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Darwinian Agriculture III

Next week I will be meeting with a publisher to talk about the possibility of writing a book on Darwinian Agriculture to be published in 2009, the 150th anniversary of The Origin of Species. (I apologize to one reader who apparently thought it was a done deal.) Here's a short draft of the first chapter, mostly about sustainable agriculture by ants and termites.

Farmers of 50,000 millennia

“We’ve been farming sustainably for three years?, read the email. I was glad to learn that my friend was farming in ways that he hoped could continue indefinitely, but how could he be sure, after only three years?

It might have been a reasonable assumption, if the farming methods he used were similar to those that other farmers have used successfully for a long time. But how similar is similar enough? And what qualifies as “a long time?? As director of “the world’s youngest 100-year experiment?, I often thought about these questions....

UC Davis’s Long-Term Research on Agricultural Systems (LTRAS) project was comparing the sustainability of ten different farming systems. Because some important soil properties change over decades, rather than years, the experiment was designed to run for 100 years. There was some initial opposition to LTRAS, from people with strong opinions about sustainability, but things calmed down after a few years. Perhaps, like the philosophers picketing Deep Thought (Adams 1979), they realized that LTRAS would stimulate interest in their opinions, whereas The Ultimate Answer might not be available until after they were dead.

When I received the email, LTRAS had been running for nine years and it was still far from clear which of our systems were most sustainable. For example, wheat grain yields (kg/hectare, roughly equal to pounds/acre) were usually similar, whether the wheat received nitrogen as synthetic fertilizer, or from decomposition of vetch disked under in alternate years (Denison et al. 2004). The vetch obtained much of its nitrogen from the atmosphere, thanks to symbiotic rhizobium bacteria in its root nodules. (I will discuss how conflicts of interest have shaped the evolution of rhizobia in a later chapter.) The good yields of wheat rotated with vetch seemed to show that the vetch was meeting 100% of the nitrogen needs of the wheat.

However, results from unfertilized wheat without vetch showed that this might not be true. Even with no external source of nitrogen, we were able to remove large quantities of nitrogen in wheat grain harvested from these unfertilized control plots: more than half as much nitrogen (kgN/hectare) as in the fertilizer or vetch treatments. This nitrogen presumably came mostly from soil organic matter, about 1% of the soil by weight, which initially contained several times the amount of nitrogen applied as fertilizer each year. This soil nitrogen would have been available to wheat in the vetch/wheat rotation also, so the vetch really only needed to make up the difference: less than half of the total. The sustainability of the vetch/wheat rotation will not be known until the soil organic matter is depleted enough that it is no longer a significant source of nitrogen, as indicated by paltry yields in the control plots.

The world’s longest-running experiment reached that point many years ago. When I visited the Rothamsted Experiment Station, in 1993, they were celebrating the 150th birthday of their Broadbalk Wheat experiment. By then, their unfertilized wheat plots were dominated, not by wheat, but by nitrogen-fixing weeds like vetch. (Agricultural scientists define a weed as a plant in the wrong place at the wrong time. Vetch growing with wheat, rather than before it, will compete with wheat for light and water, and even for nitrogen. “Volunteer? wheat growing as a weed in vetch at LTRAS was actually more nitrogen-deficient than unfertilized wheat growing alone.) In Broadbalk plots where weeds were killed, using herbicides, the wheat plants were stunted and produced very little grain. Wheat that had been fertilized for 150 years with either organic manure or inorganic fertilizers had much higher grain yields than the unfertilized control. Both also had higher yield than when the experiment began in 1843 (Johnston 1994).

A simple definition of “sustainable? is “not getting worse over time.? By this definition, few of today’s human interactions with the natural world are sustainable. Many ocean fish species are being harvested faster than they can reproduce. So are many forests. The natural replenishment rate for oil, coal, metal ores is even lower than for fish or forests; depletion of phosphorus ores is of particular concern for agriculture. Unless balanced by decreasing environmental impact per person, which must have limits, human population growth is unsustainable. Annual US federal government spending of $1000 more, per American, than is collected in taxes, may help politicians get reelected, but it is also clearly unsustainable.

What about agriculture? Given that wheat yields at Broadbalk increased over decades, with only inorganic fertilizers and continuous wheat monoculture (growing only one crop at a time in a given field), can we conclude that these practices are sustainable?

