Langellotto Gail A, Rosenheim Jay A, Williams Megan R
Department of Entomology, University of California, Davis, CA 95616, USA.
Oecologia. 2006 Nov;150(2):291-9. doi: 10.1007/s00442-006-0514-0. Epub 2006 Aug 9.
Facultative hyperparasitism is likely to be the most common form of intraguild predation among parasitoids. However, difficulties associated with studying facultative hyperparasitoids in the field have hampered a thorough understanding of their trophic ecology. In this study, we used a combination of stable isotope analysis and published natural history information to infer trophic interactions in a guild of field-collected primary parasitoids and facultative hyperparasitoids that attack a gall-making midge on Baccharis pilularis. Our three a priori hypotheses were: (1) stable isotope values should increase incrementally from the host plant to higher trophic levels; (2) the two species of ectoparasitoids should exhibit higher stable isotope signatures than the two endoparasitoids, and; (3) the two facultative hyperparasitoids should exhibit stable isotope signatures that fall between zero and one trophic level steps above that observed for the primary parasitoids. Food webs inferred from stable isotope data generally agreed with previously published accounts of community structure. As expected, both delta(13)C and delta(15)N were progressively enriched in the heavy isotope from the host plant to the herbivorous midge to the parasitic wasps. Multivariate analysis of stable isotope data revealed that the two primary ectoparasitoids occupied a similar trophic niche, but were significantly different from the primary endoparasitoids. We attribute this result to "coincidental intraguild predation" by ectoparasitoids that develop on already-parasitized midge larvae. One of the facultative hyperparasitoids, Zatropis capitis, exhibited a stable isotope signature approximately one trophic step above the primary parasitoids. Unexpectedly, the second facultative hyperparasitoid, Mesopolobus sp., appeared to be developing as a primary parasitoid at all sites. Coupled with independent assessments of community structure, stable isotope analysis validated trophic links constructed by previous researchers and identified potential taxon-specific differences in trophic interactions for two facultative hyperparasitoids in the B. pilularis gall community.
兼性重寄生很可能是寄生蜂中最常见的同类相食形式。然而，在野外研究兼性重寄生蜂存在诸多困难，这阻碍了我们对其营养生态学的全面理解。在本研究中，我们结合稳定同位素分析和已发表的自然史信息，来推断一组从野外采集的、寄生于柱状酒神菊上造瘿蠓的初级寄生蜂和兼性重寄生蜂的营养相互作用。我们的三个先验假设是：（1）稳定同位素值应从寄主植物到更高营养级逐步增加；（2）两种体外寄生蜂应比两种体内寄生蜂表现出更高的稳定同位素特征，并且；（3）两种兼性重寄生蜂应表现出稳定同位素特征，介于初级寄生蜂的特征之上零到一个营养级之间。从稳定同位素数据推断出的食物网总体上与先前发表的群落结构描述一致。正如预期的那样，从寄主植物到植食性蠓再到寄生蜂，δ(13)C和δ(15)N在重同位素中都逐渐富集。稳定同位素数据的多变量分析表明，两种初级体外寄生蜂占据相似的营养生态位，但与初级体内寄生蜂有显著差异。我们将这一结果归因于在已被寄生的蠓幼虫上发育的体外寄生蜂的“偶然同类相食捕食”。其中一种兼性重寄生蜂，头突扁腹姬小蜂，其稳定同位素特征比初级寄生蜂高约一个营养级。出乎意料的是，第二种兼性重寄生蜂，Mesopolobus sp.，在所有地点似乎都作为初级寄生蜂发育。结合对群落结构的独立评估，稳定同位素分析验证了先前研究人员构建的营养联系，并确定了柱状酒神菊瘿群落中两种兼性重寄生蜂在营养相互作用方面潜在的分类群特异性差异。
