With the increase of global commerce, biological invasions have become more frequent. Human activity is also driving other environmental changes, such as climate change and increasing nitrogen deposition, which are likely to affect the success of invaders and the invasibility of ecosystems. I predict that most of these anthropogenic changes will increase the prevalence of invaders. However, the increase in the atmospheric carbon dioxide concentration ([CO2]) is unlikely to consistently favor invaders. I studied the response of the late-season annual forb Centaurea solstitialis (yellow starthistle), a noxious weed in the western United States, to elevated [CO2]. When grown in monoculture, Centaurea responded to elevated [CO2] with a 70% increase in aboveground biomass production. When Centaurea was grown in competition with serpentine grassland species, its CO2 response was similarly positive, but nonsignificant.

Although it is commonly believed that diverse communities are less invasible than communities with few species, results of several recent experiments have refuted this notion. To examine the effects of diversity, as well as soil disturbance, on the success and impact of invaders, I introduced Centaurea into California grassland microcosms that varied in species richness and age (a proxy for soil disturbance). I found that invasibility decreased as the number of functional groups represented in a community increased, in both newly-constructed (more disturbed) and 1-year old (less disturbed) microcosms. Growth of the invader was largely dependent on the availability of soil moisture during the summer. Communities that contained late-season annual forbs (other than Centaurea) had reduced soil moisture availability in the summertime, thus minimizing the success of Centaurea. Species richness buffered grassland communities against some impacts of the invader. For instance, the biomass of resident (i.e., non-Centaurea) species generally decreased in response to Centaurea invasion. This decrease was greater in species-poor communities than in species-rich communities. Centaurea also increased late-season evapotranspiration by a greater margin in species-poor communities than in more diverse communities, probably because the species-poor communities were less likely to contain other summer-active species.

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