Widespread changes in disturbance regimes are either already being experienced or are predicted to occur within freshwater ecosystems due to human development and global climate change, which may drive shifts in community composition. Both disturbance history and type are likely to structure communities through filtering processes selecting for particular species traits. For example, the history of flooding disturbance in braided river systems likely leads to communities resistant to floods. However, these communities could be susceptible to other types of disturbance. Consequently, knowledge of the separate and interactive influences of disturbance history and type and the effect of the interaction between them on community composition is necessary to understand community resistance to future disturbances. We used an in-stream channel experiment to test how the resistance of aquatic macroinvertebrate communities were affected by changes in disturbance. Channels were placed in five stable spring-fed streams and five highly disturbed streams (disturbance history), and individual channels nested within each stream were used to manipulate disturbance type including rock-rolling to simulate flooding, low-flow and undisturbed controls.
Results/Conclusions
The effect of disturbance type (rock-rolling vs low flow) on the composition of aquatic macroinvertebrate communities was dependant on disturbance history i.e. whether streams were stable or disturbed. In stable streams, low-flow conditions had a greater effect on the composition of macroinvertebrate communities than rock-rolling, while low-flow and rock-rolling had similar effects in disturbed streams and there was no significant difference in the effect of rock-rolling across disturbance histories. In contrast, disturbance type alone determined changes in diversity with only low-flow disturbance reducing diversity, regardless of disturbance history. In addition to changes in community composition, our results also suggested that species traits influenced responses to disturbance because species groups displayed contrasting responses to disturbance. Sedentary species were significantly affected by disturbance, especially rock-rolling, while the relative abundance of mobile taxa changed very little. Therefore, our results suggest that antecedent disturbances affect community responses to further disturbance and that disturbance type and history separately influence how resistant communities are to disturbance. Overall, less frequently disturbed communities are more vulnerable to changes in disturbance regimes, and so the influence of disturbance history should be considered when managing the impacts of changes in disturbance regimes.