Understanding Eltonian biomass pyramids with size-based ecological theory

Background/Question/Methods

A central goal in ecology to understand bulk properties of communities, particularly the flow of energy as embodied in the distribution of abundance and biomass among size classes and trophic levels. Eltonian pyramids of decreasing abundance and biomass at increasing size (trophic level) are widely used for framing ecological questions relating to this goal.  While the pyramid structure is taken for granted, the detail of the possible shapes that Eltonian pyramids may take - in particular how they can be inverted - has been of particular interest, especially in reconstructing ecological baselines. Size spectra – the relationship between body-mass class and abundance or biomass, typically on log axes – are another size-based representation of community structure, and have a strong theoretical foundation. Two key parameters are known to constrain size spectra slopes: community predator-prey-mass-ratio (PPMR) and transfer efficiency (TE). We sought to explicitly link size spectra and Eltonian pyramids, and to consider what size spectra theory reveals about constraints on possible Eltonian pyramid shapes. Specifically, we ask what conditions could lead to inverted biomass pyramids, and how these conditions can be empirically assessed. We explore available empirical data with case studies from the North Sea and Pacific coral reef fish communities.

Results/Conclusions

We show that the slopes of size spectra directly reflect the shapes of Eltonian pyramids, and the possible shapes of pyramids may be understood by considering the parameter space for TE and PPMR. Inverted biomass pyramids theoretically may occur in highly efficient communities (TE 0.18 to 0.32 for realistic PPMRs of 100 to 1000), and/or at high community PPMRs (4096 to 10,000 for realistic TEs of 0.1 to 0.125). Empirically we show that such conditions may occur for subsets of assemblages (particularly in primary producer-consumer interactions), but have not been observed at the community-wide scale. Case studies indicate that inverted biomass pyramids are unlikely to represent the baseline community structure for fish communities. Instead, apparent inverted biomass pyramids reflect subsidized components of coupled food webs, such as large mobile predator aggregations and detrital communities. We encourage studies that empirically estimate PPMR and collect individual-level size-abundance data, as such data together allow powerful comparisons between theoretical predictions end empirical data for community structure.