A test of the evolution of increased competitive ability in two invaded regions

O_LIFinding patterns that predict and explain the success of non-native species has been an important focus in invasion ecology. The evolution of increased competitive ability (EICA) hypothesis has been a frequently used framework to understand invasion success. Evolution of increased competitive ability predicts that 1. Non-native populations will escape from coevolved specialist herbivores and this release from specialist herbivores should result in relaxed selection pressure on specialist-related defense traits, 2. There will be a trade-off between allocation of resources for resistance against specialist herbivores and allocation to traits related to competitive ability and 3. This shift will allow more allocation to competitive ability traits.\nC_LIO_LIWe tested the predictions of EICA in the model plant Mimulus guttatus, a native of western North America (WNA). We compared how well the predictions of EICA fit patterns in two non-native regions, the United Kingdom (UK) and eastern North America (ENA). Coupled with extensive herbivore surveys we quantified genetic variation for herbivore resistance traits and fitness/ competitive ability traits to test adherence to the predictions of EICA in a common greenhouse environment.\nC_LIO_LIHerbivore communities differed significantly between WNA, UK, and ENA populations with evidence of specialist herbivore escape in the UK, but not necessarily the ENA plants. Compared to native plants, resistance traits were lower in non-native UK plants with the exception of trichome density, while the non-native ENA plants had equivalent or higher levels of herbivore resistance traits. The UK plants had increased competitive traits than native plants while the ENA plants had equivalent competitive traits to native plants. The UK plants, but not the ENA plants, showed some signs of tradeoffs between resistance traits and fitness/ competitive ability.\nC_LIO_LISynthesis. Plants from the UK conformed to predictions of EICA more closely than those from ENA. The UK invasion is an older, more successful invasion, suggesting that support for EICA may be highest in more successful invasions. The lack of comprehensive conformity of either non-native region to the predictions of EICA also leaves room for other hypotheses that may add to our mechanistic understanding of the success of non-native plant invasions.\nC_LI