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

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 δ¹³C_corr over time. Trophic discrimination factors were calculated by subtracting the mean prey δ¹⁵N, δ¹³C, and δ¹³C_corr from that of the tiger beetle asymptotic δ¹⁵N, δ¹³C and δ¹³C_corr 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 (δ¹³C_corr), 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.