In a paper published in the Journal of Biogeography in 2001, Robert Whittaker, Katherine Willis and Richard Field attempted to develop a “general, hierarchical theory of species diversity” that overcame what they saw as weaknesses of diversity theory at the time: failures to distinguish different response variables and adequately account for geographical scale. Seventeen years after the paper was published, I asked Robert Whittaker about the origin of his interest in these issues, how the authors came together, and what we have learnt since about scale and species richness.
Citation: Whittaker, R. J., Willis, K. J., & Field, R. (2001). Scale and species richness: towards a general, hierarchical theory of species diversity. Journal of biogeography, 28(4), 453-470.
Date of interview: Questions sent by email on 29 December 2017; responses received by email on 21 July 2018
Hari Sridhar: I would like to start by asking you about your motivation to write this paper. For most of your research career up to this point, your interest seems to have been plant succession on islands, especially Krakatau. The interest in thinking about richness and diversity seems to have come sometime in the late 90s. Could you trace for us the link between your early research and the move towards richness and diversity theory?
Robert Whittaker: Alongside my long-standing interest in island biogeography, I was always interested in patterns and processes of species diversity, and in theories explaining the high diversity of the tropics. My undergraduate degree was in Botany and Geography at the University of Hull. Here I was influenced especially by John Flenley (who sadly passed away a few weeks ago), in terms of the importance of historical processes. And, I had also got interested in the importance of scale, initially with respect to geomorphological processes, but subsequently in ecology and biogeography (developed in part while studying for an MSc in Ecology at University College North Wales, where a particularly seminal moment was encountering Tim Allen, discussing hierarchy theory during a brief visit he made there). My doctoral research was an attempt to bridge population and community level processes in the study of plant succession on Norwegian glacier forelands. And alongside this, was the Krakatau work, which was simultaneously about plant succession and island biogeography. Hence, I had been thinking about multiple spatial and organisational scales and how to bridge them conceptually and analytically for the previous two decades before writing the paper.
The key collaboration that led to this paper was with my first doctoral student in Oxford, Eileen (Bonnie) M. O’Brien, who completed her thesis in 1989. Bonnie developed an original solution to the puzzle of explaining geographical patterns of species richness in what became known as ‘water–energy dynamics’. Her work suggested that these coarse-scale patterns of richness could be explained by contemporary climate. This posed something of a challenge when I had become convinced of the importance of historical processes, and in the Krakatau context, of the inadequacies of equilibrium theories to explain diversity patterns. In efforts to make sense of these seemingly conflicting streams of evidence, we engaged in long discussions between us about the importance of scale of analysis and of dealing with the problem of area. My thinking as it had developed at this time owed a lot to her work and the conviction and passion with which she argued her points.
HS: In the Acknowledgements, you mention that this paper “benefited from RJW’s participation in the NCEAS workshop ‘Foundations of Biogeography’ held in Santa Barbara, April 2000.” Could you tell us a little more about the influence of this workshop on this paper?
RW: The NCEAS workshop was an interesting moment in several ways, not least that it led directly to the establishment of the International Biogeography Society. It was focused on the development of the discipline of biogeography but happened to involve a number of influential natural scientists, most of whom I had never met before and whose ideas on diversity had already influenced my thinking. It gave opportunity to discuss and debate with them informally in the gaps between the main sessions – people such as Jim Brown, Rob Colwell, Larry Heaney, Mark Lomolino, Vicki Funk, Nick Gotelli, Dov Sax and others. These discussions helped me sharpen up my thinking and, re-reading the paper now, I can see sections that follow from these interactions. I also made a visit to Jim Brown’s lab in Albuquerque whilst we were working on the paper and spent some hours talking with him and his students (‘what is the mechanism?’ was a repeated question in discussion there): you can see that his 1999 Macroecology synthesis paper was an important reference point at several points within our article.
HS: This paper has three authors. How did this group come together and what did each author bring to this paper? Did the three of you meet often during the making of this paper?
RW: Richard Field was working with me on the Krakatau stuff and on the collaboration with Bonnie O’Brien in developing and extending her water–energy dynamics models. At the time, we met with quite a bit of resistance to lines of argument in that work and so, as I recall, Richard and I began to think of writing a separate paper to tackle some of the issues we felt needed rethinking in respect of diversity theory. Kathy Willis had fairly recently joined the department, bringing a more fully developed grasp of long-term evolutionary dynamics to the discussion. She pushed the idea of integrating temporal and spatial scale more thoroughly in the arguments. Kathy and I subsequently published a much shorter companion paper to the 2001 Journal of Biogeography paper, in Science in 2002, which was entitled ‘Species diversity – Scale matters’, which has also been quite heavily cited. I don’t recall spending much time sitting together while working on the draft of the 2001 paper and, while the gestation period for it was lengthy, I think the writing was done in a few short bursts. I am not sure the three of us ever got together in the same room to work on it.
HS: You acknowledge a number of people at the end of your paper. Could you tell us a little more about how you knew them and their contribution to this paper?
