that may
be in the area. Materials and methods section is quite
ii. Grouse were treated with antihelminthic drugs. Then, they were
1. Treated grouse stayed in their areas!! This is different from
the other paper.
iii. Estimated that they treated between 15% 50% of the grouse
1. How would this wide range of values affect that data that
they were seeing?
iv. Sample of treated birds was shot from each population and the
intensities of worm infection were compared with those of the
untreated birds to confirm that treatment reduced the worm
burdens. Numbers of harvested grouse were recorded during the
subsequent hunting season.
i. In all of the six treatments, application of the antihelminth drug
reduced the tendency of the cyclic population to crash
ii. Get someone to discuss Figure 1. (A, B, C).
1. Fig 1A: not sure what the purpose of the graph is
2. Fig 1B: shows that as the mean log worm intensity
increases, there is a decrease in the annual population
growth rate of the red grouses.
3. Fig 1C: shows that as the number of worms per grouse
increases, death during breeding increases. What does this
tell us about the effects that T. tenuis has on red grouse?
iii. Get someone to discuss Figure 2. (A,B,C)
1. Fig 2A: shows changes in population at the control sites.
There you can see the population crashes every few years
2. Fig 2B: changes in pop numbers in 2 populations with a
single treatment each
3. Fig 2C: changes in pop numbers in 2 populations with two
4. What does this show us?
d. Discussion i. What conclusion did they come to at the end of the paper?
1. Parasitic nematodes were necessary for the cyclic declines
in abundance that were observed in grouse populations.
ii. Stated that there was only one trophic interaction that caused the
1. What do you think of this based on the data?
e. “Synthesis Report”
i. Is this paper believable?
ii. What are some strengths and weaknesses that you can see in this
1. Weakness: didn’t give enough information on the sampling
iii. Did they tie everything together adequately?
iv. Did the abstract give a good sense of what was going on in the
1. Found it to be a bit lacking myself. Wished that they would
have gone into the stuff a little bit more. One sentence on
how they did it.
2. “Testing the role of parasites in driving the cyclic population dynamics of a
i. Impact of parasites on reproduction and survival b▯ eing explored
ii. Effects of parasites on population dynamics ▯not well understood.
iii. ALREADY MENTIONED THE SPECIES IN QUESTION
1. The Trichostrongylus tenuis roundworm in the Lagopus
lagopus scoticus red grouse
iv. Hypothesis: cycles generated by parasiteinduced reductions in the
host breeding production in tension with other features of the host
parasite system, including parasiteinduced mortality and the
degree of parasite aggregation
v. Other researchers removed 20% of their parasites to show that the
amplitude of the population cycle changed. Concluded that
parasites were the cause of the cyclical fluctuations. There were
some criticisms of this
1. What were the criticisms of this conclusion?
a. Grouse density estimate was based on number
killed. So, they wouldn’t have a good estimate for
the years when there was no killing b. Wasn’t possible to determine exactly what caused
the fluctuations since only small percentage of
population was treated.
vi. Studies in England and Scotland ▯suggested that the mechanisms
driving the cycles were region specific
1. What could be causing the region specificity? Will come
back to this question later.
vii. Not enough of the studies looked at the numbers of grouse in the
spring. So, the researchers tested the parasite hypothesis by
counting breeding and postbreeding densities and considering the
viii. Hypothesis of this particular study: removal of parasites from up to
81% of grouse each winter ▯should decrease the declines in
numbers to stop.
1. Hypothesis was tested on 4 populations (2 in England and 2
in Scotland). England study for 3 years; Scotland study for
a. Is it appropriate to look at studies that are being
tested with different periods of time? What if the
Scotland cycle will take more time to even out than
the England cycle? Can these two even be
i. 2 estates in England. 2 estates in Scotland.
1. Advantages/disadvantages of using wild grouse?
ii. 3 other estates = controls
iii. experiment started in winter of peak density.
iv. On estates, two 1kmsquared study plots were 2.7 km apart on each
estate. Counted the grouse on the plots and dosed them with some
v. Possibilty of mishandling or making a mistake with the taking was
1. How will this affect the data?
vi. Small tags were used to prevent the predation of the tagged grouse
to increase because of increased conspicuousness.
vii. T. tenuis parasite intensity estimates.
1. Both worm count and fecal egg counts were used. Egg
counts from birds caught in the spring. Worm counts from
birds shot in the autumn
a. Not consistent. Thoughts?
viii. Autumn counts of grouse 1. Spring counts were done. Also, autumn counts were done
right before the harvesting of the grouse was done.
2. Counted number of chicks associated with hens as well
ix. Statistical analyses
1. Ask Lordlin to talk about this with you
2. First, test effectiveness of treatment through worm count.
3. The, check if parasite removal = higher grouse densities
and numbers. Does the number of cycles matter? (the
cycles were different between the countries)
4. Compared growth rates of grouse populations in relation to
i. Within the control population, higher parasite and host numbers in
England than Scotland
ii. Grouse numbers are lower in the spring than in the summer. (what
could you use to explain this?)
iii. Average of 67% of grouse were treated with antihelminth drugs.
iv. There was movement of the birds around the study areas in early
spring; effects of the treatment could extend beyond the boundaries
of each study area
1. What effect could this have on the results?
v. Was the drug treatment effective? How do we know?
1. Look at the P value, which is below 0.05. tells us that the
differences in parasite numbers was statistically significant.
Effects of treatment were strong
vi. Why was the uncertainty of the data that they collected so high?
Does that make you trust their evaluation more or less than you did
vii. Brood size increased in the treated grouse.
viii. Grouse densities increased with the treatment.
ix. Figure 1 analysis
1. Ashows that there were higher worm loads in the controls
than in the treated grouse. This can be seen in each of the
different “grouse plots” in Scotland and in England
2. Bshows that the treated grouse were more successful at
breading than the control grouse were.
3. Cautumnal density of grouse = higher in treated grouse
4. Dtreated numbers have higher grouse densities
5. EWhat is the significance of this image? x. Evide