Invasion ecology urgently requires predictive methodologies that can forecast the ecological impacts of existing, emerging and potential invasive species. We argue that many ecologically damaging invaders are characterised by their more efficient use of resources. Consequently, comparison of the classical ‘functional response’ (relationship between resource use and availability) between invasive and trophically analogous native species may allow prediction of invader ecological impact. We review the utility of species trait comparisons and the history and context of the use of functional responses in invasion ecology, then present our framework for the use of comparative functional responses. We show that functional response analyses, by describing the resource use of species over a range of resource availabilities, avoids many pitfalls of ‘snapshot’ assessments of resource use. Our framework demonstrates how comparisons of invader and native functional responses, within and between Type II and III functional responses, allow testing of the likely population-level outcomes of invasions for affected species. Furthermore, we describe how recent studies support the predictive capacity of this method; for example, the invasive ‘bloody red shrimp’Hemimysis anomalashows higher Type II functional responses than native mysids and this corroborates, and could have predicted, actual invader impacts in the field. The comparative functional response method can also be used to examine differences in the impact of two or more invaders, two or more populations of the same invader, and the abiotic (e.g. temperature) and biotic (e.g. parasitism) context-dependencies of invader impacts. Our framework may also address the previous lack of rigour in testing major hypotheses in invasion ecology, such as the ‘enemy release’ and ‘biotic resistance’ hypotheses, as our approach explicitly considers demographic consequences for impacted resources, such as native and invasive prey species. We also identify potential challenges in the application of comparative functional responses in invasion ecology. These include incorporation of numerical responses, multiple predator effects and trait-mediated indirect interactions, replacement versus non-replacement study designs and the inclusion of functional responses in risk assessment frameworks. In future, the generation of sufficient case studies for a meta-analysis could test the overall hypothesis that comparative functional responses can indeed predict invasive species impacts.

Community ecologists generally recognize the importance of species – such as pollinators – that have clear positive effects within ecosystems. However, parasites – usually regarded in terms of their detrimental effects on the individuals they infect – can also have positive impacts on other species in the community. We now recognize that parasites influence species coexistence and extirpation by altering competition, predation, and herbivory, and that these effects can, in turn, influence ecosystem properties. Parasites and pathogens act as ecosystem engineers, alter energy budgets and nutrient cycling, and influence biodiversity. Equally, because ecosystem properties – such as biodiversity – affect parasite populations, there is the potential for feedback between parasitism and ecosystem states. Using examples from animal and plant systems, we examine this potential bidirectional interdependence and challenge the conventional wisdom that parasites have only negative or inconsequential impacts on ecological communities.