Invasive non-native species attributes and invasion extent: examining the importance of grain size

Questions: Do differences in species attributes explain the variation in invasion extent among non-native plant species? Which attributes of the invasion process exhibit scale dependency andwhich do not? Location: Hokkaido prefecture, northern Japan. Methods: We calculated the invasion extent of 316 non-native plant species based on gridded maps at three different spatial scales ( grain size of approximately 2 km 9 2 km, 10 km 9 10 km and 80 km 9 80 km). Relationships between the invasion extent of 316 non-native plant species and their attributes that are likely to enhance introduction ( human use and introduction period), establishment ( height, clonality, longevity and life form) and expansion ( seed size, introduction period) were evaluated using phylogenetic regression. Results: Irrespective of grain size, non-native species with clonality, those used as fodder or for landscaping, and those with a historically earlier introduction period had significantly larger invasion extents than species without such attributes. In contrast, seed size, which is related to successful expansion, was negatively correlated with invasion extent and was significant only at the finest grain. Conclusions: Our results suggest that the presence of scale-dependent responses of the species attributes on their grain size may be caused by the invasion process to which the attribute is related. When developing general rules for invasiveness, researchers must consider the grain size at which the invasion extent is calculated.