Urbanization and exotic plants in northern Sydney streams

The vegetation and sediment of urban and non-urban streams in the northern Sydney region were compared to examine the possible effects of urbanization on within-stream vegetation. Many sediment characteristics were significantly different in urban streams. At least one exotic plant species was found in each urban stream sampled, but none were found in the non-urban streams. The presence of exotic species led to the overall number and abundance of plant species being significantly higher in urban streams. Interestingly, the number and abundance of native species at the urban sites were the same as non-urban sites, but a different suite of species was usually present. This suggests that urban streams favour exotic plants and certain native plants that are adapted to the modified conditions. The differences between the plant communities in the urban and non-urban streams appeared to be associated with the increased level of nutrients in the urban stream sediment. Several multivariate techniques were used to assess the relative importance of individual nutrients, but no nutrients were directly associated with the observed differences. In particular, total phosphorus levels were less important in explaining the vegetation patterns than a combination of nutrients. It is therefore likely that the general increase of nutrients in stream sediment has enhanced exotic invasion and altered stream plant communities in Sydney streams.