89

Invasive Alien Plant Species in Riparian Areas of Japan:
The Contribution of Agricultural Weeds, Revegetation Species
and Aquacultural Species
Shigenari MIYAWAKI1 and Izumi WASHITANI2
1

Civil Engineering and Eco-Technology Consultants Co. Ltd.
Higashi-ikebukuro, Toshima-ku, Tokyo 170-0013, Japane-mail: miyawaki@kensetsukankyo.co.jp
2
Institute of Agricultural and Life Sciences The University of Tokyo
Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

Abstract
In Japanese riparian habitats, invasion by alien plants constitutes one of the most serious threats tobiodiversity through the displacement of native plants. This paper presents a list of “invasive alienplant species” in Japanese riparian habitats. Invasive alien species, defined here as dominant orcodominant species in certain plant communities, can be categorized as “agricultural weeds,” “revegetation species,” or “aquacultural species.” In riparian areas, the category of agricultural weeds contains
significantly more invasive alien species than would be expected by chance. The same result was
found for introduced revegetation species. To prevent the introduction of new invaders into riparian
areas, appropriate alien plant management is needed for both natural habitats and agricultural areas of
the catchments. The use of alien pasture species for revegetation or control of soil erosion should be
avoided in order to conserve biodiversity and native ecosystems.
Key words: biological invasion, geographical origin, plant families, propagule pressure,statistical trends

1. Introduction
Invasion by alien plants constitutes one of the mostserious threats to biodiversity through the displacement of native plants (Coblentz, 1990; Vitousek et al.,
1996; Simberloff et al., 1997). Owing to frequentdisturbances, riparian areas are particularly vulnerable
to invasion by alien vascular plants (Stohlgren et al.,
1998; Hood & Naiman, 2000; Planty-Tabacchi et al.,
2001; Washitani, 2001). In Japanese riparian habitats,the total area occupied by vegetation in which alienplants were dominant had reached 11,000 ha by 1999,
which amounted to 15% of the total vegetation area ofrivers administered by the Japanese Ministry of Land
Infrastructure and Transport (Miyawaki & Washitani,2004).
Riparian ecosystems are important for maintaining
local biodiversity because of their environmental
uniqueness. Riparian vegetation is established under
the influences of unique environmental conditions
such as flooding disturbance regimes. The vegetation thus established in turn influences the dynamics
of water flow and the movement of sediments andnutrients into rivers (e.g., Peterjohn & Correll, 1984;
Global Environmental Research8(1)/2004: 89-101
printed in Japan

Décamps, 1993; Nakatsubo, 1997), and it featureshigh species diversity (Gould & Walker, 1997;
Naiman et al., 1993; Ward et al., 1999).
One explanation for the high species diversity ofriparian ecosystems is that floods periodically destroy
vegetation cover, create bare ground for recolonization,and produce a shifting mosaic of vegetation patterns
and landforms that create diverse habitats (Malanson,
1993; Hood & Naiman, 2000). In addition, riparian
areas serve as landscape corridors facilitating thedispersal of propagules by water flow, which isindispensable to range expansion and maintenance of
metapopulation dynamics of riparian plants (e.g.,
Staniforth & Cavers, 1976; Schneider & Sharitz,
1988; Campbell et al., 2002).
The same factors that maintain a diversity of plant
species may equally increase vulnerability to invasion
by alien plants in riparian areas (Washitani, 2000).
Disturbances to create unoccupied sites and dispersalof propagules by water flow in riparian areas may
promote invasions by alien plant species for some ofthe same reasons that they sustain native species
diversity.
In Japanese riparian habitats, approximately 40%©2004 AIRIES90

S. MIYAWAKI and I. WASHITANIof invasive alien species belong to the largest plantfamilies: Compositae, Gramineae, and Leguminosae(Washitani, 2002b). The Compositae contain a highnumber of widely distributed agricultural weeds(Heywood, 1989; Cronk & Fuller, 1995), and theGramineae and the Leguminosae contain all the
pasture species intentionally introduced into Japaneseriparian areas for revegetation (Washitani, 2002a).
Most weeds of agricultural systems have competitive ruderal adaptations that are suitable for taking
over disturbed habitats (Grime, 1977). Consequently,
environments favoring these species include sparse
meadows that are subject to seasonal disturbancessuch as floods, as well as roadsides and agricultural
fields (Washitani & Morimoto, 1993). These characteristics of agricultural weeds make them potential
invaders of riparian habitats.
Pasture grass species are a major source of plant
invaders in many parts of the world (Heywood, 1989;
Lonsdale, 1994). In Japanese riparian areas, there isa long tradition of introducing alien pasture grasses toprevent soil erosion or provide revegetation after
construction work (Ecological Society of Japan, 2002;
Imamoto et al., 2003). It is not surprising, therefore,that these plants, which were chosen because of theirrapid growth, adaptation to sunlit environments, andtenacity, have also shown markedly high invasiveness(Washitani, 2002a; Myers & Bazely, 2003).
Although the terrestrial habitats of riparian areas
are diverse, the aquatic environment is relatively
uniform (Ashton & Mitchell, 1989). Accordingly,
many species, particularly submerged and floating
plants, are widely distributed throughout the world.
This holds for alien aquatic plant species such as
water hyacinth (Eichhornia crassipes (Mart.)) SolmsLaub.) and water lettuce (Pistia stratiotes L.)
introduced into Japanese riparian areas as ornamentals(Committee for Investigating the Effects of andCountermeasures against Riparian Exotic Species,
2003).
In response to the expanding ranges of and increasing damage caused by alien species, the control
of invasive alien species has become a priority forenvironmental management and an integral component of many habitat conservation efforts in Japaneseriparian areas (Committee for Investigating the Effects
of and Countermeasures against Riparian Exotic
Species, 2003). At least 444 alien species had been
added to the Japanese riparian flora by 1999
(Committee for Investigating the Effects of andCountermeasures against Riparian Exotic Species,
2001). However, alien species do not always become invasive. To conserve native species andecosystems appropriately, we must judge which alien
species should be given the highest priority as“invasive alien species” in management programs.
This paper presents a list of invasive alien plant
species in Japanese riparian habitats for use in such a
judgment process. Using the list, we tested the

