Biological Invasions as Agents of Change on Islands Versus Mainlands

Abstract

Over the past decade, the Scientific Committee on Problems of the Environment (SCOPE) has devoted considerable attention to the ecology of invasive organisms (e.g., Brockie et al. 1988; Loope et al. 1988; MacDonald and Frame 1988; MacDonald et al. 1988; Usher 1988; Drake et al. 1989). Conclusions emerging from the workshops that fostered these publications include: (1) perhaps most importantly — biological invasions are now recognized as a global phenomenon that can affect both structure and function in ecosystems; (2) even nature reserves, typically pieces of land set aside because they represent intact native species-dominated ecosystems, are subject to serious biological invasions; (3) island ecosystems typically have a higher representation of alien species in their flora and fauna than do mainland systems; and (4) the severity of invasions on islands increases with isolation of the island. The latter two conclusions are consistent with what has become dogma in discussions of invading species — that island communities are more invasible than mainland ones. Investigators have often expressed this viewpoint with statements such as “because it evolved in isolation, this native fauna [of islands] is highly susceptible to the introduction of alien species” (Richardson 1992). This statement does not distinguish between the possibilities that alien species might establish more readily on islands versus that alien species might have more of an effect (population, community level, etc.) on islands than they do on continents