Maybe not. The Agdell experiment, also at Rothamsted, was eventually abandoned, because nitrogen fertilizers gradually acidified the soil, leading to serious root disease in turnips, one of four crops grown in rotation there (Johnston 1994). The first yield decreases from this problem were detected only after 40 years. Although nothing similar has been seen in the Broadbalk wheat plots, yet, could there be slow trends underway that will eventually cause serious problems? 150 years is a long time for an experiment, but it is only two human life-spans. If we are concerned about long-term sustainability, we may need data spanning centuries or more.

No formal experiment has lasted longer than those at Rothamsted, but some fields around the world have been farmed for 1000 years or more. Although none of these farms used synthetic pesticides or transgenic crops, prior to the 20th century, can we at least determine the sustainability of past agricultural practices, many of which would meet today’s “organic farming? guidelines?

Archaeologists have done so, in a few cases. Crops have been grown in Mesopotamia (now Iraq) for about 6000 years. 5500 years ago, wheat and barley seem to have been equally important, based on counting impressions made by their seeds in pottery. By about 4000 years ago, however, only barley was being grown. Ancient documents show that yields decreased greatly over the same period (Jacobsen and Adams 1958).

The most likely explanation for these trends is the accumulation of salt from irrigation water. All water applied to fields contains some salt. When the water evaporates, from the soil or through crop leaves (transpiration), the salt remains. Unless the salt is removed, through natural or artificial drainage systems, it will build up in the soil. Some farm fields in California, irrigated for less than 100 years, are already salty enough that only salt-tolerant crops, like barley, can be grown.

A trend of decreasing yields over centuries, as in Iraq, would be hard to detect without written records and a population that could read and believe them. Long-term trends can be obscured by year-to-year variability in weather. Stories of higher yields in past generations might be considered myth rather than fact. Even if a downward yield trend were recognized, causes and solutions might not be obvious. Are there some witches that need to be killed? Would sacrifices to a different god help? Did the king suppress the findings of Mesopotamian scientists? Consider how long it has taken some politicians today to recognize human impacts on climate!

LTRAS and other long-term experiments were designed, in part, as early-warning systems to detect subtle but dangerous long-term trends. Increased soil acidity at Agdell experiment could have been detected long before yields started to decline, if modern methods for measuring acidity had been available in 1880! Archival samples have allowed analysis of such trends after the fact, by methods not invented when the samples were collected. For example, the polymerase chain reaction (PCR) was used to measure minute amounts of fungal DNA from archival wheat samples from Rothamsted, showing how the abundance of different pathogens varied over decades (Bearchell et al. 2005). Archival samples collected at LTRAS should be equally valuable. We already know how to measure salinity, however. Will organic and conventional systems, for example, differ in salt accumulation over decades?

Yield declines like those seen in Iraq are probably not inevitable. The agronomist Franklin King studied farm fields in Asia, where rice and other crops have been grown successfully for up to 4000 years, and described many of their practices in his book, “Farmers of Forty Centuries? (King 1911). This is an impressive span of time, relative to LTRAS or even Rothamsted. The real experts on sustainability, however, are the fungus-gardening ants, which have been refining their agricultural practices for 50 million years (Aanen and Boomsma 2006).

50 million years is a long time, even for those evolutionary biologists who study the history of life. (Other scientists focus on evolutionary changes over much shorter periods of time: sometimes less than one day, for bacteria with generation times of less than an hour.) Humans only invented agriculture about 10 thousand years ago, not long after the first humans arrived in the Americas. About 3 million years ago (MYA), the last series of ice ages began and the isthmus of Panama closed, blocking an important connection between the Atlantic and Pacific. The common ancestor of humans and chimps lived around 6 MYA and the common ancestor of humans, other apes, and monkeys 40 MYA. 50 million years ago, the Earth would have looked quite different from today, even from space. The South Pole was ice free, South America and Africa were as close to each other as to North America and Eurasia, and India was about to collide with Asia, leading to the rise of the Himalayas (Freeman and Herron 1998).

But the antiquity of ant agriculture is not the main reason that it deserves our attention. Not all “ancient wisdom? is equally reliable, as will be explained in later chapters. The important thing is that ant agriculture has been continuously tested and improved over the millennia, by natural selection.

Ant colonies compete against neighboring ant colonies, at least occasionally. Over their 50 million year history, colonies that harvested leaves (to feed their fungi) mostly in the morning must sometimes have competed against colonies that harvested all day. Whether competition involved direct aggression or not, a leaf harvested by one colony was not available to the other. Colonies that “weeded? their fungus gardens more must sometimes have competed, for leaves or other resources, against those that weeded less, and so on. These behaviors were influenced by genes that worker ants inherited from their mother, the queen. As the queens of winning colonies produced more female workers and male drones, the winning genes became more common. Therefore, we can assume, even without understanding the details, that evolution has improved the agricultural practices of ants, by the criteria used by natural selection. The relevance of these criteria to human agriculture is the theme of this book.

