Holling (1959b) subsequently defined three basic functional response types: the rectilinear type I, H1, (often linear without satiation, H0), the hyperbolic type II, H2, and the sigmoid type III, H3 ( Figures 1A– C Table 1). “…there must be a functional response to (say) an increase in the host density, because of the increased availability of victims: as host density rises, each enemy will attack more host individuals,…”. Solomon (1949) introduced the idea by stating: The functional response concept ( Solomon, 1949 Holling, 1959a, b) is a cornerstone of population and food-web ecology and is still widely relevant (e.g., DeLong, 2021 Gobin et al., 2022). Researchers use functional responses to quantitatively describe the interactions of consumers and resources, including, but not limited to, predator-prey, parasitoid-host, parasite–host, or filter-feeder-plankton interactions. Finally, we provide guidelines on how the field should move forward based on these considerations.ġ. We summarize how three different and mutually independent intricacies bedevil the empirical documentation of type III responses: (1) challenges in statistical modeling of functional responses, (2) inadequate resource density ranges and spacing, and (3) biologically meaningful and realistic design of experimental arenas. To put these studies into context, we also discuss the various biological mechanisms that may lead to the emergence of type III responses. We found 107 studies that reported type III responses, but these studies ranged across various taxa, interaction types, and ecosystems. Here, we review this evidence from laboratory- and field-based studies published during the last two decades. While theoretical and practical advances make identifying type III responses easier today, decades of research have brought only a limited number of studies that provide empirical evidence for type III response curves. However, experimentally proving that type III functional responses exist, whether in controlled laboratory systems or in nature, was challenging. Due to its potential to stabilize consumer-resource population dynamics, the sigmoid type III functional response immediately became a “holy grail” in population ecology. Initially, three types of responses were defined: linear (type I), hyperbolic (type II), and sigmoid (type III). More than 70 years after its introduction, the framework of resource density-dependent consumption rates, also known as predator-prey functional responses, remains a core concept in population and food web ecology. 6Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden.5Living Earth Collaborative, Washington University in St.4School of Biological Sciences, University of Nebraska, Lincoln, NE, United States.3Ecological Networks, Technische Universität Darmstadt, Darmstadt, Germany.2Senckenberg Biodiversity and Climate Research Centre, Frankfurt, Germany.1Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany.
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