What are the Attributes of an Aggressive Invader and How Does Heracleum mantegazzianum Compare with Other Invasive Species

Extensive stands with a high cover

For any species, there are several assumptions that need to be met if it is to be viewed as an aggressive invader. The first assumption is the capability of creating large and dominant stands. Although in Central Europe two-thirds of H. mantegazzianum records relate to plants scattered in invaded vegetation without being dominant (see Thiele et al., Chapter 8, this volume), it often forms dominant stands with a high cover (Fig. 19.1). Further, once suitable environmental conditions are met and the species starts to dominate the invaded vegetation, the stands can be very extensive. In the Slavkovsky les study area, Czech Republic (see Perglova et al., Chapter 4, this volume), on the basis of aerial photographs it is estimated that in ten 60-ha landscape sections, populations of H. mantegazzianum at present completely cover 41.9 ha (see Pysek et al., 2005; Chapter 3, this volume). This represents 7% of the total land in the region. These figures can be related to biomass, using the data from Tiley et al. (1996), who report a yield of 57 t/ha dry mass (including roots), based on measurements in the west of Scotland.

High rate of spread

Extensive populations can only result from a high rate of spread in the past, but these two measures, although closely related, do not necessarily indicate the same pattern of invasion. A species can be locally abundant and form extensive stands in a geographically limited region, yet it may not be widespread over a large region. Easy dispersal and rapid spread in the past also indicate a potential for the invasion to continue in the future, because they allow a species to spread over considerable distances from source populations and reach new areas. Quantitative information on the rate of H. mantegazzianum spread is available for the second half of the 20th century from the

Fig. 19.1. In the Slavkovsky les study area, Czech Republic, populations of H. mantegazzianum cover approximately 7% of the landscape. Photo: P. Pysek.

Czech Republic for both local and regional scales (Fig. 19.2). Interestingly, the pattern and rate of spread is similar at both scales; if the increase in the invaded area at local scale and in the number of squares at country scale is compared statistically, there is no difference in the slopes of both plots (for details, see Pysek et al., Chapter 3, this volume). This indicates that H. mantegazzianum spreads at the national scale at the same rate as locally in the region of its introduction to the country, and that the constraints imposed to its spread by landscape features and availability of suitable habitats are similar at both scales. That the invasion at the country scale is of a similarly high rate as that in the very suitable region of the Slavkovsky; les (Pysek et al., 2005; Chapter 3, this volume) indicates that the species seems to be little constrained by environmental settings. When suitable habitats are available, it spreads at a high and constant rate.

Comparing the rate of Heracleum mantegazzianum invasion with that of other species

Heracleum mantegazzianum regularly appears on global lists of the most invasive species (e.g. Cronk and Fuller, 1995; Weber, 2003) and is listed in the Global Invasive Species Database (Lowe et al., 2000) among the 100 worst invasive alien species. What is its position among plant invaders of global significance expressed in quantitative terms? Exact data are unfortunately

Year

1960

Year

Year

Fig. 19.2. Heracleum mantegazzianum spread was very fast at both national and local scales in the Czech Republic in the second half of the 20th century. (A) The pattern of increase in the cumulative number of squares ca. 11 x 12 km from which the species has been recorded up to the given time is similar to that in the actual population size in the Slavkovsky' les study area. (B) The population size is given in hectares, estimated as pooled value from ten sites of 60 ha each, monitored by using aerial photographs (Mullerovâ et al., 2005; Pyëek et al., Chapter 3, this volume). Thus, for example, 30 ha corresponds to 5% of the total landscape occupied.

Year

Fig. 19.2. Heracleum mantegazzianum spread was very fast at both national and local scales in the Czech Republic in the second half of the 20th century. (A) The pattern of increase in the cumulative number of squares ca. 11 x 12 km from which the species has been recorded up to the given time is similar to that in the actual population size in the Slavkovsky' les study area. (B) The population size is given in hectares, estimated as pooled value from ten sites of 60 ha each, monitored by using aerial photographs (Mullerovâ et al., 2005; Pyëek et al., Chapter 3, this volume). Thus, for example, 30 ha corresponds to 5% of the total landscape occupied.

limited, but recent reviews make it possible to compare the rate of invasion in the past.

The maximum rate of spread during the exponential phase of invasion was compared with that of other invasive neophytes in the Czech Republic (see Pysek et al., Chapter 3, this volume; Williamson et al., 2005). On a regional scale and with global focus, there are two records of how fast the geographical distribution of H. mantegazzianum was increasing (Fig. 19.3). Using a simple measure of the number of grid squares divided by the period between initial and final mapping shows that the geographical range of major invasive species increases by hundreds to thousands of square kilometres a year (Pysek and Hulme, 2005). The invasion of H. mantegazzianum in

Alliaria petiolata (N America) Heterotheca latifolia (USA) Heracleum mantegazzianum (Lower Saxony) Solidago gigantea (Europe) Solidago canadensis (Europe) Heracleum mantegazzianum (Ireland) Lythrum salicaria (N America) Impatiens glandulifera (Lower Saxony) Elodea nuttallii (Netherlands) Prunus serotina (Lower Saxony) Fallopia japonica (Lower Saxony) Cortaderia selloana (Sardinia) Solidago graminifolia (Europe) Solidago gigantea (Lower Saxony) Fallopia sachalinensis (Lower Saxony) Solidago canadensis (Lower Saxony) Viscum album (USA) Calluna vulgaris (New Zealand)

500 1000 1500 2000 2500 3000

Rate of spread (km2/year)

Fig. 19.3. The rate of increase in distribution of H. mantegazzianum at a regional scale compared to other species. The rate of spread in various regions is expressed as the number of grid squares divided by the length of the period over which the distribution was compared. For example, over the 16 years covered by the study of Schepker and Kowarik (1998), the distribution range in Lower Saxony, Germany, was increasing by 1089 km2/year. Based on data from Pysek and Hulme (2005).

