Science1 publisher2 min readPublished
Bogong moths swarm across Australia again after an estimated 99.5% population crash
Bogong moths are swarming across Australia again, nearly a decade after drought, pesticides and habitat loss cut their numbers by an estimated 99.5%. The return rests on sightings, and its likely cause, wetter weather, faces a severe El Nino this summer.
The Scientist · Science desk
What happened
- In a good year, 4 billion or more bogong moths flew from the inland heat to shelter in the alpine caves of eastern Australia.
- The migration kept going after the crash, but so few moths made the journey that researchers found the caves nearly empty.
- The moths are a key food source for the critically endangered mountain pygmy possum and are thought to help plants spread pollen.
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Why it matters
- constraint Biodiversity trackers can record the bogong return as renewed swarms, but cannot yet express it as a share of the 4 billion baseline or of a low that may have been near 20 million.
- decision Because researchers credit wetter weather for the rebound, managers weighing pesticide or habitat measures learn little from it about whether either pressure has eased.
- exposure If a hot El Nino summer reverses the rebound, the mountain pygmy possum loses a key food supply along with the moths.
Apply the 99.5% estimate to a good year's flight of 4 billion moths and the population left would be on the order of 20 million [1]. Treat that as a rough figure. It assumes the estimate was measured against a year of that size [1][3].
The evidence for the return is what people can see. During the crash years moths still reached the caves, only very few of them [4]. "People described going into the caves and finding barely any moths, and just kind of being in shock," said Eleanor Drinkwater, an insect researcher at Western Sydney University [5]. Now, said Marissa Parrott, a conservation biologist at Zoos Victoria, "People are noticing those big swarms again." [10] The report does not include a current population estimate, so the rebound cannot yet be placed anywhere between that low and the old 4 billion [3][1].
On cause, the researchers are cautious. They are still working out what drove the bounce-back and believe a spate of wetter weather helped [6]. That fits the account of the collapse, in which drought was one of three named pressures alongside pesticides and habitat destruction [1]. A rebound that follows the rain is consistent with drought as a driver. It does not test whether the other two pressures have eased.
Heat is the risk this summer. A severe El Nino is forecast to send temperatures soaring, and scientists hope the rebound is not short-lived [7]. "It's great they're back, and we're really excited they're back," Drinkwater said. "But we're still worried about them." [8]
Every moth in this season's swarms is making the trip for the first time, because each bogong makes it only once [14]. How they find the way was the subject of a lovely experiment last year. Researchers tethered captive moths, projected stars onto a wall and showed that the insects steer by the Milky Way [13]. They were the first invertebrates shown to use celestial navigation on trips that long [13].
What to watch
- A published count or population estimate from the alpine caves that places this season's swarms against the 4 billion good-year baseline.
- Moth numbers in the caves after the severe El Nino summer now forecast.
- Research isolating what drove the rebound, in particular whether wetter weather alone accounts for it or pesticide and habitat pressures also changed.