Do invasive species have bigger seeds? Evidence from intra- and inter-specific comparisons

Shifts in seed mass associated with the process of plant invasion may potentially enhance the competitiveness of introduced species and contribute to invasiveness. Here we test this premise using two complementary approaches. Firstly we compare the seed mass of 114 species from 31 families in both their native and introduced ranges. Secondly we compare the seed mass of 376 co-occurring native and invasive species from two families (Asteraceae and Poaceae) from California. Our results demonstrate that across the 31 families there is a significant tendency for seed mass to increase from the native to invasive ranges. In addition, the analysis on the two families revealed that such a shift in seed mass may contribute to invasive species having, on average, a higher seed mass than co-occurring native species in the same family. Consequently, these results suggest that invasion-associated upward shifts in seed mass may foster plant invasions by increasing the competitiveness of invasive species relative to natives.