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 versus 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 were almost two times that of the natives on invasives. The effect of plant size on competitive outcomes was complex. The size of invasive species correlated with their effects on natives but not 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 assumptions about 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.