In regard to the negative v. natural progression that accompanies invasive species, I find it helpful to think of invasive species in the broader context of global change. When focusing on the singular effects of invasive species, it is easy to understand their incrimination due to biodiversity loss and homogenization. However, if you consider the many other challenges & changes that our ecosystems are facing (eutrophication, climate shifts, etc), I find that invasives can take on new and fascinating roles. For example, consider the loss of a native habitat-forming plant due to human-induced abiotic shifts, resulting in degraded habitat for refuge-seeking native species. When a new habitat-forming plant emerges as an invasive in that system, do you mark it as another problem even if it is providing alternative habitat functionally equivalent to the missing native? At this point, is the invasive species simply another sign of the incredible degradation of the native ecosystem, or is it a sign of the functional re-emergence of a shifting (perhaps even adapting) ecosystem? Of course, it is important to note that in this scenario, the invasive species is not the ultimate reason for the native species loss, although it may reinforce a spiral of decline.
Thus, it seems to me that the relative role of invasive species as harmful pests v. architects of emerging ecosystems is strongly dependent on the broader context of global change. (A growing base of literature on novel and emerging ecosystems explores these ideas further.) Cora -- C. Johnston Doctoral student Behavior, Ecology, Evolution, & Systematics University of Maryland, College Park Gruner Lab <http://grunerlab.umd.edu/> CJohnston Research Site <http://ecocora.blogspot.com> "Science is the process that takes us from confusion to understanding in a manner that's precise, predictive and reliable - a transformation, for those lucky enough to experience it, that is empowering and emotional. We must embark on a cultural shift that places science in its rightful place...as an indispensable part of what makes life worth living." -B. Greene
