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Invasiveness and impact of 48 exotic plant species in native grasslands

Date: July 17, 2015


Location of sites in western Montana sampled to determine the invasiveness and impact of 48 exotic plants in the bluebunch wheatgrass habitat type.
Location of sites in western Montana sampled to determine the invasiveness and impact of 48 exotic plants in the bluebunch wheatgrass habitat type.
We quantified and ranked invasiveness and impact of 48 exotic plants based on surveys in the bluebunch wheatgrass habitat type in western Montana. Our 31, 1-hectare sites spanned 20,000 km2 of grasslands. Our results provide valuable data for managers when prioritizing exotic plants for control in the bluebunch wheatgrass habitat type. We calculated invasiveness scores as the product of a species' range (number of plots occupied) and local abundance (mean percent cover per plot). Impacts were measured as reductions in native plant cover associated with increasing abundance of each invader while controlling for the abundance of other exotic plants within the grassland. 

An average of 13 exotic species inhabited each grassland, and the average total exotic cover/grassland was 25 percent. Twenty-five percent of the total identified flora was exotic plants. Of the 48 invaders, 11 showed significant impacts on native plants, five of which are classified as noxious weeds by the state of Montana (spotted knapweed, St. John’s wort, Dalmatian toadflax, Canada thistle, and oxeye daisy). The highest impact invader was cheatgrass, which is currently listed as a regulated plant but not a noxious weed in Montana. 

These data are specific to the bluebunch wheatgrass habitat type; the same invaders might have greater or lesser impacts in other community types. 

Publication:

Pearson, D.E., Y.K. Ortega, O. Eren, J.L. Hierro.  In press.   Quantifying “apparent” impact and distinguishing impact from invasiveness in multispecies plant invasions. Ecological Applications.

 

Table (below). Components of exotic species invasiveness and impact based on surveys of n=620 1-m2 plots in 31 grasslands across west-central Montana, USA. Invasiveness scores, calculated for 48 exotic species detected, are the product of R=range (number of plots occupied), and A=local abundance (mean percent cover per plot). Impact scores, calculated for those 25 species with sufficient data for analysis, incorporated E=per unit effects of each invader on native species abundance, as estimated by the slope term in a mixed model regressing native cover against focal invader cover while controlling for cover of other invaders (see text for details). Species are categorized according to life form and growth form, with A=annual, B=biennial or P=perennial, and F=forb or G=grass; * = listed as a noxious weed by the state of Montana. 

Slope estimate, E, not significantly different from 0 at P≤0.05.

Includes V. arvensis, a less common ecological equivalent that is difficult to distinguish.

§ Includes A. desertorum, a less common ecological equivalent that is difficult to distinguish.

 

Exotic species

 

Type

Invasiveness

 

Impact

R

A

Score

Rank

 

E

Score

Rank

Cheatgrass

(Bromus tectorum)

AG

461

14.9

6860.5

1

 

-0.39

2676

1

Spotted knapweed*

(Centaurea stoebe)

PF

233

6.7

1555.0

2

 

-0.72

1120

2

Leafy spurge*

(Euphorbia esula)

PF

87

12.8

1111.0

3

 

-0.53

589

3

Sulphur cinquefoil*

(Potentilla recta)   

PF

148

6.3

936.5

4

 

-0.50

468

4

Bulbous bluegrass

(Poa bulbosa)

PF

74

7.7

567.5

5

 

-0.06

0

-

Spring speedwell

Veronica verna

AF

180

3.1

564.0

6

 

-0.57

322

5

St. John’s wort*

(Hypericum perforatum)  

PF

47

7.8

367.0

7

 

-0.44

162

10

Kentucky bluegrass

(Poa pratensis)

PG

38

9.4

357.5

8

 

-0.64

229

7

Dalmatian toadflax*

(Linaria dalmatica)

PF

84

3.8

315.5

9

 

-0.75

237

6

Moth mullein

(Verbascum blattaria)    

BF

40

6.7

268.0

10

 

-0.59

158

11

Thyme-leaved sandwort

(Arenaria serpyllifolia)

AF

114

2.0

232.0

11

 

-0.16

0

-

Canada bluegrass

(Poa compressa)

PG

48

4.8

229.0

12

 

-0.79

181

9

Salsify

(Tragopogon dubius)

BF

166

1.4

227.5

13

 

