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Salmon-derived nitrogen in terrestrial invertebrates from coniferous forests of the Pacific Northwest.

Hocking MD, Reimchen TE - BMC Ecol. (2002)

Bottom Line: We observed increasing delta15N levels in our invertebrate groups with increasing consumption of dietary protein.Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat.From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.

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Affiliation: Department of Biology, University of Victoria, B,C, V8W 3NS, Canada. morganhocking@hotmail.com

ABSTRACT

Background: Bi-directional flow of nutrients between marine and terrestrial ecosystems can provide essential resources that structure communities in transitional habitats. On the Pacific coast of North America, anadromous salmon (Oncorhynchus spp.) constitute a dominant nutrient subsidy to aquatic habitats and riparian vegetation, although the contribution to terrestrial habitats is not well established. We use a dual isotope approach of delta15N and delta13C to test for the contribution of salmon nutrients to multiple trophic levels of litter-based terrestrial invertebrates below and above waterfalls that act as a barrier to salmon migration on two watersheds in coastal British Columbia.

Results: Invertebrates varied predictably in delta15N with enrichment of 3-8 per thousand below the falls compared with above the falls in all trophic groups on both watersheds. We observed increasing delta15N levels in our invertebrate groups with increasing consumption of dietary protein. Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat. From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.

Conclusions: Enrichment of delta15N in the invertebrate community below the falls in conjunction with the absence of delta13C enrichment suggests that enrichment in delta15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than from direct consumption of salmon tissue or salmon tissue consumers. Salmon nutrient subsidies to terrestrial habitats may result in shifts in invertebrate community structure, with subsequent implications for higher vertebrate consumers, particularly the passerines.

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δ15N values in four trophic groupings of litter-based invertebrates collected above and below waterfall barriers to salmon migration on the Clatse and Neekas Rivers, British Columbia. Invertebrates are ranked (left to right) based on increasing consumption of animal protein (see methods). t-test results: ** denotes p < 0.01; *** denotes p < 0.001.
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Figure 1: δ15N values in four trophic groupings of litter-based invertebrates collected above and below waterfall barriers to salmon migration on the Clatse and Neekas Rivers, British Columbia. Invertebrates are ranked (left to right) based on increasing consumption of animal protein (see methods). t-test results: ** denotes p < 0.01; *** denotes p < 0.001.

Mentions: Invertebrate trophic groups varied predictably with respect to δ15N. The nested ANOVA analysis demonstrated that the majority of variance in δ15N was due to falls within watersheds (F = 9.191; p = 0.031; R2 = 0.819) and taxonomic group within all other factors (F = 13.71; p < 0.001; R2 = 0.689). Variation in δ15N that occurred between watersheds or distance of collection from the stream contributed little to total variance and was insignificant in the model (See methods for violations). Invertebrates were enriched by 3–8‰ along salmon spawning reaches compared to similar groups collected above the falls, and showed a gradient of increasing values with increased trophic level at both salmon and non-salmon sites (Figure 1). There were highly significant differences in δ15N (t-tests: p < 0.01) above and below waterfalls for all trophic groups at both watersheds. Multiple comparison tests (Tukey's post hoc) revealed distinct trophic separation in δ15N between at least two invertebrate groups depending on site of collection (Table 1). Millipede detritivores had higher δ15N values than root feeding weevils on all sites but only on the Clatse above the falls was this trend significant. Carabid beetles demonstrated higher δ15N values than millipedes at all sites with significant differences on the Clatse River below and above the falls and on the Neekas River above the falls. Spider predators were significantly more enriched than carabid beetles on the Neekas River on both salmon and non-salmon sites, but demonstrated only marginally higher δ15N values than these beetles on the Clatse River. Carabid beetle omnivores and spider predators demonstrated significantly higher variance in δ15N below the falls than above on both watersheds (Carabidae Clatse: F14,6 = 14.61, p < 0.005; Carabidae Neekas: F21,6= 21.94, p < 0.001; Araneae Clatse: F18,16 = 5.41, p < 0.002; Araneae Neekas: F17,11 = 4.94, p < 0.02) (F-ratio tests).


