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.