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A sticky drop of nectar clinging to the tip of a hummingbird’s beak drips into the next flower the bird visits. With that subtle change, the microbes within that drop are now in a new environment, teeming with other microbes. This small example of species forced to coexist in the real world has helped researchers unravel the relative importance of two theories scientists have had about how species can live together. It turns out that a less popular theory, having to do with the way organisms respond and contribute to environmental fluctuations, likely plays a bigger role than ecologists had thought. This concept, called relative nonlinearity, maintains that coexistence can occur when one species thrives off fluctuation in resources, the other thrives off stability in resources, and each species’ use of resources contributes to the state -- fluctuation or stability -- that benefits the other. By creating thousands of microcosms, each growing one species of nectar yeasts, the researchers gathered high-resolution data about the complex ways in which the yeasts respond to environmental conditions. Looking at their findings, the big surprise was that there were instances where a lack of relative nonlinearity led to one species dying out. This contradicts a common assumption among ecologists: the assumption that another concept, called the storage effect, is the main mechanism. Shown here: Cells of Metschnikowia gruessii, one of the four species of nectar-inhabiting yeasts that were used to study how species coexist.