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An invasion of the leafy kind

  • Assessing the impact of the European Gorse rooting its way into Horton Plains

By Buddhika Samaraweera

No doubt not only nature lovers, but almost everyone has heard of “invasive plants”. But what exactly are they, and why are they considered to be such a major issue?

Many species are introduced to locations outside their typical distribution through natural and man-made processes, and they are known as “invasive”, “non-native”, or “alien” species. These are species that have arrived from afar and taken over native species’ habitats and resources. They rapidly expand their populations at the expense of the native flora, and are divided into two groups, namely; natives and aliens.

Native plants and animals can become invasive when their normal population multiplies dramatically, smothering other species due to changes in environmental conditions or other factors. Aliens are species that have been brought in from the outside, either intentionally or unintentionally. In many cases, these invasive species cannot or will not be devoured by other species, reducing the amount of food available to the food chain. Many other species’ habitats are then deteriorated and diminished, leading to a negative impact on their numbers. Invasive species flourish, and are now one of the key issues in the management of various forest types.

Many protected areas in the country are dealing with invasive species, both flora and fauna. Among them, “European Gorse” is an invasive plant on the Horton Plains.

Horton Plains is situated in the highest elevation of the island (1,750-2,384 metres) with distinct weather patterns, cold climatic conditions, and highly range-restricted endemic flora and fauna. Horton Plains is located on the southern plateau of the central highlands of Sri Lanka. It is also a popular tourist destination and was a part of a large system of plains and forest cover that included Agra-Bopats, Moon Plains, and Elk Plains. The average temperature of the area is around 15 °C and gets to near-zero levels at times, according to the report of the Department of Wildlife Conservation (DWC). 

This highland plain is a mosaic of montane cloud forest patches and scattered grasslands. Grassland habitat (wet patana) is one of the prominent habitat types of the Horton Plains. This habitat is dominated by native species like Chrysopogon nodulibarbis, Garnotia exaristata, Andropogon polyptychs, and Dwarf Bamboo (Arundinaria densifolia), and has lately been invaded by fast spreading alien invasive species like European Gorse (U. europaeus), Aristea ecklonii, Ageratina riparia, and Austroeupatorium inulifolium.

The unique weather condition that prevails in the area creates a home away from home for the opportunistic and notorious Gorse plant. Horton Plains is the only locality in the country that sustains a population of European Gorse. Due to the spreading of the invasive European Gorse, the integrity of species’ composition and community structure of these montane grasslands remain uncertain, even after the eradication programmes of 2016-2017 (the first large-scale attempt following several unsuccessful eradication programmes in the recent past). In said programme, European Gorse removal was conducted using a large number of human labour hours, uprooting the shrubs and burning the vegetative parts in successive periods. The previous programmes were mostly one-time efforts, conducted haphazardly.

In this background, a research team comprising Dulan Jayasekara, Dr. Sahani Chandrasiri, Chathuranga Dharmarathne, and Chamara Prabhath conducted a long term study (2015-2021) under the supervision of University of Sri Jayewardenepura Zoology Department Head Prof. Dharshani Mahaulpatha on the ecological influence of this European Gorse eradication programme in the Horton Plains National Park.

The evergreen shrub Ulex europaeus, commonly known as “Gorse”, native to the Western Europe-Atlantic coast of Europe, has been identified as one of the 100 worst alien species of the world. It is a thorny leguminous perennial shrub that belongs to the family Fabaceae which grows up to seven metres in height. The stem of the plant is armed with conspicuous spines to deter herbivores. European Gorse produces acicular evergreen leaves that develop to scales or spine-like phyllodes with maturity. This plant produces pea-like dark yellow flowers and ciliated green pods that become dark brown at maturity. Each pod contains five to nine brown seeds.

European Gorse plants can live for up to 29 years and large seed banks can be observed in infected areas. Seeds of European Gorse can remain viable for more than 10 years in the soil or even a couple of decades and the optimum germination temperature is at 10-15 °C. The geographical distribution of European Gorse extends to more than 50 countries/island groups throughout the world, with climates similar to its native range in the Western Europe. Interestingly, in the central mountains of the tropical Indian Ocean island Sri Lanka, European Gorse has found a home in an unlikely place.

Commenting on the matter, research team member Dr. Chandrasiri mentioned that since the introduction of European Gorse into the area by the British colonists in 1888, several forest and grassland faunal species got attracted to the plant as behavioural adaptations. In some of its native ranges, a bimodal flowering onset has been observed in European Gorse. 

In this tropical montane climate, European Gorse has been observed flowering almost throughout the year in the absence of seasonal variation in climate. This phenomenon attracted several pollinator species, as well as their predators, to this plant. The flowers of this plant do not produce nectar.

