When bees arrive late, flowers may vanish

For flowering plants, the choice of a visiting insect or bird can be a matter of evolutionary life or death. A recent study reveals that even brief mismatches between when flowers bloom and when pollinators arrive can flip the direction of natural selection, favoring one floral form one week and a different form the next.

The work, led by biologist Liedson Carneiro from East Tennessee State University, shows how fast these “rules” can change inside a single flowering season – and hints at what may lie ahead in a warming world.

No bees, no seeds

The team focused on an oil-producing vine – nicknamed the Amazonvine – that climbs through Brazil’s dry tropical forests. The vine cannot self-pollinate; it relies on specialized oil-collecting bees that grasp the petals, extend their legs, and scrape off nutrient-rich floral oils.

Those fingerprints – tiny scrapes on the petal surface – let researchers track exactly which flowers were visited and when.

The fieldwork took place in Parque Nacional do Catimbau, Pernambuco, during the species’ 2020 flowering season. Two snapshots were taken: an early census during the vine’s normal bloom peak and a second one four weeks later.

The contrast was stark. At the first visit more than 100 flowers were open, yet bee traffic was nearly absent. By the second survey, pollinator activity had surged. Ninety-four percent of late-blooming flowers bore the telltale scrape marks, compared with just 7.5 percent at peak bloom.

Bees flip flower priorities

Because flowers leave seeds only if they receive pollen, the researchers could link each plant’s “fitness” to its floral display.

During the quiet early period, vines with the largest blossoms set the most fruit: bees apparently chose the biggest targets on their rare visits.

A month later, the tables turned. With bees everywhere, plants bearing smaller flowers now produced more seeds. When every blossom attracted attention, the value of extra-large petals evaporated; large flowers may even have cost the plant energy that could have gone into seed production.

Statistical models showed the relationship between flower size and fitness did not merely weaken – it actually reversed direction between the two censuses.

Averaged over the entire season, the contradictory pulses canceled each other, leaving only weak overall selection on blossom size.

Evolution in real time

The results matter because they confirm that evolutionary pressures can flip within weeks, not millennia.

“Our findings show that even within a single flowering season, temporal mismatches between plants and pollinators can shift how traits like flower size relate to reproductive success,” Carneiro said.

“These short-term dynamics may influence evolutionary outcomes, help maintain trait diversity, and prevent rapid trait change in plant populations.”

That diversity can be a long-term boon, giving populations a preexisting mix of shapes and strategies to weather future shifts. But it also means conservationists cannot assume pollination patterns seen on one date will hold the following month – let alone the following year.

Climate disrupts pollinator timing

Much discussion of plant-pollinator mismatches focuses on year-to-year effects: warming springs coax flowers to open earlier, while migrating hummingbirds and overwintering bees may lag behind.

The new study adds another layer of concern. In regions where rainfall, temperature, or day length fluctuate sharply over a few weeks, the “sweet spot” of overlap can shrink even inside one season.

If climate change makes those swings more extreme – stronger droughts, heavier downpours, hotter heatwaves – then the brief windows of perfect synchronization could become rarer.

In the Brazilian vine, peak bloom arrived before most bees became active. In other species, pollinators might appear before nectar flows. Either way, the study illustrates that a plant’s evolutionary trajectory can hinge on such timing errors.

Timing insights for conservation

The research also highlights the value of detailed field observation. By sampling the same population twice, the scientists captured a live-action picture of selection that would have been invisible in a single survey.

For ecologists monitoring threatened habitats, revisiting plots through the season could reveal hidden stresses – or hidden resilience.

In practical terms, knowing how selection fluctuates could guide which genotypes to conserve or propagate in restoration projects.

If climate change is likely to lengthen dry spells, for example, managers might favor individuals whose floral schedules naturally lags behind the average, ensuring flowers and bees still meet.

Bees and flowers misaligned

Plant-pollinator partnerships are often portrayed as timeless mutualisms, but they are, in fact, dynamic negotiations subject to abrupt renegotiation.

The new work from northeastern Brazil delivers a clear message: evolution does not wait for the next Ice Age. It is happening now, blossom by blossom, as flowers and their faithful couriers dance to a rhythm increasingly disrupted by a changing climate.

The study is published in the journal Annals of Botany.

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