Spatial structure in invasive Alliaria petiolata reflects restricted seed dispersal

Identification of processes that shape the spatial structure of invasive species has important implications for understanding population regulation. In invasive Alliaria petiolata we quantified (1) spatial scale of seed dispersal under field conditions, (2) spatial structure of rosette and adult density and (3) the relative importance of environmental filtering, propagule pressure and adult-rosette interactions in shaping rosette and adult density. We conducted a seed dispersal experiment under field conditions; and in a 3 year field survey we measured quadrat-level rosette and adult density, adult seed production and soil moisture and light along transects in three A. petiolata populations at Koffler Scientific Reserve in Southern Ontario, Canada. Propagule pressure was estimated based on quadrat-level seed production and neighbourhood seed production. We found that seed dispersal of A. petiolata followed a lognormal distribution with a mean dispersal distance of 1.82 m, and that spatial structure in both adults (Moran’s I: 0.30–0.56) and rosettes (0.48–0.57) was significant up to 2 m. Propagule pressure and adult-rosette interactions were significant processes associated with rosette density, whereas propagule pressure was the most important process associated with adult density. However, due to the observational nature of the data, it is difficult to determine whether the observed negative association between adult and rosette density is related to competition or to temporal variation in density-dependent seed production. Overall, we conclude that spatially restricted seed dispersal is the most important factor underlying this species’ patchy distributions.