How diverse can croplands be?

A guest post from Fernando Aramburu Merlos on his recent paper with friend-of-the-blog Robert Hijmans.

Four species (wheat, rice, maize, and soybean) occupy half the world’s croplands. It has been argued that this means we cannot increase crop species diversity much without changing what we eat ((Renard, D. & Tilman, D. Cultivate biodiversity to harvest food security and sustainability. Curr. Biol. 31, R1154–R1158 (2021) )). Radically shifting our diets is a tall order, not just because changing habits is a challenge but also because we are so good at growing and processing the major crops. It’s an unfair race in which the major crops have a head start of millions of dollars and research hours.

We wanted to know how much crop diversity can be increased without changing the global food supply ((Aramburu Merlos, F. & Hijmans, R. J. Potential, attainable, and current levels of global crop diversity. Environ. Res. Lett. 17, 044071 (2022) )). So we estimated the attainable crop diversity, which is the highest level of crop species diversity you can get without changing the total production of each crop. To compute this, we “shuffled the cards and dealt again”: over 100 crops were distributed across the worlds’ existing croplands by allocating each to the most suitable land while considering the inter-specific competition for land.

It turned out that tropical and coastal regions can reach much higher levels of diversity than temperate and continental areas. Perhaps that is not especially surprising, but one implication is that we should not assume that all countries can achieve the same maximum levels of crop diversity ((This assumption was made for the agrobiodiversity index proposed by Jones, S. K. et al. Nat. Food 2, 712–723 (2021))). We also noted that attainable diversity cannot explain current diversity patterns very well. For example, the diversity gap,  the difference between the current and the attainable diversity, is much higher in the Americas than in Europe and East Asia.

Diversity gaps, expressed as a percentage of the attainable diversity, are greater than 50% in 85% of the world’s croplands. Thus, in principle, crop diversity could double in the vast majority of the world without changing our heavy reliance on a few staple crops. So there must be strong forces at work that make farmers and regions specialize. For example, at the farm level, a high crop diversity may be difficult to manage, reduce economies of scale, and be costly if it comes at the expense of the most profitable crops.

It would be interesting to better understand what specific factors limit diversification in the regions with the largest crop diversity gaps, and how to reduce them. But more important questions need to be answered first. How much diversity is enough diversity? And is that the same for all regions? Some very low diversity systems appear to be highly sustainable (the flooded rice systems in Asia come to mind). A more spatially explicit and species-specific, functional understanding of the effect of diversity at the field scale would be helpful. Without that, diversity gaps are just an interesting emergent property of specialization, but not something that necessarily must be reduced.


The most valuable fruit introduction yet

The Sacramento Bee has a nice piece by David Boulé ((Author of ‘The orange and the dream of California‘. You can see him speak about oranges here.)) about the history of the ‘Washington’ Navel Orange in California, the world’s second most common orange variety (after ‘Valencia’).

Navel oranges have been known in Spain and Portugal for centuries. They made their way from there to Brazil, where, in Bahia, a seedless and easy-to-peel variety of great taste and color was discovered. It was probably a sport (mutant) from the Portuguese variety ‘Umbigo’, which is said to be described in the Histoire naturelle des orangers by Risso and Poiteau (you can get your own copy).  I could not find it in that book, but I did enjoy Poiteau’s botanical drawings, like this one ((Perhaps that is the one, it looks a lot like the navels from my backyard.)):oranger_portugais

From Bahia the tasty navel went to Australia in 1824 and to Florida ((Where it tends to produce poorly and the fruit is generally large, coarse-textured, and of poor quality.  It is clearly not well adapted to hot, semitropical climates)) in 1835, and from Australia to California. But the introduction that led to adoption of the name ‘Washington’ and to its commercialization in California and around the world occurred in 1870, when William O. Saunders of the USDA received twelve trees from Bahia (twigs in an earlier shipment had been dead on arrival). They were planted in a greenhouse in Washington D.C. and propagated for distribution ((,,

On 10 December 1873, Eliza Tibbets of Riverside, southern California, traveled by buckboard to Los Angeles to pick up two of these trees, delivered by stagecoach from San Francisco ((Where they had arrived after a four weeks train trip from Washington D.C.)). Their fruits won first price at a citrus fair in 1879, and the ‘Washington’ navel spread rapidly after that — there was a citrus gold rush going on after the recent completion of the transcontinental railroad, which allowed selling to markets back east. Oranges were commonly propagated by seed in California, but the seedless ‘Washington’ had to be grafted. The Tibbets sold cuttings at a dollar each, earning as much as $20,000 a year.

In 1903, one of the original trees was transplanted to a location in front of the Glenwood Hotel in central Riverside, with president Roosevelt shovelling some of the dirt. ((Here is the photograph that was used to make this postcard.))


That must have been about here (the name of the hotel was changed to Mission Inn). Alas, the tree died after a couple of years. But it was there long enough to be used in marketing:


The other ‘parent tree’ was planted a couple of miles south of the Glennwood Inn, in Low Park. It is still there, see for yourself, about 145 years old, despite its dire state 50 years ago:

for some years past it has been declining in vigor, and in 1967 seemed unlikely to survive much longer.

