|Project 1: Trophic Hierarchies Illuminated via Amino Acid Isotopic Analysis
Steffan, S.A., Y. Chikaraishi, D.R. Horton, N. Ohkouchi, M.E. Singleton, E. Miliczky, D.B. Hogg, V.P. Jones. 2013. Trophic Hierarchies Illuminated via Amino Acid Isotopic Analysis. PLoSONE 8(9):e76152 (2013).
Introduction: Stable isotopic analysis has been an indispensable tool of food web ecology, primarily because the isotopic composition of an organism encodes aspects of its biogeography, physiology, and trophic tendency. As matter and energy are transferred among trophic levels, there is discrimination among isotopes at cellular and molecular levels, not only through fractionation but also via isotopic routing. Assimilated isotopes may be effectively stockpiled in certain tissues or certain molecules, while being randomly incorporated within others. The inter-trophic shift in a consumer’s isotopic composition relative to its diet has been termed the trophic discrimination factor and represents the net effects of enrichment or depletion resulting from fractionation and/or routing.
Figure 2: Trophic position estimates, controlled feeding study.
Abstract: Food web ecologists have long sought to characterize the trophic niches of animals using stable isotopic analysis. However, distilling trophic position from isotopic composition has been difficult, largely because of the variability associated with trophic discrimination factors (inter-trophic isotopic fractionation and routing). We circumvented much of this variability using compound-specific isotopic analysis (CSIA). We examined the 15N signatures of amino acids extracted from organisms reared in pure culture at four discrete trophic levels, across two model communities. We calculated the degree of enrichment at each trophic level and found there was a consistent trophic discrimination factor (~7.6‰). The constancy of the CSIA-derived discrimination factor permitted unprecedented accuracy in the measurement of animal trophic position. Conversely, trophic position estimates generated via bulk-15N analysis significantly underestimated trophic position, particularly among higher-order consumers. We then examined the trophic hierarchy of a free-roaming arthropod community, revealing the highest trophic position (5.07) and longest food chain ever reported using CSIA. High accuracy in trophic position estimation brings trophic function into sharper focus, providing greater resolution to the analysis of food webs.
Figure 3: Trophic position estimates, apple orchard.
Discussion: Using compound-specific isotopic analysis, we provide the first evidence that 15N enriches consistently among trophic levels 2.0, 3.0, and 4.0. This range of trophic activity represents the majority of global fauna (i.e., all herbivores, omnivores, strict predators, and most tertiary predators).
Our findings suggest that the trophic tendency of an animal can be measured with high accuracy when 15N analyses are confined to glutamic acid and phenylalanine. Limiting the analyses to these two amino acids effectively screens what would otherwise be a very heterogeneous mix of 15N signatures.
Project 2: The Impacts of Detritus on Trophic Cascade Strength
Background: Biological pest control is predicated on the concept that plants can be protected effectively when carnivores suppress herbivore populations. This represents the classical type of trophic cascade, in which plant biomass is indirectly rescued by the activities of predators (Carpenter et al. 1985). However, ecosystems are subject to a variety of factors which may influence community interactions, and food webs in nature consist of a complex interplay of interacting individuals. Carnivorous species, for instance, are known to frequently attack and eat fellow carnivores—a behavior known as intra-guild predation (Polis 1991). The prevalence of omnivory and intra-guild predation in a system may diminish the strength of the trophic cascade (Polis and Strong 1996, Finke and Denno 2005). When a cascade is dampened, the ostensibly “beneficial” species become much less effective, and the resultant crop protection can be greatly undermined.
On the other hand, these trophic cascades may be strengthened by changes that increase beneficial predator populations (Polis 1991), or that force the predators to subsist on a more narrow diet of only herbivores. An awareness of the complexities of these community interactions can provide a more complete understanding of situations in which the use of biological control is most applicable (Halaj and Wise 2002). Additionally, it could be possible to augment or diminish the strength of a trophic cascade, and thus the effectiveness of biological control agents, through the addition of allochthonous subsidies or the removal of autochthonous materials.
Anticipated Results and Significance: The results of this research will illuminate the population dynamics of carnivores, herbivores, and detritivores throughout the summer in flooded cranberry marshes, as compared to non-flooded marshes. It is expected that, in accordance with Hypothesis 2 outlined above, we will see an immediate drop in population densities of all three categories immediately following the flood. Then, throughout the summer we expect to see a gradual increase in populations of all three trophic guilds, with carnivores being the quickest to recolonize. We also expect that generalist carnivores will be equally likely in flooded beds to attack herbivores and detritivores, so will significantly control pest populations. Conversely, in non-flooded beds we expect that an overwhelming majority of detritivore biomass will inhibit predation on herbivores, thus allowing pest populations to build up throughout the course of the summer