Mai Po Biodiversity: Freshwater Ponds
by Thomas Gomersall
Concentrated at the southern end of the reserve, the freshwater ponds of Mai Po were created by WWF-Hong Kong in the 1990s to help make up for the loss of an ecologically important natural marsh to the development of Fairview Park. After obtaining six brackish-water commercial fish ponds from the government, WWF drained the ponds and allowed them to fill naturally with rainwater, which is still their main means of replenishment today.
Stocked with plants like water spinach, knot grass and of course, water lilies, on its bunds or in the water itself, the ponds provide food for migratory birds, as well as a breeding ground for a wide variety of freshwater specialists such as dragonflies and amphibians, which might otherwise not be found in Mai Po due to their inability to cope with the gei wai’s brackish conditions.
Water Lily (Family: Nymphaeaceae): Whatever your feelings about their wetland habitats, nobody can deny that water lilies are beautiful, especially those in the pond outside the education centre at Mai Po. But these plants do more than brighten the scenery with their flowers. Their floating leaves also provide important resting and walking platforms, most famously for frogs but also for insects and even some birds. During the summer, the shade these leaves provide helps to cool the water beneath them, which in turn reduces the likelihood of harmful algal blooms (Rogers, 2011).
Common Bluetail (Ischnura senegalensis): As its name suggests, this damselfly is usually green with a bright-blue tip at the end of its abdomen. However, some females also have an orange morph. It is common in a wide range of aquatic habitats in Hong Kong from freshwater streams to brackish waters. However, it most prefers well-vegetated freshwater bodies with little to no water flow, especially as a habitat for its larvae (Tam et al, 2011, p. 114). As such, the lily pond outside the education centre is a good place to look for it.
Gunther’s Frog (Rana guentheri). This large, common frog ranges in colour from pale-brown to greyish-green and can be identified by the prominent folds running down both sides of its back. It lives in a wide range of habitats from ponds to lowland streams to irrigation ditches, but has been known to venture far from water and even climb trees. When threatened, it secretes a toxic, bad-smelling fluid from its skin that can kill smaller animals when exposed to it in confinement. It feeds mainly on insects but will also eat smaller frogs (Karsen et al, 1998, p. 56).
Variegated Flutterer (Rhyothemis variegata): Of all the many dragonfly and damselfly species zipping around above the ponds of Mai Po, none catch the eye quite like the variegated flutterer. Resembling a cross between a dragonfly and a butterfly with its opaque black and yellow wings, it also flies like a butterfly, flapping its wings a lot to remain airborne (Hong Kong Wetland Park, 2019). It is often seen in groups flying over weedy ponds and marshes, particularly towards the end of the summer (Tam et al, 2011, p. 326; Hong Kong Wetland Park, 2019).
Reeve’s Terrapin (Mauremys reevesii): Found mainly in lowland ponds, reservoirs and slow-flowing streams, the Reeve’s terrapin will also frequently venture on to land to bask. It was once the most common freshwater turtle in Hong Kong. Today however, it has been outcompeted in much of its former range here by the red-eared slider, an invasive terrapin from North America that has colonised Hong Kong waterways (including Mai Po) mainly via abandonment in the wild by irresponsible pet owners. It is thought that the two species compete for the same food, which includes fish, frogs and plants. Reeve’s terrapin particularly favour pond weed of the genus Hydrilla, (Karsen et al, 1998, p. 73, p. 76)
Walk for Nature takes places on 7–8 November 2020 at Mai Po Nature Reserve, with the theme “Our Habitat, Our Home”, spotlighting the rich biodiversity of this Ramsar site.
· Hong Kong Wetland Park, Variegated Flutterer, [website], 2019, https://www.wetlandpark.gov.hk/en/biodiversity/beauty-of-wetlands/wildlife/rhyothemis-variegata-arria , (Accessed: 10 July 2019).
· Karsen, S.J.,Lau, M. and A. Bogadek. 1998. Hong Kong Amphibians and Reptiles: Second Edition. Provisional Urban Council, Hong Kong. 56pp., 73pp., 76pp.
· Rogers, K., ‘The Life of a Lily Pad’, Encyclopedia Britannica Blog, [web blog], 20 July 2011, http://blogs.britannica.com/2011/07/life-lily-pad/ (Accessed: 11 July 2019).
· Tam, T.W., Leung, K.K., Kwan, B.S.P., Wu, K.K.Y, Tang, S.S.H, So, I.W.Y., Cheng, J.C.Y., Yuen, E.F.M, Tsang, Y.M., Hui, W.L. 2011. The Dragonflies of Hong Kong, first edition., Friends of the Country Parks, Hong Kong. 114pp, 326pp.