By Athayde Tonhasca
In winter I get up at night
And dress by yellow candle-light.
In summer, quite the other way,
I have to go to bed by day.
Bed in Summer, Robert Louis Stevenson
R.L. Stevenson’s light-hearted rant may have been inspired by a youth’s frustration with the vicissitudes of the seasons, but these variations are vital for life on Earth. Many biological events such as migration, egg laying, spawning, flowering and hibernation follow cyclical or seasonal timing: the reproduction and survival of countless organisms depend on this natural calendar. These periodic biological phenomena and the branch of science dealing with them are known as phenology.
For some species, life cycle events are triggered by the onset of rainfall, or by a day-length threshold (photoperiod); genetics also plays a role. But the great seasonal regulator is temperature, which affects so much of organisms’ activity and metabolism. It is no surprise, then, that phenology is considered to be one of the best tools to assess the effects of climate change.
Many spring and summer phenological events have advanced in the calendar for many groups of organisms, and these changes have been consistent with increasing average temperatures. Among these effects, the onset of blooming is well documented; several plants seem to be responding to a warmer world during the last 20–50 years by flowering earlier. In Britain, flowering has advanced by almost one month since 1986, and many species are flowering nearly one week earlier in the south and in lower elevations when compared to northern areas and higher elevations, respectively. These changes have been relentless: between 1952 and 2019, the British flora has experienced an average advancement of 5.4 days every ten years.
These variations can have many consequences because plant phenology influences the abundance and distribution of organisms, ecosystem services, food webs, and cycles of water and carbon. Even people can be directly affected by these deviations, as for example by the timing of planting, fertilizing, and harvesting crops, and the production of allergy-triggering pollen.
Of course insects could not be immune to climate change. In the Northern Hemisphere, warming has caused species distributions to shift northwards or upwards in mountainous areas. Indeed, the abundance of many butterflies, beetles, dragonflies and grasshoppers have contracted at low latitudes and elevations, and increased in northern areas and at higher altitudes. Some bumble bees do not appear to track the weather, but their spring flight times have advanced.
Plants and pollinators are undergoing rapid variations in their phenological patterns. But if these changes happen at different rates or magnitudes, pollination may become out of sync, with temporal or spatial mismatches. For flowers, there could be reduced visitation rates and pollen deposition; for pollinators, there could be less nectar and pollen.
Plant-pollinator asynchronies may cause concern, but there are few studies assessing their effects. This is not surprising because long-term field observations are afflicted by ‘noise’ such as unusual weather spells, pollinators’ travels from one site to another, and difficulties in recording the onset of phenological events. Long-term data are needed. This requires a clipboard, a pencil, compliant subjects, and a great reserve of patience.
The orange-legged furrow bee (Halictus rubicundus) was just the ticket. This bee is widely distributed throughout Britain and the Northern Hemisphere. It builds its nest in the ground, often in large aggregations, and in a variety of habitats. It visits many flowers, especially composites such as ragwort, thistle, and knapweed.
A population of orange-legged furrow bees in the American state of Utah was monitored daily for 22 years for the detection of the first day of nesting and its possible relation with the bee’s environment. It turned out that time of nesting varied widely (a range of 44 days), but was much more determined by temperature than calendar date.
Most importantly, bees’ activity and blooming in the surrounding area remained roughly synchronised: bees and plants seem to respond to the same environmental cues. Similar results were obtained from long-term observations (12 to 14 years) for the Eastern cucurbit bee (Peponapis pruinosa), the South-eastern blueberry bee (Habropoda laboriosa) and the tawny mining bee (Andrena fulva), a common British species that seems to be spreading rapidly into Scotland.
Other studies have suggested that plant-pollinator interactions are resilient to climate change: flowers and their pollinators seem to be keeping pace with each other. Even more puzzling, the seasonal timings of some species have become closer instead of further apart. This apparent tolerance for a rapidly changing environment is helped by the flexibility of pollination interactions: most pollinators visit multiple plant species, and most plant species are visited by many pollinators; one to one relationships are rare.
Plant-pollinator mismatching doesn’t seem to be occurring to a significant degree, which is a rare case of climate-related good news. But like anything else in nature, there are limits. If temperatures and the frequency of extreme weather events do not relent, the fine-tuned relationships between plants and their insect pollinators may start to unravel. And that would mean trouble for all of us.