Salmon-derived nitrogen in terrestrial invertebrates from coniferous forests of the Pacific Northwest.

Hocking MD, Reimchen TE - BMC Ecol. (2002)

δ15N values in four trophic groupings of litter-based invertebrates collected above and below waterfall barriers to salmon migration on the Clatse and Neekas Rivers, British Columbia. Invertebrates are ranked (left to right) based on increasing consumption of animal protein (see methods). t-test results: ** denotes p < 0.01; *** denotes p < 0.001.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC101382&req=5

Figure 1: δ15N values in four trophic groupings of litter-based invertebrates collected above and below waterfall barriers to salmon migration on the Clatse and Neekas Rivers, British Columbia. Invertebrates are ranked (left to right) based on increasing consumption of animal protein (see methods). t-test results: ** denotes p < 0.01; *** denotes p < 0.001.
Mentions: Invertebrate trophic groups varied predictably with respect to δ15N. The nested ANOVA analysis demonstrated that the majority of variance in δ15N was due to falls within watersheds (F = 9.191; p = 0.031; R2 = 0.819) and taxonomic group within all other factors (F = 13.71; p < 0.001; R2 = 0.689). Variation in δ15N that occurred between watersheds or distance of collection from the stream contributed little to total variance and was insignificant in the model (See methods for violations). Invertebrates were enriched by 3–8‰ along salmon spawning reaches compared to similar groups collected above the falls, and showed a gradient of increasing values with increased trophic level at both salmon and non-salmon sites (Figure 1). There were highly significant differences in δ15N (t-tests: p < 0.01) above and below waterfalls for all trophic groups at both watersheds. Multiple comparison tests (Tukey's post hoc) revealed distinct trophic separation in δ15N between at least two invertebrate groups depending on site of collection (Table 1). Millipede detritivores had higher δ15N values than root feeding weevils on all sites but only on the Clatse above the falls was this trend significant. Carabid beetles demonstrated higher δ15N values than millipedes at all sites with significant differences on the Clatse River below and above the falls and on the Neekas River above the falls. Spider predators were significantly more enriched than carabid beetles on the Neekas River on both salmon and non-salmon sites, but demonstrated only marginally higher δ15N values than these beetles on the Clatse River. Carabid beetle omnivores and spider predators demonstrated significantly higher variance in δ15N below the falls than above on both watersheds (Carabidae Clatse: F14,6 = 14.61, p < 0.005; Carabidae Neekas: F21,6= 21.94, p < 0.001; Araneae Clatse: F18,16 = 5.41, p < 0.002; Araneae Neekas: F17,11 = 4.94, p < 0.02) (F-ratio tests).

Bottom Line: We observed increasing delta15N levels in our invertebrate groups with increasing consumption of dietary protein.Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat.From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology, University of Victoria, B,C, V8W 3NS, Canada. morganhocking@hotmail.com

ABSTRACT

Background: Bi-directional flow of nutrients between marine and terrestrial ecosystems can provide essential resources that structure communities in transitional habitats. On the Pacific coast of North America, anadromous salmon (Oncorhynchus spp.) constitute a dominant nutrient subsidy to aquatic habitats and riparian vegetation, although the contribution to terrestrial habitats is not well established. We use a dual isotope approach of delta15N and delta13C to test for the contribution of salmon nutrients to multiple trophic levels of litter-based terrestrial invertebrates below and above waterfalls that act as a barrier to salmon migration on two watersheds in coastal British Columbia.

Results: Invertebrates varied predictably in delta15N with enrichment of 3-8 per thousand below the falls compared with above the falls in all trophic groups on both watersheds. We observed increasing delta15N levels in our invertebrate groups with increasing consumption of dietary protein. Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat. From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.

Conclusions: Enrichment of delta15N in the invertebrate community below the falls in conjunction with the absence of delta13C enrichment suggests that enrichment in delta15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than from direct consumption of salmon tissue or salmon tissue consumers. Salmon nutrient subsidies to terrestrial habitats may result in shifts in invertebrate community structure, with subsequent implications for higher vertebrate consumers, particularly the passerines.

Show MeSH
Related in: MedlinePlus