Invasive plant species in Portugal: an overview
Portuguese native flora is threatened by invasions of exotic plants. During the last two centuries, and especially in recent decades, the number of introduced plant species has increased extensively. Alien species now represent more than 15% of the total Portuguese vascular flora, which comprises approximately 3200 taxa. Their presence has increased probably by more than 1000% during the last two centuries, reaching nowadays around 500 species. Almost 40% of the listed species are actually or potentially invasive, including agricultural weeds and invaders of natural habitats, and around 7% are considered as dangerous invaders. Fabacae and Asteraceae provide the largest numbers of problematic species. Australia seems to have supplied the most dangerous and aggressive invaders to Portugal, including several Acacia and Hakea species.
In 1999, specific legislation (Decreto - Lei 565/99) was prepared in Portugal to address this problem. A list of introduced invasive alien plants has been established and it is prohibited to introduce any new plant species unless it has been shown to be not harmful (with a few exceptions granted for forestry and agricultural purposes). Penalties will be applied to those using any listed invasive species. It is planned that the list will be regularly updated. The implementation of these new regulations will imply inspections of the horticultural sector (e.g. nurserymen, landscape designers, gardeners) and adequate training of technicians.
The species listed as invasive by this law are: Acacia dealbata, Acacia karroo, Acacia melanoxylon, Acacia mearnsii, Acacia longifolia, Acacia pycnantha, Acacia retinodes, Acacia saligna, Ailanthus altissima (EPPO List of Invasive Alien plants), Arctotheca calendula, Azolla filiculoides (EPPO List of Invasive Alien plants), Carpobrotus edulis, Cortaderia selloana, Eichhornia crassipes, Elodea canadensis, Hakea sericea, Hakea salicifolia, Ipomoea indica, Myriophyllum aquaticum (EPPO List of Invasive Alien plants),, Oxalis pes-caprae, Pittosporum undulatum, Robinia pseudoacacia, Spartina densiflora, Tradescantia fluminensis.
Potentially invasive plants are also listed: Agave americana, Acacia baileyana, Acacia cyclops, Acacia decurrens, Arundo donax, Cortaderia selloana, Lantana camara, Opuntia ficus-indica, Ricinus communis.
Concerning aquatic ecosystems, Aguiar et al. (2006) state that several plants only invade rice fields and their vicinity (e.g. Ammannia x coccinea, Echinochloa oryzicola), while others such as the water-hyacinth (Eicchornia crassipes) and the parrot-feather (Myriophyllum aquaticum, EPPO List of Invasive Alien plants) affect multiple ecosystems including watercourses, drainage and irrigation channels, reservoirs, swamps and marshes, and can be very detrimental both as environmental weeds and to human activities. Some species are widely distributed in Portugal, such as the giant-reed (Arundo donax) and the knotgrass (Paspalum distichum), whereas others have a restricted distribution (e.g. Eryngium pandanifolium) or develop plant outbreaks under specific environmental conditions (e.g. Azolla filiculoides).
Sources
Aguiar FC, Ferrerra MT, Moreira L, Duarte MC (2006) Are invasive plant species a problem in aquatic ecosystems of Portugal? In: Invasive plants in Mediterranean Type Regions of the World (Ed. by S Brunel). Council of Europe publishing. Strasbourg. p. 358.
Marchante H (2006) Invasive plant species in Portugal: an overview. In: Invasive plants in Mediterranean Type Regions of the World (Ed. by S Brunel). Council of Europe publishing. Strasbourg. p. 147-155.
Decreto Lei 565/99 (Invasive plants in Portugal). Diário da República – I Série A, no. 295, 1999-12-21. http://www.uc.pt/invasoras/dec_lei_invasoras.pdf