Preventing and managing plant invasions on oceanic islands
Volume 8 Number 2 - July 2011
Oceanic islands are infamous for the extent and impacts of invasions by non-native species. Globally, hundreds of different invasive plant species have invaded island ecosystems and threaten native biodiversity, and the same is true for other groups of non-native organisms (e.g. Caujapé-Castells et al., 2010; Kueffer et al., 2010a). Management of invasive plants on islands is confronted with the challenges of (i) preventing further introduction of potentially invasive non-native species, (ii) containing the spread of already introduced non-native species, and (iii) mitigating the impacts of established invasive species. Comprehensive documents that treat these different management phases specifically for islands are readily available on the internet (e.g. “Guidelines for Invasive Species Management in the Pacific” or Kueffer and Loope, 2009) and I will therefore in this article only focus on selected aspects that are in my view particularly relevant for an effective prevention and management of plant invasions on oceanic islands.
If not botanic gardens, who else?
On islands, botanic gardens have a particular responsibility for invasive plant management because they are often the only organisations with substantial specialised botanical expertise and related international connections. A reliable taxonomy of established non-native plants and a rapid identification of new introductions are the basis of effective prevention and management of plant invasions. Many botanic gardens on islands maintain a continuously updated database of the native and non-native flora (for a compilation of examples of such online databases see Caujapé-Castells et al., 2010, pp. 113).
As key players in the horticulture sector, botanic gardens should be role models of good practice, and the St. Louis Codes of Conduct can serve as a guideline. The example of the Conservatoire Botanique National de Mascarin (CBNM) illustrates how botanic gardens can take on a leadership role in education, awareness building and policy formation. The CBNM was for instance a leading organisation in the preparation and implementation of the invasive species management strategy of La Réunion . As the following paragraphs highlight, the involvement of botanic gardens in invasive species management can include a broad range of activities such as the promotion of the use of native plants, ex situ propagation of native plants, restoration of invaded habitat, networking actors, and outreach to the general public.
“Botanic gardens have the necessary expertise and networks to be key players in controlling invasive species”
Although plant protection is not the focus of this article, it is also important that botanic gardens act responsibly with respect to the prevention of new introductions of plant diseases and pests, which can spread rapidly across small islands and affect the majority of the individuals of a vulnerable native plant species in a short period of time (Caujapé-Castells et al., 2010).
Most non-native plants are friends not foe, and sometimes they are both at once
While some non-native plants are indeed an important threat to island biodiversity, most of them are not problematic. In fact, the presence and abundance of many non-native plants on islands are due to past anthropogenic habitat destruction or the result of deliberate planting for forestry, restoration of degraded land or landscaping, rather than an active invasion of undisturbed natural areas. Many non-native plants remain important for agriculture, forestry and daily life on islands, and some of them have become allies of nature conservation; for instance by stabilising soils, preventing invasions by other and potentially more problematic non-native plants or providing food to native fauna (Kueffer et al., 2010b). Managers and scientists on islands increasingly recognize the beneficial role of non-native plants in heavily altered “novel ecosystems”. Sometimes the same non-native plant species can play a beneficial role in one habitat and yet have major negative impacts in a neighbouring habitat, thus posing new challenges to invasive species management. Non-native cinnamon (Cinnamomum verum) for instance is beneficial in the Seychelles for biodiversity management in mid-elevation forests (Kueffer et al., 2010b) but is a major problematic invader of nearby montane cloud forests.
Prevention – pragmatic solutions and innovations are needed
It is generally believed that the prevention of new introductions of potentially invasive non-native organisms is more effective than the later management of problematic invasions. Indeed, eradication or containment of a spreading non-native species and the management of established invasive species is very difficult and costly. However, prevention is very challenging too and new innovative biosecurity solutions and continuous learning to improve existing good practice approaches are needed. Above all, preventative measures need to be simple and take the limited financial and human resources of island societies into consideration.
“Few, if any, oceanic islands have yet implemented an effective biosecurity system that is able to substantially reduce the rate of new introductions of non-native organisms”
Increased travel and transport mean that introduction rates of new organisms to islands have become so high that effective control at borders is almost impossible. Daily, non-native organisms are transported to islands around the world and inevitably some of these species will not be detected at borders. How prevention can work in an increasingly globalised world is not evident. A solution may lie in developing multi-layered biosecurity systems based on shared responsibilities among many agencies and citizens, with post-introduction detection as a second important filter after border control. Possible invasive non-native species that pass through border control must be detected as early as possible after introduction while they are still localized and eradication is still feasible (Kueffer and Loope, 2009). This will require regular systematic early detection surveys as exemplified by the Hawaiian example (Kueffer and Loope, 2009) but depends also on the collaboration and awareness of other agencies – e.g. those involved in road maintenance, landscaping, forestry and agriculture – and the general public.
