Secondary invasion: The bane of weed management

Abstract Exotic plant invasions present a global threat to natural ecosystems, yet the efficacy of management efforts in mitigating invader impacts remains unclear. A rapidly emerging problem is that of secondary invasion — an increase in abundance of non-target exotics following treatment of targeted invasive plants. Here, we present a global literature review and meta-analysis directed at quantifying the magnitude of secondary invasion effects and identifying possible causes. Of 168 studies examining the efficacy of exotic plant management in terrestrial habitats, 29% quantified community responses sufficiently to evaluate secondary invasion. Meta-analysis of 60 cases from 38 studies showed that control efforts strongly reduced target invader abundance overall, but secondary invaders increased, with a mean effect size double what was found for native plants, which increased only weakly. Moreover, 89% of the secondary invaders identified were classified as noxious or invasive plants. Increases in secondary invaders were correlated with target invader reductions, but control method and target invader growth form failed to explain variation in secondary invader responses. These results suggest that target invader suppression is the key factor driving release of secondary invaders. However, management side effects, target invader legacy effects, provenance effects, and shifting environmental conditions may all facilitate secondary invasion. Invasive plant management often successfully suppresses target invaders, but the result is largely secondary invasion. Addressing this problem requires management strategies that anticipate and suppress secondary invaders while rapidly restoring native plants to fill the space vacated by the target weed. Accomplishing the latter will require improved re-vegetation techniques.