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 (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.

Keywords: Carabidae; Curculionidae; Pacific salmon; body condition; body size; insects; isotope; marine‐derived nutrients; nitrogen.

FIGURE 1

(a) Zacotus mathewsii and (b) Steremnius carinatus, representing ground beetles (Coleoptera: Carabidae) and weevils (Coleoptera: Curculionidae), respectively (Photo credit: Sung Jo)

FIGURE 2

Study area along the Q̓íɫcutkv (Kunsoot) River (N52°8′50.8″, W128°0′10.9″) near Bella Bella on the central coast of British Columbia, Canada

FIGURE 3

The study area along the first 180 m of the Q̓íɫcutkv (Kunsoot) River. Pitfall traps were set in the riparian area on the left bank (Photo credit: Emily Yungwirth)

FIGURE 4

(a) Soil δ¹⁵N in relation to distance from the river. Points indicate raw data, and the line represents model predictions with 95% confidence intervals while accounting for soil moisture and its interaction with distance from the river. (b) Coefficient plot for the soil δ¹⁵N model, which shows each parameter estimate with 95% confidence intervals. Coefficients with confidence intervals that do not cross zero are shown in black

FIGURE 5

(a) Weevil (Steremnius carinatus) body δ¹⁵N in relation to distance from the river. (b) Ground beetle (Zacotus matthewsii) body δ¹⁵N in relation to distance from the river. Points indicate raw data, and the line represents model predictions with 95% confidence intervals

FIGURE 6

(a) Weevil (Steremnius carinatus) elytron length in relation to body δ¹⁵N. (b) Ground beetle (Zacotus matthewsii) elytron length in relation to body δ¹⁵N. Points indicate raw data, and the line represents model predictions with 95% confidence intervals