The demographic determinants of human microbiome health

• Metabolic interactions drive demographic feedbacks between species.
• Demographic feedbacks strengthen or attenuate underlying metabolic interactions.
• Demography is a key determinant of microbiome development and functioning.
• Treatment strategies must consider the dynamical properties of species interactions.

The human microbiome is a vast reservoir of microbial diversity and increasingly recognized to have a fundamental role in human health. In polymicrobial communities, the presence of one species can modulate the demography (i.e., growth and distribution) of other species. These demographic impacts generate feedbacks in multispecies interactions, which can be magnified in spatially structured populations (e.g., host-associated communities). Here, we argue that demographic feedbacks between species are central to microbiome development, shaping whether and how potential metabolic interactions come to be realized between expanding lineages of bacteria. Understanding how demographic feedbacks tune metabolic interactions and in turn shape microbiome structure and function is now a key challenge to our abilities to better manage microbiome health.