What are the agricultural practices of ants, maintained by natural selection?

First, ants practice monoculture. Each ant colony grows only a single, genetically uniform fungal clone, although different colonies may grow different fungi (Mueller et al. 2005). Termites, which apparently evolved fungus-gardening more recently than ants – less than 40 million years ago -- also practice monoculture, with less between-colony diversity. Fungus-gardening ants are found only in the Americas, fungus-gardening termites in Africa. Ants and termites use different fungal species. So they appear to have invented agriculture independently, yet monoculture was apparently favored by natural selection in both cases.

Second, ants use confined feeding, analogous to industrial animal production in the US. Fungi are actually more closely related to animals than to plants. That is, fungi and animals share a more recent common ancestor than either shares with plants. Unlike plants, neither can photosynthesize, so they need to digest other living, or once-living things. Industrial chicken farmers keep hens indoors and bring them grain (seeds). Similarly, ants bring food (mostly leaves) to fungi kept in underground chambers.

Third, ants use toxins to control pests. The most serious pest in ant fungus gardens – maybe I should call them “feedlots? – is another fungus, Escovopsis, which greatly reduces the growth of the fungi the ants use for food. Ants control Escovopsis using toxins (antibiotics) produced by certain bacteria. These bacteria are housed within special structures (“crypts?) on the bodies of the ants themselves and fed by excretions from specialized ant glands (Currie et al. 2006). This is analogous to spraying crops with the BT toxin produced by the bacterium, Bacillus thuringensis, raised in vats. It is less similar to traditional “biological control? methods, where a predator or parasitoid species feeds on, and coevolves with, the pest it controls.

These practices all seem more similar to industrial agriculture than to traditional or organic practices. Before blindly copying the agricultural methods of ants and termites, however, can we first understand why these approaches were favored by natural selection? Some of the reasons may not be relevant to human agriculture. To see why, we need to explore when natural selection does, and does not, further human interests. That is the topic of the next chapter.


Sounds like an interesting book. What level audience are you aiming for? Are you talking to university presses?

Yes, but hoping to reach nonspecialist audience.

Off-topic comment moved to Troll Refuge.

Enjoyed your 1st chapter. Have had some interesting conversations over the months with former Dean of Agriculture Muscoplat. I've been on this kick about people being driven to practice "sustainable agriculture." That perhaps there is a genetic diversity that causes us to have different practices within our population. Dean Muscoplat said he had visited every farm featured in Alternative Agriculture book and those farmers did what they did sometimes without reason. I suggested that the reason was a diversity drive. I was just in LaMoure North Dakota visiting the folks at Northern Plains Sustainable Ag Society and frankly am glad to have folks like that on the landscape. If we ever need to call upon some recessive farming gene-- some throwbacks to 1910-- we have seed savers in the middle of North Dakota and every few hundred miles across the prairie.

I may be among those driven to do things differently. To me it seems a little to thin of a line lately-- a thin line between me (and my children's) and our food supply. Too thin a line.

Your chapter about the ants is chilling-- though you don't extrapolate to modern farming in support of monoculture. Frankly-- it is not pretty. Except for the seas of corn in the fall-- monoculture crops, animals, dirt is rather ugly. I'm looking towards beauty in the systems as my gold standard-- my inituitive measure of sustainability.

Back to diversity--- I've operated by the principle that diversity is good. Period. Whether in the ecosystem or my investment portfolio. That's worked from a portfolio point of view-- although if I had 50 million years I may be able to focus on the real big winners and leave the REITs alone.

Best. Kathy. Resettling Big Stone county with an office on the St. Paul campus.

Thanks for your comments. I agree that diversity is good, but I'm not sure about the "Period." The world relying on three crop species so much seems risky. I also agree that having people around with the knowledge and genetic resources to farm in different ways is valuable in various ways. I'm always impressed when I visit my brother's organic farm. One of the questions the book will explore is how to best deploy the available crop diversity in space and time. "Diversity is good" doesn't provide much guidance there.

Regarding the ants, I don't want to provide a "plot spoiler", but note that natural selection has operated on each species in this system and not necessarily always for the benefit of the ants.

Ok, you've got me interested. I hope to hear more about this project as time goes on!

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