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• CIRAD , France
• University of Antananarivo , Madagascar
• School of Geography and Environmental Science , Monash University, Australia

Madagascar is one of seventeen countries that together account for 80% of global biodiversity. In particular, the island has 12 000 plant species, of which 70 to 80% are endemic. However, this biodiversity is also somewhat fragile: because these treasures developed in isolation, they are vulnerable to competition from invasive species.
More than 400 species of forest trees and bushes have been introduced within a network of arboretums across the island, primarily in the 1950s. CIRAD researchers and their Madagascan and Australian partners have studied the list with a view to determining which species are most likely to become invasive and may thus pose a threat to biodiversity. They looked at the number of arboretums in which each of these forest species was introduced locally, concentrating on species considered invasive in at least two other countries, amounting to twenty-one species. Of those, nine are invasive in at least three countries and the other twelve in at least two. When compared with field observations, their results showed that five species found in these arboretums were likely to spread, to the detriment of the indigenous vegetation and its intrinsic biodiversity.
However, for the researchers involved, the heart of the issue is still man and manmade activities. In effect, the drive to diversify incomes often leads to the introduction of living organisms that are more appropriate and perform better. As we know, this process sometimes results in unexpected invasions that damage local biodiversity. However, it is not enough to point the finger at the species responsible: it is important to look at the advantages and disadvantages of an invasion. An invasion, in the form of a species introduced for the purpose of producing something (wood, fruit, fodder) may prove useful for poorer populations, and thus hold a degree of economic interest. It may also provide economic services such as helping to control erosion. In the highlands of Madagascar, Acacia dealbata , for instance, spontaneously colonizes and enriches fallow land, thus improving its fertility, and also provides farmers with firewood and protects the hills from gullying.
Faced with the growing phenomenon of plant invasions, management plans are required on every level in countries such as Madagascar. For the researchers involved, it is necessary to prioritize and then implement communication, research, early detection and preventive control operations so as to limit the emergence of new invasions. If the invasion has already begun, it is important to provide diagnostic information that reflects its impact not only on biodiversity, but also on all local production goods and environmental services.

A longer version of this article was published in issue 299 of the journal Bois et forêts des tropiques .

Literature

Tassin J., Bellefontaine R., Roger E., Kull C., 2009. Evaluation préliminaire des risques d’invasion par les essences forestières introduites à Madagascar. Bois et forêts des tropiques, 299: 27-36
Tassin J., Rakotomanana R., Kull C., 2009. Gestion paysanne de l'invasion de Acacia dealbata à Madagascar. Bois et forêts des tropiques, in press.
Kull, C. A., Tassin J., Rambeloarisoa, G., Sarrailh, J.-M., 2008. Invasive Australian acacias on western Indian Ocean islands: a historical and ecological perspective. African Journal of Ecology, 46(4): 684-689.
Kull C., Tassin J., Rangan H., 2007. Multifunctional, scrubby, and invasive forests? Wattles in the highlands of Madagascar. Mountain Research and Development, 27(3): 224-231.