What is an Invasive Species? David M Richardson & Petr Pyšek David M Richardson Institute of Plant Conservation, Botany Department, University of Cape Town, 7701 Rondebosch, South Africa. E-mail: rich@botzoo.uct.ac.za Petr Pyšek Institute of Botany, Academy of Sciences of the Czech Republic, CZ-252 43 Pruhonice, Czech Republic. E-mail: pysek@ibot.cas.cz April 2004 1. Summary 2. Background 3. Native alien 'pest' species 4. When is an organism native or alien? 5. The history of problems with alien species 6. When does an alien species become feral, naturalized, invasive? 7. What terminology should research scientists, policy makers, conservationists and quarantine staff use for practical applications in the future? 8. References 1. Summary Escalating transport of organisms around the world and the rapid increase in problems with invasive alien organisms worldwide, demands improved precision and objectivity in categorizing non-native species. This is essential for the effective implementation of various policies aimed at reducing impacts of currently problematic non-native species and those aimed at preventing introductions of other species with a high risk of causing problems. Problems that exist in this regard relate to, among other things: (1) poorly resolved systematics for many groups of organisms, especially microscopic organisms for which the geographic origin of many taxa is unknown; (2) the fact that scientific study of the determinants of invasiveness is in its infancy; (3) criteria for defining organisms as feral/naturalized, invasive, etc. have evolved differently in different parts of the world, and for