hypothesis that agricultural weeds and revegetation
species contribute highly to the number of invasive
alien plants. We also discuss desirable management
strategies for suppressing the invasion of alien plant
species in Japanese riparian habitats.

2. Terminology
We define “alien species” as those introduced after
the mid-19th century, when Japan abandoned its
national isolation policy (Washitani & Morimoto,
1993; Murakami & Washitani, 2002). Only about 20
alien species were identified up to the beginning of the
Meiji era (1868), since international trade wasrestricted to only selected routes for many years
during the Edo period by the feudal government(Enomoto, 1999). The Ecological Society of Japan(2002) has listed more than 1,500 species of alienplants recorded in Japan. It is not clear, however,
how many of these species persist in Japan.
The categories “invasive alien species,” “agricultural weeds,” “revegetation species,” and “aquacultural species” often overlap with each other, andconsequently a species can belong to more than one
category. An “invasive alien species” is an alien
species whose introduction or spread threatens
biological diversity (Ecological Society of Japan,
2002; UNEP, 2002). We define an “invasive alienplant” as a dominant or codominant species present ina certain vegetated riparian area. It can safely be
assumed that these species are invasive since theyoften competitively exclude native plants (e.g.,
Miyawaki & Washitani, 1996; Muranaka & Washitani,2001). Therefore, alien plants that do not form thedominant vegetation should not be considered as“invasive.” We also define the species that form thedominant vegetation along at least 12 of 123 investigated rivers as “widely invasive.”“Agricultural weeds” are undesirable plants that
invade highly disturbed agricultural land, where they
reduce the productive capacity of crops (Pimentel,1986). Some agricultural weeds are native to Japan,
but we consider only alien species, in this study.
We define “revegetation species” as species actively introduced to prevent soil erosion or to promote
revegetation after various types of construction work.
In Japan, many pasture grass species and several
woody leguminous species have been introduced forthis purpose (Washitani, 2002a).
“Aquacultural species” are defined as floating or
submerged plants introduced as ornamentals. This
group contains plants used for water purification such
as E. crassipes and P. stratiotes (Fujita et al., 2001).

3. Data Sets and Analyses
We assembled separate data sets on alien agricultural weeds, revegetation species, aquacultural species,
alien species, and invasive alien species in JapaneseInvasive Alien Plant Species in Riparian Areas of Japan

riparian areas. The data on the last two categories
came from the Committee for Investigating the Effects
of and Countermeasures against Riparian Exotic
Species (2001), which listed 444 alien species identified during National Censuses on River Environments(NCRE) that have been conducted by the Ministry ofLand, Infrastructure and Transport (MLIT) of Japan
since 1991 (Ministry of Construction, River Bureau,
River Environment Division, 1996, 1997a, 1997b,
1998, 1999, 2000; MLIT, River Bureau, River Environment Division, 2001). The NCRE is a periodic
survey of the fauna and flora of the 123 rivers (109river systems) within the jurisdiction of the MLIT
(Ikeuchi & Kanao, 2003). Life forms and origins of
the alien species were given by the Ecological Societyof Japan (2002) and Shimizu (2003).
We identified 87 invasive alien riparian species as
dominant or codominant on the NCRE vegetation
maps. We counted the number of rivers and total
vegetation area dominated or codominated by eachspecies. Twenty-two of these species were widelyinvasive (i.e., forming the dominant or codominant
vegetation on at least 12 rivers).
Data on agricultural weeds came from the Livestock Technology Association (1994), which listed
172 major alien weed species in Japan. Of these species, 140 were listed in the alien flora of Japaneseriparian habitats and were used in this analysis.
Data on revegetation species were obtained fromYano (1988), Enomoto (1999), and the EcologicalSociety of Japan (2002). Forty revegetation species
were listed among the alien flora of Japanese riparian
habitats and were used in this analysis.
Data on aquacultural species came from Kadono(1994). Nine aquacultural species were listed among
the alien flora of Japanese riparian habitats and wereused in this analysis.

