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Title Experimental N and P additions alter stream macroinvertebrate community composition via taxon-level responses to shifts in detrital resource stoichiometry
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1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

Contributors Lee M. Demi, Jonathan P. Benstead, Amy D. Rosemond and John C. Maerz

Demi, LM, Benstead, JP, Rosemond, AD, Maerz, JC. Experimental N and P additions alter stream macroinvertebrate community composition via taxon-level responses to shifts in detrital resource stoichiometry. Funct Ecol. 2019; 00: 1– 13.

Key Words community structure, detritus, ecological stoichiometry, nitrogen, phosphorus, stream
File Date 2019
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