Hong Kong's butterfly population hit its highest level in at least 14 years in 2019, the warmest year on record in the city, causing a local green group to suggest the increase in numbers could have been linked to climate change.
The environmental group Green Power recorded 127 species of butterfly and more than 7,800 individuals across Shing Mun Country Park and Tai Po Kau Nature Reserve, both areas under environmental protection. The locations were selected to allow the survey to focus on the effect of climate on butterfly populations and minimise other factors from human activity.
"Our 2019 survey showed climate change had induced a sharp increase in the butterfly population. In the long term, this may not be a good thing," said Matthew Sin Kar-wah, senior environment affairs manager at Green Power, which has been surveying butterfly numbers annually since 2005, and has never counted a bigger population than last year's.
Green Power found 113 species in 2018, and last year's numbers were 50% higher than the average population from 2005 to 2018.
Butterflies act as pollinators for plants to help them reproduce, and as food for birds and other animals. Any significant changes in their populations could in turn affect the local ecosystem, Sin said.
Fluctuations in temperature could change the time when caterpillars hatch, which could lead to them eating more plants or young birds having less food, if they hatch early or late.
Average temperatures in February and March 2019 were 4.4C higher than usual. During those two months, there was a drastic rise in the butterfly population, particularly of the Satyrinae, Danaidae, Lycaenidae, Papilionidae, Nymphalidae and Pieridae species, the group's survey found.
Two species of very rare butterflies were recorded for the first time in the survey: the Sullied Sailer and Tiny Grass Blue. The Tailless Lineblue, last seen a decade ago, was also spotted, along with the Swallowtail and Banded Awl, both ranked as rare by the Agriculture, Fisheries and Conservation Department, and both also spotted for the first time in the survey.
However, Cheng Luk-ki, director of Green Power, pointed out native butterflies would be the first to face headwinds if climate change caused the city's temperatures to stay warm for longer periods of time.
"Most of Hong Kong's native butterflies have specialist caterpillars that only eat certain types of native plants. If warmer temperatures cause big changes to the local plant population, that would be bad for the butterflies too," he said.
He added that the population increase should be treated with caution, as it could be a normal fluctuation in butterfly numbers.
"While this year's numbers were certainly larger than we've seen before, there needs to be more in-depth studies to conclusively say climate change has impacted the butterfly population," he said.
Separately, the Hong Kong Observatory said on Thursday that the weather this March was warmer than usual, with a monthly mean maximum temperature of 21.3C, 2.2C higher than normal. The lowest temperature of the month, 16.5C, was the highest absolute minimum temperature on record for March.