The bacterial-host mutualism is ubiquitous and is seen even in the simplest of heterotrophic species [Thomas et al. 2010]. The pervasive and extensive mutualism between gut bacteria and their heterotrophic hosts points to the possibility that gut bacteria could play an essential role in enhancing environmental adaptation of their hosts [ Norris et al. 2013 ].
From the point of view of their role in the host-food dynamic, gut bacteria occupy a strategic position in the autotroph to heterotroph flow of biomass. The flow of biomass in the exchange between autotrophs and heterotrophs occurs through the guts of the heterotrophic species. From this unique vantage point gut bacteria can sense the changing nature of food supply from autotrophs (i.e., changing diets of heterotrophs) as well as the internal states of the heterotrophic species [Norris et al. 2013]. This could enable them to modulate the absorption and utilization of resources in the gut of the host based on changing composition of food supply and internal states of the host. Hence, gut bacteria can determine how much of the consumed food-energy the host can assimilate and how much passes out back to the ecosystem.
Gut bacteria are known to modulate host appetite as well as their internal state of satiety [ Norris et al. 2013 ]. Research provides evidence that gut bacteria can even determine metabolic phenotypes in humans [Li et al. 2008 ]. Gut bacteria have also been shown to modulate growth rates and reproduction rates in some species [Storelli et al. 2011] . The species composition of gut bacteria biome can influence the level of absorption of nutrients. For instance, in human populations certain species of bacteria are associated with higher fat absorption and lead to obesity in their hosts [ Turnbaugh et al. 2006 ; Ley et al. 2006 ].
One of the most prominent food classes whose absorption is influenced by gut bacteria is carbohydrates [ Hooper et al. 2002 ; Flint et al. 2012 ]. Most mammals cannot digest and absorb complex polysaccharides from plants. Gut bacteria digest these sugars in the gut and break them down into short chain fatty acids that are readily absorbed by the host. By modulating carbohydrate metabolism gut bacterial biomes can significantly influence the calories available to aspecies [ Flint et al. 2012 ].
The species composition of gut bacterial biome is known to be dependent on the diet of the host [ Jumpertz et al. 2011 ]. Further, changes in diets produce changes in species composition [ David et al. 2014 ]. Such changes in species compositions of gut bacteria could be associated with changes in growth rates in host species [Storelli et al. 2011] . Hence it is possible that the bacterial colonies in the gut can sense changing patterns in food production (from changes in dietary intake of its hosts) and change its species composition and growth rates of species. Gut bacteria can also affect the nutritional state, motivational state [Norris et al. 2013] and activity levels in host species.
Certain species of gut bacteria also produce plant hormone like substances when faced with nutritional stress and such signaling could influence metabolic states in plants [Goffin et al. 2010] . Recent findings suggest that gut bacterial compositions play a vital role in the evolutionary emergence of new species [Brucker and Bordenstein 2013] , hence defining metabolic niches for species in an ecosystem. Interestingly, gut bacterial biomes show geographical variation even within the same species [Suzuki and Worobey 2014] . This evidence broadly suggests that ecological factors could influence composition of gut bacterial biomes and such biomes could play a role in defining metabolic niches for species.
Evidence presented above suggests that changes in the composition of colonies of gut bacteria can significantly modulate the energy and nutrients available, activity levels, and reproduction rates in heterotrophic species. In doing so, could gut bacteria serve to modulate species compositions in autotrophic-heterotrophic exchange networks in ecosystems? Further, could this also modulate the distribution of plant production between competing heterotrophic species, so as to maximize the overall benefit to the entire autotroph-heterotroph ecosystem? While definitive answers require further research, preliminary findings point to an important role of gut bacteria in modulation of flows of biomass between competing ecosystem exchange networks.