Agricultural
weeds
Revegetation
species

4. Results
4.1 Number of invasive alien species
We use Venn diagrams to show the overlap
between sets of species by category in the flora ofJapanese riparian habitats (Fig. 1(a)). Sixty-fourpercent of the riparian invasive alien species (56 of
87) are agricultural weeds (hereafter referred to as“invasive agricultural weeds”) or revegetation species(“invasive revegetation species”). Only 5% of theinvasive alien species (4 of 87) are aquacultural
species (“invasive aquacultural weeds”). Figure 1(b)Agricultural
weeds

Revegetation
species

7

24

17

9
12

5
Aquacultural
species4827Invasive
alien species

239

1

Aquacultural
species

118

4

5

35

Alien species in Japanese riparian habitats(a)Fig. 1

We used a Monte Carlo simulation procedure described in Daehler (1998) to test for differences between the observed and expected numbers of invasive
alien species, widely invasive alien species, and invasive agricultural weeds by life form, origin, and family.
In testing invasive alien species of each life form, forexample, each iteration of the simulation involved
choosing 87 invasive alien species (the number ofdominant or codominant species) at random, and without replacement, from all 444 alien species and summing the number of these randomly chosen species.
We repeated the simulation 50,000 times for each test
and used the average number of species as the expected number of invasive species. We also used
these simulations to generate test statistics for eachfeature to judge two-tailed statistical significance.
To produce test statistics for the right (or left) tail, we
divided the number of simulations that produced avalue as high (or as low) as, or higher (or lower) than,the observed value by 50,000 (i.e., the total number ofsimulations). These test statistics were comparedwith a sequentially derived Bonferroni critical value tojudge statistical significance at the α = 0.05 level(Holm, 1979).

94

2

91

11
1
Widelyinvasive
alien species

265

Alien species in Japanese riparian habitats(b)

Venn diagrams showing the overlap of the sets of (a) invasive alien species, agricultural weeds, revegetation
species, and aquaculture species, and (b) widely invasive alien species, agricultural weeds, revegetation
species, and aquaculture species in flora of Japanese riparian habitats. The numbers in each sector are thespecies counts. The total number of species is 444.

92

S. MIYAWAKI and I. WASHITANI

illustrates the overlap between sets of widely invasive
species by category. Ninety-one percent of thewidely invasive species (20 of 22) are agriculturalweeds or revegetation species. Only 4.5% of thewidely invasive alien species (1 of 22) areaquacultural species.
Thirty-one percent of agricultural weeds (44 of
140) and 52.5% of revegetation species (21 of 40) are
riparian invasive alien species. The set of agricultural weeds contains significantly more invasive
alien species than expected by chance (Fisher’s exact
test, P < 0.001; Table 1). The same result was foundfor revegetation species (Fisher’s exact test, P <0.001; Table 2). Although 44% of aquacultural
Table 1 Numbers of invasive alien species versus agricultural weeds in Japanese riparian habitats.
Agricultural weeds
Invasive alien species

Yes
No

Yes

No

44 (9.9%)
96 (21.6%)

43 (9.7%)
261 (58.8%)

Fisher’s exact test, P < 0.0001
Table 2 Numbers of invasive alien species versus revegetation species in Japanese riparian habitats.
Revegetation species
Invasive alien species

Yes
No

Yes

No

21 (4.7%)
19 (4.3%)

66 (14.9%)338 (76.1%)

Fisher’s exact test, P < 0.0001

species (4 of 9) are riparian invasive alien species, thedifference from the expected number is notstatistically significant (Fisher’s exact test, P = 0.08).
Eleven percent of agricultural weeds (15 of 140)
and 22.5% of revegetation species (9 of 40) are widelyinvasive species. The set of agricultural weeds
contains far more invasive alien species than expected(Fisher’s exact test, P < 0.001). The same result was
found for revegetation species (Fisher’s exact test, P <0.001). Although 44% of aquacultural species (4 of9) are invasive alien species, the difference from theexpected number of widely invasive alien species is
not statistically significant (Fisher’s exact test, P =0.350).
Invasive agricultural weeds, revegetation species,
and aquacultural species dominated (or codominated)
more rivers than did other invasive alien species(Fig. 2). Among these four categories, the number ofrivers dominated (or codominated) by invasive alien
species is significantly different (Kruskal-Wallis test,
P < 0.001).
The majority (83 of 87, 95.4%) of the invasive
alien plants in Japanese riparian areas are herbaceous(Table 3). Polycarpic herbaceous perennials are themost common (45 of 87, 51.7%), followed by annuals(35 of 87, 40.2%). The category of monocarpic
herbaceous perennials has fewer invasive alien species
than expected on the basis of our simulations (P <0.05 after Bonferroni correction). Although only two
widely invasive species are trees, this number isgreater than expected (P < 0.05 after Bonferroni
correction). The category of invasive agriculturalweeds contains annuals (25 of 44, 56.8%) and poly-

Aquacultural species(n=4)

Revegetation species(n=21)

Agricultural weeds(n=35)

Others(n=27)
0

20

40

60

80

100

Number of rivers

Fig. 2

Number of rivers invaded by invasive alien species (revegetation species,
aquacultural species, agricultural species, and others) among 123 rivers up
to 1999. The nine species contained in the overlap between invasive
revegetation species and invasive agricultural weeds (Fig. 1(a)) are included
in invasive revegetation species. Each box is bounded by the 25% and 75%
quartiles and is divided at the median; the horizontal lines indicate theextreme values within 1.5 times the interquartile range from the upper andlower values of each box. The circles indicate values more than 1.5
interquartile ranges away from the upper values of each box.

