The severity of the disturbance and whether it is natural or caused by humans can influence the recovery rate of streams.
Stream restoration normally focuses on the restoration of waterways that have been degraded by human-induced (anthropogenic) impacts rather than natural disturbances (such as flooding and volcanic activity). Anthropogenic activities can disrupt water flow and hydrology, and potentially affect connection within and between freshwater habitats.
Higher levels of urbanisation, industrialisation and agriculture place increasing strain on our waterways. Toxic spills and pollution can destroy all plants and animals along an entire stream, but less severe disturbances such as localised flooding, particularly in low-order streams, can leave remnant habitats like the hyporheic zone, boulders and large woody debris relatively intact. Areas of low to moderate disturbance may be suitable for recolonisation and oviposition of the disturbed area by local invertebrate colonists or by organisms that have dispersed from upstream, downstream or even lateral sources.
Similarly, disturbances to riparian vegetation such as herbicide application may be more controlled and localised, impacting only a particular section of a stream. Herbicides often target terrestrial bankside vegetation but can also be used for control of in-stream macrophytes.
Although the use of insecticides (pyrethroids) controls insect pests on farms, they are extremely toxic to aquatic organisms. Recent research has shown that not only can insecticides affect aquatic invertebrate communities, but herbicide application may also alter certain invertebrate and vertebrate communities. However, it is thought that provided the application of insecticides and herbicides are not continuous, many of the affected communities can successfully recover.
Points to consider
Evaluation of the impacted area is essential for the rehabilitation and rate of recovery. In evaluating a site for restoration, the following should be considered:
- the duration of the disturbance and probability that it will recur
- the effect of disturbance on physical habitat which may be harmful for a longer period of time than for chemical effects. Also consider the effects of chronic, slow release toxicity if sediment contamination has occurred
- the state and ability of riparian vegetation to recover, and perhaps the time taken to re-shade the stream and provide overhanging habitat and refugia for fish
- changes in hydrology that may result in reduced or enhanced success for various colonising species.
Impact of Insecticides and Herbicides on the Biodiversity and Productivity of Aquatic Communities (Ecological Society of America)