Host preference, niches and fungal diversity.

Ectomycorrhizal fungi occur in remarkably species-rich assemblages. One of the prevailing hypotheses to explain this diversity is niche differentiation; by occupying distinct ecological niches within a site, multiple fungal species are able to co-occur (Bruns, 1995). In this issue of New Phytologist (pp. 000–000), Ishida and colleagues make a significant contribution to our understanding of niche differentiation by showing that co-occurring host species have distinct mycorrhizal communities, reflecting both host taxonomy and, arguably, successional status. Although host specificity is a well-known phenomenon (Molina & Trappe, 1982), it has not previously been clear to what extent cooccurring species of plants support different species of ectomycorrhizal fungi. Using individual root collections from co-occurring plants, Ishida and colleagues have effectively demonstrated that host specificity (or, more accurately, host preference) is an important factor in local diversity. Regrettably, statistical power issues prevent a robust determination of whether host preference is more common at the family than at the genus level. Nonetheless, there are strong indications that both host family and successional status are important in determining plant–fungal associations.