It doesn’t take two to tango: increased capacity for self-fertilization towards range edges of two coastal invasive plant species in eastern Australia

Understanding how selection pressures change during the course of an invasion is a key question in invasion biology. Shifts to greater reproductive success and output are expected to occur towards range edges of expanding invasive species as a means of increasing colonization opportunities and accelerating further spread. In a glasshouse experiment, we investigated shifts in reproductive traits (allocation to reproduction, seed number vs. seed size, capacity for self-fertilization) across multiple populations spanning the entire range of two coastal exotic invasive plant species (Gladiolus gueinzii Kunze and Hydrocotyle bonariensis Lam.) in eastern Australia. Although there was no significant increase in allocation to reproduction towards range edges or changes in seed provisioning, range edge populations displayed an increased capacity for self-fertilization in the absence of pollinators for both species. For H. bonariensis this entailed an increase in the probability of fruit production towards range edges while for G. gueinzii it was an increase in the probability of forming developed seeds towards range edges. Greater capacity for self-fertilization may facilitate further range expansion as it alleviates any reliance on external factors for pollination at the range edge. Our results suggest that increased capacity for self-fertilization towards range edges may be a key factor in promoting range expansion in some invasive species.