Naturalised and invasive alien plant species in the Caribbean Netherlands: status, distribution, threats, priorities and recommendations
The Netherlands are signatories of the international Convention on Biological Diversity (CBD). This implies that the nation will protect biodiversity on its territory. This includes the protection of natural fauna and vegetation from negative impact caused by invasive alien species (see 2.1. for a definition). By 10-10-2010 the BES islands (Bonaire, St Eustatius and Saba) became ‘special municipalities’ of the Netherlands. They together form “Caribisch Nederland” (Caribbean Netherlands, Hulanda Karibe). Due to this stronger link to the Netherlands many responsibilities have moved from the Antillean government to the Netherlands. This includes important responsibilities with respect to the protection of nature.
The present study was financed by the Dutch Ministry of Economic Affairs, Agriculture and Innovation and included a literature study, a field trip and writing of the present document with main observations, conclusions and recommendations. A major part of the report consists of an alphabetical list of (known) invasives with their current status (4.1.1.). Apart from the three islands belonging to Caribisch Nederland, for completeness, some attention is given to Aruba, Curacao and St. Maarten as well (esp. in 4.1.1. and Appendix II).
Stages of invasion
In order to define the problem of invasive alien (non-native) species of plants more accurately it is relevant to recognise the following categories:
Exotic: Species that are not part of the natural indigenous vegetation are called exotics. Examples are introductions as ornamental or agricultural species. If contained within the confines of gardens and farms, these species are not considered problematic.
Established: Species that occur ‘in the wild’, i.e. outside the control of cultivation or husbandry and are able to reproduce themselves resulting in new individuals, we call established (present). Species can stay in this phase, the ‘lag phase’ (see 2.1), for quite some time. It is the stage in which the species adapts to its new environment using its genetic flexibility. At this stage complete eradication is still an option, because the number of individuals and locations is limited. This means that the costs can be relatively low, compared to eradication at a later stage.
Naturalised: If given enough time, species may start to adapt genetically to the new environment, by optimising its physiology and/or growth habit. As a result the species will start spreading more rapidly and effectively and becoming part of the natural flora. In most cases this is not considered a major problem; the plants will get their own function within the ecology of the island and will not replace indigenous species entirely. Moreover, the costs of complete eradication have become prohibitive at this stage, so only containment is an option.
Invasive: It is generally believed that about one in one thousand exotics becomes really problematic, e.g. with respect to environmental, ecological or economical impact (Williamson 1995). They start to grow out of control, massively invade natural habitats and reduce or eliminate native species. They have broken down the dispersal barrier and have become invasive. At this stage one can only try to achieve a stage of equilibrium, of mitigation, by intensive control measures. These are usually limited by financial resources, and can normally only be successful with commitment of the local society, e.g. shown by the enthusiastic support and hand labour of many volunteers.
Invasives of the Caribbean Netherlands
In this report 65 species of invasives are enumerated (4.1.1.) with their history and properties, based on a literature survey and completed with experience and findings of the authors. Four of the main problematic species are treated more extensively in 4.1.2. These are the Coral vine (Antigonon leptopus) which poses a great threat to nature, especially in St. Eustatius; the Rubber vine (Cryptostegia grandiflora) which is able to overgrow and smother shrubs and trees and is especially spreading on the Leeward Islands; the neem tree (Azadirachta indica) which is planted for shade and medicinal purposes, but is escaping on Bonaire; and ‘Donna grass’ (Bothriochloa pertusa) which is a very problematic species replacing the more palatable local grasses on the Windward Islands, most notably on St. Eustatius.
In a complementary list a further 80 species that need more investigation are mentioned (4.1.3.). This list is not complete but it enumerates species that are present on at least one of the islands. They need special attention because it is best to prevent them from entering at all or to eliminate the few plants or populations that have established themselves. Some species in this list are already present at some scale, like some of the arable weeds, but need careful monitoring to prevent them from entering nature.
A general problem are the free-roaming animals, cows, donkeys and especially goats (all non- native species) that are destroying nature in an uncontrolled way. Their presence has a detrimental effect on biodiversity, eating young seedlings and trees, and thereby preventing the natural regeneration and succession. Moreover, the bare soils that result are susceptible to water and wind erosion; material that is deposited in the surrounding seas.
Before an exotic has been introduced prevention is the most important action, i.e. keep the chance that exotic species may be introduced as low as possible. As soon as a first introduction has been realised and the exotic still occurs at low densities at few sites, eradication after first observation will be the most important action. Finally, if an exotic has already spread over different sites or even different habitats and has increased in densities, eradication might not be an achievable option anymore. Then containment and population management will be the most relevant actions to minimise the negative impact (mitigation). In general, prevention will generate the most cost-effective options to avoid problems due to invasive exotic plants (Davis 2009). The main observations are:
Prevention: Prevention plans need to be developed with regulations restricting the import of exotic species. This includes the development of ‘Black lists’ for the Leeward and Windward Islands respectively. Public awareness (customs and other officials, general public, landscapers, new inhabitants) must be raised and alternatives for imported exotics must be offered. Agricultural departments and customs offices on all islands are understaffed and not able to control the many routes through which exotics enter.
Eradication after first observation: Rapid first observation of an exotic plant after introduction into the wild is essential for the success of an eradication action. Therefore a ‘Watch list’ or ‘Grey List’ needs to be developed. Since the difference in climates, these watch lists will partly differ between islands and differ even more between the Leeward and Windward islands. Also knowledge about the natural flora and invasives must be increased through education, at schools as well as for professionals (rangers, customs personnel, agricultural department, etc.). Floras for the Windward Islands are outdated and not accessible.
Containment/population management: Management plans need to be developed for the control Antigonon, Cryptostegia and neem to be able to stop further spreading and to mitigate the impact on nature. Research on the life cycle of invasives and experiments for their control have to be carried out. The problem of roaming animals must be tackled. Small island communities are not able to do this without outside assistance. If chemical control is considered, special Dutch Caribbean regulations apply based on restricted import permissions for crop protection agents.