Marasmius magnus A.C. Magnago & J.S. Oliveira, in Magnago, Oliveira & Neves, Phytotaxa 266(4): 275 (2016).

This species is the largest Marasmius described (reddish pileus up to 12 cm diam), and it is known from the subtropical Atlantic Forest in Brazil and Argentina.

Marasmius magnus is endemic to the Atlantic Forest found in Brazil and Argentina. The Atlantic Forest is unique, highly threatened ecosytem. Marsamius magnus is known from only three sites, but it is likely to be more widespread occurring in high quality Atlantic Forest areas, with a total population estimate of around 5,000-10,000 mature individuals. Based on the severe habitat decline within the area, we infer a population decline around at least 15% in the last three generations (30 years) and this decline is ongoing. It is assessed as Endangered C1.

The species is currently known from subtropical dense ombrophilous Atlantic Forest. Distributed in eigth localiton: six localities in southern Brazil: at Morro Santana (Rio Grande do Sul); at Trilha de Naufragados, Morro da Lagoa da Conceição and Parque Municipal da Lagoa do Peri (Santa Catarina) (Magnago et al 2016); at Parque Estadual Turístico do Alto Ribeira and Parque Estadual da Cantareira (São Paulo) (unpublished data); and two localities in Missones, northeastern of Argentina: at Parque Provincial Salto Encantado, and at Parque Provincial Saltos del Moconá (Ramírez et al 2021). It is expected that the species is distributed along the dense ombrophilous Atlantic Forest towards north of Brazil. However, after many areas of the Atlantic Forest have been explored in search of the species, but so far withou succes, indicates that the species might be rare to find.

At the moment there are thirteen collections of the species deposited in fungaria. Most of the collections (8) are from Florianópolis island (SC); one collection is from Morro Santana (RS) (Magnago et al. 2016), one collection from Parque Estadual Turístico do Alto Ribeira (SP), and one from Parque Estadual da Cantareira (SP) (unpublished data). Two collections are from Missiones, Argentina. Each collection consists of one to six basidiomes. This is a species with medium to high detectability, the pileus is rusty orange to reddish brown, with extreme margin generally whitish cream. It is the largest Marasmius species described (pileus up to 12 cm diam.)
There are eigth known sites, but the species is expected to be distributed along the dense ombrophilous Atlantic Forest with occurrence up to 500-1000 additional potential sites, each supporting around 10 mature individuals. This gives a total population estimate around 5.000-10.000 mature individuals. The longest distance between known collecting sites does not exceed 800 km.
The Atlantic Forest is been deforested over decades, and the remaining fragments are suffering from biomass and biodiversity erosion, being one of the most fragmented tropical/subtropical forests in the world, and around 28% percent of the original forest in Brazil is left, much of it in small, unconnected fragments (Rezende et al. 2018). Based on extensive loss of suitable habitat and the putative influence that habitat degradation has on species occupation in a given environment (Berglund & Jonsson, 2003; Haddad et al., 2015), we precautionarily assume a population decline of at least 15% within the past three generations (30 years) and this decline is likely continuing into the future.
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The species grows on soil/litter in the dense ombrophilous Atlantic Forest, solitary or in small group (2-6 basidiomes) during rainy season.

The extraction of timber, intensive land use including tourism, urban expansion, industrialization and fuelwood harvesting are some of the reasons for deforestation in the Atlantic Forest. Only 28% of its natural coverage remains, largely composed of small forest fragments and secondary forests (Tabarelli et al., 2010; Rezende et al., 2018). Changing temperature and rainfall is causing habitat shifts and alteration. These factors are the biggest threat to the Marasmius magnus, impacting directly on its habitat and life.

The main action to preserve the species is the protection of its habitat and creation of new conservation areas to harbor the probable habitats to which the Atlantic Forest may be restricted in the future. The preservation of pristine forests could be critical for the maintenance of this species, since it has only been found in preserved areas. Also, forest protection policies must be taken to assure that the protected Atlantic Forest areas reach a mature state.

More collections are ncessary to try to understand the real geographic distribution of the species.

No use/trade.