Assessing trophic interactions in a guild of primary parasitoids and facultative hyperparasitoids: stable isotope analysis

Abstract

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.

Stable isotopes analysis to assess the trophic role of ants in a Mediterranean agroecosystem

“…Although there is a general consensus about such assumptions as, typically, the enrichment in heavier isotopes with each trophic exchange is limited for carbon and more pronounced for nitrogen, there is a growing body of evidence showing deviations from this simple model. Unexpected increases in 13 C enrichment between resources and consumers have been observed in a number of recent investigations (Blüthgen et al , 2003; McCutchan et al , 2003; Mooney & Tillberg, 2005; Langellotto et al , 2006; Sagers & Goggin, 2007), whereas evidences of depletion were observed in other studies (e.g. Blüthgen et al , 2003).…”

Specific bottom–up effects of arbuscular mycorrhizal fungi across a plant–herbivore–parasitoid system

“…However, changes in plant nutrient contents C, N, and P) were not driving the observed performance alterations, as these values were equal between mycorrhizal and non-mycorrhizal plants. Therefore, food choice experiments (Prince et al 2004 ) and stable isotope probing (Langellotto et al 2006 ) would be useful approaches for monitoring changes in preferences and nutrient fluxes. The observed changes in the trophic interactions due to AMF inoculation emphasize that belowground interactions can have strong implications for aboveground food webs (van der Putten et al 2001 ).…”

Trophic ecology of the invasive argentine ant: spatio-temporal variation in resource assimilation and isotopic enrichment

“…Because an organism’s isotopic composition provides a record of resources assimilated, isotopes allow the tracking of nutrients through food webs and can also be used to estimate trophic position (Langellotto et al 2006; Layman et al 2007; Post 2002). For animals with feeding behavior that is difficult to observe or to quantify, isotopic analyses also permit insights into the relative consumption of different food items.…”