No evidence of increased competitive ability among three widespread alien weeds in their introduced range

It has long been hypothesised that introduced species can evolve to become better competitors, which in turn will enable some of them to become invasive. The evolution of increased competitive ability hypothesis (EICA) gives a possible answer for why some introduced plants become invasive by stating that they can escape natural coevolved enemies (pests, pathogens and herbivores) in their new environment, thus allowing them to shift resource allocation from producing expensive chemical defences, towards a higher growth rate and competitive ability. In order to test if there is evidence for increased competitive ability in introduced populations, we examined the performance of three Rumex species (R. obtusifolius, R. crispus and R. conglomeratus, Polygonaceae) from their native (United Kingdom) and introduced ranges (New Zealand), when grown alone and in competition with a conspecific from the same or a different provenance. Based on the predictions of EICA, we hypothesised that plants from the introduced provenance would: (i) have a faster growth rate and a larger biomass at harvest; and (ii) would perform better in competition with a conspecific from the native provenance than one from the introduced provenance. Intraspecific competition reduced biomass by as much as 50%. However, contrary to expectations, we found no difference between the performance of plants from native and introduced provenances when grown in direct competition with each other. Plant performance when grown with a conspecific from the same provenance was similar to performance when paired with one from a different provenance, showing that there was no provenance effect. These results were consistent for all three Rumex species. Our findings contradict the predictions of the EICA hypothesis suggesting that other factors are needed to explain the success of Rumex species in New Zealand.