Kefir – a traditional fermented milk drink with a long history – is a fascinating example of ecological success at community-scale. The kefir microbial community stands out amongst other fermented food communities with its richness (>40 species), robustness, and spatial organisation. I will present my lab's findings unravelling the intricate spatio-temporal interactions in the kefir fermentation, discuss implications for understanding principles of community assembly and evolution, and show examples of practical applications. I will also present recent work on gut bacterial communities where similar principles operate and discuss our efforts in modelling community dynamics and evolution.