Stable isotope enrichment (δ15N and δ13C) in a generalist predator (Pardosa lugubris, Araneae: Lycosidae): effects of prey quality.

Analysis of the natural variations in stable isotope ratios in animal tissue may be a powerful tool to reveal the trophic position and feeding preferences of generalist predators such as lycosid spiders. In the present study, changes in 15N/14N and 13C/12C ratios in the lycosid spider Pardosa lugubris (Araneae: Lycosidae), fed with prey of different quality, were investigated. Experimental food chains included three trophic levels: prey media, prey organism and predator. In order to analyse the time course of stable isotope enrichment, different life stages of P. lugubris feeding on Drosophila melanogaster were studied. The 15N content of hatchlings of P. lugubris was significantly lower than that of their mothers, indicating the existence of nitrogen pools with different 15N signatures in female P. lugubris. With duration of feeding and progressing development, age and body weight, the 15N content in spiderlings increased. Starvation resulted in 15N and 13C enrichment in P. lugubris. Being fed with prey of different quality resulted in varying patterns of stable isotope enrichment. 15N but not 13C content consistently increased when fed on high quality prey (Heteromurus nitidus, D. melanogaster and a mixed diet consisting of H. nitidus and D. melanogaster). In contrast, low quality prey (Rhopalosiphum padi, Folsomia candida and a mixed diet consisting of H. nitidus, D. melanogaster and F. candida) resulted in deviations from postulated patterns of stable isotope enrichment with 15N content being similar to that of starving spiderlings. When fed on high quality prey, 15N enrichment of P. lugubris was close to ca. 3‰. It is concluded that the analysis of variations in the natural abundance of stable isotopes is particularly helpful in revealing the trophic structure of terrestrial food webs in which polyphagous feeders predominate, as is the case in litter and soil.