Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative-fitness-based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best-studied mutualisms. We find that although there are numerous observations of low-quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative-fitness-based definition of cheating.

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The “hierarchy of factors” hypothesis states that decomposition rates are controlled primarily by climatic, followed by biological and soil variables. Tropical montane forests (TMF) are globally important ecosystems, yet there have been limited efforts to provide a biome‐scale characterization of litter decomposition. We designed a common litter decomposition experiment replicated in 23 tropical montane sites across the Americas, Asia, and Africa and combined these results with a previous study of 23 sites in tropical lowland forests (TLF). Specifically, we investigated (1) spatial heterogeneity in decomposition, (2) the relative importance of biological factors that affect leaf and wood decomposition in TMF, and (3) the role of climate in determining leaf litter decomposition rates within and across the TMF and TLF biomes. Litterbags of two mesh sizes containing Laurus nobilis leaves or birchwood popsicle sticks were spatially dispersed and incubated in TMF sites, for 3 and 7 months on the soil surface and at 10–15 cm depth. The within‐site replication demonstrated spatial variability in mass loss. Within TMF, litter type was the predominant biological factor influencing decomposition (leaves > wood), with mesh and burial effects playing a minor role. When comparing across TMF and TLF, climate was the predominant control over decomposition, but the Yasso07 global model (based on mean annual temperature and precipitation) only modestly predicted decomposition rate. Differences in controlling factors between biomes suggest that TMF, with their high rates of carbon storage, must be explicitly considered when developing theory and models to elucidate carbon cycling rates in the tropics. Abstract in Spanish is available with online material.

The phylogenetic position of the genus Pseudoparmelia was addressed using molecular data from five loci (mtSSU, nuLSU, ITS, Mcm7, RPB1), generated from three species and aligned with sequences from 293 samples representing all major clades of Parmeliaceae. Pseudoparmelia species form a well-supported monophyletic group that is the sister group of a clade consisting of the genera Relicina and Relicinopsis. These three genera share a thallus with a pored epicortex, isolichenan as cell wall polysaccharide, and relatively small ascospores. Morphological and chemical characters that distinguish Pseudoparmelia from the closely related Relicina and Relicinopsis are discussed. To further elucidate the relationships of these three genera, we assembled a second dataset including 15 additional samples of Relicina and Relicinopsis using three loci (mtSSU, nuLSU, ITS). All three genera are monophyletic but monophyly of Relicina lacks support and, in the mtSSU single locus tree, the genus is paraphyletic with Relicinopsis nested within. Additional studies including more Relicina species are necessary to test delimitation of the genera Relicina and Relicinopsis.