  Microbial ecosystems are commonly modeled by fixed interactions between
species in steady exponential growth states. However, microbes often modify
their environments so strongly that they are forced out of the exponential
state into stressed or non-growing states. Such dynamics are typical of
ecological succession in nature and serial-dilution cycles in the laboratory.
Here, we introduce a phenomenological model, the Community State model, to gain
insight into the dynamic coexistence of microbes due to changes in their
physiological states. Our model bypasses specific interactions (e.g., nutrient
starvation, stress, aggregation) that lead to different combinations of
physiological states, referred to collectively as "community states", and
modeled by specifying the growth preference of each species along a global
ecological coordinate, taken here to be the total community biomass density. We
identify three key features of such dynamical communities that contrast starkly
with steady-state communities: increased tolerance of community diversity to
fast growth rates of species dominating different community states, enhanced
community stability through staggered dominance of different species in
different community states, and increased requirement on growth dominance for
the inclusion of late-growing species. These features, derived explicitly for
simplified models, are proposed here to be principles aiding the understanding
of complex dynamical communities. Our model shifts the focus of ecosystem
dynamics from bottom-up studies based on idealized inter-species interaction to
top-down studies based on accessible macroscopic observables such as growth
rates and total biomass density, enabling quantitative examination of
community-wide characteristics.
