Acacia longifolia
Acacia longifolia

What are invasive plants?

Simply put, invasive plants are those that, being exotic, have adapted perfectly to local conditions and, due to their great capacity for reproduction and propagation, cause negative environmental and economic impacts. But not all alien/exotic species are potentially invasive.

In ecological terms, invasive species are responsible for altering the natural dynamics that occur in any given place. They compete with native species for space, water and nutrients and inhibit their development.

They end up reducing biodiversity, altering the physical and chemical structure of the soil. Also, as Nature works in a larger interconnected system, they not only influence the flora and fauna, but modify the functioning of the ecosystem itself, the habitat and the landscape.

Their presence and spread primarily affect natural systems but also have socio-economic consequences. These species can be the source of new parasites and diseases, some can invade agricultural, forest or fishing areas, seriously harming these activities and affecting food production.

Pampas grass - Photo JLPC - WIKIPEDIA
Pampas grass – Photo JLPC – WIKIPEDIA

Species with high water needs reduce the availability of groundwater, affecting the supply of this increasingly scarce resource and, finally, alter the aesthetic and cultural value of the landscape. Many invasive plants from Australia are highly combustible, such as Eucalyptus globulus and the acacia species, and they form a false monoculture.

Many invasive species were first introduced as ornamentals in parks and gardens. For this reason, their presence seems normal, and we accept their invasive character, even using them in gardens. We should not, however, be tricked into this way of thinking.

The gall wasp (Trichilogaster acaciaelongifoliae) is a natural control (biological) for acacia and is a specific method that can only affect the target species – in this case, the invasive Acacia longifolia and species that are closely related.

It is active in the Algarve and trees carrying the galls should not be cut down as they are an aid to dispersion of the control. It should be noted that the gall wasp is a very specific organism which has undergone scientific trials here in Portugal. There are currently nine species of Acacia on the invasive list.

The galls on acacias
The galls on acacias

In Portugal, many of the species that behave today as invasive were introduced in past times with the objectives of preventing erosion (ex. chorão-das-praias, acácia-de-espigas), slope stabilization (ex. mimosa), use of the wood (ex. mimosa, austrálias) or of tanning barks (acácia-negra), hedges (ex. háquias), feeding (ex. lagostim-vermelho) or simply ornamental (pampas grass). Beyond these intentional introductions, others have occurred accidentally, but with equally serious consequences.

In the case of some of the most common ones, please do not plant them and, if you already have them, dispose of them properly. In some cases, culling is not enough, and it is necessary to resort to other techniques to inhibit propagation (crushing, burning, etc.). There is more on the very informative website noted below, in both English and Portuguese.

Water hyacinth

Pampas grass (Cortaderia selloana) is a common invasive seen along road verges. It is a perennial grass that can exceed 4 metres in height and 3 metres in diameter. The flat leaves have a cutting edge and can reach close to 2 metres in length. The flowers are known as plumes; each plume can produce thousands of seeds per reproductive cycle.

This species also reproduces vegetatively, as it has the ability to regenerate after cutting or fire and to root from root fragments that are in contact with the soil. Even if you do not find seedlings on your land, they will invade on nearby sites.

There are also many aquatic invasive plant species. Water hyacinth (Eichornia crassipes) is very expensive to remove and clogs streams and rivers. Aquatic invasive plants are often introduced, so it is very important to be alert and report these and other species that you see spreading.

Become a citizen-scientist and help to map invasive plants at BioDiversity4All (iNaturalist).

For more information, consult the INVASORAS.PT platform  

With grateful thanks to

By Rosie Peddle
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