The interactions described above, within granular ecosystems, will support the growth of a succession of micro-organisms as the nutrient availability and microenvironment changes with time. The rate of mould growth is also determined by the ability of the micro-organism to compete with other species. Some species, including those of Aspergillus, Penicillium and Fusarium, can occur both in the field and in storage. Secondary metabolites are those compounds, produced by living organisms, which are not essential for growth.
Some secondary metabolites produced by moulds are highly toxic to animals, humans and plants.
Any activity which disturbs the stability of an ecosystem will increase the production of secondary metabolites, including mycotoxins. Such activities include the widespread use of fertilizers and pesticides, high yielding plant varieties and the cultivation of a limited number of plant species with restricted genetic variation. The normal practices of harvesting, drying and storage also, of course, significantly disturb the ecosystems of grains established before harvest. Aspergillus moulds can occur both before and after harvest, whereas Fusarium and Penicillium moulds occur predominantly before and after harvest respectively. In general, Aspergillus is associated with the tropics and Penicillium with temperate climates, whereas Fusarium moulds occur world wide.