Intraspecific competitive ability declines towards the edge of the expanding range of the invasive vine Mikania micrantha.

The evolution of competitive ability plays an important role in plant invasions. While many studies of the evolution of invasive species have compared populations from native and invaded ranges in terms of their performance, little attention has been paid to the evolution of intraspecific competitive ability within the invaded range during range expansion. In addition, whether the proportional change in the amount of invasive litter influences the intraspecific competitive ability among invasive populations of different ages has not yet been investigated. Here we selected Mikania micrantha H.B.K., a highly invasive vine in south China with a well-documented invasion history, as the study species. We manipulated competition among populations of different ages from the core of the range to its edges under four litter treatments in a common garden experiment. We found that during its 30-year invasion, intraspecific competitive ability was rapidly selected against towards range edges, which may be driven partly by the decline in population density. However, litter source did not influence the outcome of the competition among populations of different ages; it instead functioned more like a supply of nutrients. We suggest that stage-specific conditions such as population density should be incorporated into the experimental design when examining the evolution of invasive plants, especially when invasive populations are subject to selection on a small geographic scale. This approach can reduce sampling bias and thus improve the ability to infer the mechanisms responsible for the evolution of invasive populations.