Making a microbiome: the many determinants of host-associated microbial community composition.

The composition of many host-associated microbial communities is characterized by seemingly contradictory features: strong selection for specific taxa by the host, but substantial variability among hosts and over time within one host. Recent advances have revealed that both deterministic and stochastic processes operating across multiple spatial scales shape the composition of host-associated microbial communities. Although most research has focused on deterministic processes within individual hosts, the microbiota within each host is increasingly recognized to contribute to a wider metacommunity maintained by transmission between individual hosts and dispersal between host-associated and free-living microbial communities. By applying a community ecology perspective encompassing the microbial metacommunity, the many determinants of host-associated microbial community composition can be identified, guiding the directions of future research.