Invasive Alien Plant Species in Riparian Areas of Japan

carpic perennials (19 of 44, 43.2%). The category of
monocarpic herbaceous perennials has fewer invasive
agricultural weeds than expected (P < 0.05 after
Bonferroni correction). Among invasive revegetation species, polycarpic herbaceous perennials are themost common (15 of 21, 71.4%), followed by annuals(3 of 21, 14.3%). All four aquacultural invasive
species are polycarpic herbaceous perennials.
With regard to their origins, 36.8% of the invasive
alien plants are native to North America (32 of 87;
Table 3), followed by Eurasia (13 of 87, 14.9%),
Europe (10 of 87, 11.5%), and South America (10 of87, 11.5%). Temperate areas are especially importantsources. The number of invasive species native to
North America is greater than expected on the basis of
our simulations (P < 0.05 after Bonferroni correction).
In contrast, the number native to Europe is less than
Table 393

expected (P < 0.05 after Bonferroni correction).
Among widely invasive alien species, 63.6% arenative to North America (14 of 22), far more than
expected (P < 0.05 after Bonferroni correction).
Many of the invasive alien plants come fromfamilies whose representatives are common invaders
in temperate climates (Pyšek, 1998): Compositae (24species), Gramineae (21), Leguminosae (9), Polygonaceae (5), and Onagraceae (4). Among widelyinvasive alien species, the Compositae (9), Gramineae(7), and Leguminosae (3) account for 86.4% (19 of
22). Of the invasive agricultural weeds, the largest
number come from the Compositae (18 of 44, 40.9%;
Table 3), followed by the Gramineae (9 of 44, 20.5%).
The invasive revegetation species belong to either theGramineae (14 of 21, 66.7%) or the Leguminosae (7of 21, 33.3%).

Comparison of numbers of invasive alien species, widely invasive alien species, agricultural weeds, revegetation species,
and aquacultural species in Japanese riparian habitats. Numbers in parentheses indicate expected species numbers basedon simulations. *P < 0.05; ** P < 0.01 (all P values are Bonferroni-corrected)
Invasive alien
species(n = 87)

Widelyinvasive alien
species(n = 22)

Agriculturalweeds(n = 44)

Revegetation
species(n = 21)

Aquacultural
species(n = 4)45 (38.2)
3 (11.2)**
35 (34.9)9 (9.6)0 (2.8)10 (8.8)

19 (16.9)0 (4.4)*25 (22.7)

15 (14.2)
1 (2.6)3 (3.1)4 (4.0)0 (0.0)0 (0.0)1 (1.7)3 (1.0)1 (0.4)2 (0.3)*0 (0.0)0 (0.0)1 (0.5)1 (0.5)0 (0.0)0 (0.0)

Origin
North America
Eurasia
EuropeSouth America
Tropical America
Eastern Asia
Mediterranean
AfricaUnknown

32 (19.8)**
13 (13.1)
10 (19.6)*
10 (6.6)
8 (7.1)7 (7.1)4 (6.7)
1 (2.5)2 (4.5)14 (5.0)**
1 (3.3)1 (5.0)3 (1.7)0 (1.8)1 (1.8)1 (1.7)1 (0.6)0 (1.1)21 (14.8)
3 (5.3)5 (8.8)6 (4.1)5 (5.3)2 (1.2)
1 (1.9)0 (0.9)1 (1.6)4 (2.6)6 (7.3)3 (4.2)2 (1.1)1 (1.0)1 (0.5)2 (2.1)1 (1.1)1 (1.0)1 (1.3)0 (0.0)0 (0.4)2 (0.9)1 (0.4)0 (0.0)0 (0.4)0 (0.4)0 (0.0)

FamilyCompositae
Gramineae
Leguminosae
Polygonaceae
Onagraceae
Scrophulariaceae
Rubiaceae
Iridaceae

24 (16.1)
21 (13.7)
9 (5.3)5 (2.9)4 (2.0)2 (2.8)2 (0.6)2 (1.2)9 (4.1)7 (3.5)3 (1.3)0 (0.7)0 (0.5)0 (0.7)0 (0.2)0 (0.3)18 (12.9)9 (4.1)*3 (1.6)4 (1.6)4 (1.9)1 (2.8)0 (0.0)0 (0.0)0 (0.0)14 (14.2)
7 (6.8)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)Hydrocharitaceae
Cruciferae
Chenopodiaceae
Amaranthaceae
Others2 (0.4)2 (5.5)2 (1.0)2 (2.8)10 (32.9)**

1 (0.1)0 (1.4)0 (0.3)0 (0.7)2 (8.3)**0 (0.0)1 (2.8)1 (1.3)1 (2.5)
2 (12.6)**

0 (0.0)0 (0.0)0 (0.0)0 (0.0)0 (0.0)2 (0.9)0 (0.0)0 (0.0)0 (0.0)2 (3.1)

Life form
Herbaceous plants
Polycarpic perennial
Monocarpic perennial
AnnualWoody plants
ShrubsTrees

94

S. MIYAWAKI and I. WASHITANI

occupied 62.5%, 45.6%, and 1.1% of the area, respectively. Sixty percent of the area was occupied bypolycarpic herbaceous perennials. Seventy-two percent of the area was occupied by species native to
North America. Three large families, the Compositae, Gramineae, and Leguminosae, accounted formost of the invasive alien species (62.1%; Table 3),
and amounted to 95.5% of the total alien vegetation
area (Table 4).

