Shifting mechanisms of biotic resistance following invasion control: Consumers, competition and legacy effects

Biotic resistance to plant invasion is driven by competition with the resident vegetation and antagonistic interactions with other biota in the community, but how these mechanisms operate after invasion control remains poorly understood. We assessed whether the resistance to reinvasion by lodgepole pine ( Pinus contorta ) and to secondary invasion by radiata pine ( Pinus radiata ) and Douglas fir ( Pseudotsuga menziesii ) differs between uninvaded grassland and sites where P. contorta had been controlled. We set up a factorial field experiment in the New Zealand High Country combining animal exclusion and vegetation removal treatments in grassland areas and previously controlled stands of P. contorta . Seeds of all three conifer species were sown in two cohorts, and we monitored seedling emergence, survival and resulting establishment over an 18 month period. Seedling emergence and survival were differently influenced by animal access and resident vegetation, and the effects differed between habitats. Animal access reduced seedling emergence, particularly in controlled pine stands, showing early consumer‐driven biotic resistance. In the grassland, intact vegetation increased seedling emergence, suggesting facilitation under stressful conditions, while no such effect occurred in the controlled pine stands. In contrast, seedling survival was consistently lower in the presence of vegetation across both habitats, showing biotic resistance driven by plant competition. The resulting seedling establishment was lowest in open and undisturbed plots in controlled pine stands, indicating increased biotic resistance as a buffer against reinvasion and secondary invasion. Our results show that the components of biotic resistance following invasion control can shift over time and can be modified by the legacy of the primary invader. Consumers primarily limit establishment during the early seed‐to‐seedling transition and plant competition thereafter, together finally providing resistance against reinvasion and secondary invasion. These findings highlight that post‐control legacies can impact habitat invasibility, with implications for the prioritisation of non‐native conifer management. Synthesis and applications . Post‐control areas are less vulnerable to reinvasion when vegetation structure and consumer communities are maintained. Managers should prioritise preventing establishment in uninvaded grasslands, while using grazing and retaining woody debris to strengthen natural biotic resistance and improve long‐term, cost‐effective invasive conifer control.