Impacts of Heracleum mantegazzianum on invaded vegetation in Germany
Heracleum mantegazzianum (Apiaceae, EPPO List of IAS) is commonly regarded as a hazardous invader. In Germany, field studies were conducted in 20 areas (1 km² landscape sections) to evaluate which habitats are the most likely to become invaded; what are the impacts on local plant communities and regional flora; what are the other environmental impacts; and what is the plant’s potential to conflict with nature conservation.
Which habitats are most likely to become invaded?
A logistic regression was used to analyse which factors could explain the presence or absence of H. mantegazzianum in a habitat. Results showed that:
- occurrences of the invasive plant are spatially auto-correlated: habitats adjacent to invaded sites have an increased probability of invasion due to local seed dispersal.
- the vegetation structure of habitats affect the probability of invasion: woody habitats (i.e.>10% tree or shrub cover) have a lower probability of being invaded than completely herbaceous ones.
- at the landscape scale, the probability of establishment decreased with increasing distance from rivers and road corridors. Regarding roads, only agricultural ones had a significant effect.
Impact on local plant communities
The stand coverage of H. mantegazzianum varied from 1 to almost 100%. A high cover percentage can be expected to affect plant communities substantially.
An analysis of 202 sampling plots (25 m²) with H. mantegazzianum from 20 study areas in Germany revealed a negative relationship between H. mantegazzianum cover and the number of vascular plant species. This suggests that increasing H. mantegazzianum cover generally reduces resident species richness. In order to thoroughly assess impacts of H. mantegazzianum on plant species diversity, it is necessary to distinguish different community types and to consider mechanisms of impact. Parameter estimates from this study predicted that the number of species was reduced by 4.8 in tall-herb communities compared to the average and that an increase in H. mantegazzianum cover by 50 percentage points decreased the number of species by 2.4. Moreover, negative trends in species numbers due to H. mantegazzianum cover were confined to ruderal grasslands and to other open community types.
The main mechanism by which H. mantegazzianum can outcompete other plant species is by shading out lower-growing species. Pyšek and Pyšek (1995) found significant differences in the number of vascular plant species between uninvaded vegetation and dominant stands of H. mantegazzianum. This is in agreement with the results from this German study, where species numbers of tall-herb communities, regardless of H. mantegazzianum cover, were considerably lower than in other community types. Altogether, it is stated that the negative relationship between H. mantegazzianum cover percentage and the number of vascular plant species per unit area is attributable to generally decreasing species numbers in the course of succession from low-growing and light-demanding vegetation types towards tall-herb stands, and finally woodlands. This reduction in species numbers is mediated by native tall herbs as well as H. mantegazzianum or other neophytes. Thus, loss of plant species diversity in such cases is a general symptom of successional changes rather than a particular effect of invasive species.
Impact on regional flora
The assumed impact of invasive plant species that attain high cover in indigenous vegetation are the suppression and possibly, local exclusion of native plant species. On the regional scale, a dominant plant invader could cause a decline of regional populations of native species. To make a native species endangered, in the sense of a high risk of regional extinction, would require that the invasive species dominates a large proportion of the habitat area of a particular indigenous population. Thus, since fairly common species normally co-occur with H. mantegazzianum, they would only be regionally endangered if the invader (i) attains high rates of habitat occupancy (i.e. percentage of suitable habitats invaded), (ii) builds up extensive stands and (iii) commonly attains dominance which would, altogether, result in a (iv) high habitat saturation (i.e. percentage of total habitat area covered by the invader).
In the 20 locations in Germany, as species that co-occur with H. mantegazzianum are generally widespread and abundant, it appears that regional populations of associated plant species have not been endangered at the current level of invasion. However, the invasion pattern of H. mantegazzianum in the study areas is merely a snapshot and does not provide a way to predict further development.
Other environmental impacts
In addition to impacts on plant communities and populations, dense stands of H. mantegazzianum can lead to riverbank erosion, mediated through the suppression or exclusion of native species, which play an important role in riverbank stabilization. Deposition of eroded silt can alter substrate characteristics in rivers and, for example, render gravel substrates unsuitable for salmonid spawning.
Potential for conflicts with nature conservation
No rare habitats, communities or co-occuring plant species were found associated with H. mantegazzianum during the field study in Germany. Analysis of preferred site conditions showed that H. mantegazzianum is barely capable of invading sites offering suitable conditions (drought, wetness, poor nutrient status) for rare species and communities and, if so, it would be constrained to low abundances. Therefore, it seems that H. mantegazzianum cannot endanger plant communities and plant species of concern for nature conservation. However, in a few cases, H. mantegazzianum was found at sites that formerly featured protected community types (e.g. nutrient-poor chalk grassland), but these sites had degenerated due to abandonment of appropriate management, eutrophication or other reasons. In such situations H. mantegazzianum, is not the cause, but is rather a symptom of habitat deterioration caused by human activity.
Sources
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Thiele J, Otte A (2007) Impact of Heracleum mantegazzianum on invaded vegetation and human activities. In Pyšek P, Cock MJW, Nentwig W, Ravn HP (eds) (2007) Ecology and management of Giant Hogweed (Heracleum mantegazzianum). CAB International. P. 144-156
Pyšek P, Pyšek A (1995) Invasion by Heracleum mantegazzianum in different habitats in the Czech Republic. Journal of Vegetation Science, 6(5):711-718.
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