Individual-level Data Reveal High Prevalence of Positive Size-trophic Position Relationships for Vertebrates in Temperate Streams

1. A positive relationship between body size and trophic position is often assumed in ecology, but efforts to confirm the generality of this relationship for freshwater vertebrates have produced mixed results. Some authors have tested for among species relationships, using species-level estimates of average size and trophic position. Others have used individual-level body size and trophic position data to test for within-species relationships. However, no study has yet estimated prevalence, defined here as the fraction of total standing stock biomass within a given ecosystem that consists of taxa exhibiting positive size versus trophic position relationships.
2. Individual-level estimates of body size and relative trophic position (inferred from bulk-tissue nitrogen stable isotopes) were collected for vertebrates in six temperate streams. In each stream, all locally occurring vertebrate taxa were collected and closed population depletion samples were used to obtain standing stock biomass estimates.
3. Ordinary least squares regression was used with the individual-level data to test for positive relationships between body size and relative trophic position (r-STPRs). A separate r-STPR model was tested for each vertebrate species collected at the six study sites. Linear mixed-effects modelling was then used to test for differing r-STPRs among species.
4. Prevalence of the r-STPR was calculated for each site by summing the standing stock biomass of all taxa that exhibited a statistically significant, positive STPR at a given site, then dividing this number by the total standing stock biomass of vertebrates at the site.
5. Significant, positive r-STPRs were detected in 15 of 22 species × site regression models. Prevalence of species with positive r-STPRs ranged from 91 to 100% of the total vertebrate biomass in western streams and from 45 to 66% in eastern streams.
6. Results confirm that positive r-STPRs are characteristic of many of the vertebrate taxa considered in this study. Furthermore, species that exhibit positive r-STPRs comprise a clear majority of the standing stock biomass in five of six study streams. By using individual-level data to account for prevalence, a more complete understanding of size-dependent trophic dynamics should be possible in freshwater ecosystems.