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

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