Data from: Competition on a neutral playing field: Invaders still win and size still matters, sometimes

Exotic invasive plant species commonly outcompete native species. However,
a great deal of the evidence for this comes from experiments conducted on
an uneven playing field – in substrates containing soil biota from the
non-native ranges of the exotics, which should give them a competitive
advantage.  In competition experiments with nine pairs of
non-native invasive vs. native species in neutral substrates composed of
sterilized soil, we found that the competitive effect of invasive species
on natives was approximately five times greater than the reverse, and
gram-per-gram competitive effects of invasives on natives was almost two
times that of the natives on invasives. The effect of plant size on
competitive outcomes was complex. The size of invasive species was
correlated with their effects on natives but was not correlated with their
tolerance to competition from natives. The size of natives was not
correlated with either aspect of competitive ability. This is important
since the tolerance of invaders to competition from natives is thought to
be essential for successful invasion. Our results also suggest that
size-based evidence for the evolution of competitive ability in non-native
ranges is reasonable, and that even without the advantage gained from
escaping soil biota, invaders still win.