• Biocontrol of Calluna vulgaris is associated with limited soil and vegetation change. • Herbivory severity is associated with variation in soil pH and available Fe. • Soil bacterial communities are structured by edaphic factors rather than herbivory. • Alien plant cover was associated with herbivory rather than with soil properties. • Biocontrol shows minimal indirect impacts in the Central Plateau ecosystem. Classical biological control is widely used to manage invasive weeds, yet its broader ecosystem consequences remain less understood. Post-release monitoring typically focuses on agent establishment and vegetation responses, while potential indirect effects on soil properties and microbial communities are rarely assessed. Secondary invasion may also follow suppression of a dominant invader through gap creation, resource redistribution, or indirect changes to soil conditions. Here, we examined spatial associations between herbivory severity by the biocontrol agent Lochmaea suturalis on the invasive shrub Calluna vulgaris and variation in soil chemical properties, soil bacterial community composition, and vegetation composition in New Zealand’s North Island Central Plateau. Using an observational, transect-based design, we assessed whether differences in herbivory severity were associated with soil chemistry, soil bacterial communities in relation to underlying edaphic gradients, and patterns of alien plant cover indicative of secondary invasion. Plots experiencing greater herbivory were associated with lower soil pH and higher acid oxalate–extractable Fe, although the single-time-point nature of the study precludes inference about causality. Soil bacterial community composition was structured primarily by abiotic soil properties, particularly pH, and showed no detectable association with herbivory severity. Alien plant cover was significantly higher in plots experiencing moderate to severe herbivory. Overall, our findings indicate that biocontrol of C. vulgaris is associated with limited and context-dependent changes to soil chemistry and vegetation, while soil bacterial communities remain governed by underlying edaphic conditions. These results support biocontrol as a low-impact weed management approach in systems with strong abiotic constraints and persistent native vegetation.
McGrannachan et al. (Fri,) studied this question.
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