Insects, salmon and their microbiomes: the influence of resource pulses on ecological networks

Resource subsidies are recognized to cross ecosystem boundaries and influence the structure of ecological networks, especially in systems where animals migrate to reproduce and then die – creating ephemeral carrion resources subsidies. This is particularly true in settings with relatively predictable nutrient pulses, such as salmon-bearing streams of Alaska. While several groups of interconnected and interacting organisms, such as bacteria, insects, and vertebrate scavengers, are associated with decomposing organic material, few studies have evaluated the internal bacterial communities of insects associated with carrion. Given that insects directly affect vertebrate decomposition they may be important biological mediators of microbial succession. The objective of this study was to assess the internal microbiome of carrion flies (Diptera: Calliphoridae), which have been shown to move carcass nutrients into the riparian forest, salmon carcasses, and aquatic insects (Ephemeroptera and Plecoptera) found within various watersheds of southeast Alaska.

Microbial communities were collected from five streams (salmon bearing and non-salmon bearing) surveyed in Juneau, AK, USA, and characterized using Illumina MiSeq. Decomposing post-spawning salmon carcasses (Oncorhynchus keta) naturally deposited on the stream bank were characterized and assessed by dipteran larvae mass activity; those carcasses with third instar masses were sampled for matched larvae collections and adults attracted to the carcasses. Proteobacteria were the dominant taxa of the internal microbial communities in both mayflies and stoneflies regardless of sampling location. However, there was an increase in unique operational taxonomic units (OTUs) detected in mayfly (1,505) and stonefly (478) larvae collected from salmon bearing streams compared to mayfly and stonefly larvae collected in non-salmon streams, 1,317 and 324, respectively. The most abundant blow fly species attracted to and colonizing vertebrate carcasses was Calliphora terraenovae. Firmicutes and Proteobacteria dominated the internal microbiome communities in adult C. terraenovae. Initial results describing the third instar internal microbiomes of C. terraenovae revealed that larval microbiomes were substantially different than those of the adult blow flies, and from those of the carcass. Additionally, C. terraenovae adults had an increase in unique OTUs (5,400) when compared to C. terraenovae larvae (1,515) developing on salmon carcasses, which suggests possible important salmon carcass effects on the insect microbiome.

Our data demonstrate unique shifts in the microbial community of the insects and resources found within a cadaver network. These data are foundational in describing resource subsidy-driven network responses to resource pulses with potential bottom-up effects on food web structure.