THE ROLE OF DISPERSAL IN PREDATOR-PREY METAPOPULATION DYNAMICS

1. We report the role of dispersal in the metapopulation dynamics of a protist predator-prey pair, the predaceous ciliate Didinium nasutum M uller feeding on the bacterivorous ciliate Colpidium cf. striatum Stok es. In previous work we showed that this extinction-prone pair persist ed as metapopulations in subdivided habitats. 2. An experiment assesse d the effects of habitat subdivision on persistence and dynamics. Undi vided habitats were 270 or 750 mL in volume, and subdivided habitats ( arrays) were sets of nine or 25 linked 30 mL bottles (270 or 750 mL to tal volume), each replicated three times. Undivided microcosms allowed maximum dispersal, whereas subdivision reduced dispersal. Within arra ys, bottles with more connecting tubes allowed more dispersal. Nine an d 25 bottle arrays also differed in the mean number of connections per bottle. The effects of dispersal on predator-prey dynamics were teste d by comparing subdivided vs. undivided microcosms, bottles with diffe rent numbers of connecting tubes, and nine vs. 25 bottle arrays. 3. We tested the following predictions from metapopulation theory. (i) Pred ator and prey persistence and predator abundance will be greatest at i ntermediate dispersal rates. (ii) Prey abundance, local population var iability and asynchrony in population fluctuations will be greatest at low dispersal rates. (iii) Predator:prey ratios will be greatest at h igh dispersal rates. 4. Predictions were confirmed, except for the fol lowing. (i) Two measures of synchrony differed in whether they showed the expected pattern. Spatial synchrony (estimated via correlation of densities among patches within sampling dates) showed high variance an d did not vary with dispersal rates. However, spatial variability (CV of density across adjacent pairs of linked bottles), showed the predic ted decrease with increased dispersal. (ii) Evidence that dispersal in creases predator:prey ratios was inconclusive. Predator:prey ratios we re lower in undivided 750 mL microcosms than in 750 mL arrays, possibl y because predators over-exploited prey in undivided microcosms, so th at both became scarce. Conversely, within arrays, predator:prey ratios were greatest in bottles that allowed the most dispersal, as predicte d. 5. This work generally confirms the predicted effects of dispersal on predator-prey metapopulation dynamics. It also demonstrates the nee d for models to include more realism, e.g. the possibility of over-exp loitation with very high dispersal.