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.