An invasive Braunsapis puangensis (right) and native Homalictus fijiensis (left). Photo credit: James Dorey.

An invasive Braunsapis puangensis (right) and native Homalictus fijiensis (left). Photo credit: James Dorey.

The archipelago of Fiji is more than just a tropical paradise – it is home to at least 30 species of bee, with most making their homes in the mountains above 800 m. ‘Homalictus fijiensis is the only endemic species of bee that commonly occurs in the lowland region of Fiji’, says Carmen da Silva, from Monash University, Australia. However, H. fijiensis’s low altitude supremacy has recently been challenged. ‘Within the last two to three decades, two invasive stem-nesting Apidae bee species [Ceratina dentipes and Braunsapis puangensis] were unintentionally introduced to Fiji’, says da Silva; and the luscious island is much to their liking. However, the insects’ idyllic ecosystem is one of the most vulnerable on the planet. Wondering how the imposters and H. fijiensis will fare as climate change takes a grip and temperatures climb, da Silva, with PI Vanessa Kellermann and colleagues from Monash and Flinders Universities and the South Australian Museum travelled to Fiji's largest island, Viti Levu, to investigate.

The team joined local scientists Marika Tuiwawa and Stephen Galvin, from the University of the South Pacific, to collect the insects on the island in April and again in September–October 2019. Returning quickly to their impromptu hotel-laboratory with the bees, da Silva, Sarah Barker, Nicholas Congedi, James Dorey and Kellermann tested how well the insects coped with high and low temperatures, in addition to how well they tolerated dry conditions. ‘Those traits are often closely linked with species distribution and survival capabilities in different environments’, da Silva explains. In addition, Julian Beaman and Lesley Alton measured the insects' metabolic rate and da Silva recalls a wild goose chase in search of soda lime to scrub CO2 out of the air when the experiments were at risk. ‘In the end, we had to buy 10 kg from someone who was no longer able to use it for SCUBA diving’, she chuckles, adding ‘Conducting experiments in the field is often tricky’.

As the team feared, the invaders are far more robust than the indigenous bees. Ceratina dentipes and B. puangensis can withstand temperatures of over 47°C while the local Fijians can't survive temperatures above 44°C. da Silva was also astonished by the invaders’ resilience during a dry period. She describes how three of the four C. dentipes survived more than 30 h without water – the experiments often required staying awake for up to 2 days with only brief naps – whereas the Fijian bees only lived for up to 6.7 h. Ceratina dentipes was also the most resilient when pulling out of a coma – caused by being chilled at –1°C for 2h – managing it in just under 12 min. In contrast, B. puangensis and H. fijiensis took almost 17 min longer to recover.

However, when it came to adjusting to the seasons, the local bees had the upper hand. ‘Homalictus fijiensis seemed to have the greatest capacity to shift their critical thermal maximum with changes in season’, says da Silva, explaining that the local insects were able to survive temperatures up to 45°C during the warmer wet season, falling to 43°C during the milder dry season.

‘The main result is that the two invasive species are more tolerant to high temperatures and dry environments than endemic H. fijiensis’, says da Silva, who is concerned that the odds may be stacked against the indigenous bees as the temperatures rise, with potentially damaging consequences for the rest of the island if the intimate relationship between the insects and the plants they pollinate is disrupted.

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