How to Imbue the Basic Models of Biology With a Positive Vision of the Future

Can Ecology and Economics be unified?

Should the fruit flies continue to multiply at their initial compound interest rate, it can be shown by computation that in a relatively few weeks the number would be considerably greater than the capacity of the bottle. Very little thought and examination of the facts should suffice to convince one that in the case of the production of coal, pig iron, or automobiles, circumstances are not essentially different.

M. King Hubbert

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The basic behavior mode of the world system is exponential growth of population and capital, followed by collapse.

The Limits to Growth

Economics is sometimes called the dismal science. Thomas Malthus’ work, “The Population Principle,” is not the etymological origin of this nickname, but it is the spiritual one. Malthus argued that since population grows faster as it gets larger, and agricultural output could at best increase by a constant amount each season, England would inevitably fall into a cycle of growth followed by famine.

Malthus was a forefather of both economics and ecology. Economists inherited the dismal name — but they are happy to laud economic growth and to optimistically extrapolate it far into the future — ecologists kept more of the dismal outlook. The Club of Rome, Paul Ehrlich, William Vogt, and many more of the modern intellectuals behind the most urgent warnings of environmentalism are grounded in the mental models of biology and ecology, but this connection is not inevitable.

The same biological models that underpin The Population Bomb can also describe a positive vision of the future with unlimited growth and prosperity.

Biological Models

It is wrong to flippantly dismiss prophets of finite limits like Ehrlich, Malthus, and Meadows. They are not motivated by malice or apocalyptic superstition, but rather by an extremely successful mathematical model of the world: logistic growth.

There are two parts to the logistic model. The first is exponential growth. A single bacterium divides into two, those two each divide again making four, and so on. This feature of population growth was what worried Malthus: The bigger it gets the faster it grows, how can anything keep up with that? Scientists can easily verify this exponential growth in experiments, just set up an agar plate with a few bacteria and watch them grow. But performing these experiments inevitably introduces them to the second feature of logistic growth: a finite limit. Whether it is paramecia in a dish, fruit flies in bottles, or rabbits in Australia, the exponential growth of organisms always slows down and eventually tops out.

These two elements are sufficient to describe the basic logistic model, but we need one additional piece to explain the ecologist’s dismal vision of collapse. The long run maximum population, or carrying capacity, of an environment is determined by the rate at which resources are replenished. In a controlled experiment you might give your fruit flies a slice of peach every morning. At first, one slice can feed far more fruit flies than are in the bottle and the population grows exponentially. But as the population gets larger, competition becomes more fierce and growth slows down. Eventually the growth converges to zero leaving only as many fruit flies as can be fed by one slice of peach a day, maybe about 100. In this simple situation the population of fruit flies never exceeds the long run carrying capacity.

Now consider what would happen if you maintained your ration of one slice per day, but you started the fruit flies off with 3 whole peaches. This stockpile of resources can sustain exponential population growth for much longer. After a few breeding cycles the population soars above what can be fed on just one slice a day. The fruit flies could number in the thousands before finishing the last scraps of the peaches they started with. Thousands of fruit flies anxiously buzz in their bottle that night and the single slice of peach is gone just a few minutes after you drop it in their enclosure in the morning. It will not be long until there are only 100 fruit flies left.

Imagine studying this model and watching it play out exactly as predicted in hundreds of examples at every level of the animal kingdom. It is no wonder why ecologists see the patterns of the logistic growth model so clearly in human society. The Earth is a very large petri dish, so we’ve been able to grow exponentially for centuries, but we are bootstrapping off of finite stockpiles of resources just like the fruit flies with their peaches.

The economist’s positive vision of the future requires exponential growth to continue for many more centuries. But how can humanity count on this future of indefinite growth given the finite limits of these powerful biological models and our reliance on finite stockpiles of resources like fossil fuels? To answer, we have to move away from controlled laboratory experiments and towards some examples that are more relevant to human society: I of course mean the evolutionary history of the plant enzyme Rubisco.

Intensive Growth: Rubisco

The logistic model we developed above assumes that the carrying capacity of an environment is a function of the resources flowing in: one slice of peach a day supports 100 flies, and if you don’t change your ration then the long run maximum population of flies stays at 100.

But there are exceptions to this rule. Organisms can sustainably increase their long run maximum population without accessing any new resource flows. A good example of this is a plant enzyme called Rubisco. Here’s how Charles C. Mann describes it:

Rubisco is the essential catalyst for photosynthesis. Like military recruiters who induct volunteers into the army and then return to their work, rubisco molecules take carbon dioxide from the air, insert it into the maelstrom of photosynthesis, then go back for more.

