Heterogeneities in host populations Notes
By: Victory Obiefuna
1. “Patterns of macroparasite abundance and aggregation in wildlife populations: a
a. don't usually talk about aggregation in the case of microparasites.
Aggregation applies to macroparasites a lot more.
b. Things that are aggregated tend to have a negative binomial distribution.
(most people have a few worms. Only a few individuals have a lot of
worms and are superspreaders)
c. Variance increases rapidly when there are higher rates of aggregation
i. Concepts that were to be discussed in the background info were
clearly explained. Aggregation was defined as the estimated
variance to mean ratio
1. Widely observed aggregation in parasite burden arises from
heterogeneities in the host populations or in infection
2. “These heterogeneities are generated by changes in the
climate over time of space, genetic differences between
hosts in terms of parasite induced host mortality,
heterogeneity in infection levels, or even variation in the
number of infectious stages encountered per infection
ii. Aim of the study was stated. Not particularly specific though:
1. “aim is to establish whether there are general underlying
trends in observed patterns of abundance and aggregation
and question whether specific taxonomic or ecological
groups appear to differ from these underlying trends.”
iii. The relevance of the study was also stated. The researchers wished
to detect associations between biological factors and worm burdens
iv. Hypothesis and predictions are not made. Perhaps that is related to
the fact that this is a review paper, and they are just looking at data
that has already been collected to get a sense of the general trends.
e. Materials and Methods
i. Started off with an explanation of the statistical analysis methods
that were used in the study.
ii. Researchers state that the degree of aggregation can’t really be
found, but they use a value defined as k as an estimate for the
degree of estimation.
1. Used an unpublished paper for this. Can we give validity to
this method if the reference that they were using had not
been published at that point?
2. Not really sure of why this was used if they couldn’t
explain the basis for its use. iii. Obviously, the accurate maximum likelihood estimation was not
entirely accurate. Didn’t mention how their statistical analyses
would take that into consideration. Made assumptions based on
their inaccurate methods. Not sure of how I feel about that
iv. In order to examine the differences in the parasite burdens, they
removed groups that were too similar to each other. This was a
v. The equation that was used to examine the degree of aggregation is
foreign to me. Don’t really understand it. If someone could explain
that to me, that would be great.
vi. Sample size was not mentioned. The number of studies that would
be examined was not mentioned. Randomizing information was
vii. Stats used was unclear. More clear explanation should have been
given, especially since this is a biology paper and not a
viii. Information on the database
1. Did not mention the kinds of organisms/organism names
that would be used in the study. That would have been
2. Indirect worm burden counts were not used. This is good
3. Each value set within the database had a different sample
size. There was no mention of how the variations in the
sample sizes may affect the trends that would be observed.
Could skew the data in different ways
4. Researchers stated that “we have to operate within an
unbalanced database, where not all types of interaction are
represented equally. This lack of balance will in turn
determine which analyses can be performed and the
conclusions that should be drawn”
a. May have been a better idea to just group the
different classes of organisms and explain the
conclusions of the study based on that. Then, those
groups could be compared. Makes more sense than
just lumping them all together right at the
5. Stated that the database does not represent the full range of
parasitic infections. Is it appropriate to extrapolate these
results to all parasitic infections that are possible then?
i. Stats seems legit.
ii. Graphs are really hard for me to figure out.
iii. Not understanding the graphs makes it hard for me to understand
how they came to the conclusion that they came to. g. Discussion
i. Stated that most parasitic helminthes and ectoparasites are
aggregated in their distribution in their natural host populations.
ii. Mentioned the evolutionary constraints on the degree of
iii. Not understanding the graphs makes it difficult for me to be able to
say whether or not the discussion makes sense.
h. Synthesis report
i. Strengths = good background
ii. Weakness = statistical methods of analysis were not explained,
failings in their method of analysis were not addressed (unbalanced
database, different sample sizes and their effects etc)
iii. Significance of the paper: Aggregation stuff.
iv. Limitation: you can make generalization because the range of
parasites that were used was not adequate.
v. Abstract is well written.