The presence of invasive alien plant species is often related to the intensity of human activity. Since river bank developments very often require excavation and the use of construction equipment, it can be assumed that they have all been subjected to a disturbance level of comparable intensity and to equivalent propagule exposure.
Riverbank with riprap invaded by Japanese knotweed. © Léon Ducasse.
IRSTEA researchers therefore looked at whether all riverbank management techniques are colonized in the same way by invasive alien species. They found that all types of structures housed on average an equivalent number of invasive alien plants.
On the other hand, and as this picture illustrates, these researchers found that the frequency of encounter of invasive alien species was significantly greater on civil engineering works (riprap) than on other types of structure, whether they were constructions using pure soil bioengineering or mixed techniques. Invasive alien species thus spread more with riprap than with plant techniques.
This result can be explained by two factors:
- The invasive alien plants identified are characterized by strong growth that gives them a competitive advantage in pioneer environments. The invasive alien plants found on structures resulting from civil engineering (initially devoid of any vegetation) have thus found ideal ground to express their invasive potential.
- It has also been shown that the relative abundance of invasive alien plants can be explained by biotic interactions, including competitive ones. The presence of competing species on soil bioengineering constructions, such as willow species with high growth dynamics, limits the vigour and spread of invasive alien species present.
When soil bioengineering techniques are used on riverbanks, it is likely that the development of invasive alien species is limited by the high density of willow cuttings used.
To elaborate
Cavaillé, P., F. Dommanget, N. Daumergue, G. Loucougaray, T. Spiegelberger, E. Tabacchi and A. Evette (2013). Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers. Ecological Engineering 53(0): 23-30.
Evette, A. et P. Cavaillé (2011). Aménager les berges : vive le génie végétal. Espaces Naturels 34: 37-38.
