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.

Seed Collection

We used nine pairs of co-existing native and invasive species that commonly occur in grasslands in the western US.  In fall 2013, we collected the seeds of one invasive species and one coexisting native species from each nine undisturbed sites across the western United States.  Different pairs of species were collected at each site.  For each species, we collected seeds from at least 30 maternal plants and stored the seeds in a refrigerator prior to the greenhouse experiments.  We also collected the soil from the native species population that has not yet been invaded adjacent to the site of seed collection. At each site we collected a pooled soil sample that was the mixture of 10-20 samples taken across a site, at a depth of 5-20 cm. In total we collected 8-10 L of soil from each site.  Soil was placed in zip-lock bags and kept in a 4℃ refrigerator prior to use.  We used this natural soil because the species collected at a particular site might preferentially occur on soils with particular textures, mineral contents, or nutrients, but sterilized soils to eliminate biota.

Experimental setup

In October 2013, soils were sterilized by autoclaving once per day on three successive days. Seeds were germinated in petri dishes containing sterilized sand which were kept moist with distilled water.  Two weeks after germination, seedlings of each species were either grown alone or grown in competition together in sterilized soil sample from the same site.  The growth substrate was a 60:40 mixture of sterilized soil and sterilized sand.  Seedlings were planted in 250 mL pots.  For each treatment combination (9 sites×3 planting treatments, alone and competition), n=10 resulting in a total n of 270 pots for the experiment. All the pots were randomly placed in a greenhouse of the University of Montana, watered daily, and grown for 120 days.  All plant materials were dried at 60°C for 3 days and weighed.  

Quantification of competitive ability

We calculated the competitive effects of invasive species on native species and competitive responses of invasive species to native species using the relative interaction intensity (RII) index (Armas et al. 2004):

RII_effect,i = (Bw_native,i - Bo_{native mean})/ (Bw_native,i + Mean Bo_{native mean})

RII_response,i = (Bw_invasive,i - Bo_{invasive mean})/ (Bw_invasive,i + Bo_{invasive mean})

Where RII_effect,i and RII_response,i are the competitive effect and response of the i^th individual of an invasive species growing with a neighbour, respectively; Bw_invative,i and Bw_native,i are the total biomasses of the i^th individual of an invasive species and its paired native competitor when grown together, respectively; Bo_{invasive mean} and Bo_{native mean} are the mean total biomass of the focal invasive and native species when grown alone, respectively; Values of this metric range from 1 to −1. A negative value indicates inhibition, and a positive value indicates facilitation.  We also calculated the gram per gram RII after Besaw et al. (2011):

Gram per gram RII_effect,i = RII_effect,i / Bo_{invasive mean}

Gram per gram RII_response,i = RII_response,i / Bo_{native mean}

Additionally, we quantified the competitive advantage of invasive over native species with competitive inequality in a similar manner as Golivets and Wallin (2018):

Inequality_RII,i = RII_effect,i - RII_response,i

Inequality_{Gram per gram RII,i} = Gram per gram RII_effect,i – Gram per gram RII_response,i

In the above equations, a negative value corresponds with competitive advantage of the invasive over the native species, whereas a positive value corresponds with competitive disadvantage for the invasive.