Pollinators like moths are increasingly missing from a warming subarctic

Temperatures in the subarctic are soaring as the planet heats, but what effect is that having on ecosystems? Leana Zoller and colleagues recently published an article examining plant-pollinator networks in Finland. They found that there has been a dramatic change in pollinator networks, with flies replacing moths. That could be disastrous for plants relying on specialist pollinators.

Ecologists would ideally like earlier work to refer to when studying climate change, so they can see the results of warming and work out what changes have already happened. However, because few studies have investigated the interaction between plants and different pollinator groups over longer periods of time, it is difficult to say whether and to what extent such developments are already underway. This makes the more than 120-year-old data from Finland on which the new study is based all the more exciting. Between 1895 and 1900, around Kittilä (a village about 120 km north of the Arctic Circle), forester Frans Silén systematically recorded which insects visited which flowers and how often. In their article Zoller and colleagues write:

Silén’s aim was to observe as complete a set of insects visiting a focal plant species as possible, making his dataset a valuable benchmark for studying changes in plant–pollinator interactions. Silén classified focal plant species according to Knuth’s ‘Handbuch der Blütenbiologie’ (for example, as ‘bumblebee flower’ or ‘syrphid flower’). However, he recorded insect visitors that did not necessarily fit into these categories, for example, he observed hoverflies visiting bumblebee flowers or butterflies and solitary bees visiting syrphid flowers. This minimizes probability of potential taxonomic biases. Silén’s dataset was collected during 6 years but the individual plant species represented in it were typically observed for only 1–3 years… In his publication, Silén states that most of the observations were made in the vicinity of Kittilä village, from where his excursions extended about a mile to the north and as far south. One sampling location is exactly specified in Silén’s records (the churchyard of Kittilä), while all other sampling sites are described approximately (for example, ‘Kittilä town’ or ‘Aakenusjoki’, a river close to Kittilä). While for most observations no time of day was recorded, Silén noted that certain typically night-pollinated plant species (D. superbus and S. vulgaris) were observed around midnight.

Zoller et al. 2023

Last author, Prof. Tiffany Knight, said in a press release: “I am passionate about working with historical datasets like this. If you repeat the historic studies again today, it’s often the only way to learn about long-term ecological processes. I am trying to understand what motivated the people who collected the data in the past and what challenges they faced. This information can then be used to plan a comparable modern study.”

The scientists started by looking around Kittilä for sites where Silén had also made observations – and where the 17 plant species he studied best still grow today. The team repeated the pollinator census at these sites in 2018 and 2019. The area remains sparsely populated, and little has changed regarding land use. However, it has not escaped the consequences of climate change. “We have noticed drastic changes in the networks of pollinators”, says author Leana Zoller. Only 7% of the flower visits observed involved the same species of insects and plants as back then. “That is surprisingly little”, says Zoller.

For example, hoverflies and moths appear much less frequently on the flowers around the village today than they used to. This is probably not good news, and that’s because these two groups have some particularly effective pollinators among them. These include the bumblebee hoverfly (Volucella bombylans) – a large, furry fly resembling a bumblebee. In Silén’s time, this species was the most frequent visitor to the Arctic raspberry (Rubus arcticus) and the woodland geranium (Geranium sylvaticum). The bumblebee hoverfly could effectively transfer these species’ pollen from one plant to the next.

Also, moths use a physical advantage during pollination: their long proboscis can reach the nectar from the base of tubular flowers. This is why they used to be the most frequent visitors of the fringed pink (Dianthus superbus) and bladder campion (Silene vulgaris), both of which have such flowers.

Whilst these insects have become rarer, the flowers around Kittilä are now getting considerably more visits from bumblebees and certain flies. Whether these animals work as effectively as the earlier pollinators is not yet known. One trend in particular concerns the researchers. There are now considerably fewer insects that are specialists for specific flower shapes. These have been replaced by flies of the genus Thricops, which visit many different plants. Such generalists are often more robust regarding environmental changes; if one of their host plants is lacking, they can easily switch to others. But they also carry the pollen of various other plant species onto a flower, potentially providing a less effective pollination service than the specialists.

Of particular interest is the broad range of pollinators in the study. In their article Zoller and colleagues write:

Our research provides a comprehensive examination of long-term changes in interactions involving several pollinator taxonomic groups, which produces key findings that are likely to be of general relevance and points to important knowledge gaps that need to be addressed in future ecological research. All of the previous long-term studies focused exclusively on bees but the most dramatic changes in relative abundances we observed were in fly and moth pollinators. As land use change has been minimal over time but climate warming has advanced in this region, climate change is a possible mechanism that might explain the dramatic patterns we observe. Thus, our study may be a harbinger of what to expect in other regions as climate change progresses.

Zoller et al. 2023

“So far, the pollinator network in our study area still seems to be working well”, says Zoller. “There is no evidence so far that the plants are getting too little pollen and are thus less capable of reproducing”. But according to the scientists, this can change in the future if changes in the insect communities continue. So far, the flies there seem to be coping with the rising temperatures. But further north in the high Arctic, one study has revealed a massive decrease in the number of flies. “If this also happens in our study area, it could become a problem”, says Zoller. Because at some point, the plants will no longer be able to compensate for the losses in their pollinator network.

📰 Press release at Eurekalert
🔬 Plant–pollinator network change across a century in the subarctic is available from Nature Ecology & Evolution.