Introduction

Shared resources like ground-water tend to be over-exploited (Hardin, 1968)…
… but not necessarily (Ostrom, 1990).
Similarly, natural selection drives over-investment in roots (Zhang et al., 1999).

When can plant breeders improve on natural selection?

Our crops inherited traits from their wild ancestors that had been improved by millions of years of natural selection. Natural selection is so powerful that – for example, watergrass evolved as a weed in rice fields to look more like rice than its own recent ancestor, barnyardgrass, in 1000 years or so (Barrett, 1983) and evolved herbicide resistance in 4 years (Hill et al., 1994) – that there may be limited scope for plant breeders to improve traits that consistently benefit individual plants.

Fortunately, natural selection only improves individual traits, sometimes undermining group benefit (Levin and Kilmer, 1974)(Gardner and Grafen, 2009). That leaves opportunities for humans to select for group-benefiting traits, such as hens that lay more eggs rather than fighting (Muir, 1996).

Tradeoffs between individual-plant and whole-crop traits as opportunities for improvement.

Maize (corn) over-invests in tassels, which reduce photosynthesis by shading leaves (Duncan et al., 1967) but plant breeders have reduced tassel size over decades, as a side-effect of selecting for yield (Duvick and Cassman, 1999).

Colin Donald argued that there are often tradeoffs between individual-plant competitiveness and plant-community productivity, and suggested “more-cooperative” traits, such as shorter stems, erect leaves, and less branching (Donald, 1968). Peter Jennings had already identified short stems and erect leaves as key to rice yield (Jennings, 1964) and followed through by developing IR-8, the first Green-Revolution rice. An analysis of wheat varieties released over several decades confirmed Donald’s claim that high-yield varieties need to be short (Austin et al., 1980), whereas competition studies in both rice (Jennings and de Jesus, 1968) and barley (Suneson, 1949) confirmed Donald’s prediction that high-yield varieties will be less competitive.

Plant breeders have also selected for more-erect leaves in maize, again as a side-effect of breeding for yield (Duvick and Cassman, 1999). Like horizontal leaves, leaves that track the sun get more light, increasing their own photosynthesis, but they cast larger shadows, reducing the photosynthesis of lower leaves. We found that solar tracking actually decreases overall photosynthesis in dense alfalfa canopies (Denison et al., 2010), but the shadows of solar tracking leaves, like those of maize tassels, often fall on competing neighbors.

Despite claims from biotech companies, “drought-tolerant” corn doesn’t necessarily yield more (Castiglioni et al., 2008) (Roth et al., 2013), perhaps due to tradeoffs . the afternoon, when soil water is used less efficiently (Kumar et al., 1999).

Selecting more-cooperative symbionts

Symbiotic rhizobia bacteria in legume root nodules face a tradeoff between respiring carbon they get from the plant to power nitrogen fixation (benefiting the plant and all its rhizobia, including competing strains) versus hoarding that carbon as the lipid, PHB (Hahn and Studer, 1986)(Cevallos et al., 1996). PHB can support rhizobial reproduction and survival after they return to the soil (Ratcliff et al., 2008), but diverting plant resources from nitrogen fixation to PHB risks host sanctions that reduce rhizobial reproduction in nodules (Kiers et al., 2003)(Kiers et al., 2006)(Oono et al., 2011).

We have never seen evidence of “negotiation” among plants or rhizobia. Sanctions don’t make rhizobia cooperate, they just kill those that don’t, making “cheaters” less abundant in the next generation. Similarly, cooperation among plants (shorter stems, not tracking the sun, etc.) will only come from selection imposed by plant breeders.

Cooperation among humans

Cooperation among humans is key to sustainable management of shared resources like ground-water, but also to management of agricultural pests. The spatial scale at which crop diversity decreases pests and favors beneficial insects can be larger than individual farms (O’Rourke et al., 2011), so diversity-based approaches to pest management would require cooperation among farmers. Three examples of pest-management programs whose success was undermined by lack of cooperation are citrus pest control by Vedalia beetles, which die out when neighbors spray insecticides (Grafton-Cardwell and Gu, 2003), Brassica-free periods to control Brassica-specific pests (Zalucki et al., 2009), and voluntary contributions from growers to control backyard pests that spread to farms (Kruger, 2016).

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