Resolution of Respect:Evelyn Chrystalla (Chris) Pielou (1924–2016)

Photo 1. Evelyn Chrystalla ‘E.C.’ Pielou. Photo credit: Sharon Niscak. Evelyn Crystalla (also known as Chris or E.C.) Pielou (1924–2016) was one of the most prominent ecologists and biogeographers of the 20th century owing to her pioneering work in applying mathematical and statistical rigor to ecological and biogeographical patterns. She effectively founded the field of Quantitative Ecology. Pielou was born on February 20, 1924, in Bognor Regis, England, and 2024 would be the 100th anniversary of her birth. Not much is known about her childhood, but she had an untraditional entry into science, as did many women at the time. In 1942, at the age of 18, she earned a certificate in radio-physics from the University of London. Soon thereafter, Pielou joined the Royal Navy, serving in the Second World War for 3 years. In 1951, she received a B.Sc. in botany from the University of London. Over the next decade or so, Pielou raised three children and worked largely in isolation and without supervision. During this time, however, she published several papers, mostly focused on patterns in plant populations and communities, how to describe them quantitatively, and how to infer processes rigorously from patterns. Based on this work, which she had again completed independently and with no supervision of note, the University of London granted her a Ph.D. in 1962. Pielou then joined the Statistical Research Service in the Department of Forestry (1963–1964), and the Department of Agriculture (1964–1967) for the Canadian government. In 1968, Pielou started her first academic job, as a Full Professor, at Queen's University in Ontario, Canada. By this time, Pielou had been publishing papers in top-tier journals for nearly 15 years on topics ranging from spatial pattern analysis, quantifying species diversity, and Robert H. MacArthur's interpretations of relative abundance models. She also soon published the first of many books, An Introduction to Mathematical Ecology (1969). After only a few years, Pielou moved to Dalhousie University in Novia Scotia, Canada, where she stayed for 10 years and produced some of her most important and synthetic work, including the foundational books Population and Community Ecology: Principles and Methods (1974), Ecological Diversity (1975), and Biogeography (1979). Few, if any, ecologists have ever matched that level of productivity over a decade. In 1981, Pielou moved to the University of Lethbridge in Alberta, Canada, where she became Professor Emerita in 1986. She received many honorary degrees and accolades, including being named Eminent Ecologist by the Ecological Society of America in 1986. Pielou retired to British Columbia, Canada, in 1986, where she remained active in local environmental issues and began publishing natural history books to bring science and an understanding of nature to the general public. Throughout her long, unconventional career, Pielou was at the very front of the vanguard of quantitative ecology and made ecology and biogeography more quantitative and rigorous sciences. Pielou died in 2016, but both her approach to science and her many books continue to inform and inspire. Unfortunately, there are no definitive biographies on Pielou. Langenheim (1996) devotes considerable page space to Pielou in her exhaustive review of the early history of and progress of women in ecology. Jacqueline Gill (https://contemplativemammoth.com/2012/10/16/happy-ada-lovelace-day-honoring-dr-evelyn-chrystalla-pielou/) and Simberloff, Sanders and Peres-Neto (https://methodsblog.com/2017/03/10/ec-pielou/) wrote blog posts honoring Pielou's many contributions to ecology, biogeography, and paleoecology. In addition, a summary of her contributions was published in the Bulletin of the Ecological Society of America (ESA) in honor of her being awarded the ESA's Eminent Ecologist Award in 1986 (Bentley 1987). Pielou was only the second woman to win the award since its inception in 1953 and one of three women in the first 40 years of the award's history (Ruth Patrick in 1972 and Margaret Davis in 1993 were the other two). Much of plant ecology in North America prior to the 1950s focused on describing (and subsequently arguing about) plant associations, successional patterns, and describing techniques to assess plant communities. Pielou's first paper (Pielou 1952) was in that vein. Subsequent papers would add quantitative rigor where it had largely been absent. Pielou (1952) was based on extensive fieldwork in the Rukwa Rift Valley in what is now Tanzania. She spent the rainy season of 1946–1947 describing spatial and temporal variation in the distribution and abundance of plants among three main habitat types. The work is largely descriptive and lacks much in the way of the quantitative approaches Pielou would later pioneer. But this approach was typical of much of ecology at the time, and she was about to move the field in a more quantitative direction with a series of papers that became her dissertation. Ecologists are concerned with the study of