Bottom-up versus Top-down regulation of pond ecosystems with the invasive Gambusia affinis

Abstract

Introduction of predators or the increase in nutrient additions can influence pond food webs through “bottom-up” and “top-down” regulations. Understanding the synergistic effects of invasive species and nutrient additions is important if the threat of these issues is to be managed effectively. This study examines how nutrient additions of nitrogen and phosphorus influence the invasive fishes, Gambusia affinis life histories, feeding ecology and trophic interactions within pond ecosystems. This was conducted through the combination of a field survey, smaller scale manipulations (laboratory feeding and excretion experiments) and a larger scale experiment (mesocosm experiment) to elucidate the mechanisms driving the top-down and bottom-up controls of these systems. The field survey studied G. affinis life histories and diets within 10 urban ponds. Phosphate concentrations and specific conductivity were the important determinants in pond ecology for these sites. Although, the interaction of bottom-up and top-down control was only evident on zooplankton trophic ecology. In the factorial designed mesocosm experiment, nutrient concentrations were manipulated under the presence and absence of G. affinis. Observed effects on zooplankton and phytoplankton communities showed how bottom-up and top-down interactions are both important in regulating these systems. Over time bottom-up control effects were uncontrollable by top-down consumers, thus nutrient enrichment became the primary control. This suggests that managemental decisions should focus on reducing nutrient inputs as a primary concern, although these decisions should incorporate the combination of reducing both nutrient enrichment and invasive species to have the desirable environmental impacts. Top-down and bottom-up influence by G. affinis was investigated individually through feeding and excretion experiments. Their bottom-up regulation was examined through their Phosphate (PO₄⁺) and Ammonia (NH₄⁺) excretion rates. Body mass and site nutrient concentrations were positively correlated with excretions rates. Top-down influence was investigated through feeding rates and dietary preferences against nutrient enrichment. Feeding rates were positively correlated with nutrient enrichment.