Host fitness should be the broadest scale of microbial function in the context of
host-associated microbial communities. Microbes can increase host fitness by providing nutrients, defense, regulating development, or extending the host’s phenotype, but they can also decrease fitness when cheaters or pathogens invade
communities. As I describe below, the host can have some control over microbial
assembly with enough filtering to avoid cheaters and pathogens but permissive
enough that beneficial species can be recruited and thrive. Importantly, there are at
least two limits to the fitness effects of microbial communities on hosts. First, there
are limits to the fitness effects of microbial communities on hosts equivalent to life
history trade-offs (Zera and Harshman 2001). Gould et al. (2018) found that fruit fly
microbiome diversity accelerated development and reproduction. However, these
flies that reproduced more also died sooner, suggesting the effects of microbiomes
on host fitness share the same life history constraints traditionally considered in
biology (Zera and Harshman 2001).
A second trade-off occurs when the same microbial community can result in
contrasting fitness outcomes, demonstrating that each functional category does not
happen in isolation. In endophyte–plant associations, for example, there seems to be
a trade-off between pathogen defense and tolerance to environmental stress. Diverse
communities of endophytes can protect the plant by out crowding fungal pathogens
(Bae et al. 2009; Alabouvette et al. 2009; Rodriguez Estrada et al. 2011), yet more
endophyte species can also result in water loss, compromising survival in drought
scenarios (Arnold and Engelbrecht 2007). Similarly, Wolbachia is an intracellular
endosymbiont that associates with insects, providing essential metabolites, but
interferes in the host’s reproduction by biasing sex ratios (Brownlie et al. 2009;
Correa and Ballard 2016). Perhaps here we can consider the dynamic nature of hostassociated microbial communities, and the possibility that different assembly processes can result in functional outcomes that better fit the host’s condition and its
surrounding environment. Whether hosts or microbes control these dynamics may
vary, depending on the specific assembly mechanisms underlying each microbial
community.
17.3.3 Assembly Contributions to Diversity and Function
When an individual host is born, its tissues must be colonized by microbes incoming
from the surrounding environment and conspecifics. Thereafter, microbial communities are the result of the interplay between the host tissue selecting for microbial
species, microbe–microbe interactions, and continued colonization of the surrounding environment or neighboring hosts (Fig. 17.3, Adair and Douglas 2017, Miller
et al. 2018). A useful approach to explain how these processes influence diversity
and function is to study mechanisms of community assembly (Shafquat et al. 2014;
Adair and Douglas 2017; Leibold et al. 2017). Although studies are beginning to
consider the relative contribution of different assembly processes on host-associated
communities (Costello et al. 2012; Miller et al. 2018; Reese and Dunn 2018),
17 Diversity–Function Relationships and the Underlying Ecological Mechanisms in. . .
311
host-associated microbial communities. Microbes can increase host fitness by providing nutrients, defense, regulating development, or extending the host’s phenotype, but they can also decrease fitness when cheaters or pathogens invade
communities. As I describe below, the host can have some control over microbial
assembly with enough filtering to avoid cheaters and pathogens but permissive
enough that beneficial species can be recruited and thrive. Importantly, there are at
least two limits to the fitness effects of microbial communities on hosts. First, there
are limits to the fitness effects of microbial communities on hosts equivalent to life
history trade-offs (Zera and Harshman 2001). Gould et al. (2018) found that fruit fly
microbiome diversity accelerated development and reproduction. However, these
flies that reproduced more also died sooner, suggesting the effects of microbiomes
on host fitness share the same life history constraints traditionally considered in
biology (Zera and Harshman 2001).
A second trade-off occurs when the same microbial community can result in
contrasting fitness outcomes, demonstrating that each functional category does not
happen in isolation. In endophyte–plant associations, for example, there seems to be
a trade-off between pathogen defense and tolerance to environmental stress. Diverse
communities of endophytes can protect the plant by out crowding fungal pathogens
(Bae et al. 2009; Alabouvette et al. 2009; Rodriguez Estrada et al. 2011), yet more
endophyte species can also result in water loss, compromising survival in drought
scenarios (Arnold and Engelbrecht 2007). Similarly, Wolbachia is an intracellular
endosymbiont that associates with insects, providing essential metabolites, but
interferes in the host’s reproduction by biasing sex ratios (Brownlie et al. 2009;
Correa and Ballard 2016). Perhaps here we can consider the dynamic nature of hostassociated microbial communities, and the possibility that different assembly processes can result in functional outcomes that better fit the host’s condition and its
surrounding environment. Whether hosts or microbes control these dynamics may
vary, depending on the specific assembly mechanisms underlying each microbial
community.
17.3.3 Assembly Contributions to Diversity and Function
When an individual host is born, its tissues must be colonized by microbes incoming
from the surrounding environment and conspecifics. Thereafter, microbial communities are the result of the interplay between the host tissue selecting for microbial
species, microbe–microbe interactions, and continued colonization of the surrounding environment or neighboring hosts (Fig. 17.3, Adair and Douglas 2017, Miller
et al. 2018). A useful approach to explain how these processes influence diversity
and function is to study mechanisms of community assembly (Shafquat et al. 2014;
Adair and Douglas 2017; Leibold et al. 2017). Although studies are beginning to
consider the relative contribution of different assembly processes on host-associated
communities (Costello et al. 2012; Miller et al. 2018; Reese and Dunn 2018),
17 Diversity–Function Relationships and the Underlying Ecological Mechanisms in. . .
311