Stoichiometry and Life-History Interact to Determine the Magnitude of Cross-Ecosystem Element and Biomass Fluxes

Abstract

Ecosystems are linked through the transfer of materials and energy. Studies examining material fluxes across habitat boundaries frequently quantify unidirectional flows of nutrients and energy. However, material fluxes can be multidirectional, and we lack a conceptual framework to describe how their quantity and stoichiometry influence the net transfer of individual elements between ecosystems. Here we develop a zero net transfer isocline (ZNTI) framework that integrates the relative mass and stoichiometry of fluxes into and out of an ecosystem. We then use case studies with amphibians and salmon to elucidate how life history, ontogenetic shifts in stoichiometry, and trophic interactions shape relative fluxes of nutrients between aquatic and terrestrial ecosystems. Because they increase in both size and Ca content from ova to metamorphs, amphibian life histories strongly bias them toward net Ca export into the terrestrial environment. Because amphibian biomass, C, P, and Ca ZNTIs do not overlap, there is no value of survivorship where the net flux of biomass, C, P, and Ca are simultaneously balanced between terrestrial and aquatic habitats. The degree of iteroparity and semelparity in salmon strongly affects both the magnitude of net biomass and P flux between riverine and marine environments. While the net direction of biomass flux generally remains strongly biased toward import into the riverine system, net P flux can reach net export into the marine environment because of increasing adult breeding survival leading to reduced mass and %P of what they deposit in rivers (e.g., ova vs. whole carcasses). These examples highlight how ontogenetic shifts in body size and stoichiometry result in asymmetric fluxes of elements and biomass that can lead to simultaneous net imports and exports of different elements within the same system. Furthermore, they demonstrate how changes in life-history characteristics and stage-specific survivorship can lead to changes in net elemental transport between ecosystems.

Understanding variation in salamander ionomes: A nutrient balance approach

“…Similar to previous amphibian studies (Luhring et al, 2017;Milanovich, Maerz, & Rosemond, 2015), we found strong differences in the elemental composition of our study species. Consistent with stoichiometric predictions, the species with the smaller body size and faster developmental rates, A. opacum, had higher body P content than A. talpoideum.…”

“…Consistent with stoichiometric predictions, the species with the smaller body size and faster developmental rates, A. opacum, had higher body P content than A. talpoideum. In addition to these traditional stoichiometric elements, we also confirmed species differences in salamander Ca and S content found in previous work (Luhring et al, 2017;Milanovich et al, 2015) and identified differences in several other essential elements. In addition to these traditional stoichiometric elements, we also confirmed species differences in salamander Ca and S content found in previous work (Luhring et al, 2017;Milanovich et al, 2015) and identified differences in several other essential elements.…”

“…Throughout ontogeny, frog and salamander carbon (C) content generally decreases while %N, P, calcium (Ca) and sulfur (S) tend to increase (Capps et al, 2015;Luhring, DeLong, & Semlitsch, 2017).…”

“…C, Ca, N, P and S;Capps et al, 2015;Tiegs et al, 2016;Luhring et al, 2017). Based on stoichiometric theory, we hypothesised that the ionomes of each species should vary due to differences in life-history and ontogeny, and we sought to identify linkages among elements, including those already known to vary significantly (i.e.…”

Context dependency of animal resource subsidies

“…In these cases, the magnitude and direction of net flux may be element‐dependent (i.e. the net flux of C, N and P may not always be in the same direction) and may depend on characteristics of the animals, such as adult population size, juvenile recruitment, ontogenetic shifts in stoichiometry, and mobility (Jackson & Fisher, ; Capps et al, ; Luhring, DeLong & Semlitsch, ), as well as characteristics of the ecosystems, such as size, perimeter to area ratio and boundary characteristics (Jackson & Fisher, ; Regester, Lips & Whiles, ). Theory suggests that gradual changes in ecosystem productivity or consumer mortality can lead to abrupt regime shifts in ecosystems coupled by consumers with complex life histories (Schreiber & Rudolf, ).…”