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Apex consumers are declining worldwide. While the effects of apex predator declines on ecosystems are widely documented, the cascading effects of apex scavenger declines are poorly understood. We evaluated whether disease‐induced declines of an apex scavenger, the Tasmanian devil (Sarcophilus harrisii), increased carrion use by invertebrate scavengers. We manipulated devil access to 36 carcasses across a gradient of devil density from east to west Tasmania and measured carcass use by invertebrates. We found the amount of carcass removed within 5 days was 3.58 times lower at sites with the lowest devil densities. Adult carrion beetle (Ptomaphila lacrymosa) and blow fly (Calliphoridae) larvae abundances were two times higher at open‐access carcasses at low‐density sites than at intermediate‐ and high‐density sites. Adult beetles persisted for 10 days at the low‐density site but declined after 5 days when devils had access to carcasses in intermediate‐ and high‐density sites. Blow fly larvae abundance was not affected by devils in the low‐density site but decreased with devil access in intermediate‐ and high‐density sites. Our results suggest that apex scavenger declines may increase invertebrate scavenger abundance and their contribution to carrion decomposition, with potential cascading effects on nutrient cycling and ecosystems. 

Global apex scavenger declines strongly alter food web dynamics, but studies rarely test whether trophic downgrading impacts ecosystem functions. Here, we leverage a unique, disease‐induced gradient in Tasmanian devil (Sarcophilus harrisi) population densities to assess feedbacks between carcass persistence, subordinate scavenger guilds, and biogeochemical cycling. We further explored interkingdom and seasonal interactions by manipulating carcass access and replicating experiments in warmer, drier summer versus cooler, wetter winter periods. We show Tasmanian devil declines significantly extend carcass persistence and increase the flux of carcass‐derived nutrients belowground (e.g., by 18–134‐fold for ammonium). Greater nutrient availability reduces soil microbiome diversity by up to 26%, increasing the relative abundance of putative zoonotic pathogens. Nutrient subsidies also shift microbial communities toward faster‐growing taxa that invest less energy in resource acquisition, with implications for soil carbon sequestration. Rates of carcass decomposition were reduced in the winter, dampening soil biogeochemical responses and interkingdom competition. Notably, while less efficient scavenger guilds clearly facilitate carcass consumption, they were not able to fill the functional role of apex scavengers. Our study illustrates how trophic downgrading effects can ripple across all levels of ecological organization.