Grain stored in bulk forms a man-made ecosystem with a limited, non-regenerating energy supply. However, frequent interference results in an immature, unstable ecosystem with a relatively simple structure and composed of species with limited specialisation, and high growth and reproductive rates. Microbial colonisation commences soon after ear emergence, by what have been termed “field” fungi. These usually have a minimum water potential for germination of −22.4 MPa (> 0.85 aw), have to contend with large diurnal changes in environmental conditions and seldom develop further in store. Sometimes, especially in humid tropical climates, species more typical of stored grain (“storage fungi”) also develop before harvest, and even produce mycotoxins. This differentiation between fungi characteristic of field or store is historical and their classification on ecological requirements would be preferable.

Isolates of the tropical fungus Crinipellis perniciosa obtained from cocoa and lianas in Ecuador were grown on media ranging from −0.45 MPa to −9.0 MPa water potential and their responses compared to those of other agarics and Xylaria species isolated from cocoa. Antagonism between mycelia of C. perniciosa and these isolates was also assessed over the same range of water potentials. It is hypothesised that C. perniciosa and other canopy inhabiting fungi of tropical forests are adapted to water stress, but are not competitive with fungi which colonise later, following litterfall, from lower litter and soil horizons, where water availability is higher. Xylaria spp. proved to be aggressive antagonists of C. perniciosa over a range of water potentials.

Evidence now points to the existence of common patterns of functional specialisation in green plants and fungi. It is predicted that further research will reveal the basis for a universal functional classification of organisms which can be used to analyse the assembly rules of communities and ecosystems.