Friday, September 3, 2010
Alien plant species on the increase
Most introduced species do not survive in nature and if they do they disappear after a short time but some manage to establish themselves and even become common sometimes to the detriment of indigenous species.
Alien species arrive in the Maltese islands for a variety of reasons. Some are introduced for agriculture and aquaculture. The sulla (silla) is grown for fodder but can also be found on clay slopes. Some such as the castor oil tree (siġra tar-riċċnu )have been introduced as ornamental plants while the cape sorrel (ħaxixa ngliża) which is now the commonest plant in Malta was originally grown in the Argotti Botanical Garden in Floriana.
Several species of plants were imported with bird seeds and crop seeds these include the canary grass which has become naturalised.
Successful aliens usually do not have natural enemies, they are able to disperse easily and are good opportunists and find an empty ecological niche which they can occupy. They often manage to make a foothold in stressed areas such as disturbed land and agricultural land. The shrub tobacco (tabakk tas-swar), another introduced species, is common in building rubble.
Many species of plants which have become part of the Maltese countryside were not always present. Carob, fig and almond trees have been in Malta since antiquity. The prickly pear was introduced in the 16th century for its fruit and as a hedge plant.
Some plants such as the castor oil tree can become problematic weeds. This species which has striking large leaves and small greenish flowers was probably imported as an ornamental plant and now grows in valley watercourses where it competes with indigenous species.
This article was published in The Times on 03.02.2010