Department of Information and Tourism, Taipei City Government

The four-spotted midget damselfly (Mortonagrion hirosei) has an extremely restricted distribution and a very small population. It was therefore assessed as Critically Endangered in Taiwan's Odonata Red List published in March this year. Following years of investment by the Taipei City Government's Hydraulic Engineering Office (HEO) in habitat restoration and conservation of the species, the latest survey conducted in July brought encouraging results: the monitoring team recorded abundant adults and nymphs at the Huanggang River estuary.

The HEO noted the four-spotted midget damselfly is a tiny odonate measuring just over two centimeters in length. Since it was first discovered at Wugu Wetland in 2005, it has been recorded at only three locations in Taiwan, all within the Tamsui River watershed: Wugu Wetland, Shezi Island Wetland, and the Huanggang River estuary.

To conserve this rare species associated with estuarine marsh wetlands, the HEO launched population monitoring and habitat surveys at Shezi Island Wetland and the Huanggang River estuary. Drawing on published research and findings from these surveys, the Office established a restoration habitat beside the Huanggang River estuary in 2018, creating a tidal pond at the site's off-road bicycle course specifically to support the species' recovery.

The HEO also noted that, through continuous microhabitat management and monitoring by the site management team and the Society of Wilderness, the four-spotted midget damselfly population at the Huanggang River estuary restoration pond grew from none at all to a stable population by 2022 and 2023, with adults or nymphs recorded during every monthly survey. However, continued sediment accumulation and terrestrialization of the pond began to seriously affect the survival of the aquatic nymphs, with monitoring data showing a significant decline in their numbers.

In response, the HEO carried out habitat improvement work in early 2024, enlarging and deepening the pond while removing overgrown trees and shrubs. These measures improved tidal exchange and water retention, but also created another challenge: the dense reed marshes used by adult four-spotted midget damselflies for shelter and habitat had disappeared. Larger potential predators, including species of skimmers and darners, subsequently moved into the area in considerable numbers.

To accelerate the recovery of suitable habitat for the four-spotted midget damselfly, the HEO worked with the Society of Wilderness, the Dragonfly Association of Taiwan, and the ecological monitoring team. Following multiple site inspections, discussions, and planning sessions, the partners carried out two rounds of supplemental planting of reeds and sedges, in September 2024 and February 2025. Sandbags were also placed at the pond's inlet and outlet to regulate water levels.

Through these collaborative efforts, the restoration pond has gradually transitioned from an open habitat with sparse vegetation back into the dense marsh vegetation preferred by the four-spotted midget damselfly. Ongoing monitoring of adult odonates and aquatic nymphs has also shown that, as the vegetation has recovered, the restoration pond at the Huanggang River estuary has gradually returned to an odonate community in which the four-spotted midget damselfly is the dominant species.

In the latest survey completed this July, the monitoring team recorded abundant adults and nymphs at the Huanggang River estuary, with mating pairs also observed repeatedly among the reeds. The results represent not only a successful example of the HEO's sustained investment in habitat restoration and species conservation, but also a rewarding outcome for the monitoring teams who have spent long summer days in waders conducting surveys through tidal channels and muddy wetlands.