Category: Climate change

This is the final part of a two-part review. You can find part 1 here.

So is there anything I don’t like about Darwinian Agriculture? Actually, yes. Intercropping and polyculture, which is usually taken to mean mixing different crop species in space (and sometimes animals too), is, in Denison’s view, not proven to be more productive than growing the component species on their own in rotation. Fair enough; too often the long-term comparisons over a full cycle or two of rotations haven’t been done. But he doesn’t consider how polyculture might affect year-on-year stability; in marginal systems, especially, that may be a more important consideration than total productivity. The same goes for what I would argue is one of the clearest ways in which imitating Nature might be a good idea: mixtures of varieties of the same crop.

Denison explores the reasons for the 1971 epidemic of Southern Corn Leaf blight in maize, and uses it to make some telling points. But having said that “this disaster is often used, rightly, to show the need for more genetic diversity within crop species,” and promising that he would return in later chapters to the question “which is more useful, diversity within fields or diversity among fields?” he doesn’t, at least not for diversity within a species.

Of course it isn’t fair to criticise an author for not covering a topic close to my own heart, but I do wish he had given at least some space to the question of intra-specific diversity as a perfectly good Darwinian response to the threat posed by pests and diseases: lack of diversity in the response to a threat is risky. This idea comes into the management of resistance to GMO plants, with a good discussion of the need for refugia of non-engineered plants as part of a strategy to delay the emergence of resistance. But as a more general mechanism, it gets short shrift, which is a shame. And it isn’t as if the evidence is lacking, at least for damping down yearly yield variability.

You could get the impression from this review that Ford Denison is long on the problems with agriculture and short on solutions. You would be wrong. In fact, some of his proposed solutions are so exciting I honestly wished I were just starting on a long research career. Boost the cooperation between legumes and nitrogen-fixing bacteria? Count me in. Figure out how to use plant-animal signals more effectively than always-on alarm pheromone? You bet. Ask in detail how this year’s crop could benefit next year’s? Yes please. Just because trade-off blind biotechnology and unthinking mimicry have so little to offer is no reason to despair.

Perhaps the most important thing Denison has to ask, in what is either true humility or unbridled academic chutzpah, is “What if my proposed core principles turn out to be wrong?” This is the central argument for his rider, on the need for a bet-hedging approach that allows ideas to compete. This, too, appeals to my confirmation bias. Simplistic, whizz-bang approaches suck the air out of the room, leaving less shiny ideas gasping, and nutrition, breeding and sustainability are three particularly vulnerable areas. Right now a squillionaire philanthropist, with, say US$30m to invest, has a couple of choices. Increase the donor budget for simplistic whizz-bangery by somewhere between 1% and 10% – nice, but hardly a big deal. Or double the budget for “other” approaches, which could indeed make a real difference.

You’ll have gathered that I think Darwinian Agriculture is the best non-fiction I’ve read in a long, long time. Anecdotally, others in the field seem to agree. The big question is, how to get those who make the big decisions, and who clearly don’t understand either ecology or evolution as they apply to agriculture, to pay attention. Answers in the comments, please.

The variation of animals and plants under domestication was a major source of inspiration for Charles Darwin’s ideas about natural selection. Ford Denison repays the debt in Darwinian Agriculture: how understanding evolution can improve agriculture. The book had its genesis in a 2003 paper, and offers a wake-up call to some of the more starry-eyed optimists out there. “When can humans find solutions beyond the reach of natural selection?” the paper asked. The book expands on that and suggests a shortish answer: “not often”.

There are two main thrusts to Denison’s argument, and a rider that is possibly of greater value still. First, natural selection is unlikely to have left any trade-off free improvements unexplored. Secondly, ecosystems, unlike individual organisms, have not generally competed against one another, and so the mere fact that they have persisted is no guarantee that they are in any sense optimal. Finally, Denison advocates both a greater diversity of crops and a greater diversity of research to hedge bets against future uncertainty.

On the first point, it is surprising how seldom breeders and, even more so, biotechnologists, acknowledge that natural selection has had ample opportunity to try out almost anything they can think of. If it isn’t around today, that’s probably because it hasn’t conferred a long-lasting evolutionary advantage in the past. Denison offers many examples, one of the simplest being shorter stems – the fundamental underpinning of the wheat and rice varieties that gave us the green revolution. A short stem does two things. It enables the plant to divert more of its resources into grains, rather than stems, and it is structurally stronger, supporting weightier seeds without buckling. But we can reap the benefits of a short-stemmed plant only when it grows in the company of other short-stemmed plants. A short-stemmed mutant in a field of taller-stemmed plants is likely to be shaded and outcompeted, while a tall-stemmed mutant in a field of short-stemmed plants, even though it may not be able to devote as much to grains, is likely to have more resources in total, because it intercepts more light.

Similar arguments apply to many of the improvements that produced modern, extremely productive agriculture. One of the most urgent is the question of increasing the efficiency of photosynthesis. Scores of scientists and millions of dollars are chasing this tempting, and so far extremely elusive, goal. The story is quite complex. In essence there are two photosynthetic pathways, C4 and C3. They differ in their efficiency, one reason being that the C4 pathway does not “waste” energy in fixing oxygen instead of carbon. At the same time, C4 plants need less water, typically around a third less, to produce the same amount of photosynthate, one reason why C4 seems to be most common in plants that can withstand high temperatures and drought, such as maize, millet and sorghum. The C4 pathway involves a complex suite of changes in biochemistry and leaf anatomy, but despite the complexity of these changes, seems to have evolved independently around 40 times. But not in rice or wheat.

Denison does a brilliant job of explaining the differences between C3 and C4 and why, years after extravagant claims about how “relatively simple” it would be to make C4 plants, we’re still waiting. And while greater efficiency in either photosynthesis or water-use would be an undeniable long-term benefit, there are good reasons to suppose that they will be extremely difficult, if not impossible, to achieve. And the short-term benefits of biotechnology are just that, short-term, because pests and weeds continue to evolve, perhaps even faster under the intense selection pressure offered by genetically engineered crops than they might otherwise.

I found the first part of Denison’s book exciting and entertaining, most likely, some would say, the result of confirmation bias. The second part, where he turns to what he calls The Misguided Mimicry of Natural Ecosystems was, naturally, a little harder to swallow. If anything, however, it was even more instructive. The crucial point here is that unlike individual plants or animals, ecosystems do not compete against one another and so are quite unlikely to be in any sense optimal in the same way that a particular plant height might be optimal in a given environment. Merely copying, say, the spatial arrangement of plants in an ecosystem is therefore no guarantee that the resulting system will be more productive. On consideration this seems right, but it does take a bit of consideration. There are plenty of examples to demonstrate the point – indeed a problem with reviewing Darwinian Agriculture is the temptation to just repeat those examples – but take just one.

Ecosystems provide services, and those services would be really expensive if we had to pay for them, therefore ecosystems are worth preserving. This is becoming an article of faith in some agro-ecological circles. But it can be a hostage to fortune when the economic basis of the calculation shifts. Forest fragments of 147 ha provided pollination services worth an estimated $60,000 a year to a neighbouring 1065 ha coffee farm in Costa Rica. Then the price of coffee dropped, the farm switched to growing pineapples, and pineapples don’t need pollinators. “Did natural forests suddenly become much less valuable?” Denison asks.

That’s a particularly cute example, and far from making an argument against conservation, Denison is at pains to point out that one of the best arguments to conserve ecosystems is that it gives us the opportunity to study them properly, and that only with proper study, rather than well-meaning imitation, can we hope to benefit from Nature’s wisdom.

Part 2 of this review is here.