Location: Vegetable Crops ResearchTitle: Trophic hierarchies illuminated via amino acid isotopic analysis Author
Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2013
Publication Date: 9/25/2013
Publication URL: http://handle.nal.usda.gov/10113/58276
Citation: Steffan, S.A., Chikaraishi, Y., Horton, D.R., Ohkouchi, N., Singleton, M., Hogg, D.B., Miliczky, E., Jones, V. 2013. Trophic hierarchies illuminated via amino acid isotopic analysis. PLoS One. 8(9):e76152. Available: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0076152. Interpretive Summary: Enrichment or depletion of isotopes within an organism often reveals elements of its trophic history. Isotopic analysis of biomass, therefore, has rapidly become a new lens with which to examine individuals and populations within their respective communities. Variability in the background signatures of isotopes, however, may exceed the isotopic shifts deriving from consumption/assimilation of biomass, particularly for 15N:14N ratios. Further, when the nitrogen isotopes of a sample are analyzed in bulk, the trophic enrichment factor can vary widely, rendering trophic level estimates inaccurate and/or imprecise. Compound-specific isotopic ratio analysis (CSIRA) has emerged as an alternative way to estimate the trophic niche of an organism while addressing background heterogeneity and variable trophic enrichment factors. The CSIRA approach measures 15N:14N ratios of specific amino acids within an individual, and has been investigated in marine, aquatic, and terrestrial systems. We tested the accuracy of this approach using plant and arthropod species of known trophic levels. Trophic levels 1.0 (plants), 2.0 (herbivores), 3.0 (strict carnivores), and 4.0 (tertiary carnivores) were established in a controlled feeding study and then assayed using CSIRA. We found that the trophic enrichment factor was consistent across all trophic levels, and that a recently published formula for trophic level estimation in C3-plant food webs was accurate. We then determined the relationship between the results of bulk 15N analyses and CSIRA analyses, which facilitated trophic level estimates for many individuals, thereby generating trophic spectra for the focal species in a terrestrial predator community.
Technical Abstract: This research addresses the problem of discerning whether natural enemies in agricultural systems are actually suppressing pest populations (or simply eating other predators). Knowing the ecological function of natural enemies (particularly arthropods) is an integral part of biological control programs. Currently, there are a number of methods, many of which involve trace isotopes, but these have suffered from a variety of logistical issues. A new method has emerged, referred to as compound-specific isotopic ratio analysis (CSIRA). This method has proven robust and accurate, but questions have emerged as to whether the trophic enrichment factor compresses for species feeding at higher trophic levels. Our study addresses the consistency of the enrichment factor, as well as the accuracy of the newer compound-specific approaches. We pose the question: what approach is most appropriate when using isotopic methods? Compound-specific or bulk-15N? Given the greater accuracy of CSIRA methods in our data as well as other past work (Chikaraishi et al. 2009, DeNiro & Epstein, Post et al. 2002), compound-specific may be superior in most instances. While the CSIRA methods may be more accurate and largely validated for TLs 1, 2, and 3, there have been questions as to the validity of the method for higher-order consumers. The basis of the problem centers on the possible compression of the TEF among higher-order consumers. Recent findings suggest that for top marine predators, the TEF may become significantly reduced (i.e., the assimilation of animal-derived amino acids enriches 15N in the consumer less than when plant-derived proteins are consumed). A critical component of the CSIRA approach is the consistency and universality of the trophic enrichment factor across all trophic levels. While it is relatively simple to confirm the validity of the TEF for pure (non-parasitized) plant and herbivore cultures, it has proven difficult to confirm the TEF for higher-order consumers, particularly for species with a tendency toward opportunistic intraguild predation. Our study examined the TEF of pure cultures of TL-2, -3, and -4 animal species. We determined the TEF for animals consuming only plant biomass, animals consuming only herbivore tissues, and animals consuming only predator biomass. Our data suggest that the TEF was the same for plant feeders as higher-order consumers. This is the first evidence of the consistency of the TEF across 4 trophic levels, suggesting that the 7.6 TEF is robust across multiple trophic levels in terrestrial arthropods.