The long wait for bees to return to restored grasslands

(Press-News.org) Recovered grasslands need more than 75 years of continuous management to regain their biodiversity because specialized pollinators are slow to return. Kobe University's finding underscores the importance of preserving old grasslands as reservoirs of biodiversity, even if it is just as ski slopes.

Grasslands worldwide are rapidly disappearing due to land-use conversion and abandonment, leading to a well-documented loss of grassland biodiversity. Restoring abandoned grasslands by removing woody vegetation and resuming traditional land management practices has positive effects on biodiversity. However, it is also known that this diversity lags behind that of old grasslands that have been under continued management for up to several millennia. The Kobe University ecologist USHIMARU Atushi says, “The reasons for this are not really clear and satisfying solutions have not been proposed.”

Tipped off by a recent study that showed that insect-pollinated plant species take longer to recover compared to wind-pollinated ones, Ushimaru and his doctoral student HIRAYAMA Gaku decided to turn their attention towards the pollinators themselves. Looking at which insects pollinate plants and with what success on grasslands of different age, from very recently recovered to continuously managed for at least 300 years, they turned to ski slopes in Nagano Prefecture as their study site. “There is no place better suited to survey restored grasslands with very different management durations within a relatively small area than ski slopes,” Hirayama explains the choice.

Their results published in the Journal of Applied Ecology paint a consistent picture. It takes 75 years of continuous management for the plant diversity in recovered grasslands to finally reach levels comparable to ancient grasslands. However, that’s still not enough for the pollinator community. Even after 75 years, pollinators are still less specialized and less successful at pollinating the plants, although the community continuously shifts towards higher specialization and successful pollination as grasslands get older. Hirayama sums up saying, “The finding shows that once valuable old grasslands are lost, their restoration cannot be achieved quickly.”

What drives this lag is not a lower diversity of pollinators in itself, it is the identity of the pollinators. Recently restored grasslands are visited mostly by flies and hoverflies that are generalists and thus often carry pollen from one plant to flowers of a different species. On old grasslands, on the other hand, pollinators are to a larger extent bees and butterflies that specialize in one species at a time, thus guaranteeing that the pollen is transferred to the right plant species.

The study therefore shows the importance of considering pollinator communities for efforts targeting the conservation of threatened grassland plants. First of all, this means that maintaining ancient grasslands as reserves for specialized pollinators should be a top priority for conservationists. But Ushimaru points out something else, saying “It may also indicate that grassland restoration should not just be left to nature, but could require active human involvement such as by sowing seeds or by planting seedlings of native bee- and butterfly-pollinated grassland plants.”

This research was funded by the Japan Science Society, the Fund of Nagano Prefecture to Promote Scientific Activity, the Japan Society for the Promotion of Science (grants 19H03303 and 22K06400), and the Environmental Restoration and Conservation Agency (grant JPMEERF20234005). It was conducted in collaboration with researchers from the University of Tsukuba and the University of Toyama.

Kobe University is a national university with roots dating back to the Kobe Higher Commercial School founded in 1902. It is now one of Japan’s leading comprehensive research universities with nearly 16,000 students and nearly 1,700 faculty in 10 faculties and schools and 15 graduate schools. Combining the social and natural sciences to cultivate leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society’s challenges.

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