Species interactions are crucial to understand the control of population growth and community structure. This chapter presents a brief and critical review of what is known about the effects of UVR (280-400 nm) on species interactions in aquatic ecosystems with emphasis on competition and predation/herbivory. Information on other species interactions such as symbiosis, parasitism, and disease are also briefly reviewed. The existing information indicates that UVR acts as a selective force in pioneer communities of transparent and shallow ecosystems strongly influencing competition output between species at the base of the food web and community structure. However, whether more UV-tolerant species could replace sensitive ones in established communities of natural environments remains uncertain. Examples of positive and negative feedbacks between populations of prey and predators/grazers caused by UVR have been found, but the present information does not ascertain as to whether these mechanisms are widespread in natural ecosystems. Despite the important advance during the last years in our understanding of how ambient and enhanced levels of UV-B radiation (280-320 nm) influence species interactions and trophic relationships, there is still a major gap of knowledge, which is partially attributed to the complexity and biological variability of the species response to UVR, but also to methodological caveats. Consequently, many of the scenarios and hypotheses stated shortly after the discovery of the stratospheric ozone reduction still remain in dispute. Without further research on this topic and the use of more realistic ecological approaches, our assessment of the impact of UVR at the community and ecosystems levels will remain fragmentary and recommendations for sound policy decisions impracticable.

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