4.2 Vegetation area dominated by invasive alien
species
The total riparian area occupied by vegetation
dominated (or codominated) by alien plant species(“total alien vegetation area”) reached approximately
11,900 ha by 1999 (Miyawaki & Washitani, 2004).
Seventy-seven percent of the total alien vegetation
area was occupied by widely invasive alien species(Table 4). Invasive agricultural weeds, invasive revegetation species, and invasive aquacultural species
Table 4

Percentage cover of invasive alien species, widely invasive alien species, agricultural weeds, revegetation
species, and aquacultural species in vegetation dominated by alien plant species in Japanese riparian habitats
Invasive alien
speciesWidelyinvasive alien
species

Agricultural
weeds

Revegetation
speciesAquacultural
species

Polycarpic perennial

60.3

39.8

44.6

22.2

1.1

Monocarpic perennial

0.8

0.0

0.0

0.6

0.0

Annual

18.3

16.9

17.9

2.4

0.0

Life form
Herbaceous plants

Woody plants
Shrubs

1.9

1.9

0.0

1.9

0.0

Trees

18.7

18.7

0.0

18.5

0.0

North America

72.3

65.2

51.1

22.7

<0.1

Eurasia

10.3

0.0

0.1

10.1

0.0

Europe

1.5

0.5

<0.1

1.3

0.0

South America

5.5

3.7

4.4

1.5

1.0

Tropical America

1.3

0.0

1.0

1.0

<0.1

OriginEastern Asia

0.8

0.2

0.6

0.5

0.0

Mediterranean

4.7

4.6

4.6

4.7

0.0

Africa

3.2

3.2

0.0

3.2

0.0

Unknown

0.5

0.0

0.5

0.5

0.0

Compositae

47.8

41.6

47.3

0.0

0.0

Gramineae

25.4

11.8

11.8

23.3

0.0

Leguminosae

22.3

20.9

0.6

22.3

0.0

FamilyPolygonaceae

0.2

0.0

0.2

0.0

0.0

Onagraceae

0.7

0.0

0.7

0.0

0.0

Scrophulariaceae

<0.1

0.0

<0.1

0.0

0.0

Rubiaceae

0.1

0.0

0.0

0.0

0.0

Iridaceae

<0.1

0.0

0.0

0.0

0.0

Hydrocharitaceae

1.1

1.0

0.0

0.0

0.0

Cruciferae

<0.1

0.0

<0.1

0.0

1.0

Chenopodiaceae

<0.1

0.0

<0.1

0.0

0.0

Amaranthaceae

0.3

0.0

<0.1

0.0

0.0

Others

2.1

2.0

1.8

0.0

<0.1

Total

100.0

77.3

62.4

45.6

1.1

Invasive Alien Plant Species in Riparian Areas of Japan

5. Discussion
5.1 Taxonomic pattern of invasive alien species
In Japanese riparian areas, the set of invasive alien
species contained 22 families and 87 species (Table 5),
with strong representation by species of the Compositae (27.6%), Gramineae (24.1%), and Leguminosae(10.3%), which are typically well-represented in alien
floras worldwide. In addition, alien plants belonging
to these three families occupied 95.5% of the alienvegetation area of the investigated river systems.
Several recent studies focused on the taxonomic
pattern of plant invasions (e.g., Daehler, 1998; Pyšek,
Table 5951998). The results of these studies led to the sameconclusion: the largest plant families (Compositae,
Gramineae, and Leguminosae) contribute most to thetotal number of alien species in local flora.
From an evolutionary point of view, the Compositae are considered one of the most advanced families,
and it contains numerous agricultural weeds, many ofwhich are remarkably successful and have become
distributed extensively throughout temperate zones
worldwide (Heywood, 1989; Cronk & Fuller, 1995).
The presence of invasive alien plants of the Gramineae and Leguminosae in Japanese riparian areas islargely the result of deliberate introduction of revege-

Invasive alien plant species identified along 123 rivers (109 river systems) in Japan.

Species

FamilySolidago altissima L.
Robinia pseudoacacia L.
Erigeron canadensis L.
Paspalum distichum L.
Sorghum halepense (L.) Pers.
Panicum dichotomiflorum Michx.
Ambrosia trifida L.
Conyza sumatrensis (Retz.) Walker
Egeria densa Planch.
Eragrostis curvula (Schrad.) Nees
Xanthium occidentale Bertol.
Sicyos angulatus L.
Amorpha fruticosa L.
Trifolium repens L.
Ailanthus altissima (Mill.) Swingle
Andropogon virginicus L.
Paspalum dilatatum Poir.
Ambrosia artemisiifolia var. elatior L.
Aster subulatus Michx.
Bidens pilosa L.
Festuca arundinacea Sch.
Stenactis annuus (L.) Cass.
Eichhornia crassipes (Mart.) Solms-Laub.
Lolium multiflorum Lam.
Helianthus tuberosus L.
Myriophyllum brasiliense Cambess.
Bidens frondosa L.
Dactylis glomerata L.
Aster subulatus Michx. var. sandwicensis A. G. Jones
Agrostis alba L.
Paspalum urvillei Steud.
Oenothera biennis L.
Diodia teres Walt.
Verbena brasiliensis Vell.
Solidago gigantea Aiton var. leiophylla Fernald
Elodea nuttallii (Planch.) H. St. John
Amaranthus patulus Bertol.

