When is a mutualist a cheater? A synthesis of conceptual and data-based perspectives on the causes and consequences of variation in mutualist quality

A mutualism is an interaction between species in which each partner benefits through the exchange of resources or services. Many species rely on their mutualist partners to provide protection from enemies, transportation, or nutrients that they cannot obtain on their own. For example, reef fish will seek out specialized cleaner fish to remove their parasites and some plants house ant colonies that defend them against herbivores. Flowering plants often depend on animals to distribute their pollen and seeds, offering sugar-rich nectar and fruit as rewards. Bacteria and fungi associate with all plants and animals and can provide their hosts with novel biochemical abilities, such as rhizobium bacteria that inhabit legume roots and convert nitrogen from the air into ammonium fertilizer. However, mutualists face a major problem: their partners may be able to cheat them. Importantly, if cheaters gain an advantage over more beneficial partners, the mutualism could break down. Currently, researchers define cheating in different ways, making it difficult to assess how commonly cheating occurs or to identify patterns of who cheats and why. Our working group brings together diverse scientists, studying a wide range of mutualisms, in order to: - achieve a consensus on how to define and measure cheating - compile a database on partner quality across a range of mutualisms - analyze our database to address fundamental questions about cheating in mutualisms By eliminating confusion over basic terminology and laying out standard methods, we will facilitate faster progress in mutualism research. The database we develop will allow different kinds of mutualisms to be compared quantitatively, enabling us to determine the species characteristics and environmental factors that make mutualisms vulnerable to cheaters. This will be important for our understanding of how mutualisms are being, and will continue to be, reshaped by global change. By forging consensus between early and later career scientists working in multiple systems, our working group has the potential to transform the study of mutualism from a collection of examples into a unified field.