A beetle introduced in Kerguelen, perpetually voracious | Polarjournal
Every spring, southern elephant seals gather in colonies on the sand spit at Baie Charrier and many others in the Kerguelen archipelago. This is where the females give birth on the sand just behind the high water mark. This ecosystem cannot function correctly without the little flies that clean up the organic matter. Photo: Camille Lin / TAAF / French Polar Institute

A beetle introduced to Kerguelen, Merizodus soledadinus, is so active that it is thought to be capable of eradicating the archipelago’s native flies. Scientists at the French Polar Institute have discovered that its predatory activity does not diminish as its population grows and its prey diminishes.

A small beetle (Merizodus soledadinus) measuring six millimetres in length has been preying on the larvae of native flies in the Kerguelen archipelago since 1913 and, to satisfy its voracious appetite, eats an equivalent of its size every three days.

According to David Renault – a biologist at the University of Rennes and scientific coordinator of the French Polar Institute – and his colleagues, the voracity of Merizodus has remained constant since the territory was first invaded. Their findings have just been published in the journal Scientific Reports at the beginning of September.

“We wondered whether the species had been more voracious on arrival and whether this might have declined over time,” explains David Renault. Some populations of introduced predators are known to exert less pressure when competition between predators increases.

Two Merizodus soledadinus fight over a fly larva – it can be a tough battle. Photo: David Renault / French Polar Institute / Écobio

“For example, they could have shrunk to expend less energy,” he adds. “But it still eats just as much.” In addition to consuming fly larvae, it kills them without necessarily eating them. Even if the competition is tough, they’re still very aggressive, so the impact on the diptera is major.

These flies decompose organic matter, such as washed-up seaweed and dead birds and animals from large colonies of penguins, albatrosses and southern elephant seals.

The wings of Anatalanta aptera, Amalopteryx maritima and Calycopteryx moseleyi – of no use to them in Kerguelen because of the strong wind – have shrunk and turned into a fat reserve.

The flies are still present in areas where there is a lot of organic matter, but they are in decline. They have been eradicated locally by the beetle, and the latter could lead to their disappearance. “If they were to disappear, it would be detrimental to the performance of the ecosystem,” notes the biologist.

Merizodus soledadinus reproduces every year, with a female laying around eight eggs, buried in the top 10 cm of soil. Hatching peaks in midsummer.

The larvae settle comfortably in aerated soil with 80-90% humidity. To ensure that their offspring develop, the adults also prey on those of their fellows.

Overwinterers from the French Polar Institute scour parts of the main island in search of Merizodus soledadinus to monitor its dynamics and distribution. Photo: Mateo Tolosano / French Polar Institute

“At the base, it is possible to collect 4,000 individuals in 45 minutes with seven or eight people,” explains the researcher. Its presence is well established in the east of the island. In the west, it is too humid, with rainfall of between two and three metres a year.

The insect is native to Patagonia and the Falklands. It has also been introduced into South Georgia, where the same problem exists, but to a lesser extent,” says David Renault. There is a native species that attacks its larvae.

In the French archipelago, the beetle was introduced to the Port Couvreux experimental farm in 1913, with the arrival of farmers, their sheep and fodder from the Falklands.

“A single date, a single episode,” he points out. “This shows that a single mistake can wreak havoc.” The distribution of the Merizodus is now such that the only thing that can be done is to avoid introducing the species to other islands in the Southern Ocean.

Camille Lin, PolarJournal

Link to the study : Géron, C., Cuthbert, R.N., Hotte, H., Renault, D., 2023. Density-dependent predatory impacts of an invasive beetle across a subantarctic archipelago. Sci Rep 13, 14456. https://doi.org/10.1038/s41598-023-41089-2

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