David L. Hawksworth

David L. Hawksworth is a British mycologist and lichenologist whose name is tied above all to the question of how many species of fungi there are. In 1991 he published in Mycological Research the paper "The fungal dimension of biodiversity: magnitude, significance, and conservation", the source of the 1.5-million-species estimate quoted for the following quarter of a century.

The method was simple, and useful for that reason. Hawksworth took the ratio of known fungal species to species of vascular plants in well-studied regions and applied that proportion at a global scale. A construction like that lets anyone follow the reasoning, point at its weak spot and propose their own numbers — which turns an estimate into a tool rather than a declaration.

A second thread in his work is nomenclature and taxonomic practice. Hawksworth took part in the debates on the rules for naming fungi, including the move away from separate names for the sexual and asexual states of one species — a change that came into force after the Melbourne congress and has its own entry in this section.

In 2017, with Robert Lücking, he published a revision of his own estimate, giving a range of 2.2 to 3.8 million species based on environmental sequencing data. Changing one's own widely cited number is the situation in which an author has the most to lose — which is exactly why it belongs in a biography.

The consequences of that estimate reach beyond taxonomy. If only a few per cent of existing species have been described, then every threat assessment covers a small slice, and most fungi will either go extinct or not without our knowing. The case for fungal conservation today rests largely on that gap — and Hawksworth was the first to present it as a number rather than an impression.

It is also worth noticing how a number behaves once it is loose. "One and a half million" lived on in textbooks and in the press long after the authors themselves had published a higher value. Numbers released into the world age more slowly than the knowledge that produced them — one reason why every statistic on this site carries a period and a date of checking.

An estimate, not a census

Hawksworth and Lücking’s 2017 paper puts a range—2.2 to 3.8 million species—in its title rather than offering one “correct” total. The authors combine several lines of evidence: fungus-to-plant ratios in studied regions, fungi associated with particular hosts, and environmental sequencing. Each carries different uncertainty. A sequence detected in soil or a leaf does not automatically become a formally described species, and sampling is uneven across regions and habitats.

The range should therefore be presented as a model that changes with evidence, not a counter of hidden taxa. Kew’s profile shows Hawksworth’s broader practice in lichenology, nomenclature and fungal diversity. His contribution was not merely the value itself, but publishing assumptions that other researchers can criticise and recalculate. Whenever the estimate is repeated, its date and method need to travel with the number; otherwise an explicitly revisable result hardens into an unsupported fact.