Does adding riffles to urban streams increase biological diversity? 

A common practice in stream restoration is to increase habitat diversity by re-introducing large woody debris to 'de-snagged' streams, or re-introducing pool-riffle sequences to channelized streams.  This project tested the effect of doing the latter on instream invertebrate communities.  In 1996,  riffles were added to 6 degraded, channelized streams in the eastern suburbs of Melbourne.  Invertebrates in those streams (and 3 other control streams) were monitored before and after addition of riffles.

Up to five years after the riffle addition, there was little change in community composition in the test sites.  Sensitive taxa, such as mayflies and caddisflies were collected at least once in all new riffles (not at all before and not at all in control streams), but none persisted beyond a single sample period.   This suggests that the riffles did provide enhanced habitat for colonization, and that lack of available recruits was not a limiting factor in colonization of the riffles.  It is hypothesized that these sensitive taxa were unable to persist in these streams because of water quality (or hydraulic) disturbances associated with urban stormwater.

This hypothesis is consistent with associated work, which pointed to catchment-scale factors associated with urban stormwater being the environmental factors that best explained patterns of community composition.  Macroinvertebrate communities of most sites in the Melbourne metropolitan area are severely degraded, even at very low levels of total catchment urbanization, probably because of impacts associated with the efficient stormwater drainage system of the metropolitan area.

Project team

Chris Walsh, Leader
Edward Tsyrlin, Research Associate (2001)
Kristy Brooke, Research Assistant (2000-2001)
Melissa Aalbers, Research Assistant (1996 - 1997)
Jessica Miller, Research Assistant (1997)

Further reading

• Walsh, C. J. and Breen, P. F. (1999).  "Urban stream rehabilitation through a decision-making framework to identify degrading processes and prioritize management actions."  In Rutherfurd, I. and Bartley, R. (eds.)  Proceedings of the Second Australian Stream Management Conference, Vol. 2.  Adelaide, South Australia.  pp. 673-678. (abstract)
• Walsh, C. J. and Breen, P. F. (2001).  A biological approach to assessing the potential success of habitat restoration in urban streams. Verhandlungen Internationale Vereinigung  fur Theoretische und Angewandte Limnologie 27, 3654-3658. (abstract)
• Walsh, C. J., Tsyrlin, E. and Breen, P. F. (ms in preparation).  An experimental assessment of physical habitat restoration in small streams degraded by catchment urbanization.