Empirical methods of identifying and quantifying trophic interactions for constructing soil food-web models

Introduction Food-web models, which depict the trophic relationships between organisms within a community, form a powerful and versatile approach to study the relationships between community structure and ecosystem functioning. Although food-web models have recently been applied to a wide range of ecological studies (Memmott, 2009; Sanders et al., 2014), such approaches can be greatly improved by introducing high-resolution trophic information from empirical studies and experiments that realistically describe topological structure and energy flows (de Ruiter et al., 2005). Over the last decades major technological advances have been made in empirically characterizing trophic networks by describing, in detail, the connectedness and flows in food webs. Existing empirical techniques, such as stable isotope probing (SIP) (Layman et al., 2012), have been refined and new approaches have been created by combining methods, e.g., combining Raman spectroscopy or fatty acid analysis with SIP (Ruess et al., 2005a; Li et al., 2013). These empirical methods can provide insight into different aspects of food webs and together form an extensive toolbox to investigate trophic interactions. It is crucial to recognize the potential and limitations of a range of empirical approaches in order to choose the right method in the design of empirically based food-web studies. Empirically based food webs are generally classified according to the type of input information that is required. In the following lines we will provide an overview of four types of food-web model: connectedness webs, semi-quantitative webs, energy-flow webs, and functional webs. Paine (1980) introduced three of those webs, which are widely accepted and applied in food-web studies across ecosystems. We propose to add a fourth type of empirically based food web, the semi-quantitative web. All of these food webs have the same basic structure, but the conceptual webs differ in the type of trophic information they describe and represent (Figure 16.1). Connectedness webs (Figure 16.1a) define the basic structure of a food web by describing the food-web connections per se