By Peter de Lange
A recent survey of Rarotonga’s little plants – the liverworts and mosses – has established that all is not well in the volcanic island’s forests. The plants, often dubbed the ‘canaries in the cage’ of the plant world because of their sensitivity to ecosystem change, are drowning under an onslaught of weeds that have spread from people’s gardens.
This is a serious matter as these plants, known collectively as bryophytes, help prevent soil erosion and purify rainwater – still the main water source for Rarotonga’s inhabitants. With the island’s booming tourism industry, the demand for water is increasing and, coupled with global warming, means that island droughts are now a looming concern.
I was privileged to be joined on the survey team by Matt von Konrat, Head of Collections at the Field Museum in Chicago and Mereia Tabua, Scientific Officer at the University of the South Pacific in Fiji. Working closely with the Cook Island government, we completed 10 days of field work on Rarotonga between late September and early October last year. This was the first significant survey ever conducted on Rarotonga and a critical initial step toward understanding the bryophyte diversity of the island. From the island’s spectacular coastal cay, lowland forest, stream catchments, cloud forest, jagged mountains and soaring rock outcrops, we have identified more than 150 species of liverworts and mosses.
But we were disturbed to see how dry the island’s cloud forest was. Merely walking through them elicited cracks and crunches as the ground-cover liverworts and mosses had shrivelled to a point of being tinder dry. Although these plants can recover with only a little rain, sustained drought will eventually kill them and as they dry out they heighten the fire risk. More concerning was the fact that cloud forest stream systems that usually flow year round were virtually dry. These streams feed into the main water supply for the island, nourishing not only people but the island’s unique flora and fauna.
It was disturbing to note how a range of weed species that derive from historical garden escapes, accidental introductions and long-distance dispersal from other neighbouring weedy Pacific Islands, are literally pulling the forest down. Added to this, we failed to find the beautiful white-flowered shrub, the unique Rarotonga cyrtandra (C. rarotongensis). Only four plants were found in 2010 and two in 2014. Another island speciality, a fern (Asplenium schisotrichum) that grew along lowland stream sides, has now not been seen for 20 years. There is now a very real concern that these species are extinct. If so, what has else has been lost?
As for the little plants, the bryophytes that we all take for granted, it’s important to know that they have a multitude of uses, including purification of the water supply, potential medicinal treatments and, of course, the enhancement to natural beauty. If the ongoing weed invasion continues, the island’s forested ecosystems will deteriorate further, some to the point of collapse, potentially leading to polluted water sources, increased soil erosion and further deterioration of the island’s lagoon system.
The spread of garden plants is not the only issue: Two mosses (feather moss, and a species of Fissidens) were found in car parks at Avarua, the capital of the Cook Islands. They could only have arrived by human traffic, most likely from Australia or New Zealand where they are now well established. While the study has more work to do, initial indications show that unless measures are taken to tighten biosecurity, control the spread of weeds, and restrict water usage, Rarotonga’s beautiful verdant tropical vista may soon be replaced by a global weed flora.
The result may be not just the loss of its unique flora but also its iconic bird life, including the Rarotongan Flycatcher or kakarori, a species brought back from the brink of extinction over the last 30 years.
The impact of humanity is being felt all over the world and it’s vital that we make moves to protect indigenous biodiversity wherever we can. Although the survey is still in its early days, we’ll be doing further work to investigate the conservation status of the bryophyte plants and establish their ecological and cultural significance.
Peter de Lange is an Associate Professor (Botany, Ecology, Plant Conservation, Biosystematics) in the Department of Environmental and Animal Sciences at Unitec. His research interests include plant biosystematics and biogeography (with a special interest in insular flora), plant conservation and management, threat classification systems, the cytogenetics and ecology of island endemic plants and ethnobotany. He also has an interest in Quaternary Earth Sciences. He holds an M.Sc. (Hons) in Palaeoecology and Tephrochronostratigraphy from the University of Waikato, and a PhD in Plant Biosystematics from the University of Auckland. He is also an Adjunct Professor of the Department of Natural Sciences, University of Sassari, Sardegna, Italy.
He previously worked for the former DSIR Botany Division (1984-1985), Ministry of Agriculture & Fisheries in the Aquatic Plants Division (1989-1990) and Department of Conservation as a Research Scientist (1990-2017).
He has published over 170 papers in peer-reviewed literature, and 20 books – mostly on threatened plant conservation, management, biosystematics and island flora.