Defense on the rocks: low monoterpenoid levels in plants on pillars without mammalian herbivores.

This study addresses the adaptive dynamics of plant toxins that ward off herbivores, an aspect of chemical warfare in ecosystems. Thyme basil (Acinos suaveolens) growing in an area regularly grazed by mammals contains double the concentration of monoterpenoids (15.61 vs. 8.18 mg/g dry matter, P = 0.001) when contrasted with plants growing on inaccessible rock pillars and not exposed to mammalian herbivores. Thyme basil produces two monoterpenoids-menthone and R-(+)-pulegone-that show contrasting toxicity. The more toxic R-(+)-pulegone shows an increase in relative abundance (66.3% vs. 47.5%, P < 0.001) in plants exposed to browsing The results demonstrate how plant chemistry responds to both abiotic and biotic environmental conditions.