Supporting Information Appendix 1
A review and meta-analysis of nonnative plant interactions included the following 65 studies. To view data collected from each reference, please see “Supporting Information
References and Data.xls
.” Asterisks (*) indicate that data from the article was also included in the meta-analysis.
Abraham JK, Corbin JD, and D'Antonio CM (2009). California native and exotic perennial grasses differ in their response to soil nitrogen, exotic annual grass density, and order of emergence. Plant Ecology, 201 , 445-456.
Adler PB, D’Antonio CM, Tunison JT (1998) Understory succession following a dieback of Myrica faya in Hawai ’i Volcanoes National Park. Pacific Science, 52 , 69-78
Allen EB, Cox RB, Tennant T, Kee SN, Deutschman DH (2005) Landscape restoration in southern California forblands: Response of abandoned farmland to invasive annual grass control. Israel Journal of Plant Science, 53 , 237-245.
Alvarez ME, Cushman JH (2002) Community-level consequences of a plant invasion: effects on three habitats in coastal California. Ecological Applications, 12 , 1434-
1444.
Bahm MA, Barnes TG, Jensen KC (2011) Restoring native plant communities in smooth brome ( Bromus inermis )-dominated grasslands. Invasive
Plant Science and Management, 4 , 239-250.
* Belote RT, Weltzin JF (2006) Interactions between two co-dominant, invasive plants in the understory of a temperate deciduous forest. Biological Invasions, 8,
1629-1641.
* Besaw LM, Thelen GC, Sutherland S, Metlen KM, Callaway RM (2011)
Disturbance, resource pulses and invasion: short-term shifts in competitive effects, not growth responses, favour exotic annuals. Journal of Applied Ecology,
48 , 998-1006.
Brooks ML (2000) Competition between alien annual grasses and native annual plants in the Mojave Desert. American Midland Naturalist, 144 , 92-108.
Bush RT, Seastedt TR, Buckner D (2007) Plant community response to the decline of diffuse knapweed in a Colorado grassland. Ecological Restoration, 25 , 169-
174.
* Callaway RM, Kim J, Mahall BE (2006) Defoliation of Centaurea solstitialis stimulates compensatory growth and intensifies negative effects on neighbors.
Biological Invasions, 8 , 1389-1397.
* Carino DA, Daehler CC (2002) Can inconspicuous legumes facilitate alien grass

invasions? Partridge peas and fountain grass in Hawai'i. Ecography, 25 , 33-41.
Cox RD, Allen EB (2008) Stability of exotic annual grasses following restoration efforts in southern California coastal sage scrub. Journal of Applied Ecology, 45 ,
495-504.
Cox RD, Allen EB (2011) The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland. Plant Ecology,
212 , 1699-1707.
Čuda JH, Skálová H, Janovský Z, Pyšek P (2014) Habitat requirements, short- term population dynamics and coexistence of native and invasive Impatiens species: a field study. Biological Invasions, 16 , 177-190.
Cushman JH, Gaffney KA (2010) Community-level consequences of invasion: impacts of exotic clonal plants on riparian vegetation. Biological Invasions, 12 ,
2765-2776
* Cushman JH, Lortie CJ, Christian CE (2011) Native herbivores and plant facilitation mediate the performance and distribution of an invasive exotic grass.
Journal of Ecology, 99 , 524-531.
* D'Antonio CM, Hughes RF, Vitousek PF (2001) Factors influencing dynamics of two invasive C4 grasses in seasonally dry Hawaiian woodlands. Ecology, 82 , 89-104.
Davies KW, Nafus AM, Sheley RF (2010) Non-native competitive perennial grass impedes the spread of an invasive annual grass. Biological Invasions, 12 , 3187-
3194.
Dorning M, Cipollini D (2006) Leaf and root extracts of the invasive shrub,
Lonicera maackii , inhibit seed germination of three herbs with no autotoxic effects. Plant Ecology, 184 , 287-296.
* Fortner AM, Weltzin JF (2007) Competitive hierarchy for four common old-field plant
* Garcia-Serrano H, Sans
FX, Escarré J (2007) Interspecific competition between alien and native congeneric species. Acta Oecologica, 31 , 69-78. species depends on resource identity and availability. Journal of the Torrey
Botanical Society, 134 , 166-176.