Compositae
Leguminosae
Compositae
Gramineae
Gramineae
Gramineae
Compositae
Compositae
Hydrocharitaceae
Gramineae
Compositae
Cucurbitaceae
Leguminosae
Leguminosae
Simaroubaceae
Gramineae
Gramineae
Compositae
Compositae
Compositae
Gramineae
Compositae
Pontederiaceae
Gramineae
Compositae
Haloragaceae
Compositae
Gramineae
Compositae
Gramineae
Gramineae
Onagraceae
Rubiaceae
Verbenaceae
Compositae
Hydrocharitaceae
Amaranthaceae

Number of Vegetation
area2 (ha)rivers1
100
62
61
44
44
39
37
36
33
31
26
24
19
18
18
17
16
13
12
12
12
11
11
11
10
9
9
9
8
8
8
7
7
7
7
7
6

3370
2197
386
59
549
216
603
254
121
379
71
319
230
57
20
134
59
22
12
151
37
76
9
77
22
1
25
834
3
61
124
79
14
2
579
5
8

Category3
W
R
W
RW
RW
RW
W
W
A
R
W
W
R
R
W
RW
W
W
W
R
W
A
R
A
W
R
W
RW
RW
W

W
A
W

96

S. MIYAWAKI and I. WASHITANI

Trifolium pratense L.
Paspalum distichum L.var. indutum Shinners
Phleum pratense L.
Poa pratensis L.
Rumex crispus L.
Nasturtium officinale R. Br.
Melilotus officinalis (L.) Pall. ssp. alba Medik.
Festuca pratensis Huds.
Vulpia myuros C. C. Gmel.
Diodia virginiana L.
Cosmos bipinnatus Cav.
Rudbeckia laciniata L.
Paspalum notatum Flugge
Persicaria pilosa (Roxb.) Kitag.
Rumex obtusifolius L.
Celosia argentea L.
Brassica junicea (L.) Czern.
Trifolium dubium Sibth.
Sapium sebiferum (L.) Roxb.

Leguminosae
Gramineae
Gramineae
Gramineae
Polygonaceae
Cruciferae
Leguminosae
Gramineae
Gramineae
Rubiaceae
Compositae
Compositae
Gramineae
Polygonaceae
Polygonaceae
Amaranthaceae
Cruciferae
Leguminosae
Euphorbiaceae

6
5
5
5
4
4
4
4
4
3
3
3
3
2
2
2
2
2
2

91
17
63
132
8
1
63
63
16
3
4
55
115
9
4
25
1
<1
2

Oenothera erythrosepala Borbás
Oenothera laciniata Hill
Lindernia dubia (L.) Pennell
Aster novae-angliae L.
Coreopsis lanceolata L.
Hypochoeris radicata L.
Sisyrinchium atlanticum E. P. Bicknell
Boehmeria nivea (L.) Gaud.
Fagopyrum cymosum Meisn.
Rumex acetosella L.
Spergularia rubra (L.) J. Presl et C. Presl
Atriplex hastata L.
Chenopodium ambrosioides L. var. ambrosioides
Sedum sarmentosum Bunge
Aeschynomene virginica (L.) Britton, Sterns et Poggenb.
Medicago polymorpha L.
Medicago sativa L.
Ludwigia decurrens Walter
Ipomoea triloba L.
Veronica anagallis-aquatica L.
Artemisia annua L.
Aster pilosus Willd.
Bidens pilosa L. var. minor (Blume) Sherff
Coreopsis tinctoria Nutt.
Erechtites hieracifolia (L.) Raf. ex DC.
Galinsoga ciliata (Raf.) Blake
Iris pseudacorus L.
Aira caryophyllea L.
Aira elegans Willd. ex Kunth
Briza maxima L.
Bromus catharticus Vahl

Onagraceae
Onagraceae
Scrophulariaceae
Compositae
Compositae
Compositae
Iridaceae
Urticaceae
Polygonaceae
Polygonaceae
Caryophyllaceae
Chenopodiaceae
Chenopodiaceae
Crassulaceae
Leguminosae
Leguminosae
Leguminosae
Onagraceae
Convolvulaceae
Scrophulariaceae
Compositae
Compositae
Compositae
Compositae
Compositae
Compositae
Iridaceae
Gramineae
Gramineae
Gramineae
Gramineae

2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1

<1
1
<1
28
11
2
<1
1
<1
<1
<1
1
1
<1
1
4
10
<1
<1
<1
1
<1
<1
2
1
<1
<1
<1
<1
<1
82

1

R
R
R
W
RW

W
R
RW
W
W
W
W
W
W
W
W
W

W
W

RW
R
W
W

W
W
W

W

Number of rivers invaded.
Total area occupied by vegetation dominated by alien species up to 1999 (total investigated area is approximately 82,000 ha).
3
W: agricultural weed; R: revegetation species; A: aquacultural species.
2