plants growing under natural conditions, with the effects of these conditions on the plants, and with the effects of the plants themselves on one another. Their interest centres, therefore, not so much on the individual plants treated as separate entities, as on the tracts of vegetation treated as wholes. One of the properties that can be possessed only by an assemblage of plants, and not by any individual plant, is spatial distribution or spatial patterns. Pielou's second peer-reviewed paper (Pielou 1957) on the effect of quadrat size on assessing spatial distributions was really the first example of one of her defining professional traits; quantitative rigor. The paper also foreshadowed another theme that would re-emerge throughout her career; the sundering of the artificially imposed barrier between mathematics and field ecology. In particular, this paper argued that the size of a quadrat that a plant ecologist uses to survey plant communities can influence the interpretation of the spatial distribution and density of plants in that community. In some sense, this work was decades ahead of its time, in that it showed that many patterns and processes in ecology depend on scale. Pielou's next five papers (Pielou 1959, 1960, 1961, 1962b,c) focused on spatial distributions of plants within and among species and were the crux of her dissertation (Pielou 1962a). This series of papers established Pielou as a leading quantitative ecologist of her generation, and all were published before Pielou obtained a Ph.D., again, with no oversight from a graduate advisor. This set of papers progresses from a pretty basic “how does one actually go out into the field and assess the spatial distributions of plants?” (Pielou 1959) to “can one use plant-to-neighbor distances to detect competition?” (Pielou 1962a). The answers, it turns out, are “by sampling truly random individuals” and “maybe,” respectively. Soon after obtaining her Ph.D., Pielou took up positions with the Departments of Forestry and Agriculture in Ottawa, Canada. Her work was still focused on spatial patterns in plant populations and communities (Pielou 1964, 1965), but she began to apply her quantitative toolkit to practical problems, such as the distribution of diseased and healthy trees in a patchily infected forest (Pielou 1963a,b). Collectively, this set of papers exemplifies Pielou's approach. Each of them eloquently positions the work in the larger context (e.g., “Much work has been done in recent years on the spatial patterns of natural populations of plants, and it has been customary to study one species only at a time. Suppose, however, an investigator was concerned with a population of two co-dominant species” [Pielou 1961]). The papers also follow a formula that mimics Pielou's overall approach to making strides in ecology and biogeography: she states the problem, says what others have done about it and why those approaches might fall short, provides new mathematical or statistical insights, then applies those insights to real data that she collected. Once you see the basic formula for a Pielou paper, and see it again, and again, you start to think that Pielou was onto something. In fact, this same approach is evidenced in her later books, and in some ways summarizes much of her career, in that what she is best known for is developing and applying quantitative approaches to real world ecological problems and data; the unification of statistical, mathematical, and field-based ecology. Beginning in the mid-1960s and for the next decade or so Pielou's work largely focused on what we would now call community ecology. Her earliest work in community ecology pointed out mathematical errors in work on relative-abundance distributions by Robert MacArthur (MacArthur 1957, 1960) that had “aroused great interest among ecologists and been widely quoted” (Pielou and Arnason 1966). Pielou, along with A. Neil Arnason, demonstrated that the error in MacArthur's paper leads to an underestimate of the abundances of common species and an overestimate of the abundances of rare species in a community. And Pielou (1966a) also identified a mathematical error in Vandermeer and MacArthur's reformulation of MacArthur's broken stick model (Vandermeer and MacArthur 1966). Pielou went on to focus on how one goes about describing a collection of different species occurring at the same place, at the same time; a community. Her first forays were two papers in Journal of Theoretical Biology entitled “The measurement of diversity in different types of biological collections” (Pielou 1966b) and “Species-diversity and pattern-diversity in the study of ecological succession” (Pielou 1966c). “Information theory” or “information content” (e.g., Shannon and Weaver 1949, Brillouin 1962) was being increasingly applied to describe how individuals were divided among species in communities, with Shannon's Diversity Index probably the most prominent statistic at the time. Pielou's Journal of Theoretical Biology papers do a few important things. First, Pielou (1966b) points out that one can't simply apply the