Giantomasi A, Tecco PA, Funes G, Gurvich DE, Cabido M (2008) Canopy effects of the invasive shrub Pyracantha angustifolia on seed bank composition, richness and density in a montane shrubland (Córdoba, Argentina). Austral Ecology, 33 , 68-
77.
Graebner RC, Callaway RM, Montesinos D (2012) Invasive species grows faster,

competes better, and shows greater evolution toward increased seed size and growth than exotic non-invasive congeners. Plant Ecology, 213 , 545-553.
Hacker SD, Zarnetske P, Seabloom E, Ruggiero P, Mull J, Gerrity S, Jones C (2012)
Subtle differences in two non-native congeneric beach grasses significantly affect their colonization, spread, and impact. Oikos, 121 , 138-148.
Hérault B, Bornet A, Trémolières M (2008) Redundancy and niche differentiation among the European invasive Elodea species. Biological Invasions, 10 , 1099-
1107.
Hughes RF, Denslow JS (2005) Invasion by a N
2
-fixing tree alters function and structure in wet lowland forests of Hawaii. Ecological Applications, 15 , 1615-
1628.
Hulme PE, Bremner ET (2006) Assessing the impact of Impatiens glandulifera on riparian habitats: partitioning diversity components following species removal.
Journal of Applied Ecology, 43 , 43-50.
Jäger H, Kowarik I, Tye A (2009) Destruction without extinction: long-term impacts of an invasive tree species on G alápagos highland vegetation. Journal of Ecology,
97 , 1252-1263.
Jordan NR, Larson DL, Huerd SC (2008) Soil modification by invasive plants: effects on native and invasive species of mixed-grass prairies. Biological
Invasions, 10 , 177-190.
Kueffer C (2010) Reduced risk for positive soil-feedback on seedling regeneration by invasive trees on a very nutrient-poor soil in Seychelles. Biological Invasions, 12 ,
97-102.
Kulmatiski A (2006) Exotic plants establish persistent communities. Plant Ecology, 187 ,
261-275.
Larson DL, Larson JL (2010) Control of one invasive plant species allows exotic grasses to become dominant in northern Great Plains grasslands. Biological
Conservation, 143 , 1901-1910.
Leicht-Young SA, Pavlovic NB, Adams JV (2012) Competitive interactions of garlic mustard ( Alliaria petiolata ) and damesrocket ( Hesperis matronalis ).
Invasive Plant Science and Management, 5 , 27-36.
* Magnoli SM, Kleinhesselink AR, Cushman J (2013) Responses to invasion and invader removal differ between native and exotic plant groups in a coastal dune.
Oecologia, 173 , 1521-1530.

Mangla S, Sheley RL, James JJ, Radosevich SR (2011a) Role of competition in restoring resource poor arid systems dominated by invasive grasses. Journal of Arid Environments, 75 , 487-493.
* Mangla S, Sheley RL, James JJ, Radosevich SR (2011b) Intra and interspecific competition among invasive and native species during early stages of plant growth. Plant Ecology, 212 , 531-542.
* Metlen KL, Aschehoug ET, Callaway RM (2013) Competitive outcomes between two exotic invaders are modified by direct and indirect effects of a native conifer.
Oikos, 122 , 632-640.
* Molina-Montenegro MA, Badano EI, Cavieres LA (2008) Positive interactions among plant species for pollinator service: assessing the 'magnet species' concept with invasive species. Oikos, 117 , 1833-1839.
Mony C, Koschnick TJ, Haller WT, Muller S (2007) Competition between two invasive Hydrocharitaceae ( Hydrilla verticillata (L.f.) (Royle) and Egeria densa
(Planch)) as influenced by sediment fertility and season. Aquatic Botany, 86 ,
236-242.
Ogden JAE, Rejmánek M (2005) Recovery of native plant communities after the control of a dominant invasive plant species, Foeniculum vulgare : Implications for management. Biological Conservation, 125 , 427-439.
