THE ROLE OF INTERACTIONS BETWEEN TROPHIC LEVELS IN DETERMINING THE EFFECTS OF UV-B ON TERRESTRIAL ECOSYSTEMS

Understanding the potential impact of ozone depletion on terrestrial e cosystems is constrained by lack of information on the effects of envi ronmentally realistic UV-B doses on terrestrial organisms other than h igher plants. Increasing UV-B may alter interactions between plants an d consumers through direct effects on consumer organisms (herbivores, phytopathogens, decomposers, etc.). The effects of increasing UV-B on arthropods are not known. Significant UV-B effects on fungi have been reported, and may be either negative (inhibition of spore germination and mycelial growth) or positive (increased growth, induction of repro ductive development and sporulation). However, in many cases consumers are unlikely to be directly exposed to UV-B in the field. In addition , UV action spectra for fungi suggest that this major group may be les s sensitive to the effects of ozone depletion than higher plants. Host mediated effects of UV-B on consumers may include alterations in plan t chemistry. While secondary metabolites such as phenolics may increas e under increased UV-B, this is not invariably the case and evidence t hat such changes have significant effects on consumers is limited. In particular, there is no evidence that increased UV-B increases resista nce of higher plants to fungal pathogens. Indeed, increased UV-B prior to inoculation results in no significant effect or increased disease. Such responses may be attributable to UV-B effects on host surface pr operties or on compounds other than phenolics. However, such changes a re poorly known, and their potential effects on phytopathogens, herbiv ores or decomposers cannot be assessed. Understanding the effects of U V-B on terrestrial ecosystems is further limited since virtually nothi ng is known of possible impacts on higher trophic levels, i.e. predato rs, parasites or pathogens.