Effects of spawning Pacific salmon on terrestrial invertebrates: Insects near spawning habitat are isotopically enriched with nitrogen‐15 but display no differences in body size

Abstract

When Pacific salmon ( Oncorhynchus spp.) spawn and die, they deliver marine‐derived nutrient subsidies to freshwater and riparian ecosystems. These subsidies can alter the behavior, productivity, and abundance of recipient species and their habitats. Isotopes, such as nitrogen‐15 ( 15 N), are often used to trace the destination of marine‐derived nutrients in riparian habitats. However, few studies have tested for correlations between stable isotopes and physiological responses of riparian organisms. We examined whether increases in δ 15 N in terrestrial insect bodies adjacent to salmon spawning habitat translate to changes in percent nitrogen content and body size. This involved comparisons between distance from a salmon‐bearing river, marine‐derived nutrients in soils and insects, soil moisture content, and body size and nitrogen content in two common beetle families (Coleoptera: Curculionidae, Carabidae). As predicted, δ 15 N in riparian soils attenuated with distance from the river but was unaffected by soil moisture. This gradient was mirrored by δ 15 N in the herbivorous curculionid beetles, whereas carabid beetles, which feed at a higher trophic level and are more mobile, did not show discernable patterns in their δ 15 N content. Additionally, neither distance from the river nor body δ 15 N content was related to beetle body size. We also found that nitrogen‐15 was not correlated with total percent nitrogen in insect bodies, meaning that the presence of spawning salmon did not increase the percent nitrogen content of these insects. We conclude that while salmon‐derived nutrients had entered terrestrial food webs, the presence of δ 15 N alone did not indicate meaningful physiological changes in these insects in terms of percent nitrogen nor body size. While stable isotopes may be useful tracers of marine‐derived nutrients, they cannot necessarily be used as a proxy for physiologically important response variables.

Ecological effects of mosquito control with Bti: evidence for shifts in the trophic structure of soil- and ground-based food webs

“…Finally, 50-150% increases in δ 15 N values were observed in benthic macroinvertebrates along gradients of increasing anthropogenic nutrient enrichment in Sweden (Bergfur et al 2009;, similar to observations elsewhere (Clapcott et al 2010;Smucker et al 2018). Differences in δ 13 C values in the same studies ranged less widely, from 0 to 9% (anthropogenic nutrient enrichment, Bergfur et al 2009;, 0-4% (marine derived nutrients, Hocking and Reimchen 2002;Rammell et al 2021) and 0-1% (river restoration, Kupilas et al 2020). Collier et al (2002) observed a more marked 28-31% difference in δ 13 C values between an agricultural compared with forested catchment in New Zealand, which is attributable to strong differences in tree cover that were not a factor in our study.…”

“…Riparian spider δ 15 N values were 9% higher in an agricultural compared with forested catchment in New Zealand (Collier et al 2002), and river restoration was associated with 7-11% increases in δ 15 N values of riparian spiders and beetles (Kupilas et al 2020). More marked 70-400% shifts in δ 15 N values were observed in detritivorous millipedes, omnivorous beetles and predatory spiders living adjacent to rivers with marine-salmon-derived nutrient enrichment, relative to references (Hocking and Reimchen 2002;Rammell et al 2021). Finally, 50-150% increases in δ 15 N values were observed in benthic macroinvertebrates along gradients of increasing anthropogenic nutrient enrichment in Sweden (Bergfur et al 2009;, similar to observations elsewhere (Clapcott et al 2010;Smucker et al 2018).…”

Experimental addition of marine-derived nutrients affects wildflower traits in a coastal meta-ecosystem

“…The seed output of hemiparasitic Castilleja species can be influenced by their host species via changes in alkaloid uptake and herbivory-pollination feedbacks [ 47 ]. The lack of effect in three of the focal species is also consistent with research demonstrating that the presence of salmon-derived nitrogen-15 is often decoupled from meaningful biological consequence for individual organisms [ 54 , 55 ].…”

Biogeographic features mediate marine subsidies to island food webs

“…A likely explanation is that lower trophic‐level organisms (e.g., plants) uptake nutrients over smaller areas, and are therefore more affected by characteristics of their local environment, while higher trophic‐level taxa (e.g., songbirds) forage over larger areas, likely spanning multiple locations both within and among islands. Indeed, a similar study found riparian soil and herbivorous weevils to have higher levels of δ 15 N closer to salmon‐bearing streams, but δ 15 N of more mobile, carnivorous beetles was unaffected (Rammell et al, 2021). We also found that 15 N enrichment drops off more rapidly with increasing distance to shore on larger islands.…”