In analyzing the role played by UVR on species interactions, I have included five categories based on the mechanism, namely competition, predation-herbivory, mutualism, parasitism, and disease . The definition and use of these categories in the scientific literature has been flexible as asserted by the interactions between species included under the categories of mutualism, disease, and parasitism . Nevertheless, competition, predation, grazing, and parasitism are among the most important processes in ecology, because they are crucial to understand control mechanisms of population growth and community structure .
As we have seen in previous chapters, the bulk of information about the effects of UVR on aquatic organisms has been gathered in studies where species interactions were not considered. Although aquatic ecologists have obtained information on how sensitive different organisms are, including those considered as keystone species, comparatively little effort has been addressed to study potential positive and negative feedbacks caused by UVR on species interactions. Yet, this information is necessary to understand the community and ecosystem response to present and increased levels of incident UVR. Particular ly, knowledge at the community level is a pre-requisite for the application of concepts like stability and recovery  that has been sometimes wrongly applied in studies of UV impact based on single species.
In general, the characteristics of UVR as an environmental stressor differ from other toxic agents such as pesticides or other man-made chemical substances that were not previously found in the environment. As evidenced by the several strategies developed among different forms of life to obtain protection and to repair damage, solar UVR has been an important selective factor during evolution of life on Earth. Moreover, the existence of a temporal pattern in UV irradiances, of a natural vertical gradient of UVR in the water column, of physical refuges, as well as the dual role of UVR (i.e., having negative and positive effects) impart a different nature to the interaction between UVR and aquatic organisms. Although UVR may affect species interactions in a similar way as a toxic substance does, the ecological buffer or adaptive response of aquatic communities to the effects of UVR may well be larger than that to xenobiotics.
Solar UVR may affect species interactions by direct and indirect ways. Figure 1 depicts some examples of hypothetical changes in population size over time of two interacting species with different UV-sensitivity, where the resulting effect high UV SENSITIVITY low high UV SENSITIVITY low
Predation/ Herbivory (1)
Figure 1. Hypothetical changes in size population over time as affected by differential sensitivity to UVR in five types of species interactions. Size of a population is represented by small or large squares. The 0, +, and — represent the effects of one species on the other (e.g., 0 means a neutral effect of population a on population b). P: predator or parasite, p: prey, and h: host.
has been categorized into neutral, positive, or negative , Despite its simplicity and the fact that species interactions are seldom one-to-one in natural food webs, which are usually characterized by a high linkage density (i.e., the average number of interactions per species in the web), Figure 1 provides a framework to analyse the effect of UVR on species interactions and to compare these scenarios with results obtained from the scientific literature.
In this chapter, I shall critically review our knowledge of the link between UVR and the different categories of species interactions mentioned above, trying to lay emphasis on the overall net result in population size for the respective interactions, and, wherever possible, presenting general patterns, identifying major gaps, and future research directions.
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