Ecosystem consequences of enhanced solar ultraviolet radiation: secondary plant metabolites as mediators of multiple trophic interactions in terrestrial plant communities.

The potential role of ultraviolet-B (UV-B)-induced secondary plant metabolites as mediators of multiple trophic responses in terrestrial ecosystems is considered through review of the major classes of secondary metabolites, the pathways for their biosynthesis, interactions with primary and secondary consumers and known UV effects on their induction. Gross effects of UV-B radiation on plant growth and survival under realistic spectral balances in the field have been generally lacking, but subtle changes in carbon allocation and partitioning induced by UV-B, in particular production of secondary metabolites, can affect ecosystem-level processes. Secondary metabolites are important in plant-herbivore interactions and may affect pathogens. They act as feeding or oviposition deterrents to generalists and nonadapted specialists, but adapted specialists are stimulated to feed by these same compounds, which they detoxify and often sequester for use against their predators. This provides a route for tritrophic effects of enhanced UV-B radiation whereby herbivory may be increased while predation on the herbivore is simultaneously reduced. It is in this context that secondary metabolites may manifest their most important role. They can be the demonstrable mechanism establishing cause and effect at higher trophic levels because the consequences of their induction can be established at all trophic levels.