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 th..., 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 m..., , # Data from: Competition on a neutral playing field: Invaders still win and size still matters… sometimes

[https://doi.org/10.5061/dryad.vq83bk41w](https://doi.org/10.5061/dryad.vq83bk41w)

Description of the data and file structure

## **1. General file description:**

This dataset includes two data sheets and one R script for statistical analyses.

The first data sheet is named as “Data 1-RII and gram per gram RII.csv”. The second date sheet is named as “Data 2-Data for Path Analysis.csv”. The R script is named as “Data Analysis.R”. All analyses were conducted using R version 4.2.1.

## **2. Description of the data files:**

### **(1) RII and gram per gram RII.csv**

This contains data for the competitive effect of invasive species on native species and the competitive response of invasive species to native species, which were calculated with the relative interaction index (RII) and the gram-per-gram RII using the plant biomass with and without competition.

species_pair = IDs of th...