Evolution of Competitive Ability within Lonicera japonica’s Invaded Range

Factors influencing invasive taxa may change during the course of an invasion. For example, intraspecific competition is predicted to be more important in areas with older stands of dense monospecific invaders than at the margins of an invaded range. We evaluated evolution in response to predicted changes in competition by comparing the intraspecific competitive ability of North American populations of Japanese honeysuckle (Lonicera japonica) from areas established 100–150 yr ago, which make up the core of the invaded range, to populations from the northern range margin, established in the past 50 yr. Plants from populations in the core and margin regions of the invaded range were grown in two common garden studies. The first had three levels of intraspecific competition, and vines differed in their access to a lattice for climbing. The second asked whether plants from older core or newer margin populations were better competitors. Intraspecific competition decreased the size of plants from both core and margin regions. Contrary to predictions, there was no increase in climbing shoot frequency or size in plants from denser core populations, although these did increase in the presence of a lattice structure. Also contrary to predictions, plants from the margin of the invaded range were larger and stronger competitors than those from the core, regardless of competition level. Enhanced intraspecific competitive ability of plants from low-density margin populations is likely driven by selection for more rapid growth rate in the north, rather than direct selection on competitive ability. Results counter to prediction highlight the importance of considering the broader selective context to understand evolution. Furthermore, this differentiation across the invaded range is likely to enhance L. japonica’s invasive potential.