Ortega YK, Pearson DE (2010) Effects of Picloram application on community dominants vary with initial levels of spotted knapweed ( Centaurea stoebe ) invasion. Invasive Plant Science and Management, 3 , 70-80.
* Pfeifer-Meister L, Cole EM, Roy BA, Bridgham SD (2008) Abiotic constraints on the competitive ability of exotic and native grasses in a Pacific Northwest prairie.
Oecologia, 155 , 357-366.
*
Puliafico KP, Schwarzländer M, Price WJ, Harmon BL, Hinz HL (2011)
Native and exotic grass competition with invasive Hoary Cress ( Cardaria draba ).
Invasive Plant Science and Management, 4 , 38-49.
Radford IJ, Dickinson KJM, Lord JM (2010) Does disturbance, competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand?
Mechanisms of establishment and persistence, and functional differentiation among invasive and native species. Austral Ecology, 35 , 282-293.
* Rauschert ESJ, Shea K (2012) Invasional interference due to similar inter- and intraspecific competition between invaders may affect management. Ecological
Applications, 22 , 1413-1420.

Rice KJ, Nagy ES (2000) Oak canopy effects on the distribution patterns of two annual grasses: The role of competition and soil nutrients. American Journal of Botany,
87 , 1699-1706.
Shaben J, Myers JH (2010) Relationships between Scotch broom ( Cytisus scoparius ), soil nutrients, and plant diversity in the Garry oak savannah ecosystem. Plant
Ecology, 207 , 81-91.
Sheley RL, Larson LL (1994) Comparative growth and interference between cheatgrass and yellow starthistle seedlings. Journal of Range Management, 47 , 470-474.
Sheley RL, Larson LL (1995) Interference between cheatgrass and yellow starthistle at
3 soil depths. Journal of Range Management, 48 , 392-397.
Skaer MJ, Graydon DJ, Cushman JH (2013) Community-level consequences of cattle grazing for an invaded grassland: variable responses of native and exotic vegetation. Journal of Vegetation Science,
* Skálová H, Jarošik V, Dvořáčková S, Pyšek P (2013) Effect of intra- and interspecific competition on the performance of native and invasive species of Impatiens under varying levels of shade and moisture. PLoS ONE, 8, e62842. doi10.137/journal.pone.0062842.
Stephens AEA, Krannitz PG, Myers JH (2009) Plant community changes after the reduction of an invasive rangeland weed, diffuse knapweed, Centaurea diffusa .
Biological Control, 51 , 140-146.
* Stevens JM, Fehmi JS (2009) Competitive effect of two nonnative grasses on a native grass in southern Arizona. Invasive Plant Science and Management, 2 ,
379-385.
Story JM, Callan NW, Corn JG, White LJ (2006) Decline of spotted knapweed density at two sites in western Montana with large populations of the introduced root weevil, Cyphocleonus achates (Fahraeus). Biological Control, 38 , 227-232.
* Takakura KI, Fujii S (2010) Reproductive interference and salinity tolerance differentiate habitat use between two alien cockleburs: Xanthium occidentale and
X. italicum (Compositae). Plant Ecology, 206 , 309-319.
* Tecco PA, Díaz S, Gurvich DE, Perez-Harguindeguy N, Cabido M, Bertone GA (2007)
Facilitation and interference underlying the association between the woody invaders Pyracantha angustifolia and Ligustrum lucidum . Applied Vegetation
Science, 10 , 211-218.
Thomsen MA, Corbin JD, D'Antonio CM (2006) The effect of soil nitrogen on

competition between native and exotic perennial grasses from northern coastal
California. Plant Ecology, 186 , 23-35.
Truscott AM, Palmer SC, Soulsby C, Westaway S, Hulme PE (2008) Consequences of invasion by the alien plant Mimulus guttatus on the species composition and soil properties of riparian plant communities in Scotland. Perspectives in Plant
Ecology, Evolution and Systematics, 10 , 231-240.
Van Riper LC, Larson D (2009) Role of invasive Melilotus officinalis in two native plant communities. Plant Ecology, 200 , 129-139.
Von Holle B, Joseph KA, Largay EF, Lohnes RG (2006) Facilitations between the introduced nitrogen-fixing tree, Robinia pseudoacacia , and nonnative plant species in the glacial outwash upland ecosystem of cape cod, MA. Biodiversity and Conservation, 15 , 2197-2215.
