PERSISTENCE OF AN EXTINCTION-PRONE PREDATOR-PREY INTERACTION THROUGH METAPOPULATION DYNAMICS

In theory, predator-prey pairs with extinction-prone local populations can persist through metapopulation dynamics, wherein local population s fluctuate asynchronously, occasionally providing dispersers that pre vent permanent extinction in all patches. A few studies have shown tha t 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 dyna mics 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 repli cated experiment assessed how habitat subdivision affects persistence. Undivided habitats were of four volumes: 30, 180, 270, and 750 mt. Su bdivided microcosms, or ''arrays,'' were groups of nine or 25 linked 3 0-mL bottles (270 or 750 mt total volume). In arrays, predators and pr ey persisted for 130 d (602 prey and 437 predator generations), at whi ch point the experiment ended. Predators went extinct in undivided mic rocosms of equivalent volumes within a mean of only 70 d. Predators pe rsisted for a mean of just 19 d in isolated 30-mL bottles (equivalent to isolated patches of arrays). In a separate experiment, prey were dr iven extinct in four of 15 isolated 30-mL bottles, and persistence tim es of predators were broadly similar. We documented the following hall marks of metapopulation dynamics: (1) asynchronous fluctuations in dif ferent subpopulations; (2) frequent local prey extinctions and recolon izations; (3) persistence of protists in arrays, despite extinction of isolated local populations; and (4) rescue effects in predator popula tions. Other experiments measured dispersal rates and the effects on l ocal dynamics of immigrant predators and prey, and initial predator : prey ratios. Only a small fraction of protists dispersed within a gene ration, consistent with metapopulation dynamics. Immigration of predat ors increased the frequency of local extinctions of prey, and immigrat ion of prey increased the persistence of both predators and prey. High er initial predator:prey ratios decreased the persistence of prey in u ndivided volumes. Although the pair persisted regionally in arrays, da ta indicated that local extinctions of prey were common. In array patc hes, predator:prey ratios were higher and predator-prey cycles were sh orter than in undivided volumes. Dispersal made local dynamics more pr one to extinction, yet promoted regional persistence because the risk of extinction of distant subpopulations became independent.