Invasive Alien Plant Species in Riparian Areas of Japantation species. Many Gramineae spread by rapid
growth of densely spaced tillers, sometimes
accompanied by dense litter production, which
promotes monocultures (Washitani, 2002a). The
Leguminosae also appear to have a high numberof invasive species, including serious invaders(Heywood, 1989). The nitrogen-fixing ability ofleguminous species may help them to invade
nutrient-poor habitats (Richardson et al., 2000).
Many leguminous invasive species are rapidly
growing shrubs or trees, with abundant production of seeds that often have the ability to survive
long periods of dormancy (Heywood, 1989;
Cronk & Fuller, 1995). These characteristics
probably contribute to the success of these threefamilies as riparian invaders.
We may have underestimated the invasiveness ofsome families, however, because we based our judgment of invasive alien species on NCRE vegetation
maps. NCRE surveys are conducted between summer and fall (Ministry of Construction, River Bureau,
River Environment Division, 1996, 1997a, 1997b,
1998, 1999, 2000; MLIT, River Bureau, River Environment Division, 2001), therefore, the invasivenessBrazil 3.2%

of families including species dominant or codominant
in vegetation in spring (e.g., Cruciferae) may beundervalued in this study.
5.2 Origins of invasive alien species
One striking aspect of the invasive alien plants
seen in the riparian areas of Japan is the surprisingly
large contribution of species native to North America(invasive, 37%; widely invasive, 64%). The climatic
similarity and intensive trade between Japan and theUnited States undoubtedly contribute to these findings(Washitani & Morimoto, 1993).
In 2002, forexample, most corn, soybeans, hay, and pasture seed
consumed in Japan were imported from the UnitedStates (Fig. 3; Plant Protection Station, Ministry ofAgriculture, Forestry and Fisheries of Japan, 2003).
The large Japanese market share of U.S. seed exports
has continued for a long period (Fig. 4). Despite theenactment of seed purity laws in many nations, alienplant seeds are still common contaminants in theinternational seed trade (Mack, 2003).
Large
amounts and a wide range of seeds of weed species
have been found in imported fodder crops and hay(Enomoto, 1999). For example, at least 50 species
were detected in shipments of crops (wheat, soybeans,

Others
4.3%
Others
8.2%
Brazil
15.9%

United States
75.9%

United States
92.5%

(a)

(b)
Others 7.4%
Canada 5.4%

Others11.2%
Australia
20.4%

Australia
19.8%

United States
68.4%

(c)
Fig. 3

97

United States
67.4%

(d)

Exporters of agricultural products to Japan in 2002: (a) corn, (b) soybean, (c) hay, and (d)pasture seed. Data from Plant Protection Station, the Ministry of Agriculture, Forestry and
Fisheries of Japan (2003).

98

S. MIYAWAKI and I. WASHITANI

sorghum, and corn) from the United States to Japan in
1994-1995 (Enomoto, 1999). The importation of
fodder crops grew rapidly until 1985 before levelingoff (Nishida & Shimizu, 1999; Ministry of Agriculture,
Forestry and Fisheries of Japan, 2002). It is clearthat large numbers of alien plant seeds have been
introduced into Japan in imported crops.
In addition, although most pasture seeds (67.4%)
were imported from the United States in 2002 (Fig.
3(d)), the geographic origins of most are in fact
Europe or Eurasia (Plant Protection Station, Ministryof Agriculture, Forestry and Fisheries of Japan, 2003).
Consequently, the U.S. contribution to the number ofand area occupied by invasive alien species spreadthrough trade is actually greater than the contribution
to origin shown in Table 3 and Table 4 (Fig. 5).
In spite of the large contribution of European
plants to the alien flora of Japanese riparian areas(Washitani, 2002b), Europe contributed far less to thenumber and quantity of invasive alien species than the

United States (Fig. 5). One reason may be that total
European seed exports to Japan were much smaller
than those from the United States. In 2002, forexample, little of the corn (0.07%), soybeans(0.004%), or hay (0.1%) consumed in Japan was
imported from the European Union (Plant Protection
Station, Ministry of Agriculture, Forestry and
Fisheries of Japan, 2003). It is clear that the European contribution to numbers of alien plant seeds
introduced into Japan in imported crops is smaller
than that from North America (the United States).
5.3 Invasiveness of agricultural weeds andrevegetation species
More than half of the invasive alien species inJapanese riparian areas were either agricultural weeds(invasive, 51%; widely invasive, 68%) or revegetation
species (invasive, 24%; widely invasive, 41%). Our
statistical analyses suggest that in Japanese riparian
habitats there is a greater tendency for alien agricultural weeds and introduced revegetation species to be
Corn
Soybeans

Percentage share of the United States

(%)
100

80

60

40

20

0

1965

1970

1975

1980

1985

1990

1995

2000

Year

Fig. 4

Changes in the U.S.’s share of crops imported to Japan.
Data from the Ministry of Agriculture, Forestry and
Fisheries of Japan (2002).

North America

North America

Eurasia

Eurasia

EuropeSouth America

South America

Mediterranean

Tropical America

AfricaEastern Asia

EuropeMediterranean

Tropical America

Pasture species
OthersAfricaEastern Asia

Unknown

Unknown

0

5

10

15

20

Number of Species

Fig. 5

Pasture species
Others

25

30

35

0

2000

4000

6000

8000

10000
(ha)

Vegetation area

Origins of the invasive alien species in Japanese riparian areas. (a) Number of invasive alien species (total number of
pasture species = 19; total number of invasive alien species = 87). (b) Total vegetation area occupied by alien species
among the 123 rivers (total alien vegetation area = 11,882 ha).

