Silent invaders encroaching on Horton Plains
Renowned for its breathtaking scenery, a blend of undulating hills draped in montane cloud forests, open pathana grasslands, and wetland habitats, Horton Plains harbours an extraordinary array of endemic flora and fauna, including the Sri Lankan sambar deer, leopard, purple-faced leaf monkey, rusty-spotted cat, grizzled giant squirrel, and European otter. It also shelters numerous endemic bird species.
The park’s flora includes iconic species such as Rhododendrons, Syzygiums, and Calophyllums, along with rare wildflowers and orchids, some found nowhere else. Additionally, it serves as the watershed for several major rivers, including the Mahaweli, Kelani, and Walawe.
Recognizing its ecological, hydrological, and aesthetic significance, Horton Plains was declared a Nature Reserve in 1969 and later became part of the UNESCO-listed Central Highlands of Sri Lanka in 2010. Today, it is managed by the Department of Wildlife Conservation.
However, this fragile ecosystem now faces a growing threat—invasive alien plant species.
A growing invasion
One of the oldest and most notorious invaders of Horton Plains is Ulex europaeus (Gorse), a thorny shrub introduced from Western Europe by the British during the colonial era. Originally planted for soil erosion control and landscaping especially around Farr Inn, a former hunting lodge, it eventually spread into the park’s native grasslands.
While gorse has long been a concern, some other alien species are now emerging at an alarming rate. The most aggressive among them include:
Mist Flower (Ageratina riparia)
Crofton Weed (Ageratina adenophora)
Blue Stars (Aristea ecklonii)
White Weed (Austroeupatorium inulifolium)
Various Pennisetum species
These species have begun colonizing pathana grasslands, disturbed areas affected by fires, walking paths, stream banks, and, in some cases, even forest undergrowth.
Additionally, Cestrum elegans (Purple Cestrum), an escaped ornamental plant, has been spotted on forest margins—an early warning sign of potential encroachment into the pristine montane forests.
New arrivals: A more alarming threat
Recent field observations have revealed the establishment of some new alien species within the park’s grasslands:
Strawberry Guava
(Psidium cattleyanum)
Native to South America and belonging to the Myrtaceae family, Strawberry Guava is a highly invasive species in many tropical regions. It has devastated native ecosystems in Mauritius, where it forms dense thickets that out-compete indigenous flora.
This species exhibits several traits that make it a formidable invader:
Prolific fruiting, attracting animals that spread its seeds
Shade tolerance, allowing it to persist under forest canopies
A clonal regenerative strategy, enabling rapid expansion
Possible allelopathic effects (chemical warfare of plants) where it releases chemicals that inhibit the growth of surrounding plants
Unchecked, Psidium cattleyanum could severely impact forest regeneration in Horton Plains, leading to long-term ecological disruption.
Loquat (Eriobotrya japonica)
A member of the Rosaceae family, Loquat is native to China but has been introduced worldwide. Known for its rapid growth and adaptability, it spreads mainly through birds and mammals that feed on its fruits. Loquats have proven invasive in cloud forests of Mexico, where they out-compete native vegetation.
Glory Bush (Pleroma urvilleanum)
This striking ornamental plant has been observed growing near some premises within Horton Plains. However, its presence raises concern especially when considering the history of Gorse that turned into the most persistent and damaging invasive species in the Horton Plains National Park.
Pleroma urvilleanum is known to be a prolific seeder, with its seeds often dispersed by birds. It can form dense stands in open pastures and natural habitats, potentially threatening native ecosystems by creating monotypic thickets that out-compete and displace indigenous plant species.
Passion fruit (Passiflora edulis)
A few passion fruit vines were observed growing near the pathways. This species has the potential to alter habitat structure and composition, thereby affecting native wildlife and ecosystem services within the national park. It is recognized as an invasive species in regions such as the Galapagos Islands, Hawaii, Raoul Island, and several other tropical oceanic islands.
Invasive potential
The spread of invasive species in Horton Plains follows a pattern observed in many ecosystems. A key example is Gorse (Ulex europaeus). British botanist Henry Trimen included a description of this plant in the Handbook to the Flora of Ceylon in 1894 as occurring around Nuwara Eliya but showed no signs of invasiveness at the time.
However, about a century after its introduction, it turned invasive. Why?
One major factor is ecosystem disturbance. Natural ecosystems, when intact, are resistant to invasions. However, disruptions –such as fires, land clearing, and human activity – create openings for opportunistic species.
Fires have devastated native pathana grasslands, making them vulnerable to gorse invasion. Near Red Bridge, a striking contrast can be observed; disturbed areas dominated by gorse, while undisturbed grasslands remain intact.
In the late 1950s, parts of Horton Plains were cleared to cultivate seed potatoes. Though the farm was closed in 1969, the land did not recover with native grasses. Instead, Pennisetum species, introduced with the potato farm, took over.
The rise in sambar deer populations due to the spread of exotic grasses altered the ecosystem. Their heavy grazing further increased the alien grass cover by replacing native grasses. Increased sambar population can prevent forest regeneration by consuming seedlings and saplings of native trees.
These examples highlight how human activity and ecological imbalances create conditions that allow invasive plants to thrive.
The silent invasion of alien species in Horton Plains underscores the urgent need for continuous monitoring and conservation action. While the impacts may not be immediately visible, the long-term consequences could be devastating.
Preventative measures, early detection and prompt removal efforts are crucial to safeguarding this irreplaceable wilderness.
(The writer is a member of the Flora Study Circle of the Youth Exploration Society of Sri Lanka)
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