But Rubisco has a huge problem: it can’t tell CO2 from O2.

As much as two out of every five times, rubisco fails to pick up carbon dioxide, fumblingly grabs oxygen instead, and tries to shove the oxygen into a chemical reaction that can’t use it. To get rid of the unneeded oxygen, plants have evolved an entire secondary process that pumps it out of the cell and re-primes the rubisco to try again for carbon dioxide. The mistakes waste energy. Rubisco’s penchant for oxygen reduces the maximum efficiency of photosynthesis by almost half.

This problem gets worse with higher temperatures so plants around the equator have evolved a workaround, called C4 photosynthesis.

C4 plants split photosynthesis in half. The light reactions take place near the leaf surface, as in ordinary photosynthesis. But something different occurs with the dark reactions, those that incorporate carbon dioxide. When carbon dioxide comes into a C4 leaf, it is grabbed not by rubisco but by a different enzyme which is then pumped into special cells in the interior of the leaf … Because they are deep in the leaf, oxygen from the air doesn’t easily slip into them … Rubisco’s inability to distinguish carbon dioxide and oxygen was not a problem, because oxygen was rare.

Once plants evolve C4 photosynthesis they can increase their biomass, population, and energy use sustainably and permanently without collecting any more resources than before, just because C4 makes more output for the same input.

The impact of C4 is evident to anyone who has looked at a recently mowed lawn. Within a few days of mowing, the crabgrass in the lawn springs up, towering over the rest of the lawn (typically bluegrass or fescue in cool areas). Fast-growing crabgrass is C4; lawn grass is ordinary photosynthesis. In addition to growing faster, C4 plants also need less water and fertilizer, because they don’t waste water on reactions that lead to excess oxygen, and because they don’t have to make as much rubisco.

This episode in evolutionary history shows that there is more to population dynamics than the usual story told by ecologists. They model carrying capacity as a function of the resource flows into an environment, but you can keep everything about your lawn the same: sunlight, water, fertilizer, soil, and the evolution of C4 photosynthesis will still increase the output. So the ecological output of an environment is not just a function of the resources flowing in, it also depends on how efficiently those resources are used. 

Rubisco and Brains

The patterns of the logistic model are everywhere, but it’s missing another powerful tool in the biologist’s toolbox: evolution. Evolutionary inventions like Rubisco allow organisms to sustainably increase their populations, even when the resources available in their environment do not change. The human brain’s capacity for innovation is part of this same fundamental process, it’s just less reliant on mutations of biochemistry.

Our population growth during the neolithic revolution was based on adaptive inventions like clothing, agriculture, and domestication, which let humans access the resource flows of every continent and use them more efficiently than their hunter-gatherer predecessors. The early stages of the industrial revolution in England relied on surface reserves of coal that were burned in crude steam powered pumps and engines. This stockpile was easy to exploit, and England’s population grew exponentially off its back. The reserve of surface level coal quickly ran out, but England’s population did not collapse. Just like plants improved the efficiency of photosynthesis by moving the Rubisco reactions to a separate part of the leaf, James Watt doubled the efficiency of steam power by separating the hot and cold parts of the engine. This pattern extends to the modern day. We get more food from less land, more energy from less fuel, and more computation from smaller circuits.

The dismal conclusions of Malthus, Ehrlich, and Meadows rely on an analogy between fruit flies and humanity, but human growth shares much more with the evolution of Rubisco. Evolutionary inventions sustainably increase the output of an organism by expanding their accessible resources and increasing the efficiency at which they are used. 

Humanity’s case is even stronger than this. The brain is an evolutionary invention that can make more inventions without waiting for beneficial biochemical mutations. Since each human is born with this invention-machine in their skull, the rate of invention increases with our population size. This positive feedback loop defines the arc of human history and secures the prospects for unlimited future growth in human prosperity.

Ecologists and economists often find themselves at odds, especially when viewing the future. One sees collapse, the other, unbounded growth. Their visions are based on dueling mathematical models of the world, but these disparate visions and different models are not incommensurable. Evolution bridges the gap. Adaptations like Rubisco create large and sustainable increases in the population or output of a species. Humans evolved a meta-adaptation which allows us to evolve with ideas, but the principle is the same.

A more complete biological model unifies the perspectives of economists and ecologists into a positive vision of unlimited growth and prosperity.