Do non-native pollinators provide services to native plants in Hawaii?

A lot of Hawaii’s plants are in trouble. A new study by Clare Aslan and colleagues says over a third are threatened or endangered. Why are they suffering? The authors decided to look at pollination. Has something gone wrong with pollination so plants aren’t reproducing as they should? To answer the question, they went on a stake-out in the Pōhakuloa Training Area on Hawai‘i Island. The team followed eight native species, half of them endangered, to see what happened with pollination.

Stenogyne angustifolia drawing
Stenogyne angustifolia Illustrationes florae insularum Maris Pacifici / Biodiversity Heritage Library

Their results were the plants were all visited by insects alone. However, they found fewer than 1 in 5 visits were by native species. Instead, honey bees (Apis mellifera) and Syrphidae flies made the visits. The visits weren’t distributed evenly though. The endangered species attracted a narrow range of insects than the other plants. Stenogyne angustifolia was visited on one solitary occasion in 120 hours of observation.

How important were the visits? The team tackled this question by isolating flowers while they were in bud. Excluding pollinators meant that any seed set would be by selfing. Then, comparing seed set for bagged flowers against open flowers would show how important pollinator visits were. They found that outcrossing pollen was important for seed set.

Clare Aslan and colleagues conclude the non-native pollinators are providing an important service to native plants. They do have some reservations. For example, they note their study was done in daylight. There might be more happening at night time that they have missed.

The results show that changes in pollinators can have an effect on plant populations, even if pollination is still happening. The authors say: “Non‐native species may carry pollen in different spatial, quantitative, and temporal patterns from those once exhibited by native pollinators, for example.” This highlights how not all pollination is the same. Clare Aslan and colleagues also note that for some endangered species, the historic pollinators are unknown. If the replacement pollinators act in a different way, this could also affect the plant populations. Conservation might require conserving co-evolved populations rather than species.

Further reading

Aslan, C. E., Shiels, A. B., Haines, W., & Liang, C. T. (2019). Non-native insects dominate daytime pollination in a high-elevation Hawaiian dryland ecosystem. American Journal of Botany. https://doi.org/10.1002/ajb2.1233