How omnivores respond to plant species richness: Analyzing gut contents with next-generation sequencing
Plant species richness affects the associated consumer community, including omnivores. They play an important role in ecological communities by linking multiple trophic levels which can stabilize population dynamics by adaptive food selection. However, how changing plant species richness affects the spectrum of the omnivores’ diet has not been answered yet because methods to direct assess feeding interactions of polyphagous consumers in species-rich ecosystems were lacking so far. A promising approach is the DNA-based classification of food remains in gut contents with a combination of next generation sequencing technologies and universal primers for common barcoding regions, allowing the detection of feeding events from a wide range of potential interaction partners. Here, we use the framework of a biodiversity experiment as a model system to directly assess feeding interactions involving the omnivorous ground beetle Pterostichus melanariuswith novel DNA sequencing technologies. We released and recaptured ground beetles into fenced plots of an experimental grassland covering a gradient of manipulated plant species richness (1-8 species). Beetle regurgitates were collected and the DNA of ingested species was sequenced with universal primers for eukaryotes (18S).
We detected DNA from organisms covering a great range of phyla at a high taxonomic resolution in regurgitates of P. melanarius. The number of invertebrate and plant taxa that likely originated from feeding events increased with plant species richness and with vegetation cover. At high plant cover, we also detected more cases of intraguild predation. Besides DNA of food remains, we detected DNA of mites that use beetles as phoretic carriers and fungal and protist taxa that are known to parasitize beetles. Most taxa found in regurgitates were examples of neutral interactions with environmental organisms that coexist with the beetles without affecting them in a particular way. The number of these neutral interactions also increased with plant cover. Our findings suggest that sequencing gut contents of consumers, even with a dietary range as broad as omnivores, is a useful tool to assess feeding interactions. Moreover, this approach allows the simultaneous integration of interaction types such as parasitism, commensalism, and even neutralism which are often ignored in ecological networks. Thus, integrating novel DNA sequencing technologies into biodiversity experiments can contribute to a deeper understanding of plant mediated effects on interconnections among species in ecological communities.