Publication Abstract

Measurement of body size and abundance in tests of macroecological and food web theory

S. Jennings, J.A.A. De Oliveira and K.J. Warr

Mean body mass (W) and mean numerical (N) or biomass (B) abundance are frequently used as variables to describe populations and species in macroecological and food web studies.  We investigate how the use of mean W and mean N or B, rather than other measures of W and/ or accounting for the properties of all individuals, can affect the outcome of tests of macroecological and food web theory. Theoretical and empirical analyses demonstrate that relationships between mean W, W at maximum population biomass (W_mb), W when population energy requirements are greatest (W_mp) and W when a population requires the greatest proportion of available energy in a W class (W_mpp) are not consistently related.  or a population at equilibrium, relationships between mean W or  W_mb, W_mp and W_mpp depend on the slope b of the relationship between trophic level and W. For marine fishes, data show that b varies widely among species and thus mean W is an unreliable indicator of a species’ role in the food web. Two different approaches, ‘cross-species’ and ‘all individuals’ have been used to estimate slopes of abundance-body mass relationships and to test the energetic equivalence hypothesis and related theory. The approaches, based on relationships between (1) log mean W and log mean N or B and (2) log W  and log N or B of all individuals pooled into log W classes (size spectra), give different slopes and confidence intervals with the same data. Our results show that the ‘all individuals’ approach has the potential to provide more powerful tests of the role of energy availability in determining slopes, but new theory and empirical analysis are needed to explain distributions of species relative abundance at W. Biases introduced when working with mean W in macroecological and food web studies are greatest when species have indeterminate growth, when relationships between W and trophic level are strong and when the range of species’ W is narrow.

Reference:

S. Jennings, J.A.A. De Oliveira and K.J. Warr (2007) Measurement of body size and abundance in tests of macroecological and food web theory. Journal of Animal Ecology, 76(1): 72-82