Implications of changes in network size in response to
environment
Bacteria with high betweenness centrality may act as hub species that
maintain the stability of a network (Röttjers & Faust, 2018). In large
fragments, Arsenophonus , Wolbachia , and Bartonellahad high betweenness centrality, but these bacterial taxa were less
central to the networks from small fragments despite maintaining high
relative abundance in flies at these sites (Figures 2 and 6). IfArsenophonus and Wolbachia are acting as primary
symbionts, decreasing betweenness centrality may be indicative of
changing symbiont-host interactions in response to microbiome community
perturbations. The primary symbionts of blood-feeding insects play an
important role in vector competence in insects (Weiss & Aksoy, 2011).
For example, in tsetse flies, primary bacterial endosymbionts in the
genus Wigglesworthia impede the invasion of trypanosome parasites
by assisting host defenses and subsequently decrease the competence of
tsetse flies to vector these harmful parasites to downstream hosts
including humans (Weiss et al. 2013). As bat flies are important
arthropod vectors of bat pathogens, changes in the structure of their
microbiomes in response to habitat fragmentation may have implications
for the disease ecology of arthropod vectored pathogens in bats.
Modules may delimit groups of bacteria with specific functional
specializations and/or groups that respond in similar ways to
environmental variables (Röttjers & Faust, 2018). Higher modularity may
protect a community of free-living organisms from invading pathogens,
because a pathogen would be isolated to one module within the community
(i.e., diversity-stability debate; (Krause et al., 2003; Stouffer &
Bascompte, 2010)). This hypothesis may be applicable to bacterial
networks if pathogens are limited in transmission by direct competition
with endogenous bacteria. However, high modularity in bacterial
interaction networks may also reflect the absence of microbiome-mediated
host defenses against pathogen invasion. If we revisit the tsetse fly
example above, Wigglesworthia and trypanosome parasites would
share an edge in a microbe interaction network becauseWigglesworthia abundance and prevalence is associated with low
trypanosome abundance and prevalence mediated by host defenses. The
absence of this interaction in a network might indicate thatWigglesworthia -induced host response to trypanosome invasion is
impaired by other microbes in the network (e.g., priority effects) or
other aspects of host health. Trypanosome transmission would be less
regulated in this instance, but modularity would be high as long as
trypanosome abundance and prevalence did not lead to changes in the
abundance or prevalence of other microbes. In the case of bat flies, the
consequences of more isolated modules for the functionality and
stability of the microbiome are unclear and merit future investigation.
Our research builds upon previous evidence that the environment
influences microbiome composition in addition to host factors (Amato et
al., 2013; Avena et al., 2016; Becker et al., 2017; Ingala et al.,
2019). However, all previous studies on the impact of the environment on
microbiome composition have been conducted on free-living organisms. The
interactions of bat flies with the broader environment are filtered
through their obligate associations with host bats, yet the signal of
environmental change is detected in the composition of bat fly
microbiomes. This indicates that environmental degradation may have
community-wide implications for the composition of microbiomes in
various organisms, possibly through microclimatic changes that alter the
pool of bacteria available in habitat patches.