* Wundrow EJ, Carrillo J, Gabler CA, Horn KC, Siemann E (2012) Facilitation and competition among invasive plants: a field experiment with Alligatorweed and
Water Hyacinth. PLoS ONE, 7, e48444. doi10.137/journal.pone.0048444.
* Yang SA, Ferrari MJ, Shea K (2011) Pollinator behavior mediates negative interactions between two congeneric invasive plant species. The American
Naturalist, 177 , 110-118.
Zarnetske PL, Gouhier TC, Hacker SD, Seabloom EW, Bokil VA (2013) Indirect effects and facilitation among native and non-native species promote invasion success along a stress gradient. Journal of Applied Ecology, 101 , 905-915.
* Zhi YB, Li HL, An SQ, Zhao L, Zhou CF, Deng ZF (2007) Inter-specific competition:
Spartina alterniflora is replacing Spartina anglica in coastal China. Estuarine
Coastal and Shelf Science, 74 , 437-448.

Supporting Information Table 1: Results from a meta-analysis of 22 published manuscripts entailing 124 observations of non-native plant interactions. We report the me an effect size and 95% confidence intervals (Hedge’s d +) and bold values when the 95% CI does not overlap zero. Mean effect sizes were calculated for the entire data set and subsets of the data that compared intra- and interspecific competition (i.e., Relative Competitive Effect, RCE) or that compared intraspecific competition with growth of a single individual plant (i.e., Relative Neighbor Effect, RNE).
N
CATEGORY
Overall
DURATION
annual (focal)
perennial (focal)
annual (neighbor)
perennial (neighbor)
perennial (focal & neighbor)
annual (focal & neighbor) 31
perennial (focal) & annual (neighbor) 17
annual (focal) 7 perennial (neighbor) 22
124
53
71
48
76
54
EXPERIMENT TYPE
greenhouse
field
FUNCTIONAL GROUP
vine (focal)
grass (focal)
herb (focal)
shrub (focal)
vine (neighbor)
grass (neighbor)
herb (neighbor)
shrub (neighbor)
88
36
2
2
60
54
8
2
57
63
All Data direction
+
-
-
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Hedge’s D+
-
-
0.37 ± 0.09
N
- 0.97± 0.26 4
- 0.44± 0.23 9
- 1.48 ± 1.11
2
70
- 0.33± 0.14 31
- 0.39± 0.11 39
- 0.47± 0.14 26
- 0.30 ± 0.11 44
- 0.26 ± 0.12 35
- 0.26 ± 0.17 22
- 0.45 ± 0.10 45
0.11 ± 0.18 25
- 0.31 ± 0.13
29
- 0.43 ± 0.12
39
0.62 ± 0.64 0
- 0.66 ± 1.01 2
- 0.46 ± 0.13
30
- 0.35 ± 0.12
34
0.35 ± 0.39 4 direction
-
-
-
-
+
+
-
-
+
+
-
-
-
+
RCE Data
NA
-
+
-
+
Hedge’s D+
- 0.01 ± 0.12 54
- 0.01± 0.19 22
- 0.01± 0.16 32
- 0.13 ± 0.19 22
0.07± 0.16 32
0.04± 0.17 19
- 0.06± 0.20 9
- 1.29 ± 0.80 13
0.34 ± 0.57 13
