Ile Plate’s Ecosystem faces challenges from unwanted Castor oil plants

Upon arriving on Ile Plate, what strikes you is not only the flat sand cays exposed at low tide and the far-reaching lagoon outlined by a foamy white barrier where the ocean meets the coral reef, but also the lush green vegetation that conceals all human habitation.

Upon closer inspection, one is surprised to note that apart from coconut trees, the greenery is mostly attributed to Castor oil (Ricinus communis) or ‘Tantan’ in Creole. This introduced species is believed to occupy an area of 5,000 square meters, which is roughly the size of a football pitch. It is considered one of the most dominant invasive species on Ile Plate, an island slightly bigger than Ile Moyenne. What is worrying to conservationists is that the plant is now widespread, and there are no indications that it is slowing down.

This has prompted the Island Conservation Society (ICS) to implement a vigorous eradication program to contain this species, which, as on other islands of Seychelles, was introduced and cultivated for its medicinal properties and is now escaping into the wild.

“We cleared this area as part of our first project on the island. And look how it is now! They’ve all grown back,” says Diary Rakotondrazara, the Conservation Officer of ICS, as she points to a growing patch of thick Castor oil. So thriving is this species on Ile Plate that a tree measuring 5.10 meters - the height of a two-storey building - has gone down as the tallest ever recorded.   

“The best way to eradicate Castor oil without disrupting the environment is to do it manually,” says Rakotondrazara, adding that this necessitates uprooting each plant one after the other. As part of their weekly routine, the team remove Castor oil plants regularly, with an ambitious target of clearing a 5-meter square area per week. “In one month, we can remove as many as 340 plants from fruiting trees to different sizes,” she adds.  With a flowering season that spans throughout the year, the task is harder than the team initially thought it would be.

So far, castor oil is found almost everywhere, but they are most abundant in the northwest to southwest of the island. Since ancient times, the medicinal properties of the Castor oil plant have been documented, and to this day, in several countries, different parts of the plant are still used for such purposes. The seeds, for example, are known to purge worms whilst the leaves can be crushed to make ointment to cure wounds. On the other hand, Castor oil is also very toxic if ingested in large quantities, and it is also used as a natural insecticide in agriculture. But when left in the wild as on Ile Plate, this species can also become very invasive.

“The seeds can stay in the soil for a very long time and re-sprout again even after a short drizzle,” says Rakotondrazara. “We have noticed that several areas which have been cleared have already been recolonized by the plants. In addition, many trees have fruits, and these are easily dispersed by birds and people but also by wind to other areas.” As part of their strategy, the team always remove the seeds first before uprooting the plants. An astounding 3 kilograms of Castor oil seeds have been collected and disposed of by ICS since their arrival on the island late last year.  

These past 6 months have been a learning curve for the team as they learnt to tackle this species and assist in restoring the biodiversity of the island. The project is also of great interest to other partners on the island, as Castor oil plants also harbour a species of caterpillars from the genus Euproctis - a yellowish-black caterpillar with stinging hairs or “hairy caterpillars,” locally referred to as “Senir plim,” which can cause incessant itching. Spraying these hairy caterpillars is not encouraged as this could also hurt insects and other biodiversity of the island, including the abundant diadem butterflies.

Such projects are also vital for other restoration projects that ICS wishes to implement on Ile Plate. Another plant species on ICS’s radar is the Coco grass (Cyperus rotundus), or “Lerb Zonyon” in Creole which is an indigenous plant that has also grown in abundance on the island. This plant species is referred to as one of the most invasive weeds in the world and is widespread on several continents. Similar to Castor oil, Coco grass is also a conservationist’s nightmare, as it is difficult to control, since its tubers can grow up to 8 inches below the ground surface, and it is also highly resistant to herbicides. 
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ICS is dedicated to safeguarding the biodiversity of Ile Plate, one of the smallest islands under its care, with a strong focus on preserving its terrestrial ecosystem. New bird species, such as the Brown Noddy (Anous stolidus), Lesser Noddy (A.tenuirostris), Wedge-tailed Shearwater (Ardenna pacifica) and the Red-tailed Tropicbird (Phaethon rubricauda), have already shown interest in recolonising the island. Improving the island's plant life is expected to further increase the likelihood of other species establishing themselves on this small yet significant island, located just a few kilometers from Mahé and at the edge of the Outer Islands group.

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