resources and exclude others) resulting in high variability across algae tufts
(Campbell et al. 2015).
In contrast to these competition-based priority effect models, cooperation should
benefit different species within the microbial community and promote coexistence,
but it has been difficult to prove in the wild. Using in vitro experiments,
Bacteroidetes thetaiotaomicron and B. ovatus isolated from the human gut were
shown to secrete extracellular enzymes to process polysaccharides like inulin, to the
benefit of other bacterial species and with reciprocal fitness effects that seem to
balance the cost of enzyme production (Rakoff-Nahoum et al. 2016). However, the
question of whether competitive or cooperative microbe–microbe interactions characterize host-associated microbial communities has been contentious (Coyte and
Rakoff-Nahoum 2019), limiting our understanding of their role in microbial function
within hosts. Some authors have suggested that microbe–microbe networks and
complementary effects, rather than individual species, maybe building blocks of
microbial function within hosts, calling for further study into species interactions
networks (Rolig et al. 2015; Gould et al. 2018).
In many cases, communities remain open to colonization throughout the life of
the host. Instead of communities dominated by initial colonizers, these hosts’
microbial communities reflect the diversity and potentially changing composition
of the pool of species available for colonization (Fig. 17.3, “Colonist pool”). For
example, neighboring corals harbor more similar microbial communities than do
coral heads that are far away from each other, indicating that neighbors exchange
more colonists and microbial dispersal is limited in space (Dunphy et al. 2019).
Similarly, nectar microbes are more similar in nearby flowers visited by the same
hummingbirds, suggesting dispersal is limited and driven by hummingbird vectors
(Belisle et al. 2012, 2014). In social hosts, this dispersal limitation results in
increased similarity in microbial communities within cohabitating humans (Song
et al. 2013) and primates living in groups (Perofsky et al. 2017). The pool of
environmental bacteria from the surrounding habitat is an important source of
microbial colonists for many hosts, including tadpoles (Louca et al. 2016a; Correa
et al. 2020), fruit flies (Blum et al. 2013), and crabs (Cuellar-Gempeler and Leibold
2018, 2019). Even with increasing evidence of the role of dispersal in shaping hostassociated microbial communities, we know little of its functional consequences.
One way to address this issue is to recognize that the influence of immigrants on
function should depend on the identity, traits, and diversity of colonists (Spasojevic
et al. 2018). If the pool of microbial colonists contains diverse species that are
functionally redundant, then hosts can benefit from permissive filters where many
different species can survive (Fig. 17.3 “Colonist pool”). However, the colonist pool
can also add species that contribute to diversity but not to function (Spasojevic et al.
2018). The addition of ineffective colonists could result in negative diversity function because incoming colonists increase diversity but decrease function by utilizing
resources. This is one way in which dispersal and colonization can cause the
decoupling of diversity and function. Hosts that do not invest in strong filters
could counteract this issue with behavioral mechanisms that result in symbiont
choice. For example, fungus-growing attine ants choose the fungus that is most
314
C. Cuellar-Gempeler
(Campbell et al. 2015).
In contrast to these competition-based priority effect models, cooperation should
benefit different species within the microbial community and promote coexistence,
but it has been difficult to prove in the wild. Using in vitro experiments,
Bacteroidetes thetaiotaomicron and B. ovatus isolated from the human gut were
shown to secrete extracellular enzymes to process polysaccharides like inulin, to the
benefit of other bacterial species and with reciprocal fitness effects that seem to
balance the cost of enzyme production (Rakoff-Nahoum et al. 2016). However, the
question of whether competitive or cooperative microbe–microbe interactions characterize host-associated microbial communities has been contentious (Coyte and
Rakoff-Nahoum 2019), limiting our understanding of their role in microbial function
within hosts. Some authors have suggested that microbe–microbe networks and
complementary effects, rather than individual species, maybe building blocks of
microbial function within hosts, calling for further study into species interactions
networks (Rolig et al. 2015; Gould et al. 2018).
In many cases, communities remain open to colonization throughout the life of
the host. Instead of communities dominated by initial colonizers, these hosts’
microbial communities reflect the diversity and potentially changing composition
of the pool of species available for colonization (Fig. 17.3, “Colonist pool”). For
example, neighboring corals harbor more similar microbial communities than do
coral heads that are far away from each other, indicating that neighbors exchange
more colonists and microbial dispersal is limited in space (Dunphy et al. 2019).
Similarly, nectar microbes are more similar in nearby flowers visited by the same
hummingbirds, suggesting dispersal is limited and driven by hummingbird vectors
(Belisle et al. 2012, 2014). In social hosts, this dispersal limitation results in
increased similarity in microbial communities within cohabitating humans (Song
et al. 2013) and primates living in groups (Perofsky et al. 2017). The pool of
environmental bacteria from the surrounding habitat is an important source of
microbial colonists for many hosts, including tadpoles (Louca et al. 2016a; Correa
et al. 2020), fruit flies (Blum et al. 2013), and crabs (Cuellar-Gempeler and Leibold
2018, 2019). Even with increasing evidence of the role of dispersal in shaping hostassociated microbial communities, we know little of its functional consequences.
One way to address this issue is to recognize that the influence of immigrants on
function should depend on the identity, traits, and diversity of colonists (Spasojevic
et al. 2018). If the pool of microbial colonists contains diverse species that are
functionally redundant, then hosts can benefit from permissive filters where many
different species can survive (Fig. 17.3 “Colonist pool”). However, the colonist pool
can also add species that contribute to diversity but not to function (Spasojevic et al.
2018). The addition of ineffective colonists could result in negative diversity function because incoming colonists increase diversity but decrease function by utilizing
resources. This is one way in which dispersal and colonization can cause the
decoupling of diversity and function. Hosts that do not invest in strong filters
could counteract this issue with behavioral mechanisms that result in symbiont
choice. For example, fungus-growing attine ants choose the fungus that is most
314
C. Cuellar-Gempeler