Recently in Polyculture vs. rotation Category

Jeremy Cherfas recently interviewed me on Eat This Podcast, which has many interesting food- and agriculture-related interviews.

Earlier, Cherfas cofounded my favorite blog, the Agricultural Diversity Weblog, so I was delighted when he was the first to review my book. My book argues that crop rotation (with both crop diversity over time and lanscape-scale spatial diversity) often makes more sense than crop diversity within a field (intercropping). Jeremy's mostly-positive review points out that I neglected to discuss diversity within a species, such as growing a mixture of two or more wheat varieties. So I talk about that some in the podcast interview.

But in asking "how should we deploy crop diversity in space and time" both book and podcast implicitly assume that total usable diversity is limited. If there are currently only two wheat varieties that can be grown profitably in a given region, you have to choose between growing the same two-variety mixture every time you grow wheat, versus rotating (alternating) between them. (Growing them as a mixture further assumes they can be managed similarly, including planting and harvesting the same day, similar irrigation, etc. A rotation that alternates different wheat varieties could also include other crops or fallow years.)

If there are dozens of suitable wheat varieties, though, you could have lots of diversity both at different spatial scales and over time: different two-variety mixtures in different fields, without having to grow the same variety in the same field in successive years.

My main worry, though, is that world food security relies so heavily on just three crops (corn, wheat, and rice). In both the book and the podcast, I argue that farmers making rational decisions about their individual risks and benefits will collectively choose less crop diversity than we need to ensure global food security. I doubt that this example made it into Robert Frank's book (see last post), but the issues are analogous to those that caused the recent Great Recession.

I don't promise to post everything people send me, but Timothy Crews (of the Land Institute) has obviously put a lot of thought into critiquing one of the main points in my book. I disagree with many of his conclusions and will respond via comments or another post, but here's what he wrote:

The following two quotes illustrate one of the most central, important and repeated points that R. Ford Dennison makes in his book Darwinian Agriculture.

p. 8 Natural selection "has bequeathed many sophisticated adaptations to individual plants and animals, but it has not consistently improved the overall organization of the natural communities where they live." The available evidence suggests that no other natural process has optimized natural communities either.(italics mine)

p. 43 "Some landscape-scale patterns in natural ecosystems have demonstrated their sustainability by persisting for millennia. However, only individual adaptations have been tested by competition among plants or animals with alternative adaptations. ...So ideas inspired by landscape-level patterns of even ancient ecosystems will require more testing, relative to ideas inspired by the competitively tested individual adaptations of wild species."

Contrary to Denison's evaluation of the available evidence, I argue that there are natural processes other than natural selection that "improve" upon native ecosystem function with respect to agricultural goals. In recent decades, ecologists have worked to understand "assembly rules" to clarify what we know and don't know about the forces that shape ecological communities (Diamond 1975, Weiher and Keddy 1999). This topic has received a lot of attention in recent years, and is complex and contentious (HilleRisLambers et al. 2012), and no one suggests that there are hard and fast "rules". But many would agree on some processes that are not natural selection, which occur predictably and consistently at the aggregate community level, and, I will argue, result in attributes that are worthy of consideration to improve agriculture.

The gradual development of niche partitioning in plant community development is a good example. Early in primary or secondary succession, when species are colonizing a new parent material or a recently disturbed site, it is common for a few (usually annual) plant species to establish and dominate the community. Whatever the composition of the initial community, propagules of new species will arrive and become established to the extent that unused soil and light resources remain, or the new species can appropriate resources better than already established ones. This process generally results in species turnover, or succession, and perennial growth forms almost always overtake annual forms. With time,later successional plant communities are commonly made up of species with differentiated niches that utilize slightly to largely different pools of resources in time and/or space.

