Non-native species with strong impacts on native communities - distinguished as being “invasive” - are a major driver of global change. Previous meta-analyses have shown convincingly that invasive species have negative ecological impacts, however, these studies have compared ecological effects when invasive species are present vs. absent. Thus, it is unknown how invasive impacts develop during the invasion process. Here, we use a meta-analysis to assess the shape (linear vs. non-linear) of community-level and individual native species responses to invasive species abundance, in both terrestrial and aquatic ecosystems. We reviewed empirical papers that measured responses of native species or communities along gradients of invasive species’ abundance. Data were extracted from a total of 222 papers that met our inclusion criteria. We summarized invasive–native relationships using effect sizes derived from partial correlation coefficients, describing linear and curvilinear aspects of response shape. We fitted Bayesian mixed effects meta-analyses (using MCMCglmm) to determine mean effect sizes of invader abundance on individual native species and community-level diversity indices. Understanding the functional shape of invasion impacts is critical for predicting the magnitude and rate of native responses to continuing invasions and improving predictions of the potential for invasive species to drive local extinction.
Across taxonomic groups, invasive species had a strongly negative, linear effect on both native species and ecological communities. This negative, linear effect was observed regardless of whether the study considered the abundance of a single native species (cover, biomass or density) or community-level response (richness, evenness, or diversity). Negative, linear effects were also significant regardless of study design (i.e., whether the data were collected across space or through time) and ecosystem type. Invader abundance did not have any significant non-linear effects on community-level diversity indices, but it did have a marginally significant non-linear effect on single native species abundance. In the latter case, native abundance was more likely to decline rapidly at low levels of invader abundance. These findings suggest that invader impacts on single species can have pronounced effects at early stages of invasion, while community-level impacts continue to occur throughout the invasion process.