Selection for size and flowering time drives invasion success in a non-native legume

Background/Question/Methods

Successful biological invasions depend on both abiotic (e.g. resource availability) and biotic (e.g. community structure and genetic variation) factors. Because we expect successful invasions to lead to new interactions among species and environments, we might also expect new selection pressures to accompany these new interactions. However, selection pressures on competitive traits should also be present in initial stages of invasion, and thus natural selection may play a key role in allowing invasives to successfully establish. We tested whether disturbance and herbivory alter selection on traits of the invasive Medicago polymorpha. Genotypes of native and invasive M. polymorpha were subject to a four-treatment, fully crossed factorial design (presence/absence of disturbance and presence/absence of insecticide). We measured fitness as lifetime fruit production, and regressed it on the following traits: flowering phenology, herbivory and plant size. 

Results/Conclusions

We found overall selection on flowering phenology and plant size, however selection did not differ between treatments. The presence/absence of disturbance and insecticide did not have an effect on selection of these traits, though it has been shown that insect herbivores select for higher levels of defense in a related species, Lotus wrangelianus. Although selection may play a role in invasion success, it appears not to be driven by disturbance or insecticide, as our results suggest that neither of these factors serve as selective agents during invasion.