Influence of invasive species and oligotrophication on the size-structure and emergent food web properties of a large river fish assemblage

Background/Question/Methods

Aquatic food webs are highly size-structured owing to metabolic scaling, competition for limited resources, and trophic transfer energy losses. Resource availability, transfer efficiency, and metabolic scaling collectively have a strong influence on food web carrying capacity and stability. Food web size-structure, stability, carrying capacity, and transfer efficiency are important emergent food web properties. Size-spectra, log abundance (or normalized biomass) regressed against log body size, are an effective approach for assessing food web stability and functioning. Size-spectra integrate bottom-up and top-down processes and are fundamentally based on the well established allometric relationships between body size, metabolism, and abundance. Size-spectra greatly simplify complex food web dynamics and provide metrics of community capacity and ecological efficiency.  Size-spectra have also been used as indices of food web health and have been shown to respond to commercial harvest and to a lesser degree nutrient pollution. However, there has been little additional empirical work examining the response of size-spectra to other natural and anthropogenic stressors, and in particular the influence of invasive species. We used a long-term (1982-2010) experimental gillnet data set to evaluate the stability of emergent food web properties (capacity and ecological efficiency) and assess the impacts of invasive species on these same properties.

Results/Conclusions

The size-spectra elevation and slope (food web capacity and ecological efficiency, respectively) were relatively stable over time (Tilman’s S (1/CV) = 5.2 and 15.0, respectively), but were both influenced by ecosystem changes.  Oligotrophication (related to declining total phosphorus and invasive dreissenid mussels) explained 42.6% of the variance in annual size-spectra elevation.  We observed three distinct time periods of the double-crested cormorant (DCC) invasion; pre-DCC, growth, and stabilization, which each had a unique impact on food web properties.  Food web capacity (size-spectra elevation) was significantly lower during the growth and stabilization periods than during the pre-DCC period (P = 0.0002) and DCC abundance described 60% of the variance (decline) in fishery capacity during the DCC-growth period.  Similarly, the size-spectra slope (ecological efficiency) was significantly more negative (less efficient) during the DCC-growth period than during the stabilization period (P = 0.0246).  Ecosystem changes and invasive species had both bottom-up (i.e. declining phosphorus and dreissenid mussels) and top-down (DCC) effects on food web capacity and ecological efficiency.  These results will help inform further development of indices of ecosystem well-being from size-spectra relationships.