Sources of variance in Δδ¹⁵N (‰).

Variance in Δδ15N as accounted for by either food chain or tropic link was established by expressing ANOVA model terms as a proportion of the null model variance. Δδ15N is based upon differences in raw δ15N between trophic levels, for all source-consumer links on wheat and nettle food chains only (to provide a balanced analysis). ANOVA was conducted twice: firstly for all source-consumer links and secondly excluding all links including a primary producer. A significant interaction explained greater variance in Δδ15N after primary producer links were excluded. Significant model terms are indicated as *** with P = <0.001.