Effects of nutrient enrichment on predator-prey metapopulation dynamics

Abstract

Summary 1. In certain predator–prey models, enrichment of nutrients produces large amplitude population oscillations that can cause extinctions; this effect has been termed the ‘paradox of enrichment’. Metapopulation models also show that despite enrichment reducing the stability of equilibria in local populations, spatially subdivided populations may show bounded fluctuations because 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 prey, Colpidium striatum Stokes. A replicated experiment compared predator–prey dynamics in low, medium and high nutrient microcosms. Microcosms consisted of subdivided arrays of nine interconnected 30 mL bottles, undivided bottles of the same total volume (270 mL) and isolated 30 mL bottles. 3. Consistent with the paradox of enrichment, isolated populations produced greater mean predator densities at high nutrient concentrations than low or medium concentrations. The proportion of replicates in which predators drove prey extinct was also greater with high than with low or medium nutrient concentrations. 4. Enrichment increased prey density in all microcosm types. This is consistent 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 enrichment 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 numbers dispersing, and spatial synchrony. Time until recolonization of individual patches by predators was also longer in enriched microcosms, suggesting a possible decrease in the likelihood of regional persistence. A published simulation model produced qualitatively similar changes in dynamics with changes in prey and predator dispersal rate. 7. This study shows that spatial dynamics can potentially explain persistence of subdivided populations for long periods despite nutrient enrichment. However, the complex influence of enrichment on spatial dynamics illustrates the need to study more natural populations.