Europe is comparable with such spectacular invasions as that of Alliaria petiolata (Bieb.) Cavara & Grande (Weber, 1998) or Heterotheca latifolia Buckley (Plummer and Keever, 1963) in North America (Fig. 19.3).

In terms of the rate of increase in population size on a local scale, H. man-tegazzianum spread by 1261 m2/year (see Mullerova et al., 2005; Pysek et al., Chapter 3, this volume), which compares it to such prominent invasions as those of Rhododendron ponticum L. in the UK (Fuller and Boorman, 1977) or Caulerpa taxifolia (Vahl) C. Agandh in the Mediterranean Sea (Meyer et al., 1998) (Table 19.1).

Rapid spread alone may not necessarily result in a dramatic invasion if the spread is not associated with the ability of invading populations to persist in invaded locations. Wade et al. (1997) surveyed H. mantegazzianum occurrence in Ireland and found that of the 96 historical sites, the species was still present at 43 in 1993. This represents 45% persistence and illustrates a remarkable ability of the species to thrive in the places once invaded. This value is lower than that reported for prominent clonally spreading invaders, such as Fallopia spp. (Pysek et al., 2001), but it needs to be noted that H. mantegazzianum was a target of control efforts in recent decades. It may be hypothesized that without locally successful eradications, the persistence would be higher.

Table 19.1. Comparison of the rate of invasion by H. mantegazzianum at a local scale with that of other species. The area covered by stands of the invading species was monitored over time or reconstructed from aerial photographs (see Pysek et al, Chapter 3). Data from Pysek and Hulme (2005) - see references to primary sources therein.

Table 19.1. Comparison of the rate of invasion by H. mantegazzianum at a local scale with that of other species. The area covered by stands of the invading species was monitored over time or reconstructed from aerial photographs (see Pysek et al, Chapter 3). Data from Pysek and Hulme (2005) - see references to primary sources therein.

Span of

Rate of

Life

study

spread

Species

form

Region

Habitat

(years)

(m2/year)

Caulerpa taxifolia

Alga

France

Sea bottom

5

2000

Heracleum mantegazzianum

Herb

Czech Republic

Pastures, disturbed

45

1260

Rhododendron ponticum

Shrub

UK

Forest, dunes

20

1100

Spartina anglica C.E. Hubb

Grass

New Zealand

Seashore

9

22

Spartina anglica

Grass

New Zealand

Seashore

4

13

Spartina anglica

Grass

New Zealand

Seashore

41

1

Impact of Heracleum mantegazzianum on resident vegetation

Although impact is difficult to measure and includes various aspects (Williamson, 1998), that of H. mantegazzianum on resident vegetation is manifested through changes in vegetation structure, cover and species composition. It depends on the type of invaded plant communities and their suc-cessional status, being especially marked in ruderal grasslands and ruderal pioneer vegetation (see Thiele and Otte, Chapter 9, this volume). H. mantegazzianum excludes resident species or reduces their abundance, hence decreases the local species richness of invaded communities (Pysek and Pysek, 1995). Nevertheless, conflicts with nature conservation are unlikely as H. mantegazzianum was not reported to invade habitats of conservation concern, nor did the regional populations of common native species seem to be endangered, as documented for Germany (see Thiele and Otte, Chapter 9, this volume). As pointed out in Chapter 9, the impact of H. mantegazzianum on resident vegetation is driven by human-induced disturbances and some native species have similar effects on species richness of plant communities if they prevail in the course of succession.

The detailed analysis carried out in Chapter 9 seems to imply that the impact of giant hogweed is overestimated in the literature. However, even if the above-mentioned effects on resident vegetation are subtle, the fact that the species is able to cover up to 10% of the landscape with dense populations must be taken as clear evidence of a serious impact. Moreover, unpublished data are available from paired invaded and uninvaded plots in the Czech Republic to demonstrate that H. mantegazzianum exerts a rather severe effect on species diversity of invaded sites, reducing their species richness to about half of the state prior to invasion, which is comparable to the effect of Fallopia spp., but stronger than that of Lupinus polyphyllus Lindl. and Impatiens glandulifera Arn. (M. Hejda, unpublished data).

To summarize, H. mantegazzianum has all the attributes of a successful invasive species (Fig. 19.4). Extensive and often dominant stands associated

Fig. 19.4. A flow chart summarizing attributes of a successful plant invader. Heracleum mantegazzianum exhibits the attributes associated with a serious invader (see text for details).

with fast spread result in a wide distribution; this is possible because the persistence in localities once colonized is high. Consequently, any alien species with a wide distribution and marked impact is potentially a serious invader.

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