Assessment Notes

Justification

Solioccasus polychromus is a hypogeous species, known from tropical and subtropical forests in Australia and Papua New Guinea. It was only described in 2013 and is likely to be undersampled due to its inconspicuous fruitbodies. Nevertheless, it is already known from approximately 20 sites, and the population size is estimated to be at least 100,000 mature individuals, with no evidence of a significant population decline. Therefore, this species is assessed as Least Concern (LC).

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Taxonomic notes

This is the type and only species in the Solioccasus genus, established in 2013 based on morphological and phylogenetic evidence (Trappe et al. 2013).

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Geographic range

This species is found in Papua New Guinea (Western Province) and in lowland tropical and subtropical forests in Australia, in the Northern Territory (Arnhem Land) and along the coast and coastal mountains of Queensland (Cape York to Brisbane) (Trappe et al. 2013).

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Population and Trends

It is known from roughly 20 sites (five in Papua New Guinea, 15 in Australia) (Trappe et al. 2013, GBIF.org 2023), though it is difficult to determine the exact number from locality descriptions. Observations and known sites are likely limited by its inconspicuous fruitbodies, which, despite being brightly coloured, are mostly hypogeous, small, and sometimes covered by forest litter. Taking this into account, as well as the recent description of the species, and large number of potential hosts, a large multiplier for unknown sites can be applied (e.g. x 500). Following guidelines by Dahlberg and Mueller (2011) the population size is therefore estimated to be above 100,000 mature individuals, assuming 10 mature individuals exist per site. There is no evidence of significant population decline.

It is possible there is some small decrease in population size due to habitat declines, but this is difficult to establish. Estimates by World Resources Institute (2023) show a decrease of 4.2% of tree cover (>30% canopy density) in the area of distribution of this species for a period of 20 years from 2001 and 2021. Nevertheless, assuming a linear decline, a decrease in population size through habitat loss is not projected to meet criteria for threatened categories in a 50 year period.

Population Trend: Decreasing

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Habitat and Ecology

Given its phylogenetic placement Solioccasus polychromus is assumed to be ectomycorrhizal but no host has been confirmed. Nevertheless, it is found in lowland tropical and subtropical forests, dominated by or in a mixture of ectomycorrhizal trees which constitute potential hosts, such as Allocasuarina littoralis, Corymbia spp. (C. dichromophloia, C. erythrophloia, C. polycarpa), Eucalyptus spp. (E. pellita, E. pilularis, E. racemosa, E. tetrodonta), Leptospermum spp., Lophostemon sp. and Melaleuca spp. The fruitbodies are hypogeous or partly emergent, gregarious, and grow in sand or soil, often covered by forest litter. Spore dispersal is thought to occur through mycophagist species, namely by mammals and birds between Papua New Guinea and Australia through the Torres Strait.

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Threats

This species may be threatened by changes in habitat quality within its distribution, namely in Queensland’s tropical rainforest region. This includes disturbances caused by habitat fragmentation caused by roads and powerline construction, presence of invasive species such as Phytophthora cinnamomi, tourism within protected areas, pollution from agricultural runoff, changes in fire regime and climate change (Williams et al. 2011, Stork et al. 2011)

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Conservation Actions

Research into the ecology of the species is needed to help establish potential threats, and determine host species preference.

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Use and Trade

There are no known anthropogenic uses

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Source and Citation

Cunha, S.P. & Gonçalves, S.C. 2025. Solioccasus polychromus. The IUCN Red List of Threatened Species 2025: e.T266000680A266018833. Accessed on 23 November 2025.

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• Country occurrence