Defence expenditure is a problem that faces plants as well as politicians. Spend too little and the aggressor takes advantage; spend too much and vital resources are wasted. Plants defend themselves from aggressive herbivores in many ways — ranging from the conventional weapons of spines and stinging hairs, to more sophisticated chemical and biological deterrents. These include toxins, or the development of mutualistic relationships with animal defenders such as ants. All of these methods cost energy, and the products of photosynthesis must be diverted from plant growth or reproduction into defence. But how can the expenditure be optimized, and what is the response of the herbivore to deterrents? Writing in Oikos, Schmidt, Brown and Morgan1 look at the herbivore end of the equation, by feeding squirrels with seeds spiked with toxins. And in the latest Journal of Ecology, Heil and co-workers2 examine the proportion of photosynthetic productivity that a plant needs to expend to support a population of mercenary ants that will protect it from grazers.
Generalist herbivores are more successful when they adopt a mixed diet. One hypothesis for this is the complementarity of food sources — as the proportion of one component of the diet falls, its value to the consumer rises, making it worth seeking out. This may be due to the nutritional content of the food resources, or the presence of different toxins in the components of the diet. Consumption of an excess of one toxin could, for example, lead the grazer to seek plants with a different chemical constitution.
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