Pinning and pulsed expansion in a spatially expanding bacterial mutualism

Range expansions occur when a population expands in space due to dispersal and growth. These expansions can result from environmental change, introduction of invasive species, or evolutionary adaptation that enables a population to move into previously unoccupied territory. Theoretical work has shown that fragmented habitats and seasonal growth (discrete space and discrete time) can result in pinning and pulsed invasions in expanding populations with an Allee effect. However, it is not clear how underlying population dynamics such as limit cycle oscillations influence these phenomena. We probe this question in an experimental model system consisting of an oscillating bacterial mutualism inhabiting discrete population patches and subject to periodic growth cycles. For low nearest-neighbor migration rates, the mutualism cannot expand, and the population is pinned in place. For high nearest-neighbor migration rates, the mutualism expands at pulsed speeds. Furthermore, our experimental results are consistent with a mechanistic model prediction that the period of population oscillations locks into specific values during pulsed invasion, emphasizing the interplay between ecological interactions and spatial structure.