A tweet from Patrick Moore to the effect that most of the pesticides present in the food we eat are produced by the crops themselves set me thinking about the “arms race” between plants to avoid being eaten and at the same time encourage herbivores to eat other competing species.
Eating or being eaten can be viewed as a very simple game. In order to survive, any plant or animal, any living organism, must eat. That is, take in from outside the substances that it needs to grow its own body and to provide energy to run its internal processes. It also must not be eaten if it is going to survive for long. Winning this leg of the game consists of convincing any predator that it cannot be eaten, to use the terminology of Stephens (1993).
Plants have developed the ‘must eat’ game virtually to its limit by now. Housing symbiotic chloroplasts that fix carbon by photosynthesis, absorbing other nutrient elements, and the metabolic pathways that produce the whole plant have been established since the Palaeozoic. Even the later innovations of CAM and C4 photosynthesis have been around for millions of years.
Dennis Stephens (1994) further suggested that the main game among plants is ‘must not be eaten’ because they have not yet evolved as far as they can go in that department. They are still in an arms race with herbivores, with pathogens and with each other. Plants may develop spines or other physically deterring outgrowths that convince hungry herbivores that they are not edible. They may use nectar to encourage ants to wander over their surface and clean up feeding insects. But most importantly, they may produce any of a vast range of chemicals (secondary metabolites) that make them bad-tasting or toxic to the particular herbivores that threaten them. These are all physical manifestations of the strategy of being inedible.
As an aside, it’s interesting that the development of toxic secondary metabolites is a speciality of the angiosperms or flowering plants that have dominated land vegetation since the Cretaceous age. Ferns can be inedible too, but they are much less rich in this kind of chemistry. And the notably toxic members of the gymnosperms are not the really ancient ones, but those that have diversified since the Cretaceous in competition with angiosperms – the cycads, Ephedra, Taxus and some related conifers.
So there is an evolutionary pressure on plants to not be eaten by convincing herbivores that they are inedible. The most obvious benefit from this – when it succeeds – is that they do not lose biomass or get killed outright by herbivory.
But there can be a second advantage for the uneatable. Consider a three-way game between two plants and a herbivore, such as sheep grazing on a pasture of grass containing thistles. A thistle’s spines make it unpalatable; either totally inedible, or so hard for the sheep to eat that it will not be nibbled as long as any edible, palatable grass remains around it.
So the harder the sheep graze, the less the grass will compete with the thistles for light, water and ground space. The combination of grazing pressure and their spiny defence against being eaten has given them a powerful strategy in their own competitive game with the grass.
It’s also interesting to consider the strategy of the grass. It might actually derive some benefit from being grazed along with broadleaf weeds that lack the thistle’s defence, since it is better equipped to regrow after grazing than they are. But that’s another story.
Stephens, D.H. (1993) Expanding on Level 5, Sex. Letter tape of 6 May 1993.
Stephens, D.H. (1994) Postulates, Self and the Obsessive IP. Letter tape of August 1994.