Impact of alien species on the distribution of native submerged macrophytes in small rivers and agricultural canals around Lake Mikata.
The distribution of submerged macrophytes and the effects of the invasion of exotic animals and plants on their occurrence were examined in small rivers and agricultural canals around Lake Mikata, Fukui, Japan. Four native and two exotic submerged species were recorded in surveys of 59 transects (each 20 m long × the stream width) conducted in 2010 and 2011. Egeria densa Planch., a non-native species, or Hydrilla verticillata (L.f.) Royle, a native species, dominated over 81% of the sites where any submerged species occurred. Both species richness and the cumulative abundance of native submerged plants were significantly lower at the sites where either E. densa or Procambarus clarkii, an exotic crayfish, occurred. Analyses of factors affecting the distribution of submerged plants using generalized linear mixed models (GLMMs) revealed that the occurrence of P. clarkii significantly negatively affected the occurrences of any native species and H. verticillata, but significantly positively affected the distribution of E. densa. These results suggest that the two aliens E. densa and P. clarkii, might facilitate each other. The control of them is necessary for effective conservation of native submerged plants.