Life Science Hour Seminar Series

Name:Shawn Steffan
 Assistant Professor, Dept. of Entomology. University of Wisconsin-Madison.
Title:Integrating microbes into food-chains: Animal trophic identity reflects rampant microbivory
Date:Friday, 7 April 2017
Location:Murie Life Science Bldg, Murie Auditorium.


Detritivory is the dominant trophic paradigm in most terrestrial, aquatic, and marine ecosystems, yet accurate measurement of consumer trophic position within detrital (= ‘brown’) food-webs has remained relatively impenetrable. Measurement of detritivore trophic position is complicated by the fact that detritus is suffused with microbes. Given that microbes and metazoans are trophic analogues of each other, animals feeding on microbe-colonised detritus ingest multiple trophic groups. This phenomenon should elevate consumer trophic position, and should be exceedingly common in brown food-webs. We tested this expectation using compound-specific isotopic (15N) analysis of amino acids extracted from both wild and laboratory-cultured consumers. Vertebrate (fish) and invertebrate detritivores (beetles and moths) were reared on detritus, with and without microbial colonisation. In the field, specimens of diverse animal taxa were collected to measure the trophic identities of fauna known to be detritivorous. We show that after being colonised by bacteria or fungi, the trophic positions of detrital complexes increased significantly over time. The ‘inflation’ of detrital trophic position was mediated by the extent to which microbes consumed the detritus. In turn, animals consuming microbe-colonised detritus registered distinctly non-integer trophic positions, which were approximately one trophic level higher than their diets. All wild-collected invertebrate detritivores in our study exhibited non-integer trophic positions. Our data indicate that the trophic position of detritus rises as microbes convert non-living resources into microbial biomass. A detrital complex, therefore, will tend to have a dynamic trophic identity. Animals consuming microbe-colonised detritus exhibit trophic inflation as a predictable function of the degree to which the animal has consumed microbial prey. For meso- or macrofauna, detritivory is effectively omnivory. By incorporating microbes into trophic hierarchies, we quantify the microbe effect within food-chains, and provide a basis to better interpret trophic position among detritivores, as well as the carnivores consuming such prey. Collectively, these findings help to unite the macro- and microbiome in food-web studies, refining the questions that may be posed regarding trophic structure and function.

About the Speaker:

Shawn grew up in California, cultivating his interests in entomology, agriculture, and ecology at UC-Berkeley, and later attended graduate school at the University of Wisconsin-Madison. After finishing his MS work on habitat modification as a means to enhance bio-control, he moved back to the West Coast where he continued to work at the intersection of entomology, plant pathology, and agroecology. He eventually went back to school to get his PhD (Washington State University, 2009), studying predator biodiversity and the ecology of fear. After postdoc'ing in Wenatchee, Washington, where he used herbivore-induced plant volatiles to measure natural enemy diversity in apple orchards, he was lured back to Wisconsin where he currently holds joint appointments as a federal scientist with the USDA-ARS, and as a faculty member in the Dept. of Entomology, University of Wisconsin-Madison. His primary research mandate is the study of arthropods in the cranberry systems of North America, and in this endeavor he uses empirical, isotopic, and metagenomic approaches to investigate heterotrophic diversity within cranberry systems, as well as the trophic ecology of macro- and microbiota in a variety of terrestrial and aquatic ecosystems. In the field of food-web ecology, he and colleagues are providing a basis to better interpret the trophic identities of carnivore and omnivore species, most of which tend to be fungi, bacteria, and micro-invertebrates (the little things that rule the world).

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