In assessments of ecological impact in invasion ecology, most studies compare un-invaded sites with highly invaded sites,
representing the ‘ worst-case scenario ’ , and so there is little information on how impact is modifi ed by the population
density of the invader. Here, we assess how ecological impact is modifi ed by population density through the experimental
development of density-impact curves for a model invasive fi sh. Using replicated mesocosms and the highly invasive
Pseudorasbora parva as the model, we quantifi ed how their population density infl uenced their diet composition and
their impacts on invertebrate communities and ecosystem processes. Th e density – impact curves revealed both linear and
non-linear density – impact relationships. Th e relationship between P. parva density and zooplankton body mass was
represented by a low-threshold curve, where their impact was higher at low densities than predicted by a linear relationship.
In contrast, whilst the relationship between density and zooplankton biomass and abundance was also non-linear, it was
high-threshold, indicating a lower impact than a linear relationship would predict. Impacts on diversity and phytoplankton
standing stock were linear and impacts on benthic invertebrate abundance and decomposition rates were represented
by s-shaped curves. Th ese relationships were underpinned by P. parva dietary analyses that revealed increasing reliance on
zooplankton as density increased due to depletion of other resources. We caution against the common assumption that
ecological impact increases linearly with invader density and suggest that increased understanding of the relationship
between invader population density and ecological impact can avoid under-investment in the management of invaders that
cause severe problems at low densities.