VI. RESULTADOS
6.3. Medidas para un buen control y almacenamiento de inventario en la cooperativa PRODECOOP,
The effect of introduced species may be neutral, positive or negative.
+ Neutral The introduced species finds an unoccupied niche and integrates well with local ecosystems, causing no disruption.
Case study: the Mediterranean chameleon Chamaeleo chamaeleon in the Maltese Islands
This arboreal lizard was introduced from North Africa between 1846 and 1865 and released in the gardens of a large house at St.Julians. From there it has spread and it now occurs in the wild all over Malta and also in Gozo and Comino. It does not seems to have caused any negative effects on any local species or ecosystem, perhaps because there is not native organism that occupies an identical or even similar niche to the chameleon and its population size has remained overall quite small.
+ Positive Some introductions are necessary and beneficial to human society, for example:
?? crop plants
?? aquaculture species
?? biological control agents
?? ornamental plants
?? economically important species
+ Negative Some introduced species have the potential for becoming nuisances. Possible negative effects result if the introduced species:
?? are pathogenic
?? are or become vectors for pathogens
?? become pests
?? seriously compete with species of economical importance
?? disrupt ecosystems which has been shaped by thousands of years of evolution (through competition, predation, parasitism, allelopathic effects)
?? cause extinction of unique variants and therefore irreparable loss of genetic diversity and loss of the natural heritage
?? adversely affect the aesthetic quality of traditional landscapes.
Hundreds of species have been introduced into the Maltese Islands over the millennia and of these relatively few have managed to establish themselves and fewer still have become fully naturalised. However, some of those that have established themselves, have had significant impacts on local biota and ecosystems.
Examples of impacts that introduced species have had on the ecosystems, habitats and biocoenoses of the Maltese Islands are given in Table 3.1.8.
Table 3.1.8
Examples of impacts which introduced species have had on the ecosystems, habitats and biocoenoses of the Maltese Islands
System impacted Alien species concerned Impacts
Garigue communities on karstland
African cornflag, Chasmanthe
aethiopica
Common freesia, Freesia
refracta
Sweet garlic, Nothoscordum
inodorum
Shrubby putterlick, Pittosporum
tobira
(a common hedge shrub that often escapes)
Century plants, Agave spp.
Become naturalised in these communities and compete with native species
Naturalised in some valleys including Xlendi Valley (Gozo), Wied Harq Hamiem and Wied il-Kbir (l/o Swieqi)
Naturalised in many parts of the Maltese Islands. Particularly invasive at Bahar ic-Caghak to Ghallis/Qalet Marku coastal strip.
Maquis communities Prickly pear Opuntia ficus-
barbarica [= O. ficus-indica]
(introduced into the Maltese
Becomes fully naturalised in these communities which are rare, especially in their primary
Islands at beginning of the 16th Century for fruit, fodder and use as a hedge plant)
Coastal wattle, Acacia karroo
form, and compete with native species
Originally introduced for ornamental reasons, and has become naturalised in many places, e.g. Ghallis and various parts of the Marfa Peninsula. Valley watercourses Xanthium strumarium
Paspalum paspaloides
False pepper Schinus
terebinthifolius
Marvels of Peru, Mirabilis
jalapa and M. odorata
Zantedeschia aethiopica
Narrow-leaved aster, Aster
squamatus
Dense populations of this adventive at Wied il-Qlejgha and Wied il-Ghasel.
Dense populations of this adventive at Wied il-Lunzjata and Wied tax-Xlendi (Gozo), Marsa and San Martin Naturalised at Wied Harq Hamiem and Wied il-Lunzjata (Gozo)
Naturalised, sometimes for- ming dense populations as at Wied Harq Hamiem (Pembroke) and Wied il-Ghajn. Recently observed at Bahrija.
Naturalised as at Bahrija and Wied Harq Hamiem.
