United we stand: evolution of increased competitive response and defense in response to crowding in an invasive plant

1. The evolution of increased competitive ability (EICA) hypothesis predicts that invasive plant could evolve to be more competitive but be less defended as a result of releasing from their natural enemies, yet this hypothesis has rarely been addressed in density-dependence.

2. Here, we grew five native (Argentina) and five introduced (USA) genotypes of perennial herb invasive plant Alternanthera philoxeroides, using an experimental setup that simulated different levels of neighbours heights and densities.

3. Our results showed that introduced and native genotypes responded differently to changes in density: when neighbours were denser, introduced genotypes showed increases in total biomass, trichome density and triterpenoid saponins than native genotypes, but constants in shade-avoidance-related traits.

4. Contrary to the predictions of EICA hypothesis, our findings contribute to a new pattern of both increased competitive response and defense in introduced populations in response to crowding, and highlight the importance of  positive density-dependence in understanding invasive plant–plant interactions.