same diversity metric to different collections of species (e.g., collections in which all individuals can be counted and identified vs. collections where not all individuals can be counted and identified). Second, Pielou (1966b) provides an early example of species accumulation curves (i.e., how species richness accumulates as individuals are sampled from a community). And finally, both papers introduce the world to what Pielou called “The Evenness Component of Diversity,” which we now call Pielou's Evenness or J. Pielou describes what evenness is in the 1966b paper and provides the formula and a worked example in the 1966c paper. We suspect most students of ecology are familiar with Pielou's Evenness. In yet another paper in 1966 (Pielou 1966d), she warns ecologists about the misuse of Shannon's Diversity index; unfortunately, few appear to have listened. In 1967 and 1968, Pielou collaborated with her entomologist husband to publish the first substantial statistical treatments of missing species combinations in local communities consisting of subsets of a regional biota (Pielou and Pielou 1967, 1968). They proposed two methods, one of which was an early exemplar of randomly distributing species into sites as a sort of null hypothesis, then applied these methods to real data consisting of insects and spiders on bracket fungi and discussed the limitations of deducing causal mechanisms directly from distributional patterns. Pielou published her first of many books in 1969, entitled An Introduction to Mathematical Ecology; she published a second edition in 1977 entitled simply Mathematical Ecology. In the preface to the first edition, she writes “The fact that ecology is essentially a mathematical subject is becoming ever more widely accepted. Ecologists everywhere are attempting to formulate and solve their problems by mathematical reasoning…The purpose of this book is to serve as a text for these students and to demonstrate the wide array of ecological problems that invite continued investigation.” Sieniutycz (2023) provides an excellent overview of the book. Reviews of the book were decidedly mixed. Feldman (1970) called it a “valuable and timely book” whereas Levin and Solomon (1971) stop just short of wondering why Pielou bothered to write a book about mathematical ecology in the first place and include a series of detailed corrections to the text. Nevertheless, the book unified a lot of what Pielou had been working on since her Ph.D. and included sections on the dynamics of populations, spatial patterns of species, and the description of communities. Though she continued to publish papers on species associations (Pielou 1972a) and niche width and overlap (Pielou 1972b), the next big milestone was her second book, Population and Community Ecology: Principles and Methods (Pielou 1974a). There were few textbooks or reference books on population and community ecology at the time. The book was generally well received (Rosenzweig [1976] called it a “valuable textbook…[and] a yeomanlike summary of most of the topics in population ecology”) and went through four editions, the last in 1983. The book builds from the growth of populations through interactions, and finally addresses patterns of diversity in space and time. Although some key concepts are largely omitted or glossed over, and there is, as Rosenzweig (1976) pointed out, a dearth of examples of experiments, someone teaching an advanced undergraduate or graduate-level course in Population and Community Ecology could certainly use this book as the backbone of the course curriculum. Much research is currently in progress on the various consequences of simple ecological models. There is also debate on whether these models are “overly” simple in the sense that, owing to the great complexity of the real world, one can never expect their assumptions to be realized in nature. The greatest defect of extremely simple models, however, is not that they cannot be “true” but that (except in the artificial conditions of a laboratory) they cannot be tested. This is because they presuppose that the processes being modeled occur in that virtually non-existent setting, a uniform (spatially homogeneous) environment. But Pielou always seemed to be able to test her models because the models were not overly complex, but neither were the natural systems she often worked in. 1975 saw the publication of yet another important book: Ecological Diversity (Pielou 1975). Ecological Diversity aimed to be a state-of-the-art book for researchers and grad students who were interested in studying (and/or conserving) diversity. In the introduction, Pielou outlines a series of questions that investigators are still examining 50 years later: why are some species abundant and some rare? Do species differ in their tolerances of environmental variation? Both approaches have drawbacks. The mathematicians run the risk of constructing interesting models divorced from reality; and the statisticians of providing clear answers to ecologically uninteresting questions. 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