Common along watercourses
Cliffs Kaffir fig Carpobrotus edulis (a South African succulent widely cultivated as a ground cover which often ‘escapes’) Century plant, Agave americana Prickly pear, Opuntia ficus-
barbarica
Competition with native species growing in this habitat, possibly the most important in the Maltese Islands
For example, at Ta’Cenc
Sand dunes Blue wattle Acacia cyanophylla (planted extensively during the 1960s and 70s; regenerates from seed)
Aptenia cordifolia
Kaffir fig, Carpobrotus edulis
Has become invasive in the sand dune of Ghadira and is
displacing native dune species
Naturalised at Ramla tat-Torri Invasive in some dunes, and is increasing in others, e.g. parts of Ramla tal-Mixquqa
Saline marshlands Narrow-leaved aster, Aster
squamatus
Invasion by this weedy species which has a high salt tolerance. Countryside in general Cape sorrel Oxalis pes-caprae
(ubiquitous weed)
Narrow-leaved aster, Aster
squamatus
(humid and sheltered soils) Glaucous Tobacco Nicotiana
glauca
(building rubble and in rubble filled crevices)
Tree-of-Heaven, Ailanthus
altissima
(planted and naturalised in various sites)
Wattles, Acacia cyanophylla and Acacia karroo
(planted and naturalised in various sites)
Castor oil Ricinus communis (disturbed ground)
Eucalypts Eucalyptus
camaldulensis and Eucalyptus gomphocephala
( planted extensively in natural contexts)
Competition with native species and prevent the re-establishment of native species on disturbed habitats.
Drain soil water and their leaf litter prevents other species from growing
Agriculture Citrus whitefly Aleurothrixus
floccosus
(introduced around 1985) Sweet potato whitefly Bemisia
tabaci
(introduced around 1993)
Important pest of citrus trees
Serious pest of tomatoes and other greenhouse crops and a vector for the tomato yellow leaf curl virus
Aquaculture Crepidula fornicata
(marine snail native to the Atlantic coast of North America introduced with oysters to the western Mediterranean and known from Marsaxlokk and Marsamxett harbours
Serious nuisance to oyster and mussel as it competes for space and food. At p resent it occurs at low population densities in Maltese waters, however if bivalve culture becomes important locally, it has the potential to become a serious pest
Others Argentine Fire Ant Iridomyrmex
humilis
(introduced and now widespread
Successful competitor and has displaced native species of ants
in the Maltese Islands)
Undisturbed ecosystems are inherently resistant to invasion by exotics, however this capacity is reduced in ecosystems that are disturbed. Such disturbance may be natural, but is usually due to human activity. Many successful introductions have appeared first in ecosystems that are not in equilibrium or are stressed, or are by nature highly labile and/ or are much subject to human interference for example:
?? agroecosystems
?? habitats suffering from periodic disturbance
?? habitats undergoing succession
?? harbours
?? polluted environments
?? engineered environments
In the Maltese Islands most exotics which have become established first gained a foothold in such disturbed ecosystems which include:
?? inhabited areas
?? gardens
?? fields
?? land cleared of its natural vegetation cover
?? harbours
?? valleys subjected to dredging
The chances of invading exotics successfully establishing themselves in the Maltese Islands are now much greater because:
?? Local ecosystems and habitats are small-scale and are easily disturbed even by minor interventions;
?? Many local habitats are already disturbed or stressed either naturally or due to anthropogenic factors;
?? Lack of knowledge often results in ‘embellishment’ projects that make use of inappropriate species and/or cause severe disturbance to natural ecosystems;
?? The opportunities for exotic species to reach the Maltese Islands have increased due to
?? increasing commercial activity;
?? more efficient transportation which has become accessible to a wider variety of people; and
?? an increase in the general standard of living
Case study: Exotic pets in the Maltese Islands
Many Maltese have acquired a taste for keeping exotic fauna and flora. The Environment Protection Department has received requests for permits to import more than 100 different species of exotic animals, including reptiles, amphibians, spiders, scorpions, land crabs, crayfish and others.
3.1.3.3 Legislation Regulating Aliens