RESULTS
The beta diversity and normalized Sackin index of parasites varied
according to the host-switching intensity (i.e. the expected percentage
of parasite individuals that switch hosts during the entire history of
the host community). To illustrate the prevalent beta diversity and
normalized Sackin index according to the host-switching intensity, we
presented the case of feather mites associated with birds (ID. 1 - Fig.
1). As expected, beta diversity decreases as host-switching intensity
increases regardless of the studied case (Fig. 1a and Fig. S14). It
occurs because the increase of host-switching promotes the interaction
of different host species with the same parasite species. Additionally,
for each value of host-switching intensity, there is a small variation
in the beta diversity (Fig. 1a and Fig. S14). The only exception was ID.
4, which resulted in a wide variation in beta diversity under high
host-switching intensity (Fig. S14).
When host-switching intensity is low (below 5%), the normalized Sackin
index (In) for the simulated parasite phylogenies
assumes exactly the same value of the one obtained from the empirical
phylogeny of the host (note the dashed line in Fig. 1b and also Fig.
S15). This low host-switching intensity does not allow the establishment
of a population in a new host. As a consequence, the simulated parasite
phylogenies have the same normalized Sackin index of the empirical host
phylogeny. For higher host-switching intensity, colonization followed by
speciation is more likely to occur and the normalized Sackin index
varies over simulations even when they are under the same host-switching
intensity (Fig. 1b and Fig. S15). The wide variation in the normalized
Sackin index for a given host-switching intensity reveals that
stochastic host-switching events, even if more likely occurring between
closely related species, can change the structure of the resulting
phylogenetic tree. The normalized Sackin index tends towards zero
(balanced tree) as host-switching intensity goes to one, regardless of
the community (Fig. S15), resembling a neutral speciation scenario Yule
model (Yule 1924; Aldous 2001).
For all cases, there is a host-switching intensity that simultaneously
reproduces the empirical beta diversity and the parasite normalized
Sackin index (Fig. 2). As mentioned before, both metrics are sensitive
to host-switching intensity, but still restricted to some combinations
of beta diversity and Sackin index (see in Fig. 2, the graphs showing
the beta diversity vs. normalized Sackin index are not fully
filled). Generally, the beta diversity and the parasite normalized
Sackin index obtained under high host-switching intensity (greater than
50%) are far from the empirical pattern (see Fig. 2, the yellow dots
rarely approach the intersection of the solid lines). Then, host
phylogeny is an important proxy for host-switching events.
The simulated host-switching intensity that simultaneously fit beta
diversity and parasite normalized Sackin index of each empirical case
(\(\pm\)5% confidence interval) recovered a range of 0.06% to 22.07%
of host-switching intensity. Within this range, the associations between
mammals and lice presented the lowest host-switching intensity (case ID.
2 with 0.07% - 1.13%), followed by that involving birds and feather
mites (case ID. 1 with 0.06% - 8. 17%), wildlife and arthropod
parasites (case ID. 3 with 0.82% - 2.69%), frogs and Polystomatidae
(case ID. 7 with 0.22% - 3.71%), frogs and Rhabdias spp. (case
ID. 8 with 1.99% - 4.94%), frogs and Oswaldocruzia spp. (case
ID. 9 with 5.29% - 5.91%), fish and Gyrodactylidae (case ID. 6 with
9.35%), - the highest intensities of host-switching were observed
between rodents and fleas (case ID. 5 with 13.47% - 16.87% and case
ID. 4 with 0.43% - 22.07%). For ectoparasites, the switching intensity
of hosts observed in the simulations was 0.06% to 22.07% and, for
endoparasites, it was in the range of 0.22% to 5.91% but no
significant correlation was observed between host-switching intensity
and parasitism type (LMM: beta= -0.007, SE= 0.018, df = 5.98, t= -0.6, p
= 0.70, ANOVA: F=0.16, p = 0.69, Fig. 3). The host-switching events are
more frequent in studies conducted in a local scale (blue color in Fig.
3) than in regional scales (salmon color in Fig. 3) (LMM: effect of
host-switching intensity on spatial scale: beta= 0.08, SE= 0.18, df =
6.12, t= 4.66, p = 0.003, ANOVA: F = 21.75, p = 0.003, Fig. 3).