Terrestrial and aquatic ecosystems represent complex, interconnected assemblages of organisms which Interact with each other and their physical environment. n understanding of how ecosystems will respond to subtle changes in the environment, as would occur under stratospheric ozone depletion, will require that consideration be given both to the direct effects on specific organisms as well as to the indirect effects mediated by species interactions, such as competition and herbivory. he few examples of indirect effects discussed in this paper serve mainly to point out the need for this broader approach to the question of how elevated UV-B irradiation will affect terrestrial and aquatic ecosystems.