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 (15N), 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 15N 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, δ15N in riparian soils attenuated with distance from the river but was unaffected by soil moisture. This gradient was mirrored by δ15N 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 δ15N content. Additionally, neither distance from the river nor body δ15N 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 δ15N 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.
Highlights
The exchange of nutrients between marine and terrestrial ecosystems can have a wide variety of biological consequences (Fariña et al, 2003; Nakano & Murakami, 2001; Walsh et al, 2020)
We tested for evidence of increased body size in an ecosystem that receives marine subsidies by asking the following questions (Table 1): First, does soil δ15N increase with proximity to a salmon-bearing river? Second, does insect body δ15N increase with proximity to the river, and does this result in an increase in percent nitrogen content? Third, are insects with elevated body δ15N larger? And lastly, are larger insects found closer to the river? We hypothesized that patterns of marine-derived nutrients would follow a gradient in both the soil and the terrestrial insects with distance from the salmon-bearing river, and tested whether near-river nitrogen enrichment translates to increased body size for weevils and ground beetles
Elytron lengths were averaged across the three insects pooled for isotope analysis. For these stable isotope models, transect was not included since data collection was constrained to a distance of 180 m upstream and inclusion resulted in a singular model fit due to constraints on the number of insects of each species captured at each collection station
Summary
The exchange of nutrients between marine and terrestrial ecosystems can have a wide variety of biological consequences (Fariña et al, 2003; Nakano & Murakami, 2001; Walsh et al, 2020). Previous work has focused on community-level impacts of nitrogen enrichment and has captured changes in abundance and diversity of riparian species alongside salmon-bearing streams (Hocking et al, 2013; Hocking & Reynolds, 2011; Wagner & Reynolds, 2019) and on estimations of the proportion of salmon- derived nitrogen in the organisms’ percent nitrogen content (e.g., Bilby et al, 2003; Helfield & Naiman, 2001) While these estimates approximate the extent to which nutrients from salmon are incorporated into organismal tissues, they do not inform us of resulting biological consequences. We hypothesized that patterns of marine-derived nutrients would follow a gradient in both the soil and the terrestrial insects with distance from the salmon-bearing river, and tested whether near-river nitrogen enrichment translates to increased body size for weevils and ground beetles We tested for evidence of increased body size (using elytron length) in an ecosystem that receives marine subsidies by asking the following questions (Table 1): First, does soil δ15N increase with proximity to a salmon-bearing river? Second, does insect body δ15N increase with proximity to the river, and does this result in an increase in percent nitrogen content? Third, are insects with elevated body δ15N larger? And lastly, are larger insects found closer to the river? We hypothesized that patterns of marine-derived nutrients would follow a gradient in both the soil and the terrestrial insects with distance from the salmon-bearing river, and tested whether near-river nitrogen enrichment translates to increased body size for weevils and ground beetles
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