0.01 ± 0.15 43
- 0.07 ± 0.23 11
- 1.48 ± 1.11
0
- 0.003 ± 0.20 28
0.01 ± 0.16 24
-
NA
0.66 ± 1.01
6.63 ± 1.59
2
0
0.14 ± 0.20 30
- 0.17 ± 0.16 20
4
-
-
NA
-
-
+
NA
-
-
+
-
-
-
-
-
-
-
-
-
RNE Data
N direction Hedge’s D+
- 0.71 ± 0.12
- 0.66 ± 0.20
- 0.74± 0.15
- 0.85± 0.21
- 0.63 ± 0.15
- 0.65 ± 0.19
- 0.76± 0.31
- 0.93 ± 0.27
- 0.59± 0.25
- 0.85 ± 0.14
- 0.17 ± 0.26
NA
- 0.54 ± 0.17
- 1.02 ± 0.18
0.62 ± 0.64
NA
- 0.91 ± 0.17
- 0.60 ± 0.19
0.05 ± 0.40

HABITAT TYPE aquatic desert disturbed dune temperate forest tropical forest grassland old field riparian shrubland wetland
LIFE STAGE adult seedling
NATIVE RANGE OVERLAP overlapping ranges non-overlapping ranges
NITROGEN FIXATION nitrogen fixer (focal) nitrogen fixer (neighbor)
MANIPULATION ambient high water high nitrogen shade
RESPONSE
Shoot mass
Total mass
Height
Number of stems
Reproduction (# seeds, etc)
N
14
45
20
16
10
2
4
4
2
3
4
49
74
85
39
5
8
75
4
20
6
37
50
13
8
3
-
-
-
-
-
-
-
-
All Data direction
Hedge’s D+
-
-
-
-
+
-
+
-
-
-
-
0.02 ± 0.50
-
0.32 ± 0.45
-
0.72 ± 0.69
-
0.18 ± 0.45
- 1.03 ± 0.74
-
0.15 ± 0.39
- 0.72 ± 0.16
-
0.35 ± 0.16
-
0.11 ± 0.23
0.23 ± 0.30
-
0.73 ± 1.00
-
-
- 0.31 ± 0.16
- 0.41 ± 0.10
-
0.44 ± 0.10
-
0.13 ± 0.18
- 0.56 ± 0.30
- 0.004 ± 0.31
-
0.47 ± 0.10
-
0.09 ± 0.54
-
0.05 ± 0.25
-
0.21 ± 0.41
-
-
+
-
-
- 0.35 ± 0.14
-
0.58 ± 0.15
0.20± 0.27
-
0.24 ± 0.35
-
0.07 ± 0.97
N
0
4
11
13
4
2
2
12
16
4
2
42
28
36
34
3
6
41
4
8
6
21
17
13
5
3
-
-
-
+
-
-
+
-
RCE Data direction
Hedge’s D+
NA
-
+
-
+
+
-
+
-
+
-
0.02 ± 0.50
0.17 ± 0.61
-
0.72 ± 0.69
NA
- 1.03 ± 0.74
0.09 ± 0.43
- 0.05 ± 0.44
0.12 ± 0.22
-
0.11 ± 0.23
0.54 ± 0.51
-
0.73 ± 1.00
-
+
- 0.21 ± 0.17
0.20 ± 0.18
-
0.02 ± 0.16
-
0.06 ± 0.19
- 0.55 ± 0.41
0.48 ± 0.43
-
0.13 ± 0.15
-
0.09± 0.54
1.16 ± 0.53
-
0.21± 0.41
+
-
-
-
-
0.07 ± 0.19
- 0.01 ± 0.31
- 0.20± 0.27
-
0.01± 0.40
-
0.07 ± 0.97
N
49
5
2
2
0
2
0
3
0
3
32
8
0
6
0
7
46
34
0
12
0
16
33
0
3
0
-
-
-
-
-
NA
-
NA
RNE Data direction
Hedge’s D+
NA
-
NA
-
NA
-
-
-
NA
+
NA
NA
-
0.87 ± 0.65
NA
-
0.18 ± 0.45
NA
- 1.35 ± 0.96
- 0.83 ± 0.17
-
0.88 ± 0.23
NA
0.07 ± 0.38
NA
-
-
- 0.84 ± 0.39
- 0.73 ± 0.13
-
0.72 ± 0.13
-
0.52 ± 0.46
- 0.58 ± 0.45
- 0.55 ± 0.45
-
0.81 ± 0.15
NA
-
0.39 ± 0.28
NA
-
-
NA
-
NA
- 0.76 ± 0.19
-
0.9 ± 0.16
NA
-
0.93 ± 0.69
NA

Supporting Information Figure 1: Funnel plot of effect sizes (Hedges’ d ) of the raw data against the average sample size of the competition treatment.
15
10
5
-10
-15
0
-5
4 5 6 17 20 8 10 11