1. Invaders can influence freshwater systems at the individual, population, community and ecosystem levels. Some of these impacts may be subtle or not easily predicted but they may be critical to understanding more obvious changes. Despite this, studies of impacts of freshwater invaders at several levels of ecological organisation are rare. Most commonly reported are changes in the distribution or abundance of populations after invasion, whereas documentation of impacts on ecosystem functioning, such as energy and nutrient flux, is rare.
2. Unlike most invaders, salmonids have been studied at multiple ecological levels. These fish can cause trophic cascades that result in increased algal biomass and production and are responsible for changes to energy and nutrient flux in both streams and lakes. The mechanisms behind these changes are different in the two systems and only become evident when information at the individual and population levels are considered. In streams, salmonids can alter invertebrate behaviour that suppresses grazing of periphyton. In lakes, salmonid feeding behaviour can stimulate phytoplankton by shunting nutrients from the littoral to the pelagic zone.
3. Simultaneous study at several ecological levels should yield a fuller understanding of the mechanisms underlying impacts of invading animals and plants, providing a sounder basis for predicting the impacts of freshwater invasive species. Species traits of the invaders that may be associated with particularly profound impacts include: a method of resource acquisition formerly lacking in the invaded system, a broad feeding niche that links previously unlinked ecosystem compartments, a feeding relationship with negative consequences for native strong interactors, physiological traits that enhance resource transformation and lead to high biomass, and behavioural or demographic traits that provide high resistance or resilience in the face of natural disturbances.