Robert McCredie May

1998 Balzan Prize for Biodiversity

For his seminal contributions to the mathematical analysis of biodiversity, in particular, his pioneering work on chaos theory and ecological systems, and the development of a variety of methods for estimating the total number and rates of extinction of species living on the earth today.

(1936 – 2020) May’s work has been hugely influential in illuminating fundamental biological problems relevont to the causes and consequences of biologcal diversity.

His earliest contributions are drawn together in the influential book Stability and Complexity in Model Ecosystems, which changed the way ecologists think about complex versus simple ecosystems. No longer is the ability to cope with disturbance (or “stability”) seen as some automatic consequence of diversity (or “complexity”), as it was in the texts of the 1970s and earlier, but rather it is understood that communities with high biological diversity (such as the tropical rainforest) are often likely to be dynamically fragile, and typically more vulnerable to disturbance than are simpler temperate systems. This has led to a still-expanding programme of research on how the persistence and ultimate conservation of communities depends on food web structures of particular kinds. May’s work has emphasised the differences between different environments, and helps explain why we may be likely to lose a larger fraction of species in disturbed tropical environments than have been lost in the past in correspondingly disturbed temperate or boreal environments. More generally, this work has set the agenda for a new generation of research on the difference between “demographic stochasticity” and “environmental stochasticity” (terms introduced by May), on food chains, on the relative abundance and rarity of species, on the relation between numbers of species or of individuals and their physical size, and on the dynamical response of complex ecosystems to specific kinds of disturbance.

Motivated by such problems arising from the study of natural populations, May showed that simple nonlinear difference equations can exhibit an astonishing array of dynamical behaviour ranging from stable points, to period doubling bifurcations that produce a cascade of stable cycles, to apparently random or “chaotic” fluctuations (his 1976 Nature review paper is still the most cited paper in this general subject). This seminal work raises important, and largely yet unresolved, questions about how we gather and analyse data about populations. Especially when spatial patchiness is important, populations can easily show erratic fluctuations, even in environmentally predictable settings. Such phenomena have major implications for the co-existence of species, or for the invasibility of communities, and thence for explaining patterns of biodiversity.

In collaboration with Roy Anderson, May has combined theoretical and empirical studies to explore the circumstances under which infectious diseases (defined broadly to include viruses, bacteria, protozoans and fungi, along with helminth and arthropod parasites) may influence the numerical abundance or geographical distribution or other ecological features of their plant or animal hosts, this work has advanced our understanding both of how infectious diseases can influence biological diversity, and of their importance in conservation biology.

In recent years, May has been a leader in developing a variety of rigorous methods for estimating the total number of species alive on earth today. Since 1994, he has also developed several new ways of assessing rates of species extinction in the recent past and likely future. This work is mainly based on comparisons between recent data sets and fossil record data; by comparing rates one can gain a degree of precision absent from attempted estimates of total numbers of extinctions. His other recent work has developed quantitative measures of the taxonomic or evolutionary uniqueness of individual species or groups. Taken together, this general body of work pioneers a new “calculus of biodiversity” (May’s phrase), which is an increasingly important tool for conservation planners.

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