High dimensionality of stoichiometric niches in soil fauna

Multi-elemental consumer-driven nutrient cycling when predators feed on different prey

Predators play a fundamental role in cycling nutrients through ecosystems, by altering the amount and compositions of waste products and uneaten prey parts available to decomposers. Different prey can vary in their elemental content and the deposition of elements in predator waste can vary depending on which elements are preferentially retained versus eliminated as waste products. We tested how feeding on different prey (caterpillars, cockroaches, crickets, and flies) affected the concentrations of 23 elements in excreta deposited by wolf spider across 2 seasons (spring versus fall). Spider excreta had lower concentrations of carbon and higher concentrations of many other elements (Al, B, Ba, K, Li, P, S, Si, and Sr) compared to prey remains and whole prey carcasses. In addition, elemental concentrations in unconsumed whole prey carcasses and prey remains varied between prey species, while spider excreta had the lowest variation among prey species. Finally, the concentrations of elements deposited differed between seasons, with wolf spiders excreting greater concentrations of Fe, Mg, Mn, Mo, S, and V in the fall. However, in the spring, spiders excreted higher concentrations of Al, B, Ba, Ca, Cd, Cu, K, P, Na, Si, Sr, and Zn. These results highlight that prey identity and environmental variation can determine the role that predators play in regulating the cycling of many elements. A better understanding of these convoluted nutritional interactions is critical to disentangle specific consumer-driven effects on ecosystem function.

Opening up new niche dimensions: The stoichiometry of soil microarthropods in European beech and Norway spruce forests

Niche theory fundamentally contributed to the understanding of animal diversity. However, in soil, the diversity of animals seems enigmatic since the soil is a rather homogeneous habitat, and soil animals are often generalist feeders. A new approach to understand soil animal diversity is the use of ecological stoichiometry. The elemental composition of animals may explain their occurrence, distribution, and density. This approach has been used before in soil macrofauna, but this study is the first to investigate soil mesofauna. Using inductively coupled plasma optic emission spectrometry (ICP-OES), we analyzed the concentration of a wide range of elements (Al, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, Zn) in 15 soil mite taxa (Oribatida, Mesostigmata) from the litter of two different forest types (beech, spruce) in Central Europe (Germany). Additionally, the concentration of carbon and nitrogen, and their stable isotope ratios (15N/14N, 13C/12C), reflecting their trophic niche, were measured. We hypothesized that (1) stoichiometry differs between mite taxa, (2) stoichiometry of mite taxa occurring in both forest types is not different, and (3) element composition is correlated to trophic level as indicated by 15N/14N ratios. The results showed that stoichiometric niches of soil mite taxa differed considerably indicating that elemental composition is an important niche dimension of soil animal taxa. Further, stoichiometric niches of the studied taxa did not differ significantly between the two forest types. Calcium was negatively correlated with trophic level indicating that taxa incorporating calcium carbonate in their cuticle for defense occupy lower trophic positions in the food web. Furthermore, a positive correlation of phosphorus with trophic level indicated that taxa higher in the food web have higher energetic demand. Overall, the results indicate that ecological stoichiometry of soil animals is a promising tool for understanding their diversity and functioning.

Multidimensional stoichiometric mismatch explains differences in detritivore biomass across three forest types

The ecological stoichiometry theory provides a framework to understand organism fitness and population dynamics based on stoichiometric mismatch between organisms and their resources. Recent studies have revealed that different soil animals occupy distinct multidimensional stoichiometric niches (MSNs), which likely determine their specific stoichiometric mismatches and population responses facing resource changes. The goals of the present study are to examine how long-term forest plantations affect multidimensional elemental contents of litter and detritivores and the population size of detritivores that occupy distinct MSNs. We evaluated the contents of 10 elements of two detritivore taxa (lumbricid earthworms and julid millipedes) and their litter resources, quantified their MSNs and the multidimensional stoichiometric mismatches, and examined how such mismatch patterns influence the density and total biomass of detritivores across three forest types spanning from natural forests (oak forest) to plantations (pine and larch forests). Sixty-year pine plantations changed the multidimensional elemental contents of litter, but did not influence the elemental contents of the two detritivore taxa. Earthworms and millipedes exhibited distinct patterns of MSNs and stoichiometric mismatches, but they both experienced severer stoichiometric mismatches in pine plantations than in oak forests and larch plantations. Such stoichiometric mismatches led to lower density and biomass of both earthworms and millipedes in pine plantations. In other words, under conditions of low litter quality and severe stoichiometric mismatches in pine plantations, detritivores maintained their body elemental contents but decreased their population biomass. Our study illustrates the success in using the multidimensional stoichiometric framework to understand the impact of forest plantations on animal population dynamics, which may serve as a useful tool in addressing ecosystem responses to global environmental changes.

High dimensionality of stoichiometric niches in soil fauna

The ecological niche is a fundamental concept to understand species coexistence in natural communities. The recently developed framework of the multidimensional stoichiometric niche (MSN) characterizes species niches using chemical elements in living organisms. Despite the fact that living organisms are composed by multiple elements, stoichiometric studies have so far mostly focused on carbon (C), nitrogen (N), and phosphorus (P), and therefore a quantitative analysis of the dimensionality of the MSN in living organisms is still lacking, particularly for animals. Here we quantified ten elements composing the biomass of nine soil animal taxa (958 individuals) from three trophic groups. We found that all ten elements exhibited large variation among taxa, which was partially explained by their phylogeny. Overlaps of MSNs among the nine soil animal taxa were relatively smaller based on ten elements, compared with those based on only C, N, and P. Discriminant analysis using all ten elements successfully differentiated among the nine taxa (accuracy: 90%), whereas that using only C, N, and P resulted in a lower accuracy (60%). Our findings provide new evidence for MSN differentiation in soil fauna and demonstrate the high dimensionality of organismal stoichiometric niches beyond C, N, and P.