OBITUARY20 March 2026The environmentalist founded the field of co-evolution, and warned that rapid human expansion would cause biodiversity collapse and famine.ByDanny Dorling0Danny DorlingDanny Dorling is a professor of geography at the University of Oxford, Oxford, UK.View author publicationsSearch author on: PubMed Google ScholarCredit: Mark Mahaney/Redux/eyevinePaul R. Ehrlich’s book The Population Bomb (1968), written with his wife Anne, made him one of the most influential, if controversial, scientists of the twentieth century. In it, he alerted the public to the possible problems of global overpopulation, including the depletion of natural resources and the deterioration of the environmental systems that support humanity. But his overemphasis on population growth at the expense of other factors also influenced oppressive policies in some of the world’s most populous countries, and has not proved to be justified. Ehrlich has died, aged 93.Ehrlich was born in Philadelphia, Pennsylvania, and attended Columbia High School, a highly rated state school in Maplewood, New Jersey. Butterflies were the childhood passion that drove him into science. Sure of this interest, at 15 years old, he joined the US-based Lepidopterists’ Society.The scale of the biodiversity crisis laid bareAfter earning a zoology degree at the University of Pennsylvania in Philadelphia and then a master’s and PhD in entomology at the University of Kansas in Lawrence, in 1959, Ehrlich secured a post at Stanford University in California. He became a full professor in 1966, after he published his most important work — a 1964 paper with botanist Peter Raven on plant and butterfly co-evolution, a concept they pioneered (P. R. Ehrlich and P. H. Raven Evolution 18, 586–608; 1964).The pair suggested that it was because of butterflies that many until-then unexplained chemicals in plants were “immediately explicable”. For instance, when milkweed butterflies extended their range after evolving such that they could feed on dogbanes (Apocynum spp.) and milkweeds (Asclepias spp.), scientists hypothesized that the plants’ bitter glycoside and alkaloid compounds were helping the animals to avoid predation. Unsurprisingly, research since then has found that there are more-important factors in butterfly–plant interaction patterns, but work in this field is often influenced by this seminal paper.Ehrlich and Anne wrote The Population Bomb in the months after they visited Delhi in 1966. Its publication in May 1968 brought Paul into the public eye. Critics labelled him a ‘population nut’ and accused him of hating children and being ignorant of human creativity. Undeterred by death threats, the couple published The Race Bomb in 1977, and included arguments against racial theories of intelligence. The same year, Stanford appointed him Bing Professor of Population Studies, and he remained at Stanford for the rest of his career.People are having fewer babies: is it really the end of the world?The Ehrlichs’ work on population had a profound impact on society. It encouraged mass sterilization programmes in India and the one-child policy in China, and influenced how children everywhere were viewed and valued. Ehrlich did not express regret over any of this, even much later in his life.More positively, The Population Bomb helped to encourage the availability of contraception worldwide and the right of women to control their fertility, including through abortion, leading to improved health and well-being.doi: https://doi.org/10.1038/d41586-026-00939-5Competing InterestsThe author declares no competing interests. The scale of the biodiversity crisis laid bare People are having fewer babies: is it really the end of the world? Global population is crashing, soaring and movingA population plunge could help to mitigate the global biodiversity crisisSubjectsHistoryScientific communitySocietyEvolutionLatest on:HistoryScientific communitySocietyJobs Group Leader Positions in Chemical and Structural BiologyIRB Barcelona invites applications for Group Leader positions in Chemical Biology (one) and Structural Biology (one).Barcelona (Provincia), Cataluña (ES)Fundació Institut de Recerca Biomèdica (IRB Barcelona)