Space use contributed to the emergence of carnivorous squirrels in a vole boom year

Abstract

Biologists have long recognized the importance of two native rodent species – California ground squirrels and voles – as important ecosystem engineers and prey for a suite of predators in California ecosystems. However, in the twelfth year of studying California ground squirrels at Briones Regional Park, for the first time, we observed ground squirrels shifting their diets from primarily granivorous to actively consuming vole prey. In 2024, our team documented a total of 74 events involving the hunting, killing, and/or consuming of voles by ground squirrels. Here we explored associations between vole density and these events. First, we extracted iNaturalist data to quantify vole numbers. Vole sightings in 2024 were regionally high at our site and seven times greater than the 10-year average statewide. Second, we created heat maps to show that vole-squirrel events occurred mostly where vole and ground squirrel burrows were the closest at our site. Our findings reveal important associations between this unusual peak in vole numbers, close spatial proximity between voles and squirrels, and the emergence of novel carnivorous behaviors by squirrels. Thus, spatial overlap with a locally abundant prey in a boom year likely contributed to the emergence of unusual dietary shift by a granivorous mammal.