The orchid that’s running out of time

Ophrys sphegodes, the early spider-orchid, is a plant with a very short pollination window. The changing climate is closing that window more and more often as the planet warms.
Ophrys sphegodes
Early Spider Orchid – Ophrys sphegodes. Photo: / Flickr

The early spider-orchid, Ophrys sphegodes, is a cunning plant. Instead of providing food for insects, it emits an alluring perfume. At least it’s alluring if you’re a male Buffish Mining-bee, Andrena nigroaenea. The bee takes flight in the warming Spring air with one thing on its mind. It’s trying to sniff out the scent of a female bee. The male bees fly earlier than the females, and so there’s a window of opportunity. The orchids flower and release copies of the bee scent into the air. The male bees arrive at an orchid and attempt to mate with the flower, in the process picking up pollen. Disappointed they leave the flower and look for a female again, but for a few days, what they’re more likely to find is another orchid. Another disappointing tryst leaves them depositing the pollen and maybe picking a new batch up.

It’s an elegant way for the orchid to get pollinated. Or at least it would be elegant if it worked. More often than not, the female bees take flight before the orchid flowers. In those years the male bees mate and, by the time the orchid is ready, the bees are no longer searching for sex. The result for the orchid is reproductive failure. It’s no surprise the orchid is rare in some places, but why would it evolve a reproductive mechanism that fails so often? A new 356-year analysis suggests that the answer is that the climate is changing so rapidly the orchid cannot adapt.

A team of scientists led by Prof Michael J Hutchings examined what it was that made the bees take flight and the orchids flower. They correlated records of bee flight and flowering against weather records that revealed the average temperature, total rainfall and total sunshine hours, along with the number of frost days in the winter before flowering.

What they found was that it was the temperature and, to a lesser-extent, the frosts that determined when the orchids flowered. However, field studies only dated from 1975, so the team went looking for more data. They found it in the herbarium specimens stored at the Royal Botanic Gardens, Kew, and the British Museum, London. The orchid only flowers for a relatively brief time, so the dates attached to the samples allowed Hutchings and colleagues to calculate the peak flowering time. This allowed them to build a record back to 1848. Similarly, they were able to track the flights of the bees thanks to specimens at the Natural History Museum, London, and the Oxford University Museum of Natural History, giving them flight dates back to 1893. From this they were able to work out that it was temperature that triggered the bee flight.

The final piece of the puzzle was the central England temperature (CET) record. The CET includes the average monthly temperature from 1659. Through using this record, the team were able to estimate what the likely dates for flight and flowering were in the past for a period of over three centuries.

Prof. Michael Hutchings, the lead author of the study said: “We regard the use of weather data going back to the seventeenth century as a very important feature of this study. Indeed, we believe that the study makes a very valuable case for the collection of long-term data in many forms. The slow and relatively small directional changes in climate and in flight and flowering phenology of the study species can only be observed by using such lengthy data sets. Moreover, it is in a sense a strong indication of the validity of the analysis that we have been able to unearth these results from independently-collected sets of data that were obtained without the slightest thought of what they might ultimately reveal in this type of analysis.”

Comparing flowering dates of the early-spider orchid against mean temperatures, using two data sets.
Comparing flowering dates of the early-spider orchid against mean temperatures, using two data sets.

Putting together a long-term study raises the question of accuracy. How confident can you be that the correlation you are seeing is real? For Hutchings and colleagues, it was the ability to examine two different types of data that showed that what they were observing was a real effect. Asked about the correlation Hutchings commented: “We checked, and the predictions of flowering date vs. temperature from the two sets of data match almost perfectly (an astonishing result).”

The results of the study are worrying.

Male bee flight, female bee flight and flowering have all been getting earlier, on average. However, they are not all responding the same way. It seems that the rising temperature is having the biggest effect on the female bees, bringing them out earlier and earlier. The orchids are flowering earlier, but their change is not as much as the bees. They are starting to fall ‘out of sync’ with their pollinators. It means that the orchids flower after the females have taken flight and this causes problems for the plants. Prof. Hutchings explained: “The flowers can only be cross-pollinated for a few days after they emerge, while they are still fresh. Indeed, the bouquet of scents emitted by a flower changes after pollination, purportedly so that bees are not attracted to already-pollinated flowers, but may find their way to other flowers. Although the emergence of female bees may not end the orchid’s chance of pollination, the female bees will at least compete with flowers for the attention of the male bees. In addition, there is no evidence that the flowers’ scent is more attractive to male bees than the scent of female bees. The most probable scenario is therefore that male bees would prefer to copulate with female bees than pseudocopulate with orchids.”

Of course, not all years are the same. Some would be colder or hotter anyway, so the orchid would not be successful or unsuccessful every year in a period. But if the orchid has to flower before the female bee takes flight, then things are getting more difficult. Prof Hutchings said: “Back in the seventeenth century, there was a much higher frequency of years in which the order of the three phenological events was male bee emergence before orchid flowering before female bee emergence. Nowadays, female bee emergence precedes flowering far more frequently. Thus, years when the pollination system could work to plan used to be far more frequent. Any fruits produced by the orchid can contain thousands of seeds, and therefore, there used to be a much higher potential for recruitment of new plants on a regular basis. ”

In pre-industrial times, the orchid got its timing right about 60% of the time. In warmer times its success has fallen to just 20% of years.

Despite producing thousands of seeds, the combination of a lack of self-pollination by the early-spider orchid and the falling odds of success for cross-pollination mean the orchid is facing a grim future. Prof. Hutchings added: “Given the very short life-span of this orchid, frequent recruitment is vital to maintain populations. When the number of years in which fruiting is less likely exceeds a certain level, and especially when several such years occur consecutively, populations are at serious risk. Climate warming could, therefore, be the final element of a “perfect storm” for this species.”

Prof. Hutchings concludes “Having very long weather records is vital for any study of climate change. As we all know – and as the graphs in our paper show – weather is very variable. Mean annual and mean spring temperature show wide variation from year to year. It is only when a long series of years can be set side-by-side that the systematic change in temperature can be seen. It is likely that many other species dependencies are also suffering from climate-induced phenological change. This study is, we believe, the best documentation we have as yet of such an effect.”

Reference List

Hutchings, M. J., Robbirt, K. M., Roberts, D. L., & Davy, A. J. (2018). Vulnerability of a specialized pollination mechanism to climate change revealed by a 356-year analysis. Botanical Journal of the Linnean Society. https://doi.org/10.1093/botlinnean/box086

Robbirt, K. M., Davy, A. J., Hutchings, M. J., & Roberts, D. L. (2010). Validation of biological collections as a source of phenological data for use in climate change studies: a case study with the orchid Ophrys sphegodes. Journal of Ecology, 99(1), 235–241. https://doi.org/10.1111/j.1365-2745.2010.01727.x

Robbirt, K. M., Roberts, D. L., Hutchings, M. J., & Davy, A. J. (2014). Potential Disruption of Pollination in a Sexually Deceptive Orchid by Climatic Change. Current Biology, 24(23), 2845–2849. https://doi.org/10.1016/j.cub.2014.10.033