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In theory, predator—prey pairs with extinction—prone local populations can persist through metapopulation dynamics, wherein local populations fluctuate asynchronously, occasionally providing dispersers that prevent permanent extinction in all patches. A few studies have shown that spatial structure can extend predator—prey persistence. However, no studies have unequivocally demonstrated the asynchrony among patches, low dispersal rates, and rescue effects that prove metapopulation dynamics extend persistence. We used a protist predator—prey pair to show that spatial subdivision lengthens persistence through metapopulation dynamics. The pair comprised the predaceous ciliate, Didinium nasutum, feeding on the bacterivorous ciliate, Colpidium cf. striatum. A replicated experiment assessed how habitat subdivision affects persistence. Undivided habitats were of four volumes: 30, 180, 270, and 750 mL. Subdivided microcosms, or arrays, were groups of nine or 25 linked 30—mL bottles (270 or 750 mL total volume). In arrays, predators and prey persisted for 130 d (602 prey and 437 predator generations), at which point the experiment ended. Predators went extinct in undivided microcosms of equivalent volumes within a mean of only 70 d. Predators persisted for a mean of just 19 d in isolated 30—mL bottles (equivalent to isolated patches of arrays). In a separate experiment, prey were driven extinct in four of 15 isolated 30—mL bottles, and persistence times of predators were broadly similar. We documented the following hallmarks of metapopulation dynamics: (1) asynchronous fluctuations in different subpopulations; (2) frequent local prey extinctions and recolonizations; (3) persistence of protists in arrays, despite extinction of isolated local populations; and (4) rescue effects in predator populations. Other experiments measured dispersal rates and the effects on local dynamics of immigrant predators and prey, and initial predator: prey ratios. Only a small fraction of protists dispersed within a generation, consistent with metapopulation dynamics. Immigration of predators increased the frequency of local extinctions of prey, and immigration of prey increased the persistence of both predators and prey. Higher initial predator: prey ratios decreased the persistence of prey in undivided volumes. Although the pair persisted regionally in arrays, data indicated that local extinctions of prey were common. In array patches, predator: prey ratios were higher and predator—prey cycles were shorter than in undivided volumes. Dispersal made local dynamics more prone to extinction, yet promoted regional persistence because the risk of extinction of distant subpopulations became independent.

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In theory, food chain length and omnivory are pivotal elements of food web structure that can affect the population dynamics of species within the web. Long food chains are thought to be less stable than shorter food chains, and omnivores are thought to destabilize food webs, although populations of omnivores may be more stable than populations of nonomnivores. In three of four simple food webs assembled from bacteria and protists in laboratory microcosms, the abundance of bacterivorous protists varied more over time when the species occurred in longer versus shorter food chains. The abundance of protists attacked by omnivorous top predators was either more or less temporally variable than in webs where top predators fed only at one adjacent trophic level, depending on the particular combination of interacting species. The abundance of omnivorous top predators varied less over time than the abundance of top predators restricted to feeding only at an adjacent trophic level. Observations of increased temporal variation in prey abundance in longer food chains and low temporal variation in omnivore abundance agree broadly with several predictions of food web theory. The observation that different species in similar trophic positions can exhibit very different dynamics suggests that stability may depend on complex interactions between species-specific life-history traits and general patterns of food web architecture.