Introduction
The gut microbiota is a community of microorganisms inhabiting an
organism’s digestive tract that can affect host physiology and health
(Gomaa, 2020; Grond et al., 2018). For example, specific gut bacterial
taxa, such as Ruminococcus , can aid in host digestion by breaking
down indigestible food compounds, such as cellulose, and increase
digestive efficiency in humans (Arumugam et al., 2011; Thursby & Juge,
2017). Furthermore, variation in diet, such as a plant-based diet versus
a meat-based diet, can shape host gut microbiota as it shifts in
response to nutrient availability (Gomaa, 2020; Zhu et al., 2015).
Additionally, the host’s immune system can interact with the gut
microbiota, specifically by shaping its community composition (Hooper et
al., 2012). For example, Kato et al., (2014) found that knockout mice
who are deficient in B-cell production have lower gut bacterial
diversity compared to non-knockout mice. The gut microbiota can also
initiate the production of immune cells to help maintain or develop an
effective immune response (Zhang et al., 2017). Studying the dynamic
relationships between physiological processes and gut microbiota will be
continually important, especially in light of increasing anthropogenic
change which can directly and indirectly affect these factors.
Parasites and pathogens, which also affect host health, can directly and
indirectly affect microbial communities in the gut (Stensvold & Van Der
Giezen, 2018). Within the gut, parasites can directly affect the
microbiota by competing with or consuming commensal bacteria (Abt &
Pamer, 2014). Parasites might also indirectly affect the gut microbiota
via the host’s immune system. For example, gut parasites (e.g.,Heligmosomoides polygyrus ) can activate a non-specific mucosal
immune response (e.g., T and Th2 cells), which can select for or against
bacterial taxa, such as an increase in gram negative Enterobacteriaceae
and a decrease in Enterococcus faecium (Rausch et al., 2018).
However, parasites located outside of the gut might also elicit a
systemic immune response that could affect the bacterial community in
the gut. For example, fish with ectoparasitic fluke (Dactylogyrus
lamellatus ) infections have lower gut bacterial diversity compared to
uninfected fish, which was likely mediated by elevated expression of
immune genes related to the IgM antibody, toll-like receptor 3, and
Major histocompatibility factor II responses (Wang et al., 2023).
Knutie, (2020) found that parasitic nest fly abundance is negatively
correlated with gut bacterial diversity and positively correlated with
IgY antibody levels, in eastern bluebirds (Sialia sialis ).
Overall, most of these non-model organism studies are correlational and
thus the causal effects of parasitism on gut microbiota are not well
understood.
Factors related to urbanization, such as changes in food availability,
can also influence the gut microbiota of wild hosts (Berlow et al.,
2021; Phillips et al., 2018; Teyssier et al., 2020). Studies have found
that urban hosts have a larger diet breadth, which can result in greater
diversity of the gut bacterial community (Littleford-Colquhoun et al.,
2017). For example, non-urban water dragons (Intellagama
lesueurii ) feed primarily on invertebrates but urban water dragons feed
on invertebrates and plant material, which results in higher gut
bacterial diversity. These effects are either because plants introduce
additional bacterial taxa or a larger diet breadth selects for different
bacteria to aid in digestion. If the latter is supported, a more diverse
gut microbial community could help hosts be more equipped to deal with
environmental change (Littleford-Colquhoun et al., 2017). In contrast,
the gut of house sparrows (Passer domesticus ) consuming a
non-urban diet, which is rich in protein, is correlated with higher
bacterial diversity compared to urban sparrows whose diet is poor in
protein
(Teyssier et
al., 2020). While increased gut bacterial diversity is generally
assumed to be beneficial to hosts, in some cases, such as in organisms
with highly specialized diets, lower bacterial diversity can be
associated with better health outcomes (Shade et al., 2017). For
example, urban coyotes consume carbohydrate-rich anthropogenic food
items and have higher bacterial diversity compared to their rural
counterparts, but are in poorer body condition and have increased
prevalence of the parasite Echinococcus multilocularis(Sugden et al.,
2020). Ultimately, these effects of urbanization and other stressors
related to human activity (e.g., invasive parasites) are complex and
could result in positive or negative implications for host health.
Given the complex effects of human activity on hosts, few studies have
examined the influence of multiple synergistic anthropogenic factors,
such as urbanization and invasive parasitism, on host gut microbiota.
The Galápagos Islands of Ecuador provide an ideal study system to
investigate these complex effects. Since 1979, the number of residents
and tourists have increased, leading to changes in the natural habitat.
These changes include the introduction of non-native species, including
parasites (Kerr et al., 2004; Wikelski et al., 2004). For example, the
avian vampire fly (Philornis downsi ; hereon, vampire fly) was
introduced to the Galápagos in the past several decades and is found on
nearly all islands, including human-inhabited islands such as San
Cristóbal. Adult flies are non-parasitic but lay their eggs in birds’
nests where the hematophagous larvae feed on nestling hosts and brooding
mothers (Fessl et al., 2001; Fessl & Tebbich, 2002). Several studies
have found that the vampire fly can have detrimental effects on the
survival of nestling Darwin’s finches (Fessl et al., 2010; Kleindorfer
& Dudaniec, 2016; Knutie et al., 2016; Koop et al., 2011, 2013; McNew
& Clayton, 2018; O’Connor et al., 2010). However, a recent study found
that urban finches on San Cristóbal Island are less affected by and more
resistant to the vampire fly than non-urban finches (Knutie et al.,
2023) who suffer up to 100% mortality due to the fly (Koop et al.,
2013; O’Connor et al., 2014). For non-urban finches, the vampire fly
does not affect the gut microbiota (Addesso et al., 2020; Knutie, 2018;
Knutie et al., 2019). However, because urban finches are more resistant
to the fly, and immunological resistance can interact with gut
microbiota, parasitism may cause a greater change on the microbiota of
urban finches compared to non-urban finches. To date, no studies have
causally explored whether parasitism and urbanization interact to affect
the gut microbiota of hosts.
The goal of this study is to compare the effects of avian vampire flies
and urbanization on the gut microbiota of nestling small ground finches
(Geospiza fulignosa ) in 2018 and 2019. Specifically, we
experimentally manipulated parasite abundance in urban and non-urban
finch nests and then characterized the gut microbiota (i.e., alpha and
beta diversity, community composition, relative abundance of taxa).
Because diet can influence the gut microbiota (Davidson et al., 2020)
and urban finches have a more diverse diet compared to non-urban finches
(De León et al., 2019), we hypothesize that urban nestlings will have a
different gut microbiota than non-urban nestlings (Loo et al., 2019).
Because immunological resistance can be linked to the gut microbiota
(Hooper et al., 2012), we predict parasitism will affect and ultimately
change the gut microbiota of urban nestlings. Past studies have found
that parasitism by vampire flies does not affect the gut microbiota of
finch nestlings in non-urban areas (Addesso et al., 2020; Knutie, 2018;
Knutie et al., 2019), but their diet is not supplemented. Therefore, we
predict that parasitism will not affect the gut microbiota of non-urban
nestlings. Further investigation will allow for a more thorough
understanding of environmental change on the gut microbiota of small
ground finch nestlings.