The remarkably different plant co-existing growth forms that can be found in the desert grass and shrub lands of southern Arizona illustrate resource partitioning in a late successional community. Extremely deep rooted mesquite trees with C3 photosynthesis are found growing in close proximity to shallow rooted perennial grasses, like Bouteloua gracilis, that employ C4 photosynthesis, which can be found near cacti such as cholla, that maintain CAM photosynthetic pathways. Burgess (1995) described three functional categories that encompass the three species I mention here, 1) extensive exploiters (mesquite)--deeply rooted woody plants that exploit deep water resources, typically recharged by winter precipitation, 2) intensive exploiters(Bouteloua gracilis)--shallow rooted grasses and shrubs that rely on erratic summer rainfall and can persist for long durations in a drought-induced dormancy, 3) water storers (cholla)--cacti and succulents that maintain relatively low leaf surface area, tend to lose out to intensive exploiters in competition for water, but re-hydrate effectively during moderate to high rainfall events, and transpire water slowly. Ecologists believe the plants with these different water uptake strategies experience less head to head competition, and therefore have a greater likelihood of co-existing than groups of species from a common functional group. A similar diversity of resource use strategies have been found to exist for other resource axes, such as soil nitrogen (Weigelt et al. 2005, Kahmen et al. 2006). An ecosystem-level outcome of greater niche partitioning through succession is that plant resources (nutrients, water, light) are more completely utilized. More complete resource utilization should, ceteris paribus, result in greater productivity, and, importantly, less leakage or loss of water and nutrients from the ecosystem.

It is also important to point out that community composition is never static-- the environment is constantly changing, and new species are constantly being introduced from the outside. New arrivals of potential community members effectively "test" the resource uptake efficiency of the extant community. In some cases, the new arrival may be able to go deeper for water, or persist at a lower soil matric potential during drought. Such advantages can lead to species replacement, and a more productive and resource efficient community as a whole.

How might agriculture benefit from studies of niche complementarity or resource partitioning in native plant communities? The somewhat obvious but nevertheless important idea is one that has interested agroecologists for some time--the deployment of diversity in space through polycultures. While Denison seems to prefer the deployment of diversity in time through rotations, the potential to substantially improve uptake of soil resources, increase productivity and reduce vulnerability to weed invasions through crop mixtures in space is worthy of further attention. Even more interesting is the potential to improve on these ecological functions when the perennial growth form that is dominant in native systems is included in the polycultures.

In Darwinian Agriculture, Dennison identifies numerous significant shortcomings of modern agriculture. Many, if not most of them involve ecological functions above the species level, such as leakiness of nitrogen or soil erosion, and thus are not easily addressed by improvements in the performance of individual annual crops. Does it make sense to look at native community and ecosystem attributes to inform how we might make improvements? Absolutely. Especially when the functioning of native ecosystems appears to be superior with respect to attributes of interest (e.g., nutrient retention), and there are no alternative human constructed systems that provide models superior to the native systems from which to learn. If native ecosystem patterns and processes can be improved on with respect to desirable agricultural goals, then all the better--those projects are worthy of funding.

References
Burgess, T. 1995.Desert grassland, mixed shrub savanna, shrub steppe, or semidesert scrub?pp. 31-67 In: The Desert Grassland. M.P. McClaran& T.R. Van Devender (eds). University of Arizona Press, Tucson, Arizona
Diamond, J. 1975.Assembly of species communities. Pp. 342-444 In: Ecology and Evolution of Communities. M.L. Cody & J.M. Diamond (eds). Harvard University Press, Cambridge, Mass.
HilleRisLambers, J. P.B. Adler, W.S. Harpole, J.M. Levine & M.M. Mayfield. 2012. Rethinking community assembly through the lens of coexistence theory. Annual Review of Ecology, Evolution, and Systematics 43:227-248
Kahmen, A., C. Renker, S. B. Unsicker& N. Buchmann. 2006. Niche complementarity for nitrogen: An explanation for the biodiversity and ecosystem functioning relationship? Ecology 87:1244-1255
Weigelt, A. R. Bol& R. Bardgett. 2005. Preferential uptake of soil nitrogen forms by grassland plant species. Oecologia 142:627-635
Weiher, E. & P. Keddy. 2001. Ecological Assembly Rules: Perspectives, Advances, Retreats. Cambridge University Press, Cambridge, UK.