Stream ecosystems often face sudden or short-term (pulse) disturbances such as spates when flow speeds increase but the stream does not flood over its banks. Such streams can be quite “flashy,” experiencing rapid changes in flow speed. Streams also experience longer-term disturbances such as catchment clearing for agriculture and drought caused by global warming (ramp disturbances). However, we do not know whether these ramp disturbances impair the ability of stream ecosystems to recover from pulse disturbances. In southwestern Australia, many streams on the Swan Coastal Plain experience pulse disturbances such as environmental releases from upstream dams, or spates from intense winter storms. Combined with extensive stream modification associated with clearing and agriculture, these pulse disturbances may place pressure on stream ecosystems. Understanding ecosystem responses to, and recovery from such disturbances across different landscapes is key to understanding how they alter ecosystem function.
Tackling this question requires the use of field experiments to create a pulse disturbance in streams under different levels of longer-term disturbance, taking samples before, during and after the pulse disturbance (to observe recovery). To simulate pulse disturbances, we constructed temporary half-weirs in eight streams: four with highly cleared catchments and four with low levels of clearing (≤ 30%). We then tracked changes in invertebrate community structure and rates of decomposition during the presence of the temporary weir and after it was removed. The temporal sampling was repeated at an upstream control site where no weir was built. The half-weirs successfully created two contrasting zones of flow: a slow-flow and fast-flow zone. This talk will describe how manipulative field experiments can be used to study disturbance, the range of variables they enable you to measure and their advantages (and disadvantages).