Background/Question/Methods . Seed size and subsequent seedling growth are critical traits in determining population dynamics, competition and the extinction risk of plant populations.  These traits have also been implicated in the success and spread of invasive species.  For instance, an endangered thistle (Cirsium pitcheri), native to the Great Lake’s region of the Midwestern United States, produces large seeds (12.5 mg) and do not exhibit vegetative reproduction.  Among its threats is an invasive thistle (C. arvense) that produces small seeds (0.08 mg) and exhibits vegetative reproduction.  Using these thistles as inspiration, we developed a game theory model for incorporating seed size and seedling competition to explore potential differences in fitness success of these two plant species, and to identify if a species is able to resist invasion. We started with the model of Schwining & Fox (1995) where competition is influenced by the density and relative sizes of seedlings – larger seedlings exert more competition on smaller seedlings, and larger seedlings experience less competition from smaller seedlings.  By letting seed-size be an evolutionary strategy, the ecological model becomes a fitness generating function (G-function) that can include both thistle species as part of a continuum of seed-size strategies.

Results/Conclusions . In the evolutionarily stable strategy or ESS, seed size increased with the size of the competitive neighborhood, with the relative competitive advantage of size, and with a decreasing cost of producing larger seeds.  If an ESS was reached, then the native thistle was resistant to invasion by the exotic thistle.  Hence, successful invasion means that the native thistle is no longer in ESS perhaps because of human disturbances.  As a non-ESS state, the model produced two possibilities.  One had the new environment favoring such a small seed that the exotic species out competed and replaced the native species, or alternatively, the new ESS represented an intermediate seed size that permitted the ecological coexistence of the two thistle species at a new steady-state. This research showed that an individual plant species might have several possible strategies that represent a balance between survival and extinction, which can have an impact on successful invasion by exotic species.