Using space-for-time substitution and time sequence approaches in invasion ecology

1. Invasion biologists use two main approaches to evaluate the effects of non-native species (NNS) on diversity of native species (DNS), namely space-for-time and time approaches. These approaches have pitfalls related to lack of controls: the former lacks pre-invasion data, while the latter often lacks data from non-invaded sites. 2. We propose a framework that combines space-for-time and time approaches and which should result in more focused mechanistic hypotheses and experiments to test the causes of invasibility and the effects of NNS on DNS. We illustrate the usefulness of our framework using two case studies: one with the submersed macrophyte, Hydrilla verticillata, in reservoir and the other with the fish, Geophagus proximus, in a large riverfloodplain system. 3. Hydrilla verticillata invaded sites with DNS similar to that found in non-invaded sites, indicating that biotic and/or abiotic factors did not influence invasion success; however, DNS increased over time in invaded sites compared with non-invaded sites, suggesting that H. verticillata facilitated natives. In contrast, G. proximus invaded sites with higher DNS than non-invaded sites, suggesting that biotic and/or abiotic factors favouring natives were important for invasion success, but DNS increased in invaded and non-invaded sites over time, indicating that an independent factor contributed to DNS increases. 4. Conclusions from both studies would have been inaccurate or incomplete if the space-for-time and time approaches had not been used in combination as proposed in our framework.