Exploring the use of targeted selective treatment and stocking density to reduce the non-target effects of ivermectin on a northern temperate decomposer community. 

Abstract

Agricultural intensification, specifically the routine use of anthelmintic drugs such as avermectins, and the loss of traditional farming practices such as seasonal movement of herds between pastures, are major drivers of decomposer insect declines, with cascading impacts on nutrient cycling, productivity, and the global carbon budget. We developed a spatially explicit, population-based simulation model to explore how targeted selective treatment (TST) and livestock stocking density influence the dynamics of a multi-guild decomposer community in a northern temperate farmland. The model incorporates three functionally diverse dung-colonising guilds: flies, endocoprid beetles, and paracoprid beetles, across four life stages. We incorporate lethal and sublethal effects of ivermectin, a common avermectin; realistic life-history parameters; and dispersal mechanisms over a 1,000-day timescale. Increasing TST generally enhances abundances with increases slowing beyond low–intermediate levels. Increased stocking density amplifies these effects, particularly for flies. There are strong guild-specific responses: as TST and stocking density increase, flies outcompete beetles across life stages due to high fecundity and rapid development, while beetle pupating populations remain suppressed even under high TST. However, high fly abundance accelerates pat degradation, constraining beetle recovery. Interaction effects indicate that benefits of reduced ivermectin treatment are contingent on dung availability, with competitive exclusion shaping community structure. These findings highlight the complex, non-linear interplay between anthelmintic use, dung availability, and species interactions, underscoring the need for multi-guild, landscape-scale perspectives of sustainable grazing strategies.

Date of Award	20 Jan 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Chris F Clements (Supervisor) & Richard Wall (Supervisor)