Partnerships with stakeholders are of pivotal importance
An important lesson learnt is that stakeholder concerns and expertise should be built into invasive species prevention and management from the very beginning. In Hawaii, for instance, a weed risk assessment system with the aim of predicting potentially invasive non-native species that should be prevented – the Hawaii-Pacific Weed Risk Assessment system (HP-WRA ) – was at first developed relatively independently of stakeholders, especially the plant industry, and consequently acceptance by stakeholders was low. Thanks to committed representatives of the plant industry a more participatory process was later initiated, which significantly increased the acceptance of weed risk assessments and preventative measures among industry partners (Kueffer and Loope, 2009).
In the Hawaiian archipelago it was also particularly effective to set up a separate Invasive Species Committee (ISC) on each of the different islands (Kueffer and Loope, 2009). These island-specific ISCs allow preventative measures: early detection, monitoring, awareness building and outreach to be tailored to the specificities of the individual islands. For instance, on a small island such as Molokai, where everyone knows everyone, effective approaches are different to those required on a highly urbanised island such as Oahu, or a large and sparsely populated island such as the Big Island.
Prevent future invasions, don’t fight the ghost of past invasions
There is a tendency in invasive species management to invest most resources on the management of those islands and habitats that are already heavily affected by invasions. Once a problem is experienced, money flows. But prevention is only effective if it is one step ahead. The most problematic future invasions will likely happen in areas that are not yet badly invaded. In contrast to already invaded areas, un-invaded ecosystems offer open ecological niches that are not yet filled and native biodiversity is not yet impacted through earlier invasions. Ecosystems situated in less developed, remoter or less disturbed regions or islands may thus require particular attention. For instance, the expansion of ecotourism onto new islands or into new habitats may be a reason for concern and should be accompanied by preventative measures, increased early detection survey activities and awareness building. With environmental change and the introductions of new types of non-native plants through land use changes, those habitats that have been resistant to invasions in the past may become vulnerable too.
Promote native and non-invasive alternative plants
A particular important strategy to support the prevention of new introductions of potentially invasive non-native plants is formulated in the St. Louis Code of Conduct cited above as follows: "5. Promote non-invasive alternative plants or, when possible, help develop non-invasive alternatives through plant selection or breeding”. Environmental degradation related to fire, droughts, deforestation, and erosion is for instance a major environmental management challenge on many oceanic islands. To combat erosion, even today non-native Acacia species, which are known invaders, are introduced to new islands. To stop such deliberate introductions that pose a severe invasion risk, alternative ways of restoring degraded sites with native or non-invasive, non-native species have to be urgently developed. For the development and promotion of native plants for use in restoration – as well as, for instance, in landscaping, horticulture or forestry, botanic gardens play a key role. Many botanic gardens on islands do already invest heavily in native plant ex situ propagation programmes.
Resurrection of native biodiversity after invasion can happen
Even in the case of heavily invaded island ecosystems, there is hope. Removal of invasive plants and exclusion of invasive animals from conservation management areas, either through mechanical or biological control measures, can have rapid and dramatic positive effects on the recovery of native plants and other biodiversity. This has for instance been shown for intensively managed conservation areas in Hawaii (Kueffer and Loope, 2009) and Mauritius (Baider and Florens, 2011; Florens et al., 2010), or native regeneration after biological control of Miconia calvescens in Tahiti (Meyer and Fourdrigniez, 2011). A striking example has recently been reported from Conservation Management Areas (CMA) in Mauritius (Western Indian Ocean). Ten years after weeding of sites formerly infested with Psidium cattleianum, juveniles of two presumed extinct, three critically endangered and four endangered native plant species were recorded, together with vigorous regeneration of many other native species (Baider & Florens, 2011). The same positive trend was also found for butterfly species (Florens et al., 2010). Island ecosystems still harbour surprisingly high levels of relict native biodiversity, but this is probably only due to a time lag effect (“extinction debt”) and much of this biodiversity may go extinct without immediate active management intervention. Exclusion of invasive species from intensively managed conservation areas is one such emergency measure.
This article has profited from discussions on the list-server of the Global Island Plant Conservation Network (GIPCN, http://www.bgci.org/ourwork/islands/). I thank Claudia Baider and Eva Schumacher for providing pictures.
Baider, C. and Florens, F.B.V. 2011. Control of invasive alien weeds averts imminent plant extinction. Biological Invasions, in press. doi: 10.1007/s10530-011-9980-3
Botanic gardens and alien invasive species
BGjournal Vol 8(2), July 2012. Download the complete issue as a pdf document here