Hornets in town: How top predators coexist

In urban environments, competing hornet species coexist by specializing on different prey species. This finding was made possible by pioneering DNA analysis of hornet larvae’s gut contents and shows that cities are fascinating model systems for how predatory species adapt to environmental stress.

City gangs brutally illustrate a principle that is a staple in ecological theory, the “competitive exclusion principle”: Two species competing for the same limited resource cannot coexist. Nature seems to find more peaceful solutions.

“The yellow-vented hornet and the Japanese yellow hornet are both considered urban adapters with nesting sites and activity periods largely overlapping. Top predators like these should not coexist. And yet, they both thrive in urban areas,” says Kobe University entomologist SAGA Tatsuya.

The details of how they achieve this, however, have remained shrouded in mystery, probably largely because visual observation of hornet feeding habits is difficult and insufficient.

Saga recounts an event that led him to come up with a solution, saying, “While working in a region where people observed the custom of eating giant hornet larvae, I had the opportunity to eat one myself. I cannot forget the unique, gritty texture of the intestinal contents. At that moment, it occurred to me that extracting the DNA from the larvae’s intestinal contents might finally reveal what hornets eat.”

And so, he developed a technique that applies “DNA metabarcoding,” sampling DNA from a specific environment and categorizing it into taxonomic groups, to larva contents acquired from hornet nests that were collected during exterminations across Kobe and its environs.

In the journal Entomologia Generalis, Saga and his collaborator (now at Kyushu University) published what they found: The two hornet species divided their prey by specializing on different targets. While the Japanese yellow hornet ate soft-bodied prey like crickets and moths, the yellow-vented hornet mostly consumed “low-value prey” such as beetles and other wasps.

Saga says, “We were surprised to see this pattern, because the yellow-vented hornet usually displaces the Japanese yellow hornet in sap-feeding experiments and is thus considered more competitive.”

All this is even more interesting since the entomologists conducted their analysis both in urban and non-urban environments and found that away from town, the two species actually consumed similar prey. However, going from non-urban to urban environments, it was the “more dominant” yellow-vented hornet that changed its food habits more drastically than the Japanese yellow hornet.

This means that resource dominance is possibly less suited to predicting a predator’s success under stress than its ability to adapt its prey choice.

“These changes show that cities act as trait filters that drive niche differentiation by selecting on traits that confer resilience under stress,” explains Saga.

Urbanization is not only a rapidly advancing, global process, it is also a process that occurs consistently across a broad range of climates and regions.

These processes include habitat simplification and resource fragmentation that make urban habitats “valuable model systems for the understanding of broader patterns of environmental change,” the researchers write.

In addition, Saga hopes his study may lead to a new understanding of hornets, saying, “The study provides the data to see hornets not merely as dangerous pests but as important ecosystem regulators.”