Nutrient Availability and the Deployment of Mechanical Defenses in Grazed Plants: A New Experimental Approach to the Optimal Defense Theory

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Abstract
Optimal defense theory predicts that plants living in nutrient poor areas will devote more of their resources to herbivore defense. Contrary to this prediction, prickly pear cactus, Opuntia stricta, produced spines in response to cutting damage whether or not the plants had been fertilized. Other studies of the occurrence and production of mechanical defenses, spines, have not found plants in areas of low nutrient availability to be well armoured. In contrast, chemical defenses are reported to be higher in plants living in areas with limited soil nutrients or light availability. The interrelationships between soil nutrients, light availability, plant growth, leaf nitrogen and phenolic compounds makes the interpretation of chemical defenses and resource availability difficult. Mechanical defenses do not suffer from these complicated interrelationships and are recommended for further testing of optimal defense theory.