A survey of freshwater fishes and their macroparasites in the ... - NINA

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NINA Report 635
observed difference between the two samples is larger than expected, that is, the populations
we sample from are different (in terms of the parasite taxa present).
This approach is first applied to the samples from location 7 (Q. Limonal), which was sampled
all three years with a relatively good number of hosts. If we start by comparing the two samples
from the dry season (1999 and 2001), we can pool the two samples and simulate 10.000 new
pairs of samples of the same sizes as the original ones. Figure 6, top panel, shows the distribution
for the number of parasite taxa in the simulated “1999-samples” (43 fish hosts drawn at
random from the 158 fish hosts in the pooled sample) with the red point indicating the number
of parasite taxa in the original sample (s99= 10). The middle panel of Figure 6 shows the distribution
from the simulated “2001-samples” (the remaining 115 fish hosts in each simulation;
s01=15). Finally, the lower panel of Figure 6 shows the distribution of simulated differences in
the number of parasite taxa. The observed difference (s99 – s01 = -5) is well within the 95% confidence
interval (blue vertical lines) with p = 0.44. Thus, we have no reason to believe that the
actual numbers of parasite taxa in the fish communities are different in the dry seasons of 1999
and 2001, the observed difference can be explained by differing sample sizes, and we may
pool the dry season samples from the two years.
No. of simulations
No. of simulations
No. of simulations
0 1000 2000
0 1000 3000
0 500 1500
1999 - Dry season
6 8 10 12 14 16
No. of parasite species
2001 - Dry season
6 8 10 12 14 16
No. of parasite species
-10 -5 0 5
Simulated diff. in parasite species richness
Figure 6 Simulated distributions of number of parasite taxa at location 7, for the two dry seasons
(top and middle panels), and the null distribution for the expected difference in number of
parasite taxa between the two samples (lower panel). The red diamond at the baseline of all
histograms shows the values observed in the original data set and the blue vertical lines in the
lower panel gives the 95% confidence limits of the null distribution.