An exercise in the natural philosophy of ecology

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Damming of rivers is one of the main human interferences in natural ecosystems. With the purpose to analyze food webs in 30 reservoirs of the state of Paraná, southern Brazil, we built standardized models (same number of compartments) for each reservoir in the software Ecopath. We tested the existence of an association among eight attributes described in Odum's theory (estimated by the models built in Ecopath and summarized in a Principal Component Analysis—PCA) and some characteristics of the reservoirs, such as age and area. Compartments were categorized in two groups: ‘non-fish’ (phytoplankton, periphyton, rotifers, copepods, cladocerans, bacteria, protozoans, benthos and detritus) and ‘fish’ (omnivorous, insectivorous, piscivorous, herbivorous, detritivorous and invertivorous). To balance the models, diet (main food items) and biomass of some groups (especially periphyton) were modified to get values of ecotrophic efficiency lower than one. We found that Axis 1 of the PCA (which represented 37.1% of the variability of the original data matrix) was significantly correlated with three categories from Odum's theory: community energetics, homeostasis and structure. Axis 2 (20.2%) was not correlated with the attributes, whereas axis 3 (15.6%) was significantly correlated with nutrient cycling and life history. Reservoir age was negatively correlated with axis 1; hence, older reservoirs seemed to be more developed (or senesced) than younger ones. Thus, it is possible to conclude that maturity (or senescence) is an inherent characteristic of reservoir aging, regardless of human interference, reservoir area or number of species. Therefore, the results presented here are additional evidence of the importance to improve Odum's central theory of ecosystem development. In spite of some limitations, we believe that the Ecopath model applied to these reservoirs is a feasible tool for interdisciplinary environmental studies.