Cost-effectiveness, chemical defensive compounds

Indirect defense Chemical compounds attract natural enemies of an herbivore to locate a suitable prey after attack (also termed top-down effects or multitrophic Interactions) Indirect defense Is normally inducible and thus cost effective under a low herbivore environment. Effective only if natural enemies are within the range of spreading signals... 

Anti-predator adaptations are often mediated or induced by chemical cues (Kats and Dill, 1998), especially in aquatic systems where visual cues are limited (Smith, 1992). Chemical cues function well in this medium as a large number of compounds can dissolve in water allowing for the production of a great number of possible signals (Hara, 1994). Research, in the past decade, has indicated that the assessment of these chemical cues is highly sophisticated (reviews Chivers and Mirza, 2001). Logically, the ability to accurately assess the risk of predation would be beneficial as each anti-predator defense has an innate cost to the user and the effectiveness of each response option is dependent on the context of the encounter and the specific predator. 

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