DOI: 10.5061/dryad.vq83bk41w
title: Data from: Competition on a neutral playing field: Invaders still win and size still matters, sometimes

description: 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.
description: <strong>Seed Collection</strong>
 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. 
 <strong>Species
 pair</strong> 
 <strong>Invasive species (field cover
 %)</strong> 
 <strong>Native species (field cover
 %)</strong> 
 <strong>Seed Origin (GPS
 coordinates)</strong> 
 <strong>1</strong> 
 <em>Agropyron cristatum</em>
 (25.7%) <em>Leymus
 cinereus </em>(18.5%) 
 Dixon (47°22'10.32"N,
 114°14'37.00"W) 
 <strong>2</strong> 
 <em>Arctium minus </em>(5.8%)
 <em>Canadanthus modestus
 </em>(2.3%) Moiese
 (47°22'18.3000"N, -114°15'29.9700"W)
 
 <strong>3</strong> 
 <em>Bromus inermis </em>(13.9%) 
 <em>Pseudoroegneria spicata
 </em>(19.8%) Stevensville
 (46°29'11.57"N, 114°09'08.57"W) 
 
 <strong>4</strong> 
 <em>Centaurea stoebe ssp.
 micranthos</em>(6.4%) 
 <em>Achillea millefolium </em>(4.6%)
 Lolo (46°48'27.33"N,
 114°05'57.53"W) 
 <strong>5</strong> 
 <em>Dipsacus fullonum</em>
 (2.0%) <em>Geum
 macrophyllum </em>(1.6%) 
 Bison Range (47°22'10.62"N,
 114°14'43.74"W) 
 <strong>6</strong> 
 <em>Lactuca serriola </em>(4.1%)
 <em>Conyza canadensis
 </em>(16.6%) Alberton
 (46°59'34.49"N, 114°26'44.50"W) 
 
 <strong>7</strong> 
 <em>Linaria dalmatica </em>(12.3%)
 <em>Erigeron speciosus</em>
 (1.5%) Missoula
 (46°52'54.30"N, 113°59'32.94"W) 
 
 <strong>8</strong> 
 <em>Tanacetum vulgare </em>(23.0%)
 <em>Solidago gigantea
 </em>(33.0%) Missoula
 (46°50'03.20"N, 114°02'38.83"W) 
 
 <strong>9</strong> 
 <em>Verbascum thapsus</em> (3.8%)
 <em>Artemisia ludoviciana
 </em>(3.6%) Lolo
 (46°43'22.81"N, 114°08'35.24"W) 
 
 <strong>Experimental setup</strong>
 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.  
 <strong>Quantification of competitive
 ability</strong> 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<sub>effect,i</sub> =
 (Bw<sub>native,i</sub> - Bo<sub>native
 mean</sub>)/ (Bw<sub>native,i</sub> + Mean
 Bo<sub>native mean</sub>)
 RII<sub>response,i</sub> =
 (Bw<sub>invasive,i</sub> - Bo<sub>invasive
 mean</sub>)/ (Bw<sub>invasive,i</sub> +
 Bo<sub>invasive mean</sub>) Where
 RII<sub>effect,i</sub> and
 RII<sub>response,i</sub> are the competitive effect and
 response of the i<sup>th</sup> individual of an invasive
 species growing with a neighbour, respectively;
 Bw<sub>invative,i</sub> and Bw<sub>native,i</sub>
 are the total biomasses of the i<sup>th</sup> individual of an
 invasive species and its paired native competitor when grown together,
 respectively; Bo<sub>invasive mean</sub> and
 Bo<sub>native mean</sub> 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<sub>effect,i</sub> = RII<sub>effect,i</sub> /
 Bo<sub>invasive mean</sub> Gram per gram
 RII<sub>response,i</sub> =
 RII<sub>response,i</sub> / Bo<sub>native
 mean</sub> Additionally, we quantified the
 competitive advantage of invasive over native species with competitive
 inequality in a similar manner as Golivets and Wallin (2018):
 Inequality<sub>RII,i</sub> =
 RII<sub>effect,i</sub> -
 RII<sub>response,i</sub>
 Inequality<sub>Gram per gram RII,i</sub> = Gram per
 gram RII<sub>effect,i</sub> – Gram per gram
 RII<sub>response,i</sub> 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.
description: # 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 the pairs of invasive and native species component = component of competitive ability (effect vs. response) nativity = species nativity (invasive vs. native) species_name = species name total_biomass_with_com = total biomass of individual with competition (g) total_biomass_without_com = mean total biomass of individuals grown alone (g) RII = relative interaction index (RII) GG_RII = gram per gram RII ### **(2) Dataset for Path Analysis.csv** This contains data used for the path model exploring the direct and indirect effects of plant size on the competitive inequality between an invasive-native species pair, which was calculated as the difference between the competitive effect and response of the invasive species. species_pair = IDs of the pairs of invasive and native species invasive_species_name = species name of the invasive species within the species pair native_species_name = species name of the native species within the species pair competitive_inequality_RII = competitive inequality between invasive and native species based on relative interaction index (RII) competitive_inequality_GG_RII = competitive inequality between invasive and native species based on gram per gram RII effect_RII = competitive effect based on RII effect_GG_RII = competitive effect based on gram per gram RII response_RII = competitive response based on RII response_GG_RII = competitive response based on gram per gram RII invader_size = total biomass of the invasive species within the species pair (g) native_size = total biomass of the native species within the species pair (g) size_inequality = size inequality between invasive and native species (g) ## **3. Description of the R script:** ### **Data Analysis.R** This contains three sections for data analyses: section 1 shows the code for calculation of the relative interaction index (RII) using Data 1; section 2 shows the code for the ANOVA model and t test for the difference between competitive effect and response for all species combined and for each pairs of invasive versus native species using Data 1; section 3 shows the code for the partial-least-square (PLS) path analysis using Data 2. To run this code, run the “Data Analysis.R” script in Rstudio. ## **4. Software:** R version 4.2.1. Running under macOS Sequoia 15.3.1. Attached packages: base, methods, datasets, stats, graphics Loaded packages: car, carData, rstatix, rsq, plspm