Comparison of competitive behaviours between native and invasive ecotypes of garlic mustard under different density conditions, presence and identity of neighbours

Invasive species in monocultures in introduced habitats experience different competitive conditions compared with their native habitats. Invasive monospecific stands can be composed of highly related individuals, creating high opportunity for kin selection in invaded habitats. We investigated the responses of North American and European ecotypes of the invasive species garlic mustard ( Alliaria petiolata (M. Bieb.) Cavara & Grande) to aspects of the competitive environment including density, presence and identity of conspecific neighbors. Several aboveground morphological and performance traits responded to density independent of the origin of the plants. Belowground, however, North American ecotypes allocated more resources to roots and particularly to the taproot portion of their root system, while petiole elongation was density-dependent with North American ecotypes showing reduced elongation in high density compared to European ecotypes. These results were consistent with the evolution of reduced competitive ability in garlic mustard and indicated better resource storage in the introduced environment. Differential responses to neighbour identity indicated the ability for kin recognition in this species. Thus, a combination of natural and kin selection favouring better resource storage and less intraspecific competition, aided by the ability for kin recognition, may be responsible for the success of garlic mustard as an invasive species in North America.