Effects of nutrient enrichment on predator-prey metapopulation dynamics

1. In certain predator-prey models, enrichment of nutrients produces large amplitude population oscillations that can cause extinctions; this effect h as been termed the 'paradox of enrichment'. Metapopulation models also show that despite enrichment reducing the stability of equilibria in local popu lations, spatially subdivided populations may show bounded fluctuations bec ause of asynchronous local density fluctuations. 2. Effects of enrichment and subdivided habitat structure were tested using two protozoans, predatory Didinium nasutum Muller and its bacterivorous pr ey, Colpidium striatum Stokes. A replicated experiment compared predator-pr ey dynamics in low, medium and high nutrient microcosms. Microcosms consist ed of subdivided arrays of nine interconnected 30 mL bottles, undivided bot tles of the same total volume (270 mt) and isolated 30 mt bottles. 3. Consistent with the paradox of enrichment, isolated populations produced greater mean predator densities at high nutrient concentrations than low o r medium concentrations. The proportion of replicates in which predators dr ove prey extinct was also greater with high than with low or medium nutrien t concentrations. 4. Enrichment increased prey density in all microcosm types. This is consis tent with published predator-prey models where dynamics are mathematically unstable. 5. Predators and prey persisted longer in all subdivided microcosms than in undivided bottles. However, there were some unexpected changes in spatial dynamics with enrichment. For prey, direct or indirect effects of enrichmen t raised numbers dispersing, spatial synchrony of density fluctuations and variability in the number of patches occupied. 6. For predators in arrays, enrichment reduced the starvation rate, total n umbers dispersing, and spatial synchrony. Time until recolonization of indi vidual patches by predators was also longer in enriched microcosms, suggest ing a possible decrease in the likelihood of regional persistence. A publis hed simulation model produced qualitatively similar changes in dynamics wit h changes in prey and predator dispersal rate. 7. This study shows that spatial dynamics can potentially explain persisten ce of subdivided populations for long periods despite nutrient enrichment. However, the complex influence of enrichment on spatial dynamics illustrate s the need to study more natural populations.