Invasive Alien Plant Species in Riparian Areas of Japan

invaders than other alien species (Table 1 & 2) and forinvasive agricultural species, invasive revegetation
species and invasive aquacultural species to be
distributed more widely than other invasive alien
species (Fig. 2).
The availability of suitable habitats and the supplyof propagules of alien agricultural weeds and revegetation species in riparian areas may account for theirinvasiveness. As we have mentioned, riparian habitats are subject to numerous and varied disturbances(e.g., construction works, floods). These disturbances are likely to increase the creation of bare areas
suitable for colonization by these species (Gregory etal., 1991; Pollock et al., 1998).
Propagule pressure is widely recognized as afundamental driver of invasion (Williamson, 1996;
Kolar & Lodge, 2001). Recent studies have shown astrong human influence on the characteristics andabundance of alien species (Lockwood, 1999; Kiritani& Yamamura, 2003). For birds, Lockwood (1999)
explained that the taxonomic distribution of alien
birds in relation to the probability of successful
introduction could be explained by differences in theprobability of deliberate introduction by humans.
Birds actively transported as pets, food, or gamespecies were much more likely to be traded, and thus
dominate the lists of aliens (Lockwood, 1999). For
insects, Kiritani and Yamamura (2003) showed that
increases in plant material imports were associatedwith an increasing number of alien insects.
In Japanese riparian areas, propagule supply fromadjacent invaded habitats (e.g., pastures, revegetatedlevees, agricultural fields, roadsides) is likely tocontribute significantly to the further invasion of alienplants. In 1999, the total area of riparian pastures
dominated by alien grasses had reached 18,000 ha,
which amounted to 12% of the total river areaadministered by the Ministry of Land, Infrastructure
and Transport (Miyawaki & Washitani, 2004; Fig. 6).

Others 21%
Forestry2%

Native vegetation
46%

Agricultural
fields11%
Pastures
12%

Alien vegetation
8%

Fig. 6 Percentage of riparian area of Japan by land
status or usage. Data from NCRE (Ministryof Construction, River Bureau, River
Environment Division, 1996, 1997a, 1997b,
1998, 1999, 2000; MLIT, River Bureau,
River Environment Division, 2001).

99

We estimated that more than half of the total leveearea was occupied by revegetation species (e.g.,
Lolium multiflorum L., Festuca arundinacea Schreb.)from NCRE (Miyawaki & Washitani, unpublished).
Furthermore, in the upper reaches of many rivers inJapan, woody leguminous species such as Robinia
pseudoacacia L. and Amorpha fruticosa L. are often
used for soil erosion control (Washitani, 2002a).
Similarly, in 1999 the total area of riparian agricultural fields had reached 17,000 ha, equivalent to
11% of the total river area (Fig. 6). In agricultural
fields in Japan, the area invaded by alien agriculturalweeds significantly increased since the 1980s(National Grassland Research Institute, 1998; Nishida& Shimizu, 1999). The majority of alien agriculturalweeds were introduced as contaminants of imported
agricultural produce (Nishida & Shimizu, 1999).
Consequently, the agricultural weeds primarily invading agricultural fields would contribute to the total
number of alien species in Japanese riparian habitats.
5.4 Control strategies for invasive alien plants inriparian areas
Alien species that form dominant (or codominant)communities are listed in Table 4. These species
should be given the highest priority for management
programs in Japanese riparian areas. Species thattypically form dominant communities and are abundant in their respective communities have a muchwider geographical distribution than those that occur
as scattered or isolated individuals (Williamson &Fitter, 1996). Moreover, the fact that a species with acertain impact in one place has a similar impact in an
ecologically similar place is widely thought to be thebest predictor of its invasive character (Williamson,
1996; Reichard, 1997).
Active control of invasive alien species started
along in some rivers before 2002 (Committee forInvestigating the Effects of and Countermeasuresagainst Riparian Exotic Species, 2003; Table 6). The
examples given are much fewer than the number ofrivers invaded and influenced by these alien species(Table 5), so to conserve biodiversity and ecosystems,
more active management is needed in Japaneseriparian areas.
In addition to discouraging the introduction ofunstudied potentially invasive species, one strategy for
reducing introductions of species that could invade
riparian areas in the future would be to scrutinize
extremely carefully any proposed introductions ofprimarily revegetation species. More than half ofriparian invaders are agricultural weeds, so appropriate management of weeds in agricultural areas
could be effective in restricting the introduction of
new invaders to riparian areas. Therefore, alien plant
management plans are needed for both natural habitats
and agricultural areas. In addition, the use of alien
pasture species for the purpose of revegetation orcontrol of soil erosion should be restrained, firstly toconserve biodiversity and native ecosystems, but also

100

S. MIYAWAKI and I. WASHITANI

Table 6 Target species for control in Japanese riparian zones
executed up to 2002. Data from the Committee forInvestigating the Effects of and Countermeasuresagainst Riparian Exotic Species (2003).
Target species
Solidago altissima
Eichhornia crassipes
Sicyos angulatus
Robinia pseudoacacia
Ambrosia trifida
Gramineae spp.

Number of cases
10
9
6
4
3
4

to avoid provoking grass pollen allergies.
There is a lack of information on the biological andecological characteristics of alien plant species inJapanese riparian habitats (Miyawaki & Washitani,2004). To enable the planning of appropriate management practices, more extensive studies of alien species
are needed. Identifying the characteristics of invasive alien species is the first step in developing
management strategies for Japanese riparian areas andwill be a stepping-stone to further detailed studies oninvasion biology and conservation ecology.

Acknowledgements
Our study is supported by the Ministry of the Environment as a Global Environment Research Programme.
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