Why do you think your work is highly cited?

In many ways that is a good question. I was surprised that the work has attracted so many citations in the last few years. The number of citations is likely due to several items:

First, the scaling work touches on many fundamental issues of biology, ranging from cellular physiology to organismal anatomy and physiology to evolutionary biology and population and community ecology all the way to large-scale variation in the flux of matter and energy through ecosystems and the biosphere. In short, the work has implications for fundamental issues in evolution, biodiversity science, and large-scale ecology in general. Thus, many of the citations are from vastly differing fields (including the fields of biomedical science, physics, ecology, genetics, population biology, global change biology, and even geosciences).

Second, another important aspect of the work is that it is inherently capable of making quantitative predictions. After the important findings from the understanding of non-linear dynamics, chaos and complex systems ecology in particular seemed to be heading down the path that concludes that "prediction is difficult if not impossible." The scaling work emphasizes law-like behavior in biology. It shows that it is possible to make quantitative predictions for numerous aspects in biology. In doing so, the work offers the intriguing hypothesis that many aspects of biology are mechanistically related by first principles. Naturally, this is an intriguing proposition. SO perhaps it is not surprising that many papers citing our work claim to test the assumptions and or predictions of the model.

Third, undoubtedly another reason the work is highly cited is that theoretical work in allometric scaling is contentious. Allometry and the mechanisms generating constancy and or variance in allometric functions have had a long history of contentious debate (well before our work). In many ways a general theory of allometry has been a Holy Grail of sorts. So, I guess it is of no surprise that the general metabolic scaling framework we and our collaborators have proposed should also come under close scrutiny.

   What are the circumstances which led you to your work?

The circumstances are all related to a wonderful convergence of many lines of thinking. My advisor Jim Brown had published many articles emphasizing that scaling and allometry seemed to offer the potential for synthesis in biology. Further, Geoffrey West, who had been thinking along similar lines, was interested in how physical scaling laws translated to biology and had published articles in scaling. When I arrived to start my graduate career I put together several datasets for plants showing that Kleiber’s relationship likely also held for plants. In addition, plants also shared many other allometric relationships with animals—including scaling properties of the vascular network and ecological scaling relationships such as the scaling of population density. In showing these graphs to Jim I wondered out loud if the similarities suggested shared mechanisms. So, we started in-depth conversations to figure out just what those shared mechanisms might be.

We had a hunch that the key mechanism was related to resource supply through vascular networks. It became increasingly clear, however, that we needed the guidance and insights of someone who was used to trafficking in physical laws. The first meeting between Jim, Geoffrey, and myself started over 10 years ago in January 1995 at the Santa Fe Institute. These meetings resulted in our 1997 Science paper that detailed the general allometric scaling model (West, G.B., Brown, J.H., and Enquist, B.J., "A general model for the origin of allometric scaling laws in biology," Science 276[5309]: 112-6, 4 April 1997). This paper showed that variation in metabolic rates between individuals was critically constrained by the topology of the vascular networks that supply resources to metabolizing cells. This was followed by research highlighting the ecological implications of metabolic scaling (Enquist, B.J., Brown, J.H., and West, G.B., "Allometric scaling of plant energetics and population density," Nature 395: 163-5, 1998; and Enquist, B.J., West, G.B., Charnov, E.L., and Brown, J.H., "Allometric scaling of production and life-history variation in vascular plants," Nature 401: 907-11, 1999). The three of us have essentially been meeting, collaborating, and publishing papers since our first meeting in 1995.

Since 1995 the scaling group has greatly increased in size and the impact and reach of the baseline scaling model has grown. I have been fortunate to collaborate with Karl Niklas of Cornell University. Karl and I have continued to flesh out the details of the implications of the plant allometric model for plant ecology and evolution. Together, we have published numerous papers on these topics.

   Would you describe the significance of this work for your field?

This is too early to tell. Currently, the work is already being used in introductory textbooks in physiology and ecology. The work was also the topic of a Gordon Conference last year (The Metabolic Basis of Ecology). However, ultimately, the field must decide if this work will be "significant."

The general hypothesis behind the work is bold. The work hypothesizes that many scaling phenomena in biology (ranging from the genome to the ecosystem) are the result of the processes that control the scaling of cellular metabolism. Further, the work hypothesizes that observed quarter-power scaling relationships in biology are the result of strong selection during the course of evolution to maximize organismal metabolic performance. If correct, then the evolution of the wonderful diversity of many scaling phenomena in biology are likely all inter-related by a single mechanism. Naturally, this is a grand statement and the assumptions and theory need to be fully assessed and tested.

If the general theoretical framework is sound then I expect that it will be used (with tweaking and modification along the way) as a basis to build a more detailed quantitative theory for the role of metabolism in influencing pattern and process in nearly all areas of biology. However, if the fundamental assumption of the model—that metabolic rates of cells are controlled by the scaling properties of the network—is violated, then the model should be abandoned. I would like to note that, regardless of the impact of the scaling work, I do not think that the plethora of scaling relationships that we observe in the fossil record and across a diverse suite of organisms are the result of chance alone.

   How has this work advanced since you first started publishing on it?

In many, many ways. The work of Jamie Gillooly has clearly added a new dimension to the scaling work by the incorporation of temperature to the general scaling model. Plus the number of collaborators in the "scaling group" (Eric Charnov, Van Savage, Andrew Allen, Karl Niklas, James Elser, Andrew Kerkhoff, Evan Economo, etc.) has continued to increase as the number of new facets of the theory has grown. For example, current work is focused on extending the scaling model to understanding whole-ecosystem net primary production, the flux of CO₂ and the nutrient budgets of terrestrial ecosystems, genome evolution, speciation rates, and the limits of vegetation height across the planet.

Last summer (2004) there was a Gordon Conference dedicated to the importance of metabolism in ecology. It was clear from the meeting that the connections yet to be made were numerous and exciting.

It is unclear just how far the scaling paradigm will be pushed. However, this is an exciting time and I expect many papers on the following topics to be integrated into the scaling framework: food webs, evolutionary dynamics, phylogenies, life history variation, large-scale ecosystem dynamics, stoichiometry, and a more robust theory for community ecology. Plus, I see the focus shifting more toward the ecological and evolutionary dynamics around allometric and metabolic scaling constraints.

Brian J. Enquist, Ph.D.
University of Arizona
Tucson, AZ, USA