Why are we so bothered with alien species? The physical isolation of oceanic islands like Seychelles has led to the evolution of very specialised and unique ecosystems. With increased human and other traffic, the role of the ocean as a barrier to plant and animal dispersal has diminished. This has increased deliberate and accidental introductions of alien species into once-remote island environments. Species on islands are very vulnerable to competition with and diseases of aggressive mainland species.
The alien invasive project on Cousin and Cousine involved three things: 1 surveying alien plant species on the islands; 2 investigating possible pathways for the arrival of new invaders; 3 identifying species present elsewhere in Seychelles that could pose a future threat.
The islands of Seychelles provide many examples of the nasty impact invasives can have. One obvious example is the forests of Mahe, now dominated by introduced tree species. Most native land bird species no longer survive in these forests, being now restricted to a handful of the smaller islands, where a safer and more intact native habitat still exists.
It was found that both Cousin and Cousine are largely free of the globally infamous harmful species. Weeds present are predominantly herbaceous, restricted mainly to disturbed or open areas. The islands have been restored to enclosed, self-sustaining native forest, and such open areas are limited. Any woody invasives have been largely controlled through active management and are closely monitored to ensure they do not regenerate.
Although herbaceous weeds tend not to be as problematic, they do still require ongoing management. Generally, these are plants that exhibit high rates of growth, and each plant produces thousands of easily dispersed seeds. The plants can rapidly smother large areas of native vegetation, and prevent their regeneration. This process potentially affects some animal species. The result over time is a very simplified ecosystem that cannot support its former diversity of wildlife.
The more problematic species subject to control programmes on Cousin and Cousine include the Rangoon creeper and the wild pea. Staff remove the invaders by hand weeding, a labour-intensive activity. More focused weed management may make total eradication of some invasive species possible in the future.
An essential part of the management of the islands involves preventing the arrival of new invaders. Tourists, transport of supplies and staff are the most likely avenues by which new invasive species could arrive, and are monitored closely. Awareness of which species are most likely to arrive is vital for quick removal – ‘nipping in the bud’ any new threats, in addition to management of the existing ones. We can keep on top of the threats, with constant effort, but we can never rest on our laurels, so to speak.
Nature Seychelles, published on the Regar Newspaper, Seychelles, 8th October 2005