In defense of a “theory of everything” in ecology

Genetics of Adaptation Graduate Seminar

Author: Francis Cartieri*

The beginning of the modern scientific era arguably began when Isaac Newton was able to explain the diverse phenomena of ocean tides, projectile motion, the movement of the moon and planets, and the seemingly chaotic oscillations in the trajectories of the stars… all with one cohesive set of mathematical formulas. It was the first great “theory of everything,” and since then, scientists and philosophers have endeavored to reproduce new and better general theories to account for groups of phenomena within the diverse array of disciplines that make up humanity’s knowledge‐gathering enterprise.

Some disciplines, namely the physical sciences such as theoretical physics and thermodynamics, are more amenable to highly general theories. The objects and relations they study are more uniform than those studied by the life sciences. As a result, thinkers have had more difficulty establishing the existence of “theories of everything” that regularly apply to such complex things as living organisms and their relations to complex ecosystems over time. A major difficulty has been providing theories that are predictive, rather than descriptive. That is, the general tenets of evolutionary science do not allow one to make a wide range of predictions about how a given ecosystem will look six million years from now. General theories in physics are composed of law‐like relations that do allow for long‐range predictions of future states, given the present state of a physical system.

It comes as quite a surprise, then, that a theory of everything of the predictive sort has been put forward, and has been convincingly defended, in ecology‐‐that discipline of boundless living complexity and contingency. The name of that theory is the Metabolic Theory of Ecology, or MTE, and it is a kind of master equation based on a set of bio‐physical principles, as well as the assumption that organisms evolve by natural selection to use resources efficiently. The applications of MTE are startlingly wide—the theory is purported to account for such diverse phenomena as individual plant and animal structure and function, the design of circulatory systems, migration patterns (be they birds or mega‐fauna), the storage of nutrients in ecosystems, and population growth rates. Incredibly, this list is far from exhaustive. “If the theory is right, it's one of the most significant in biology for a long time,” says ecologist David Robinson of the University of Aberdeen. “It would provide a common functional basis for all biodiversity.”

To say the reception of the Metabolic Theory of Ecology has been stormy would be an understatement. Many critics actually think it does damage to the study of ecology. “If they're not right, they'll have done a disservice to ecology,” says Jan Kozlowski, of Jagiellonian University, Krakow. Others are ecstatic, believing MTE represents a massive leap forward in how we can account for the frustratingly complex world of environment‐organism and organism‐organism interactions. “I've never been more excited in my life,” says Stephan Hubbell of the University of

Georgia. “Ecology now is like quantum mechanics in the 1930s—we're on the cusp of some major rearrangements and syntheses. I'm having a lot of fun.”

But the most common reaction is, perhaps understandably, cautious excitement. After all, the Metabolic Theory is only about ten years old, and it is not quite clear how one should go about testing it. Because the Theory is so abstract, one cannot conclusively claim that it is “correct” or “incorrect.” Instead, the question is whether or not the Theory is useful, whether or not it can be correctly applied, and where it can be usefully applied.

The Theory was born out of a longstanding mystery: why is the relationship between the rate an organism processes energy (base metabolic rate) and the size of an organism (biomass) more or less conserved across much of the tree of life? For the most part, there is a constant mathematical relationship between size and metabolism that holds for everything from apples to elephants. Why is this so?

The Metabolic Theory of Ecology answers that question by supposing that what matters in building a living thing is how energy is transferred; that there is an optimal way to get energy from one place to another, and that natural selection pushes living things to move energy around in the optimal way. The founders of the Theory used elegant mathematical engines called fractals to generate models of optimal energy transfer networks—networks that any living thing must use to move energy from where it is released to where it is needed.

In focusing on energy transfer, the founders of the Metabolic Theory had made a eureka insight—it turns out that their fractal mathematics produced ideal models of energy transfer systems that a huge variety of living things tend to approximate. If one knows the metabolic rate of an organism, one can use the Theory to determine when that organism will die, how many offspring it is likely to have, and how large it is or may become. Since the same equations govern energy transfer and organism growth across different species, predictions can be made using MTE for a whole community of different organisms, or a whole ecosystem of different communities. In fact, MTE can be used to make predictions about future states of living systems, as well as model how evolution has gotten a given species or group of species to their present state.

One of the things that make the Metabolic Theory of Ecology a potential “theory of everything,” is the extent to which researchers are able to add on extensional models. Extensional models are tools that allow researchers to translate the mathematics of the general Theory into the language of their particular area of interest. Say you are interested in bird migration. MTE is very abstract and says nothing about birds. But researchers have been able to translate the general things MTE predicts about mass‐metabolism relations into terms that the ornithologist can understand and make us of. In this way, MTE has been extended to a great number of research areas.

Of course, all this generality comes at a price. While the Theory is useful for predicting central tendencies of form and function (such as: what is the relationship between size and abundance in this population of water‐lilies?), its predictions are only approximate. If a researcher is interested in very specific questions about a given organism and its ecosystem, then more fine‐grained tools are necessary. Additionally, tension has been building between those thinkers who try to explain natural systems using the Theory as a substitute for fieldwork, and those researchers who believe that ecosystems are too complicated to understand without direct observation and intervention. Some even worry that MTE will lead to “armchair ecology” akin to the empirically disreputable “armchair philosophy.”

Whether the Metabolic Theory of Ecology is recognized to be a “theory of everything” or not will be the subject of great debate in the coming years. What is undoubtedly true, however, is that the Theory represents a powerful new tool in our attempts to understand the endless complexity of living, changing systems here on earth.

*Francis is a graduate student in the Dept of Philosophy, and because he was excited about the Metabolic Theory of Ecology, we agreed to let him write a on a slightly different blog topic than the other students