Calls to protect dragonfly, damselfly environment on Lantau Island, home to 67 species
Green group urges government to reference study findings and protect watercourses on city’s largest island
At least 67 species of dragonfly and damselfly – half of the total figure for Hong Kong – can be found on Lantau Island, the city’s first comprehensive survey of its kind in recent years has found.
The environmental group behind the study said the real figure could be closer to 80 but the results highlighted the importance of the relatively pristine freshwater ecosystems of the city’s largest island and the need for proper and timely protection.
For the first time, at least 15 odonate species – a predatory insect group such as dragonflies and damselflies – recorded by Green Power were seen on the mountainous island, including the common evening hawker, Hainan hooktail, black river darter and eastern lily squatter.
“The island’s large size means there is a huge diversity of freshwater ecosystems such as hill streams and abandoned farmland in lowland areas that have since become small water catchments,” said the group’s senior education and conservation officer Elaine Yuen Yan-ling.
“As they flow out to sea, they also create mangrove and reed bed habitats, which are very suitable for dragonflies and damselflies.”
The insects require freshwater habitats to lay their eggs, and their larvae exist as aquatic nymphs before climbing out of the water and morphing into their flying adult forms.
The team began conducting a baseline survey of odonates at Tung Chung River in 2012 and again in 2015 and 2016, this time, with five other sites including Wong Lung Hang, Mui Wo, Pui O, Sham Wat and Tai O.
The area with the most biodiversity was Tung Chung River, which boasted 47 species including the fiery emperor dragonfly and mangrove skimmer, which is listed on the International Union for Conservation of Nature’s Red List of Threatened Species as “vulnerable”.
This was followed by Tai O with 41 species, Wong Lung Hang with 39, Mui Wo and Pui O with 38 each and Sham Wat with 31.
In all, 67 species were found across Lantau Island, forming about half of the 120 species known to Hong Kong.
“Dragonfly larvae ... live and grow freshwater environments and require oxygen-rich water. Their survivability can be seen as an indicator of freshwater quality,” Dr Cheng Luk-ki, the group’s head of scientific research and conservation said. “It shows there is a limit to how polluted the water upstream is.”
Lantau’s hilly topography – most areas higher than 100 metres are protected within country parks – mean water quality in hill streams are relatively clean.
Cheng said the fact that dragonflies could thrive in these areas meant riverbeds were still untouched, facilitating the growth of different microbes, algae, protozoa, insects and other life forms, which form the foundations of a healthy riverine ecosystem.
But threats loom over the horizon, not least from environmental pressures of new town extensions in north Lantau and other developments.
“Apart from large developmental threats, there are some smaller ones that shouldn’t be overlooked, such as man-made destruction,” Yuen said.
“Most downstream areas of rivers have been channelised [as storm drains], for example. Then there are incompatible land uses near low-lying wetland areas such as open-air scrapyards, that can pollute wetlands.”
Waste water discharge and illegal dumping from nearby villages, as well as open-air storage sites have polluted several nearby watercourses, the group said.
They urged authorities to reference the survey’s findings and do more to protect the island’s watercourses in a holistic manner, such as by prohibiting works near natural stream beds and riverbanks, banning the connection of sewage and stormwater drains and clamping down on illegal dumping of waste near wetlands.
They also called for more water to be released into natural streams that have been previously intercepted by catchwater structures.
In December, the government’s Sustainable Lantau Office commissioned an ecological study of three priority sites on the island – Pui O, Shui Hau and Tai O – to review their existing ecology and identify other appropriate conservation measures. The other odonate hotspots identified in Green Power’s study were not included.
A spokesman for the office said the green group’s findings would serve as a useful reference for its study, which is still under way.
“Any development proposal in the vicinity of any ecologically important habitats should be subject to impact assessment and feasibility study to ensure that the project would not result in an unacceptable impact on their conservation value,” the spokesman added.