Patterns of species richness and introduced species in native freshwater fish faunas of a Mediterranean‐type basin: the Guadiana River (southwest Iberian Peninsula)

In this study, we analysed the factors affecting species richness and introduced species component patterns in native fish faunas of 30 streams of the Middle Basin of the Guadiana River. From a principal component analysis and a stepwise multiple regression analysis performed on a data matrix composed of ten hydrological and biotic variables, we showed that: (1) fish species richness increased with stream length and watershed area, (2) the number of native species in a stream declined as channelizations and river regulation (constructions of dams) are higher, whereas introduced species increased in the same way, (3) the two main negative factors affecting native ichthyofaunas affected dissimilar ecological areas: channelizations, which depend on land-use intensity of floodplain, mainly occurred in lower reaches of streams, but construction of dams mainly took place in upper sections of rivers, (4) the length of the remaining well-preserved reaches in a stream appeared to be the only factor accurately predicting native fish species richness, and (5) native fish faunas of small isolated streams are more vulnerable to habitat alteration than those of large streams. Both isolation and fragmentation of populations were recorded, so the conservation status of native and highly endemic fish fauna of the study area is extreme. Protection of the few still extant, well-preserved small streams and upper reaches, habitat restoration of channeled areas, and inclusion of the need for native fish fauna conservation in long-term public planning of water use become a priority. Fish communities appear to be a sensitive indicator of biological monitoring to assess environmental degradation. Copyright © 2001 John Wiley & Sons, Ltd.