Stoichiometry of homeostasis and growth across aquatic invertebrate detritivores Halvor M. Halvorson 1, Chris L. Fuller 2, Sally A. Entrekin 3, J. Thad Scott 4, and Michelle A. Evans-White 1 1 University of Arkansas Department of Biological Sciences 2 University of Central Arkansas Department of Biology 3 Arkansas Water Resources Center 4 University of Arkansas Department of Crop, Soil, and Environmental Sciences

Ecological Stoichiometry (ES): understanding organism responses to limiting or excess nutrients X:Y food + X:Y consumer X:Y growth + X:Y waste (Sterner and Elser 2002) 3 focal elements: Carbon (C) Carbon (C) Nitrogen (N) Nitrogen (N) Phosphorus (P) Phosphorus (P) Connects abiotic and biotic components of ecosystems

Consumer nutrient recycling models often assume animals maintain fixed body C:N:P composition, or “strict homeostasis” (Sterner 1990, Elser & Urabe 1999, Sterner and Elser 2002) ES often uses organism stoichiometry to predict growth and nutrient recycling

Body % P Major challenge for ES: Many animals exhibit dynamic, flexible body stoichiometry Flexible stoichiometric homeostasis of some taxa (Persson et al. 2010) Stoichiometric shifts during ontogeny or development (Back and King 2013) Log(Consumer P:X) Log(Resource P:X) Mass (µg)

Objective Investigate 2 drivers of organism stoichiometry: 1)Stoichiometric Homeostasis – Quantify flexibility of consumer C:N:P content when fed variable resource C:N:P content 2)Ontogeny – Compare the C:N:P contents of newly grown tissues to initial (older) tissues Diverse understudied taxa: aquatic invertebrate detritivores Photos by Robert Henricks and Kerry Wixted

Methods: Growth Experiments Lirceus sp. (LIR) Allocapnia sp. (ALLO) Amphinemura sp. (AMP) Strophopteryx sp. (STRO) Pycnopsyche lepida (PYC) Lepidostoma sp. (LEP) Tipula abdominalis (TIP) Crustacea:Isopoda Insecta:Plecoptera Insecta:Trichoptera Insecta:Diptera Laboratory growth experiments with 7 taxa: Non-metabolous Holometabolous Fed sugar maple and post oak litter incubated across 3-4 dissolved P levels  gradient of diet C:N:P 2-5 wks growth, n=40-80, 5-15⁰C

Methods: Homeostasis calculations Determined homeostasis slope term 1/H for each taxon on maple and oak diets separately: (Sterner and Elser 2002, Persson et al. 2010) Log resource X:Y Log Consumer X:Y 1/H=0.5 1/H=0.2 1/H=0 Line slope = 1/H ↑ 1/H indicates flexible homeostasis due to storage or depletion of tissue X or Y

Methods: Growth stoichiometry calculations Calculated C/N/P-specific growth for each taxon Subsequently compared molar C:N, C:P, and N:P of growth to initial tissue contents. LowHigh Grown tissue C:P = Initial Body C:P Increasing body C:P Decreasing body C:P

Results: Stoichiometric homeostasis Log resource X:Y Log Consumer X:Y Line slope = 1/H

Homeostasis plots Black solid lines indicate flexible homeostasis (slope≠0; P<0.05) Gray dotted lines indicate strict homeostasis (slope=0; P>0.05)

Homeostasis plots Black solid lines indicate flexible homeostasis (slope≠0; P<0.05) Gray dotted lines indicate strict homeostasis (slope=0; P>0.05)

Homeostasis plots Black solid lines indicate flexible homeostasis (slope≠0; P<0.05) Gray dotted lines indicate strict homeostasis (slope=0; P>0.05)

Homeostasis summary across taxa Lower, negative 1/H in C:N on oak diets compared to maple Detritivores generally more flexible in N:P (positive 1/H) than herbivorous invertebrates in Persson et al. (2010) meta-analysis Detritivores deviate slightly from strict homeostasis (1/H=0)

Results: Stoichiometry of new versus old tissues LowHigh Grown tissue C:P = Initial Body C:P Increasing body C:P Decreasing body C:P

Grown tissue C:P versus initial tissue C:P Holometabolous taxa exhibit higher, increasing C:P content during growth. May indicate growth “slow down” prior to pupal stage? Red boxes indicate growth C:N:P significantly different from initial contents (t-test; P<0.002) Decreasing body C:P Increasing body C:P

Grown tissue C:N versus initial tissue C:N Taxa preparing for immediate emergence and flight (Plecoptera) tend to exhibit low, declining C:N of growth (for N-rich muscle tissue?)

Grown tissue N:P versus initial tissue N:P Non- and hemi-metabolous taxa exhibit lower, declining N:P during growth compared to holometabolous taxa

Conclusions and implications Future work will compare effects of flexible homeostasis vs. ontogeny on organism stoichiometry Could holometabolous versus hemimetabolous taxa contribute distinctly to ecosystem nutrient cycling? Habitattemplate (↑Disturbance intensity) Life history (↑ adult mobility) nutrient cycling (↓ N excretion) Flight-dispersal animals require more N for wing muscle development and may recycle less N in streams Townsend et al Better-parameterized stoichiometric models?

Acknowledgements Evans-White Laboratory (U of Arkansas) Scott Laboratory (U of Arkansas) Entrekin Laboratory (U of Central Arkansas) U of Arkansas Graduate School and Honors College National Science Foundation Grants DEB , DBI , DBI

Questions?