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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.

Exotic species have frequently caused declines of native fauna and may contribute to some cases of amphibian decline. Introductions of mosquitofish ( Gambusia affinis) and bullfrogs ( Rana catesbeiana) are suspected to have caused the decline of California red‐legged frogs ( Rana aurora draytonii). We tested the effects of mosquitofish and bullfrog tadpoles on red‐legged frog tadpoles in spatially complex, speciose communities. We added 720 hatchling red‐legged frog tadpoles to each of 12 earthen ponds. Three ponds were controls, 3 were stocked with 50 bullfrog tadpoles, 3 with 8 adult mosquitofish, and 3 with 50 bullfrogs plus 8 mosquitofish. We performed tests in aquaria to determine whether red‐legged frog tadpoles are preferred prey of mosquitofish. Mosquitofish fed on a mixture of equal numbers of tadpoles and either mosquitoes, Daphnia, or corixids until < 50% of prey were eaten; then we calculated whether there was disproportionate predation on tadpoles. We also recorded the activity of tadpoles in the presence and absence of mosquitofish to test whether mosquitofish interfere with tadpole foraging. Survival of red‐legged frogs in the presence of bullfrog tadpoles was less than 5%; survival was 34% in control ponds. Mosquitofish did not affect red‐legged frog survival, even though fish became abundant (approximately 1011 per pond). Two mechanisms may have blocked the effects of mosquitofish on tadpole survival: (1) fish ponds contained fewer predatory invertebrates, and (2) mosquitofish preferred other prey to red‐legged frogs in laboratory trials. Red‐legged frog tadpoles suffered more injuries in ponds with fish, however, and weighed 34% less at metamorphosis. The growth decrease could have been caused by injuries or by lower foraging levels in the presence of fish. Laboratory results showed that young tadpoles were less active in the presence of mosquitofish. Although both mosquitofish and bullfrogs affected red‐legged frogs, the impact of bullfrogs on the survival of red‐legged frogs may contribute more strongly to their decline.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Interspecific competitors often colonize communities at different times, but few studies have experimentally tested whether the strength of interspecific competition hinges on the order or temporal separation of species' arrivals. We added hatchlings of two sympatric anuran species (Hyla [= Pseudacris] crucifer and Bufo woodhousii) to artificial ponds on three different dates to manipulate the order and temporal separation of their arrival. Measurement of the growth and survival of each species in ponds where the second species arrived simultaneously (day 0), or after a delay of 7 or 14 d, indicated whether differences in the history of introductions affected interspecific competition. Other ponds contained H. crucifer alone, introduced on the same three dates, or B. woodhousii alone, introduced on the first or second date. These provided controls for seasonal differences in performance in the absence of competition from the other anuran species.Introductions of tadpoles at different times produced small differences in growth rates and larval periods when each anuran species occurred alone. In ponds containing both species, differences in the order and temporal separation of introductions had complex effects on the intensity of interspecific competition.

Replicated experiments in artificial ponds demonstrated that an assemblage of aquatic insects competed with tadpoles of the frogs Hyla andersonii and Bufo woodhousei fowleri. We independently manipulated the presence or absence of aquatic insects, and the abundance of an anuran competitor (O or 150 Bufo w. fowleri per experimental pond), using a completely crossed design for two—factor variance analysis, and observed the responses of initially similar cohorts of Hyla andersonii tadpoles to neither, either, or both insect and anuran competitors. Insects and Bufo significantly depressed the mean individual mass at metamorphosis of Hyla froglets and the cumulative biomass of anurans leaving the ponds at metamorphosis. Neither insects nor Bufo affected the survival or larval period of Hyla. Insects also significantly reduced the mean mass of Bufo, showing that both anurans responded to competition from insects. The intensity of competition between natural densities of insects and Hyla tadpoles was comparable to the intensity of competition between Bufo and Hyla, as a density of 150 Bufo/1000 L.