UBC Theses and Dissertations
The propagation of top-down and bottom-up signals in heterogeneous aquatic food webs Bell, Thomas
The two experiments described in this thesis attempt to identify the mechanisms that determine the biomass of trophic levels in freshwater communities. Early work in this field suggested that trophic level biomass is determined by resource supply and levels of predation. I present the results of two studies that investigate the impact of various other factors on the biomass of trophic levels. In the first study, I describe the short-term impact of prey (phytoplankton) edibility on the phytoplankton response to nutrient enrichment and the addition of a top (third) trophic level. The results suggest that the proportion of inedible phytoplankton in the community modifies the degree to which phytoplankton biomass is regulated by bottom-up (nutrient supply) or top-down (predation) forces. Compensation by inedible phytoplankton could therefore preclude the propagation of top-down signals in these communities. As a result, experiments over longer time scales might show a progressive weakening of top-down signals. In the second study, I therefore describe the results of a long-term (4-year) experiment in which zooplanktivorous fish are either present or absent. The results of this experiment, combined with the results of an extensive literature analysis, provide evidence that there is in fact no obvious decline in the strength of top-down signals with increasing experiment duration. The results do suggest, however, that the experimental results depend to some degree on the type of system in which the study is performed.
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