Energy Flux: The Link between Multitrophic Biodiversity and Ecosystem Functioning

Abstract

Relating biodiversity to ecosystem functioning in natural communities has become a paramount challenge as links between trophic complexity and multiple ecosystem functions become increasingly apparent. Yet, there is still no generalised approach to address such complexity in biodiversity-ecosystem functioning (BEF) studies. Energy flux dynamics in ecological networks provide the theoretical underpinning of multitrophic BEF relationships. Accordingly, we propose the quantification of energy fluxes in food webs as a powerful, universal tool for understanding ecosystem functioning in multitrophic systems spanning different ecological scales. Although the concept of energy flux in food webs is not novel, its application to BEF research remains virtually untapped, providing a framework to foster new discoveries into the determinants of ecosystem functioning in complex systems.

Keywords: ecological stoichiometry; ecosystem multifunctionality; food web; interaction network; metabolic theory; trophic cascade.

Figure I

Using the ‘food web energetics’ approach, energy flux is quantified following six general steps: For a given community, individual body masses are assigned (Step 1) and used to calculate individual metabolic rates. This can be done using body mass-metabolic rate regressions available from the literature for many taxonomic groups. Alternatively, metabolic rates can be directly measured for study organisms. (Step 2). Network topology is then constructed and feeding preferences defined, either through experimentation or via literature review (Step 3), followed by the calculation of metabolic demands of each node (Step 4) by summing all individual metabolic rates of the respective group. Assimilation efficiencies are assigned (Step 5) based on the resource of a consumer, which can be measured, derived from existing literature for specific consumer types or temperatures (e.g., [31,42]), or scaled depending on resource stoichiometry if e.g. C/N content is measured for organisms in the food web (e.g., [45]). Single fluxes throughout the network are then calculated, starting from the highest trophic level and working to the bottom (Step 6). See Online Supplementary Material S1 for a worked example.

Figure II

Definition of a ‘true’ diversity effect in classical BEF experiments (A), with the translation of how one would conceptualise such an effect in ecological networks using energy flux (B). The double-headed arrow in (A) indicates the magnitude of the diversity effect on yield in a hypothetical three-species mixture. In (B), orange nodes denote three hypothetical species consuming a heterogeneous resource (consumption is indicated by coloured segments), with a complementarity effect on total energy flux depicted for the three-species mixture (all resources consumed simultaneously).

Figure III

Different measures of diversity obtained from the network (A) can be directly related to individual energy fluxes for specific functions, or to summed network fluxes to infer ecosystem multifunctionality (B). Line colours in (B) correspond to individual functions depicted in (A), with network multifunctionality indicated by these summed fluxes as ‘multitrophic flux’ in (B).

Figure 1. Energy fluxes and their relation to ecosystem services across ecosystems.

Arrows denote the directional flux of energy among functional feeding guilds, indicating how these fluxes permeate ecosystem boundaries (such as terrestrial above and below ground, freshwater, and marine systems as shown here). All energy fluxes in ecosystems are analogous to respective ecosystem functions, many of which can be directly translated to services that are beneficial to human wellbeing, as indicated by numbered examples. Soil cross-section image courtesy of Julia Siebert.