Brown-veined white butterfly (Belenois aurota) during their annual migration in South Africa. Credit: Rudi Steenkamp/ CC BY-SA 4.0
Plants and insects in South Africa are adjusting their timing in line with changing climates, a look back at 100 years of news archives, and more recently Instagram and Twitter posts too, has revealed.
Jacaranda (Jacaradanra mimosifolia) trees lining the streets in the province of Gauteng, South Africa’s economic hub, are now blooming to their full purple glory as much as two months earlier than they did in the 1920s. Similarly, brown-veined white butterflies (Belenois aurora) have begun to annually migrate eastwards in their thousands through Johannesburg in December, on their way from the dry Southern African Kalahari hinterland to the moist coast of Mozambique, rather than by February as they did in the early 1900s.
The research was carried out by Jennifer Fitchett, an associate professor of physical geography at the University of Witwatersrand in Johannesburg.
Her field is phenology, the study of the timing of annually recurrent biological events such as the appearance of carpets of spring wildflowers, jacaranda flowers and mass butterfly migrations. Others have studied changes in the timing of when cherry, apple and pear trees blossom. To properly gauge possible shifts in these events because of various environmental factors, phenologists scour long-term records collated over decades, even centuries.
By using local examples, Fitchett and others are helping people visualize the effects that rising post-industrialization global temperature increases of 1.1 °C are having. Phenological shifts represent one of the most robust bio-indicators of climate change and highlighting has the benefit of raising public awareness.
She was lead author of a paper in the International Journal of Biometeorology chronicling the appearance of brown-veined butterflies over Johannesburg between 1914 and 2020. During the short-lived event, masses of snowflake-like butterflies flutter around the city for only three days — a spectacle that attracts headlines and social media posts.
According to a 1934 local newspaper, the butterflies’ presence once held up a cricket match. And from 120 other articles, tweets and Instagram posts Fitchett learnt that the insects on average are arriving progressively earlier — statistically at an average rate of 2,9 days earlier per decade over the past century. Their Johannesburg due date has effectively advanced from approximately mid-January to mid-December over the past 100 years.
This year, Fitchett was surprised when the butterflies appeared at the end of November, much earlier than any other dates she has on record. “Because we rely on print and social media reporting, it is very possible that it is not the earliest that they have ever arrived,” she notes. “However, the 2022 occurrence is evidence of the gradual shift to the earlier arrival dates we are seeing.”
“This year, however, it wasn’t much of a spectacle. There were fewer butterflies, so that they didn’t make any news headlines at all.”
In Johannesburg, the timing of the white butterfly event occurs against the backdrop of mean annual temperature changes of between 0.2–1 °C per decade in the region over the past century.
But it’s not just rising temperatures that affects the butterflies’ schedule. Using verified local rainfall figures and daily and average temperature data from the South African Weather Service for the greater Johannesburg region, Fitchett noted how weather conditions prior to the migration influence its timing too. If it rains early in summer, the insects tend to start moving sooner, for instance.
“To talk climate change, and its influence on phenology, we need more than 30 years of data to ensure that what we see happening in nature isn’t just part of an upward or downward turn of a shorter natural climatic cycle such as El Niño,” Fitchett says.
Phenology gardens
European countries have whole networks of phenology gardens that scientists use to keep tabs on environmental changes over time. Generations of professional and amateur naturalists have been taking note of when certain trees flower, or when flocks of migratory birds return to an area.
“There’s a family estate in the UK, where data has been collected for more than 300 years,” Fitchett explains. “In South Africa we don’t have such phenology gardens. Many of our farmers make detailed records of what happens on their farms, for instance about their crops’ yield or when they start spraying against insects, but at best they tend to only keep such records for five years, for tax purposes. At least we have newspapers and social media posts to follow up on.”
