Forest diseases caused by fungi, such as Elm graphiosis and Alder phytophthora, have become a serious problem in the conservation of riparian woodland. Freshwater habitats in the Mediterranean are under diverse anthropogenic pressures; of these, the forest management in the surroundings is among the least analysed. Odonata larvae in a permanent stream in the Sierra Morena were studied for twenty-seven years, recording the changes that occurred in the stream bed and the surrounding forest. Several diversity indices were analysed and apparent differences between years in Odonata species richness were recorded. Our findings show that forest diseases, as a consequence of anthropogenic activities, can initially lead to a higher Odonata diversity in Mediterranean permanent streams. This increase in species richness is a consequence of an increase in the seasonality of the flow and luminosity in the channel from late spring to early autumn. Homogeneity of the stream bed due to the disappearance of alternate riffles and ponds of more than a meter deep, and the appearance of strongly rooted aquatic phanerogams like water-cress also plays an important role. The populations of Anisoptera species with a long life cycle, which previously were the only ones inhabiting the stream and which still are dominant today, will possibly disappear in the long term.