The ecological impacts of some of the world’s most notorious invaders are well-documented. However, the impacts of the majority of invaders may be small, whilst native species can have large ecological effects. This raises two important questions. First, are introduced species fundamentally different from natives, or do they have similar ecological impacts? Second, do native and introduced species differ in key ecological relationships, such as abundance-impact, species-area, or diversity-productivity? A lack of general criteria differentiating invasive from native species might lead to a misperception of the overall effects of invasions. For example, a recent article in the New York Times, entitled "Friendly Invaders", reviewed evidence that biological invasions have increased species diversity on islands. However, an overall increase in diversity overlooks potential changes to nutrient cycling and other ecological processes that might result in extirpation of native species from particular habitats. Knowing whether introduced species are generally different from natives is crucial to determining whether invaders are truly "friendly". Our working-group will compare the effects of native and alien species on the structure and dynamics of native populations and communities, and the mechanisms underlying these impacts. We will focus on classic ecological relationships, and attempt to determine whether they differ for native and introduced species. We will also consider whether there are important distinctions in these relationships for plants vs. animals from aquatic vs. terrestrial environments.