However, according to research team member Dharmarathne, the pollen of European Gorse can attract some insects with their beneficial properties. Pollinator species include mainly “bee” species, while one of their main predators – the Black-lipped agama/lizard – and several bird species like Dull blue Flycatcher and Pied-Bushchat have been observed to be frequenting the European Gorse shrubs to prey upon the insect pollinators. Previous researchers investigated the beneficial properties of European Gorse towards native Black-lipped agama/lizards. Bees use the pollen of European Gorse to feed their larvae, which in turn strengthens the hives, and increases overall honey production.

The consequences that were left unanswered by the rapid eradication of this invasive plant are yet to be evaluated. Therefore, the research team has conducted this research to evaluate the impact of European Gorse eradication programmes on the balance of native faunal and floral assemblage, specifically considering the impact on faunal species that had been attracted to the beneficial properties of the plant. Furthermore, the success or failure of the programme was also evaluated by comparing the pre-removal and post-removal periods.

Research team member Jayasekara mentioned that they “used several quantitative parameters, in the form of density, abundance, and object-oriented spatial analysis, for the Spatio-temporal evaluation of the changes that occurred parallel to the European Gorse eradication programmes”. “The results will indicate the amount of ecological resilience towards the (partially) human-intervened alterations to the grassland habitats of the Horton Plains,” he said.

The quantity of knowledge accessible about the species of interest, as well as its ecological connections, may determine the success or failure of invasive species management initiatives. The invasive nature of these alien species may cause novel ecological linkages in new settings that were not present in their original range, resulting in novel ecosystems, species combinations, and ecosystem function changes.

According to Prof. Mahaulpatha, the breakdown of biogeographic barriers through the global human transport of species has increased the pace of this phenomenon, in particular the spread of invasive plants. She further said that the highly competitive nature of invasive plants results in them consuming more resources than native plants. She added that the invasive plants pose a challenge to the pollination and reproduction of endemic plants. “On occasions, these invasive species may change the abundance and fitness of animal species. However, these changes may include both adverse as well as facilitative impacts to the native species. Therefore, eradication of invasive species, once established in a nonnative landscape, may trigger unexpected consequences.”

According to the researchers, this study is a comprehensive example of the impacts of invasive species in non-native landscapes. While most biological invasions are considered to be threatening to native communities, the long-term establishment of invasive species causes more complex scenarios where invasive species management could be more challenging. European Gorse had been rapidly invading the native grassland vegetation in one area of the park, growing as a large continuous stand and dominating the native vegetation types.

The pre-removal and post-removal densities of European Gorse suggest that the removal programme carried out in 2016-2017 was successful to a greater extent in the short term. However, after a few months of shrub removal, small bushes that survived the removal programme, and newly-emerging vegetative parts (probably from the buried dormant seeds), have been observed, despite the park management’s efforts of continuous removal with the help of available labour.

Research team member Prabhath mentioned that the considerable increase in European Gorse density in the post-removal periods in 2021 indicates the high regenerating capacity of the plant. It is interesting to see how long these seeds will keep producing young buds. If it lasts for a longer time, close to 30 years as the literature suggests, the mechanical removal of plants will cost a large number of labour hours and time in order to achieve complete and successful eradication of this invasive plant. Therefore, the researchers suggest that the viability of mechanical removal programmes be studied further.

Moreover, they doubt that other control methods such as herbicides, fire, and biological control used in other parts of the world could apply to the highly-sensitive ecosystems of the Horton Plains. It leaves continuous mechanical removal (uprooting) the only option of controlling this invasive plant in these habitats. However, lack of available human labour and funds make it difficult for the park management to continue the eradication programmes for extended periods, which in turn has resulted in newly emergent plants reaching maturity, producing flowers and contributing more seeds to the seed banks. The natural stream system of the park aids the dispersal of seeds.

Researchers also mentioned that, based on the findings of this study; there were both positive and negative impacts of eradication programmes of European Gorse. When long-term conservation targets are considered, it can be argued that invasive species like European Gorse should be eliminated from their non-native habitats. Furthermore, the invasive nature of the plant species itself poses a threat of unbalancing the ecosystem and community structure. The scenario observed, particularly with Black-lipped lizards and bees, gives the message of implementing correct and timely conservation and management practices before the invasive species causes irreversible changes to the habitat and community. If European Gorse was eliminated from the habitat in its early stages of invasion, it would not have affected the endemic and threatened Black-lipped lizard, or the pollinators.

Therefore, the early detection and removal of invasive species is vital for reducing their ecological impact. However, once the installation is done and the changes irreversible, the costs and benefits of eradication must be carefully weighed before undertaking a large-scale enterprise. They highly recommend considering the possible relocation of sensitive species like the Black-lipped lizard and endemic Pygmy Lizard (Cophotis ceylanica) before engaging in future removal programmes. Park management should continue strict fire prevention protocols and visitor awareness programmes. The controlled burning of removed plants should also be carried out under strict guidelines, since burning plots could become potential future nurseries for European Gorse. There is a need for research focused on identifying ecologically less damaging and more viable removal methods that could be implemented in highly sensitive areas like the Horton Plains.