Study finds bees take more than 75 years to return to restored grasslands

By Stephen Beech

Bees take more than 75 years to return to restored grasslands, warns new research.

The finding underlines the importance of preserving grassy areas as reservoirs of biodiversity, say scientists.

Researchers in Japan found that recovered grasslands need more than 75 years of "continuous management" to regain their biodiversity because specialized pollinators are slow to return.

Grasslands are rapidly disappearing around the world due to conversion and abandonment, leading to a well-documented loss of biodiversity.

Restoring abandoned grasslands by removing woody vegetation and resuming traditional land management practices has positive effects on biodiversity.

But that diversity lags behind that of old grasslands that have been under continued management for up to several millennia, say scientists.

Study author Professor Atsushi Ushimaru, of Kobe University, said: “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 Gaku Hirayama decided to turn their attention toward 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 as their study site.

Hirayama said: “There is no place better suited to survey restored grasslands with very different management durations within a relatively small area than ski slopes."

Their findings, published in the Journal of Applied Ecology, show that it takes 75 years of continuous management for the plant diversity in recovered grasslands to finally reach levels comparable to ancient grasslands.

But the study also shows 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 said: “The finding shows that once valuable old grasslands are lost, their restoration cannot be achieved quickly.”

The researchers say that what drives the lag is not a lower diversity of pollinators in itself, but rather the identity of the pollinators.

They explained that recently restored grasslands are visited mostly by flies and hoverflies that are generalists and often carry pollen from one plant to flowers of a different species.

But on old grasslands pollinators are to a larger extent bees and butterflies that specialize in one species at a time, guaranteeing that the pollen is transferred to the right plant species.

Ushimaru says the study shows the importance of considering pollinator communities for efforts targeting the conservation of threatened grassland plants.

He said that means, first of all, that maintaining ancient grasslands as reserves for specialized pollinators should be a "top priority" for conservationists.

Ushimaru added: “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.”