Prey species have evolved various defenses to lessen the impact of predation. For plant, herbivores are the predators. Plants have evolved an astonishing variety of morphological and chemical defences against herbivores. Calotropis produces highly poisonous cardiac glycosides and that is why we never see any cattle or goat browsing on this plant. A wide variety of chemical substance that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.) are produced by them actually as defences against grazers and browsers.