The new study is the result of a decade-long collaboration between Dr. Plowright and Peggy Eby, a wildlife ecologist with an adjunct position at the University of New South Wales who has spent 30 years studying the flying foxes that live in a subtropical region of Eastern Australia. They worked with an interdisciplinary group of colleagues to analyze a wide range of ecological data — including on bat roosts, bat fitness, climate, nectar shortages, habitat loss and viral spillover into horses — collected in the region between 1996 and 2020.
Historically, the local black flying foxes, which feed heavily on the nectar of eucalyptus flowers, have lived in enormous nomadic groups, winging their way through native forests in search of trees in bloom. Although the flowers are abundant in summer, during the winter and spring, the supply is much more limited. And every few years, a fluctuation in the climate, such as a strong El Niño event, disrupts winter or spring blooming, creating food shortages.
Typically, the bats have coped by splitting into smaller groups and setting up temporary roosts near more readily available food sources, such as farms or urban gardens. When the nectar shortages eased, the bats would find their way back to the forest. “As soon as the nectar started to flow again, they’d re-fuse into big aggregations and start becoming nomadic and feed in native forests again,” Dr. Plowright said.
This pattern held during the early years of the study period, from 1996 to about 2002, the researchers reported. And during these years, there were no Hendra spillovers detected in the region.
But around 2003, the pattern changed, the scientists found. When major food shortages struck, new groups of bats would still splinter off from their compatriots and set up shop near farms and cities. But now, the bats made these new habitats permanent, abandoning their nomadic forest lifestyles.
Between 2003 and 2020, the total number of roosts in the region tripled, while the size of each bat group declined. In addition, the roosts grew closer together, and the bats foraged in smaller areas.
