A non-lethal sampling method for estimating the trophic position of an endangered giant water bug using stable isotope analysis


Shin-ya Ohba, Center for Ecological Research, Kyoto University, 2-509-3, Hirano, Otsu 520-2113, Japan. E-mail: oobug@hotmail.com


Abstract.  1. We propose a non-lethal sampling method involving stable isotope analysis for estimating the trophic position of the endangered giant water bug Kirkaldyia (=Lethocerus) deyrolli (Heteroptera: Belostomatidae) in the wild.

2. Kirkaldyia deyrolli individuals were collected and their δ15N and δ13C values were measured. The δ15N and δ13C values of periphyton and particulate organic matter, the basal food sources in lentic ecosystems of rice fields, were also measured to estimate the trophic position of K. deyrolli. When individual isotopic signatures of the whole body were compared with those of their middle leg tarsus, we found strong correlations between them for both δ15N and δ13C. To estimate their trophic position without killing individuals, we constructed a regression model incorporating their middle leg tarsus’s isotopic signatures and their body size as explanatory variables. This non-lethal method revealed that K. deyrolli showed great individual variation in its δ15N which is a proxy of trophic position, ranging from 5.60‰ to 8.11‰.

3. To evaluate the negative effects of our non-lethal method on the fitness of K. deyrolli, we examined how the removal of the middle leg tarsus affected reproductive performance under laboratory conditions. A comparison between the manipulated and unmanipulated individuals revealed that the removal treatment did not have any negative effects on female clutch size or egg hatchability for males.

4. In conclusion, stable isotope analysis of the middle leg tarsus of K. deyrolli is useful for estimating its trophic position without lethal or any negative fitness effects.