RW: See the earlier answer about the NCEAS meeting and my visit to Jim Brown’s lab. I gave a draft of it to a few colleagues for comment and pre-review. We made some changes in response but as a perspective piece, it was set out the way we wanted to set it out. I still have a copy of Dov Sax’s written comments in response to the first draft. They were rather more extensive than those from the formal review process and some specific parts of the paper were added (e.g. the brief discussion of mangrove systems) or changed as a result.
HS: How long did the writing of this paper take? When and where did you do most of the writing?
RW: I think it was mostly written in Oxford over the summer of 2000. The earliest file that is a recognisable first step towards the paper is dated 8th August 2000 and the first version was submitted on 18th December 2000, with the revised version submitted on 30th March 2001. It was published by July that year.
HS: Did this paper have a relatively smooth ride through peer-review? Was Journal of Biogeography the first place this was submitted to?
RW: Yes and yes. Nicest review ever. In places the text is a bit dense – perhaps the review process was too kind and we should have been urged to improve the flow in a few places.
HS: What kind of attention did this paper receive when it was published? Today, this paper has received over 1300 citations. Do you have a sense of what it mostly gets cited for?
RW: It did generate quite a lot of interest, although it took a couple of years to build up citations. It has never really had a peak, but has been cited steadily at a rate between 60 and 100 or so each year since 2004. I think it probably mostly gets cited in relation to the importance of scale in diversity theory (that’s what colleagues tend to talk about when we discuss it) but to be honest I haven’t done the analysis to check. Re-reading the paper it is rather dense in places and covers a lot of ideas, in a way a set of linked essays, so it could be being cited for varied reasons.
HS: What kind of impact did this paper have on your career? In what ways, if any, did it influence the future course of your research?
RW: Citations apart, it was one of those papers that seemed to bring me into contact with a number of other scientists interested in similar ideas. For sure, it led to a number of collaborations with other macroecologists. The paper frames a number of specific areas of research within island biogeography and macroecology, which I have continued to work on, exploring and testing many of the arguments laid out within it. I don’t think I consciously set out to do that, but I can certainly see the links.
HS: Today, 17 years after it was published, I would like you to reflect on the following lines from your paper and tell us how far we have come in developing such a general theory of diversity:
“A general theory of diversity must necessarily cover many disparate phenomena, at various scales of analysis, and cannot therefore be expressed in a simple formula, but individual elements of this general theory may be. In particular, it appears possible to capture in a dynamic climate-based model and `capacity rule’, the form of the grand cline in richness of woody plants at the macro-scale. This provides a starting point for a top-down, global-to-local, macro-to-micro scale approach to modelling richness variations in a variety of taxa. Patterns in differentiation/endemicity, on the other hand, require more immediate attention to historical events, and to features of geography such as isolation. Thus, whilst we argue that there are basic physical principles and laws underlying certain diversity phenomena (e.g. macro-scale richness gradients), a pluralistic body of theory is required that incorporates dynamic and historical explanation, and which bridges equilibrial and nonequilibrial concepts and ideas.”
RW: I think I would still be happy to sign off on an assessment along these lines. I don’t think we are much closer to a general theory of everything about diversity but we have improved understanding of a range of topics. Hence, diversity theory has developed since this was written and I think we could add quite a bit on the historical signal and the extent to which elements of time-structured signal may in fact be emergent, providing signals that can be captured in analytical and even predictive models. I also think that O’Brien’s ideas of water–energy dynamics, which were an important component to the thinking covered in the paper, have gradually gained more traction, although I think there is still more to her ideas than has been realised to date. Unfortunately, she is no longer with us to contribute to that process herself.
To a large extent, I have continued to work on these themes, even though I have spent more time working on island biogeography in recent years. In essence, understanding diversity patterns requires us to deal with the issue of area, and this is best done either by analysing how diversity varies with area (which we do in island biogeography) or holding area constant and dealing with how patterns and processes change across scales, which was more the theme of the 2001 paper.
HS: If you were to rewrite a paper on this topic today, what would its main emphasis be?
RW: It would be interesting to revisit and update it and to broaden it to include more recent work on functional diversity, phylogenetic diversity and a lot more island material, and to make a serious effort to expand to more explicitly include aquatic and especially marine systems, but that sort of synthesis would probably require a monograph to do it justice.
HS: In the paper you indicate that your argument has a “plant-bias”. In retrospect, in the light of subsequent theoretical treatments with an animal-focus, how important do you think this bias was in influencing your argument?
RW: I think that starting with plants is the right place to start as they are the base of the food chain.
HS: Have you ever read this paper after it was published? If yes, in what context?
RW: Yes, but I have probably raided bits of it more often than I have sat and read through it. I read through it again before answering the questions.
HS: Would you count this paper as a favourite, among all the papers you have written?
HS: What would you say to a student who is about to read this paper today? What should he or she take away from this paper written 17 years ago? Would you add any caveats?
RW: Like any attempt to synthesise ideas, it was written in a particular context of ideas and experience and of involvement in arguments and debates. I don’t think I would add any caveats but I would want to point the reader to related work that has appeared since.