Tiny but mighty: Insects can drive carrion decomposition in the absence of facultative vertebrate scavengers

Scavengers play a key role in recycling animal biomass, yet the relative contributions of insects and vertebrates to this process remain poorly understood. Moreover, understanding whether one scavenger group can compensate for the absence of another is critical for predicting how carrion decomposition may respond to changes in community composition or seasonal conditions. To address this, we assessed the capacity of scavenging insects and facultative vertebrate scavengers to recycle animal biomass in alpine Australia. We measured carcass persistence (decay rates) of 64 eastern gray kangaroos (Macropus giganteus; 1709.6 kg of total biomass) across four seasons and experimental scenarios of (1) full vertebrate and insect scavenger access, and (2) vertebrate exclusion but insect access. In warmer seasons, insects were the primary consumers of carrion, with only occasional vertebrate scavenging observed. There was almost a complete lack of carrion removal in colder seasons when insects were less active, even where vertebrate scavenging rates were highest. Moreover, the presence of vertebrates did not influence the number of flies or beetles at carrion in any season, suggesting that vertebrates were not suppressing these specialist insect scavengers by rapidly consuming carrion. Our results indicate that carrion decomposition of a relatively large carcass in this alpine ecosystem is primarily driven by insects. Indeed, insect scavengers compensated for the absence of vertebrates, resulting in similar carcass removal rates across treatments. However, vertebrate scavengers did not compensate where insect scavengers were functionally absent during cooler months, with carcasses persisting for prolonged periods. These findings stand in contrast to many studies globally that demonstrate a dominant role of vertebrate scavengers in removing carcass biomass or outcompeting insects, even during colder seasons. Understanding whether insects can compensate for the absence of vertebrates to accelerate carcass biomass removal and vice versa-a form of functional redundancy-is crucial for informing when efficient carcass removal occurs and in turn how ecosystems might respond to high carcass loads during different times of the year.