-0.44

0

-

Dense silkybent

(Agrostis interrupta)

AG

100

2.1

212.0

14

 

0.14

0

-

Japanese brome

(Bromus japonicas)

AG

88

2.3

204.0

15

 

-0.06

0

-

Pale alyssum

(Alyssum alyssoides)§

AF

114

1.7

188.0

16

 

-1.17

220

8

Redstem stork’s bill

(Erodium cicutarium)

AF

68

2.7

185.5

17

 

-0.44

0

-

Tall tumblemustard

(Sisymbrium altissimum)

ABF

69

1.9

132.0

18

 

-0.74

0

-

Common dandelion

(Taraxacum officinale)

PF

81

2.3

108.0

19

 

-0.91

0

-

Strict forget-me-not

(Myosotis micrantha)

AF

151

0.7

99.5

20

 

-2.20

0

-

Jagged chickweed

(Holosteum umbellatum)

AF

158

0.6

96.5

21

 

0.69

0

-

Prickly lettuce

(Lactuca serriola)

ABF

77

1.0

77.5

22

 

-1.09

0

-

Common mullein

(Verbascum Thapsus)

BF

19

4.0

76.0

23

 

-

-

-

Crested wheatgrass

(Agropyron cristatum)

PG

7

9.1

63.5

24

 

-

-

-

Field cudweed

(Filago arvensis)

AF

72

0.5

38.5

25

 

-1.01

0

-

Tall hedge mustard

(Sisymbrium loeselii)

ABF

34

1.0

35.5

26

 

-1.69

0

-

Black medick

(Medicago lupulina)

AF

5

6.8

34.0

27

 

-

-

-

Littlepod false flax

(Camelina microcarpa)

AF

42

0.5

22.5

28

 

-7.12

0

-

Hairy vetch

(Vicia villosa)

ABF

7

3.2

22.5

28

 

-

-

-

Smooth brome

(Bromus inermis)

PG

1

20.0

20.0

29

 

-

-

-

Common sheep sorrel

(Rumex acetosella)

PF

8

2.1

17.0

30

 

-

-

-

Canada thistle*

(Cirsium arvense)

PF

3

5.5

16.5

31

 

-

-

-

Mouseear cress

(Arabidopsis thaliana)

AF

16

0.6

9.5

32

 

-

-

-

Intermediate wheatgrass

(Agropyron intermedium)

PG

3

3.0

9.0

33

 

-

-

-

Clasping pepperweed

(Lepidium perfoliatum)

AF

6

1.4

8.5

34

 

-

-

-

Nodding thistle

(Carduus nutans)

BF

4

1.9

7.5

35

 

-

-

-

Corn gromwell

(Lithospermum arvense)

AF

8

0.6

5.0

36

 

-

-

-

Piedmont bedstraw

(Galium pedemontanum)

AF

3

1.2

3.5

37

 

-

-

-

Soft brome

(Bromus hordeaceus)

AG

1

3.0

3.0

38

 

-

-

-

Timothy

(Phleum pretense)

PG

4

0.8

3.0

38

 

-

-

-

Field pennycress

(Thlaspi arvense)

AF

4

0.6

2.5

39

 

-

-

-

Poison hemlock

(Conium maculatum)

BF

2

1.0

2.0

40

 

-

-

-

Flixweed

(Descurainia sophia)

ABF

4

0.5

2.0

40

 

-

-

-

Common mouseear chickweed

(Cerastium fontanum)

PF

3

0.5

1.5

41

 

-

-

-

Sweetclover

(Melilotus sp.)

ABF

2

0.8

1.5

41

 

-

-

-

Small geranium

(Geranium pusillum)

AF

2

0.5

1.0

42

 

-

-

-

Oxeye daisy*

(Leucanthemum vulgare)

PF

1

1.0

1.0

42

 

-

-

-

Deptford pink

(Dianthus armeria)

ABF

1

0.5

0.5

43

 

-

-

-

 

 

 

 

 

 



Principal Investigators: 
Forest Service Partners: 
Bitterroot National Forest
External Partners: 
Bureau of Land Management
Confederated Salish and Kootenai Tribes
Lee Metcalf National Wildlife Refuge
Missoula Open Space
Montana Department of Natural Resources and Conservation
Montana Fish, Wildlife and Parks
MPG Ranch
National Bison Range
Plum Creek
Rimel Ranch
Research Location: 
Montana