2000; Attard et al. 2012), and that to protect itself from multiple stressors, a plant
would need to maintain a diverse community (Arnold et al. 2003; Bae et al. 2009).
However, some of these taxa are not strong competitors and would be eliminated
under normal conditions. For example, ice-nucleating bacteria that reduce the
freezing point of water (Duman and Olsen 1993) are outcompeted under nonfreezing
conditions and excluded from phylosphere communities (Lindow et al. 1996;
Stockwell and Stack 2007). Maintaining microbial protection against multiple
stressors may require host intervention including complex hormonal secretion patterns and stomatal closures (Stone et al. 2018).
17.3 How Does Diversity Result in Function?
It is increasingly clear that host-associated microbial communities range from
hundreds to single species, suggesting that sufficient function to benefit a host’s
fitness can result in different points of a diversity gradient. Ecologists have been
addressing the issue of how diversity results in function for a long time, laying a rich
body of literature. Therefore, I draw from this ecological literature to interpret the
broad range of diversity–function relationships in host-associated microbial
communities.
17.3.1 Does Diversity Lead to Function? The
Biodiversity–Ecosystem Function Relationship
One way to understand this wide range of functional communities is to use the shape
of the biodiversity–function relationship to infer the ecological mechanisms that
result in function. The biodiversity and ecosystem function (BEF) has been central to
the study of ecology for several decades (Hooper et al. 2005; Cardinale et al. 2012),
revealing several possible ways in which species contribute to function, including
positive, asymptotic, negative, and idiosyncratic relationships (Scherer-Lorenzen
2005). In this section, I review these mechanisms and the role they play in host–
microbe interactions.
Many host-associated communities seem to have positive diversity–function
relationships (Fig. 17.2a). For example, the human infant’s gut microbial community
increases in diversity and function during the first 3 years of life (Odamaki et al.
2016; Hill et al. 2017). Mechanistically, the accumulation of microbial species may
correspond to the addition of complementary functions, resource use efficiency, and
increased access to nutrients for the host (Ottman et al. 2012). These trends are
equivalent to the complementarity model in plant and animal BEF relationships,
where species with different functional traits contribute to an overarching function
(Loreau and Hector 2001; Hooper et al. 2005; Fox 2005; Balvanera et al. 2006;
17 Diversity–Function Relationships and the Underlying Ecological Mechanisms in. . .
305
would need to maintain a diverse community (Arnold et al. 2003; Bae et al. 2009).
However, some of these taxa are not strong competitors and would be eliminated
under normal conditions. For example, ice-nucleating bacteria that reduce the
freezing point of water (Duman and Olsen 1993) are outcompeted under nonfreezing
conditions and excluded from phylosphere communities (Lindow et al. 1996;
Stockwell and Stack 2007). Maintaining microbial protection against multiple
stressors may require host intervention including complex hormonal secretion patterns and stomatal closures (Stone et al. 2018).
17.3 How Does Diversity Result in Function?
It is increasingly clear that host-associated microbial communities range from
hundreds to single species, suggesting that sufficient function to benefit a host’s
fitness can result in different points of a diversity gradient. Ecologists have been
addressing the issue of how diversity results in function for a long time, laying a rich
body of literature. Therefore, I draw from this ecological literature to interpret the
broad range of diversity–function relationships in host-associated microbial
communities.
17.3.1 Does Diversity Lead to Function? The
Biodiversity–Ecosystem Function Relationship
One way to understand this wide range of functional communities is to use the shape
of the biodiversity–function relationship to infer the ecological mechanisms that
result in function. The biodiversity and ecosystem function (BEF) has been central to
the study of ecology for several decades (Hooper et al. 2005; Cardinale et al. 2012),
revealing several possible ways in which species contribute to function, including
positive, asymptotic, negative, and idiosyncratic relationships (Scherer-Lorenzen
2005). In this section, I review these mechanisms and the role they play in host–
microbe interactions.
Many host-associated communities seem to have positive diversity–function
relationships (Fig. 17.2a). For example, the human infant’s gut microbial community
increases in diversity and function during the first 3 years of life (Odamaki et al.
2016; Hill et al. 2017). Mechanistically, the accumulation of microbial species may
correspond to the addition of complementary functions, resource use efficiency, and
increased access to nutrients for the host (Ottman et al. 2012). These trends are
equivalent to the complementarity model in plant and animal BEF relationships,
where species with different functional traits contribute to an overarching function
(Loreau and Hector 2001; Hooper et al. 2005; Fox 2005; Balvanera et al. 2006;
17 Diversity–Function Relationships and the Underlying Ecological Mechanisms in. . .
305