# Establishing isotopic turnover rates and trophic discrimination factors in tiger beetle (Coleoptera: Cicindelidae) larvae through a diet switch experiment

## TL;DR

This study measures how quickly tiger beetle larvae adjust their isotopic signatures after switching diets and calculates the differences between their isotopic values and their prey.

## Contribution

The paper provides new isotopic turnover rates and trophic discrimination factors for tiger beetle larvae through a diet switch experiment.

## Key findings

- Tiger beetle larvae have a δ15N trophic discrimination factor of 1.7 ± 0.2‰ with a half-life of 11.4 days.
- The δ13C trophic discrimination factor is –0.6 ± 0.2‰ with a half-life of 3.9 days.
- After lipid correction, δ13Ccorr has a trophic discrimination factor of –0.2 ± 0.2‰ and a half-life of 4.7 days.

## Abstract

Stable isotope ratios give insight into food web interactions, but interpretation can be clouded by the timing of isotopic change associated with changes in diet and the difference in isotope ratios between consumers and their diets at equilibrium. The 15N/14N, deemed δ15N, increases with each trophic transfer as 15N becomes enriched, whereas the 13C/12C ratio, denoted as δ13C, remains relatively constant with each trophic transfer but can be influenced by lipid content. This study reports the trophic discrimination factors and isotopic half-lives in tiger beetles (Coleoptera: Cicindelidae). Wild-caught tiger beetle larvae were reared in a laboratory setting, subjected to a diet switch experiment, and sampled over time. Quadratic plateau models were used to characterize the change in δ15N, δ13C, and δ13Ccorr over time. Trophic discrimination factors were calculated by subtracting the mean prey δ15N, δ13C, and δ13Ccorr from that of the tiger beetle asymptotic δ15N, δ13C and δ13Ccorr values, respectively. The tiger beetle trophic discrimination factor for δ15N was 1.7 ± 0.2‰ with a half-life of 11.4 days. For δ13C, the trophic discrimination factor and half-life were –0.6 ± 0.2‰ and 3.9 days, respectively. After correcting for lipids (δ13Ccorr), the trophic discrimination factor was –0.2 ± 0.2‰ with a half-life of 4.7 days. Isotopic turnover was fast with short half-lives, but factors that influence metabolic rates, such as ambient temperatures and life stage, should be considered when applying these estimates to wild tiger beetles. Despite this, the trophic discrimination factors and turnover rates calculated in this article are suitable estimates which can be applied to future studies.