The tree is a ‘California historical landmark’ and has this plaque in front of it:



which states that, as of 1920, this was the

most valuable fruit introduction yet made by the USDA.

Was that a fair claim back then? And if so, is it still true? There is some economic analysis here and here.

Riverside does not boast only that tree, it also has the California Citrus State Historic Park. And after you visit that, drive east to the Coachella Valley to see the dates that were introduced a few decades later. ((Like these, which were reintroduced to Morocco, the Washington navel was reintroduced to Brazil after they had been wiped out there.))

All sweetpotatoes are transgenic

ResearchBlogging.orgTina Kyndta and collaborators ((Kyndt T, Quispe D, Zhai H, Jarret R, Ghislain M, Liu Q, Gheysen G, & Kreuze JF (2015). The genome of cultivated sweet potato contains Agrobacterium T-DNAs with expressed genes: An example of a naturally transgenic food crop. Proceedings of the National Academy of Sciences of the United States of America PMID: 25902487)) have found that all cultivated sweetpotatoes are naturally transgenic because they contain transfer DNA (T-DNA) sequences from Agrobacterium.  Gene-transfer via Agrobacterium is a naturally occurring process, that is used to make genetically modified crops in the lab. We did not know that one of our main food crops was once naturally transformed via the same process.

Kyndta et al. did not find any T-DNA in the wild relatives of sweetpotato, suggesting that the transformation(s) provided a beneficial trait that was selected for during domestication. The introduced genes are intact and expressed in different organs of the “Huachano” variety that they studied in detail, but we’ll have to wait for future expression studies to find out about the benefit of these paleo-GMOs.

The authors also suggest that, as people have been eating these swollen roots for millennia, we might now consider all transgenic crops to be “natural”. I don’t know about that. Didn’t most of these people suffer and die young? I predict that sweetpotato consumption will plummet now that the word is out.

Wild potato diversity halved

ResearchBlogging.orgDavid Spooner and co-workers have written a comprehensive overview of the systematics and genetics of wild and cultivated potato species (Solanum section Petota) ((Spooner, D., Ghislain, M., Simon, R., Jansky, S., & Gavrilenko, T. (2014). Systematics, Diversity, Genetics, and Evolution of Wild and Cultivated Potatoes The Botanical Review, 80 (4), 283-383 DOI: 10.1007/s12229-014-9146-y)). This nicely illustrated and very accessible paper is essential reading for anyone interested in potato diversity — or indeed the study of plant diversity in general.

A remarkable aspect of wild potato systematics is the way the number of recognized species has fluctuated over time. In 1956, Hawkes recognized 106 species, but in his 1990 treatment of the group this had increased to 232. This will likely be the highest number we’ll see, because it has come down drastically since, and Spooner et al.’s paper puts it at 107 — almost exactly where it was back in 1956. This does not mean that we are back to the same set of taxa though. Many new species were described after 1956, notably by Carlos Ochoa, who named about 25% of the 107 species. ((Ed.: Our thanks to Dr Spooner and his co-authors for linking to the obituary of Carlos Ochoa we published on this blog. I think this marks the first time the blog has been referred to in a peer-reviewed paper.))

The graph below shows the number of species over time, based on published compilations, and the name of the authors ((This is an update of a figure in the now somewhat obsolete Atlas of Wild Potatoes.)) .


It is not easy to determine where a wild potato species begins and where it ends. Many species look very similar, and there is “lack of strong biological isolating mechanisms and the resulting interspecific hybridization and introgression, allopolyploidy, a mixture of sexual and asexual reproduction, and recent species divergence.” A smaller number of species is not necessarily better, but, in the case of wild potatoes, Spooner et al. think it will help us move away from “a taxonomy that is unnatural, unworkable, and perpetuates variant identification” to a system that hopefully enables better conservation and use of these plants.

It also creates a mess, though, because previous analyses based on species level diversity, for example to set collection and conservation priorities, may need to be revised. Spooner et al. update some of the analysis of geographic pattern in wild potato species richness described previously.

The reduction in the number of species is in large part due to new insights from David Spooner’s incessant work on this group, through molecular and morphological studies, and observations during collecting expeditions. His kind of naturalist is a species that is also declining in numbers, or so it seems. That is not a good thing, as there is a lot of work to do.

European on farm diversification

In my previous post on the new EU Common Agricultural Policy, I missed that it promotes in situ conservation of crops. At least that is my reading of Annex IX, which provides a list of practices equivalent to crop diversification. The text is a bit confusing (why are legal documents never clear?); here’s an excerpt (my bolding):

  1. Crop Diversification
    Requirement: at least three crops, the main crop covering a maximum of 75%, and any one or more of the following applying:
    — there is a more appropriate selection of crops, such as, for example, leguminous, protein crops, crops not requiring irrigation or pesticide treatments, as appropriate,
    — regional varieties of old, traditional or endangered crop types are included on at least 5% of the rotated area.

The ‘and‘ does not make sense, and should surely be ‘or‘? Otherwise there would be no ‘equivalency.’

Perhaps it is a European thing to emphasize the old & traditional over the novel & rare? Either way, there are busy times to come for European on farm conservation buffs! But where should interested farmers get seed? Many of these varieties may not be registered, and I thought that exchanging such seed was not legal in Europe?