Fig. 1.

Alternative stable states (ball and cup model) of human gut microbial ecosystems. Here we conceptualize the ecological stability of the human gut microbiota as inferred through its species richness. The gut microbial ecosystem is represented as a ball that exists within an equilibrium represented as a cup. The depth of each cup is symbolic of the resilience of a given ecosystem and is related to species richness, where lower diversity leads to lower resilience and the greater the likelihood that the “ball” will roll out of the “cup” into a new state of equilibrium that may be less stable that the first. Perturbational stresses caused by numerous factors such as poor diet, disease state, drug use (including antimicrobials), immunosenescence as examples can impact bacterial diversity and force the ecosystem into a less-stable equilibrium state. Repeated stresses can cause a situation where function may be reduced to the point that dysbiosis and the development of disease ensue.

Adapted from (Relman, 2012) and (Folke et al., 2004).