Energy flux through ecological networks

One of the main drivers of the ongoing loss of biodiversity and the provisioning of important ecosystem functions is nutrient enrichment due to increased fertilizer use in agriculture and fossil fuel combustion. In this project we aim to assess energy fluxes through ecological food webs as an independent measure of ecosystem functioning on an island system in lakes in northern Sweden.

The ongoing loss of biodiversity across Earth’s ecosystems has prompted concern regarding the provisioning of ecosystem function and service delivery. This is because links between biodiversity at all trophic levels and multiple ecosystem functions become increasingly apparent. Recently, particularly the relationship between variation in soil fertility and biodiversity has become a major research topic – soil fertility is affected by nitrogen and phosphorus enrichment, two major global-change drivers causing substantial loss of plant diversity. While there is considerable knowledge on how plants respond to variation in soil fertility, we still know very little about consequences of nutrient enrichment for higher trophic levels or complete food webs. However, emerging evidence suggests that the relationship between biodiversity and the stability of ecosystem function delivery can only be fully understood if we consider the interactions between multiple trophic levels. Hence, if we are to understand the consequences of anthropogenic global change for natural ecosystems, studying the relationship between soil fertility, biodiversity and ecosystem functioning is crucial.

A novel and powerful way to measure multitrophic ecosystem functioning across different ecosystem types in a single, comparable currency is the calculation of energy flux through ecological networks. Such energy fluxes along trophic links can be directly related to various functions and are also indicators for ecosystem services. They provide an untapped potential to independently compare ecosystem functioning along environmental gradients. Most studies that have explored linkages between plants and higher trophic levels (herbivores, predators) and their effects on ecosystem functions have used controlled experimental approaches and fast-growing herbaceous plant species. However, there is an increasing recognition that observational approaches and ‘natural experiments’ have considerable potential for answering ecological questions over much greater spatial and temporal scales than can be achieved with conventional, small-scale experiments.

Chronosequence studies have often demonstrated how the long-term absence of major disturbances leads to declines in nutrient availability, decomposition rates and plant productivity, and would provide an ideal system to study real-world energy flux in relation to soil fertility.

In this project, we take advantage of an existing collection of invertebrate communities  sampled along a well-studied island chronosequence in Sweden, which consists of 30 islands differing in size and soil fertility due to different fire histories. Measurements of individual body length of all 22000 macroinvertebrates from these invertebrate communities will allow us to calculate individual body mass and metabolic rates. Using the recently developed food-web energetics approach to estimating community energy flux, these measurements will enable us to calculate energy flux from plants to herbivores and herbivores to predators per unit area, and scale-up these metrics to overall community energy flux per island. Together with the excellent quality of the underlying island soil fertility, vegetation and invertebrate abundance and diversity data, the new methodology makes this project a promising attempt to tackle the timely questions on the consequences of anthropogenic change on the functioning of natural ecosystems.  As such, our project will help to shed light on the consequences of anthropogenic global change for the stability of  ecosystem function and service delivery.

Anne Kempel and Eric Allan work on this project together with Malte Jochum. We are collaborating with David Wardle and Martin Gossner (project page @ WSL).

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Measurements of body length of the invertebrate community will allow us to calculate individual body mass and metabolic rates, which we can then use to estimate energy flux through ecological networks.

Header photograph taken by Anne Kempel ©.