Around half were introduced deliberately (ornamental, crop or forest species), and the other half accidentally, along with imported seeds. In addition to chemical, mechanical and more recently biological control measures, it is now crucial to intervene upstream of invasion to prevent the introduction of potentially invasive species or regulate the spread of plants that are already on the island.
Lake Gol is a particularly eloquent example. In early 2006, it was completely covered by a mixture of water hyacinths (Eichhornia crassipes) and water lettuces (Pistia stratiotes). In March 2006, cyclone Diwa cleared almost 99% of the lake by flushing it out and washing the plants into the sea, but eight months later, it was again totally covered, to the detriment of the aquatic ecosystem as a whole. These aquatic plants are estimated to produce some 250 tonnes of biomass per hectare per fortnight, a rate that rules out mechanical control and calls for biological measures, which have already proved their efficacy in many tropical countries.
Rational joint management of natural environments and rural areas
There are four main ways of intervening upstream of invasion. The first is to ban the introduction of risky plants. This means assessing the risks beforehand. To this end, analyses by CIRAD under the EU POSEIDOM* programme served to identify around fifty potentially invasive plants, most of them ornamental. A ban is due to be placed on introducing those plants in the French overseas regions. Of the ornamental species grown in the highlands of Reunion, 34 are highly invasive. Moreover, once plants are introduced, there is a latency period before they begin to multiply and spread. However, once they have spread over an area of 100 hectares, they are impossible to eradicate, which confirms the need for early intervention.
It is also vital to take account of the movement of species between the various elements of the landscape: forests, rangelands, crops and inhabited areas. This is the second line of intervention. For instance, grasslands, which contain a high proportion of species from outside (80%), can play a major role in plant movements on a landscape level, depending on how well they are managed. In sensitive areas such as the highlands, rearing herbivorous animals is one way of managing the environment. Recent work under a rangeland management project (PASTOFOR)**, showed that productive, well-kept grasslands help to maintain biodiversity in surrounding natural environments by limiting the spread of invasive species. On the other hand, grasslands in which weeds are not sufficiently controlled are a threat to neighbouring natural environments. The interfaces between different environments are key to the circulation and development of invasive species: rational joint management of natural environments and rural areas is the only way of controlling them.
Taking account of how plants spread
Classing invasive plants according to their ecological impact and ability to conquer new areas has meant that it is now possible to define the priorities more effectively. However, it is also necessary to determine how they spread depending on the ecological context and on how the environments concerned are managed. In fact, these plants may spread in different ways depending on the type of environment. For instance, the false pepper tree (Schinus terebenthifolius) grows from seed in humid environments and from suckers under dry conditions. Likewise, the giant bramble (Rubus alceifolius) bears fruit at low altitudes but only propagates vegetatively at heights of more than 1000 metres above sea level.
The results of these studies mean that it is now possible to take more effective action against the threat posed to the island's biodiversity by these invasive plants. Managing invasive species on an island means taking account of all the processes that lead to invasion and considering various levels of intervention, from preventing risky introductions, through early detection of the initial signs of invasions, to control and the subsequent restoration of environments. Lastly, over and above preventing invasion, the main priority is to make local populations aware of the problem.
* POSEIDOM: Programme of options specific to the remote and insular nature of the French overseas department.
** PASTOFOR: Management of pastoralism on the fringes of highly protected natural environments.
Helen Burford | alfa
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