The Evolution of Increased Competitive Ability (EICA) hypothesis posits that release from co-evolved natural enemies alters selection pressure on introduced species from defense to growth, resulting in a competitive advantage in the new range. To test defense and competitive ability of the widespread invasive grass Microstegium vimineum we first conducted a no-choice bioassay by feeding Spodoptera frugiperda, a model generalist herbivore, invader biomass from 8 native and 8 invasive populations. Then, we used a fully factorial greenhouse experiment with 10 native and 10 invasive Microstegium populations grown in full sun and shaded conditions, with and without competition, to determine if invasive range populations showed evidence of greater competitive ability. Based on EICA, we predicted that Spodoptera larvae would perform better on non-native Microstegium due to decreased defenses of the invader, and that introduced range populations would display greater growth and competitive ability compared to native range populations.
Results/Conclusions
There were no differences in growth or fitness of Spodoptera larvae fed Microstegium from native and invasive range populations, indicating introduced range populations are not less well defended. Similarly, there were no differences between native and invasive range Microstegium populations in their height or biomass, regardless of whether they were under competition with other individuals. However, we found significant variation among populations in growth and competitive ability, suggesting that native and introduced range populations contain sufficient genetic diversity for significant adaptation to varying environments. Predictably, shade suppressed plant growth but did not alter competitive outcomes, and among-population variation in growth and competitive ability was invariant to shade. Our results do not support EICA in that there was no evidence that Microstegium populations from the invaded range were less well defended or more competitive, thus the EICA hypothesis is unlikely to explain the invasive success of this highly problematic invader.