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In aquatic systems, the ratio of pre - dator mass to prey mass (PPMR) is an important constraint on food web structure, and has been correlated with environmental stability. One com- mon approach of estimating PPMR uses nitrogen stable isotopes (δ 15 N) as an indicator of trophic position, under the assumption that the discrimi - nation between diet and tissue is constant with increasing diet δ 15 N (an additive approach). How- ever, recent studies have shown that this assump- tion may not be valid and that there is a negative trend between the δ 15 N of the diet and the dis- crimination value (a scaled approach). Here, we estimated PPMR for a simulated food web using both the traditional additive approach and the improved scaled approach, and then tested our predictions with isotope samples from a North Sea food web. Our simulations show that the additive approach yields incorrect estimates of PPMR, and these biases are reflected in North Sea PPMR esti- mates. The extent of the bias is dependent on the baseline δ 15 N and trophic level sampled, with the greatest differences for samples with low baseline δ 15 N sampled at lower trophic levels. The scaled approach allows for the comparison of PPMR across varying δ 15 N baselines and trophic levels, and will refine estimates of PPMR.

Estimation of predator-prey mass ratios using stable isotopes: sources of errors