With accelerating rates of invasion being documented in many ecosystems,
communities of interacting invasive species are becoming increasingly common. Opposing
theories predict that invaders can either hinder or promote one another’s success.
Additionally, evidence suggests that co-occurring invaders can interact to amplify or mitigate
one another’s impacts on ecosystems. However, there has not been a quantitative review on
interactions among multiple invasive animals. Here I use a meta-analysis approach to show
that, across a global scale, the mean interaction among invaders was to reduce one another’s
performance. This pattern was consistent when considering interactions between marine
animals but interactions were neutral overall in terrestrial and freshwater ecosystems.
Crucially, individual studies showed that neutral interactions were the most common
interaction type. Further, I demonstrate that the combined ecological impacts of multiple
invaders were frequently the sum of their independent effects (additive) but the mean effect
was non-additive and less than predicted (antagonistic). In both meta-analyses, the disparity
between the most frequent and mean interaction type indicates that case studies of multiple
invasions commonly have different outcomes to global trends. These results will help predict
how co-occurring invasive animals interact and assist in developing management strategies for
problematic invaders in our changing world.
Appendices A and B are available online: http://dx.doi.org/10.1890/15-0171.1.sm