Rescaling stable isotope data for standardized evaluations of food webs and species niches

Abstract

Many human and natural events can impact aquatic populations and communities, leav- ing strong imprints as altered food web dynamics. Stable C and N isotopes in fish can record these altered trophic dynamics in an integrated way, and a new simple methodology is presented to extract measures of food web change from isotope measure- ments of fish species. Measured C and N isotope data are re-scaled as modified Z-scores to equalize the effects of average food inputs, then graphed in (x,y) space to obtain food web shapes. Pairs of food webs are compared for shifts in Euclidean distance of isotope values for individual species and pairs of spe- cies, and these shifts are used to assess overall con- stancy of species niches and fish food webs. Appli - cation of the methodology shows that (1) on average, estuarine fish species are relatively fixed in their trophic niches, with only 14 to 24% variability in the niches; (2) relatively undisturbed natural fish com- munities often show substantially different food web structure even when the same species of fish are present; and (3) factors such as species invasions and dredging result in marked differences between food webs. Isotope evaluations of possible food web shifts should be valuable in many conservation and resto- ration contexts.