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Abstract Increases in nitrogen (N) and phosphorus (P) availability are changing animal communities, partly by altering stoichiometric imbalances between consumers and their food. Testing relationships between resource stoichiometry and consumer assemblage structure requires ecosystem‐level manipulations that have been lacking to date.We analysed patterns of macroinvertebrate community composition in five detritus‐based headwater streams subject to experimental whole‐stream N and P additions that spanned a steep gradient in dissolved N:P ratio (2:1, 8:1, 16:1, 32:1, 128:1) over 2 years, following a 1‐year pre‐treatment period.We predicted that shifts in leaf litter stoichiometry would drive overall patterns of community composition via greater responses of shredders to enrichment than other taxa, as shredders dominate primary consumer biomass and experience larger consumer–resource elemental imbalances than other functional groups in stream ecosystems. Specifically, we expected litter C:P to be a significant predictor of shredder biomass given the greater relative imbalances between shredder and litter C:P than C:N. Finally, we tested whether shredder responses to enrichment were related to other taxon‐level traits, including body size and stoichiometry, larval life span and growth rate.Whole‐community composition shifted similarly across the five streams after enrichment, largely driven by increased shredder and predator biomass. These shifts were limited to the autumn/winter seasons and related to decreased leaf litter C:P, highlighting important links between the quality of seasonal litter subsidies and community phenology.Among 10 taxa that drove structural shifts, two declined while other taxa from the same functional/taxonomic groups responded positively, suggesting that specific life‐history traits may determine sensitivity to enrichment.Increases in total shredder biomass, and in biomass of several common shredders, were associated with lower litter C:P. Body C:P did not predict shredder response to enrichment. However, weak negative relationships between shredder response and body size, and larval life span, suggest that small‐bodied and short‐lived taxa may be more responsive to shifting resource stoichiometry.Moderate anthropogenic increases in N and P availability affect resource stoichiometry and can alter animal communities, influencing additional food web and ecosystem properties. We provide support for ecological stoichiometry as a framework for predicting such outcomes based on changes in the elemental composition of resource pools. Aplain language summaryis available for this article.
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Individual‐level data reveal high prevalence of positive size‐trophic position relationships for vertebrates in temperate streams
Abstract A positive relationship between body size and trophic position is often assumed in ecology, but efforts to confirm the generality of this relationship for freshwater vertebrates have produced mixed results. Some authors have tested for among‐species relationships, using species‐level estimates of average size and trophic position. Others have used individual‐level body size and trophic position data to test for within‐species relationships. However, no study has yet estimatedprevalence, defined here as the fraction of total standing stock biomass within a given ecosystem that consists of taxa exhibiting positive size versus trophic position relationships.Individual‐level estimates of body size and relative trophic position (inferred from bulk‐tissue nitrogen stable isotopes) were collected for vertebrates in six temperate streams. In each stream, all locally occurring vertebrate taxa were collected and closed population depletion samples were used to obtain standing stock biomass estimates.Ordinary least squares regression was used with the individual‐level data to test for positive relationships between body size and relative trophic position (r‐STPRs). A separater‐STPR model was tested for each vertebrate species collected at the six study sites. Linear mixed‐effects modelling was then used to test for differingr‐STPRs among species.Prevalence of ther‐STPR was calculated for each site by summing the standing stock biomass of all taxa that exhibited a statistically significant, positive STPR at a given site, then dividing this number by the total standing stock biomass of vertebrates at the site.Significant, positiver‐STPRs were detected in 15 of 22 species × site regression models. Prevalence of species with positiver‐STPRs ranged from 91 to 100% of the total vertebrate biomass in western streams and from 45 to 66% in eastern streams.Results confirm that positiver‐STPRs are characteristic of many of the vertebrate taxa considered in this study. Furthermore, species that exhibit positiver‐STPRs comprise a clear majority of the standing stock biomass in five of six study streams. By using individual‐level data to account for prevalence, a more complete understanding of size‐dependent trophic dynamics should be possible in freshwater ecosystems.
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- Award ID(s):
- 1553111
- PAR ID:
- 10453402
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Freshwater Biology
- Volume:
- 66
- Issue:
- 4
- ISSN:
- 0046-5070
- Page Range / eLocation ID:
- p. 628-639
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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