Response of introduced and native grassland species to climate variability, eutrophication and their interactions: A multi-site comparison
Climate variability and nutrient addition are both known to significantly reorganize ecological communities, in many cases enhancing the success of invasive species. While these drivers are increasing at a global scale, their rate and direction of change differ among regions. Whereas previous research has mainly focused on the effects of either eutrophication or climate variability on invasive species’ success, the direction, relative effect, and interactions between these ecosystem drivers on the success of invasive species is largely unknown. We replicated experimental manipulations of nutrients at each of 33 grassland sites spanning four different continents, and measured the richness and relative abundance of native and invasive species after 1-3 years.
Climate variability (measured as extreme events and variance in temperature and rainfall) aid introduced species. However, it is interannual climate that better explains the variation in their rate of invasion. Nutrient addition has a positive effect on invasion rates. However, when we determined the effect of eutrophication on invasive species’ success as a function of each site’s climate, we found that these interactions diminish, and sometimes reverse, the positive effect that climate or nutrient addition have on introduced species’ rate of invasion. Eutrophication has a negative impact in native communities by increasing introduced species’ success, but climate stabilizes the competitive exclusion effect fuelled by eutrophication. The interaction between climate and nutrient addition on introduced species’ success is complex. Although sites under nutrient addition often become most dominated by introduced species, variation in climate may enable coexistence between introduced and native species under this condition.