Control or re-treat? Model-based guidelines for managing established plant invasions

Established invasions have wide-ranging negative impacts but constraints relating to invader detectability, cost, and efficacy of control may hamper management efforts. One choice that managers face is whether to target control efforts at heavily invaded areas that are sources of invasive propagules or to re-treat previously controlled areas that may be cheaper to control and in which local elimination may be achieved. We developed a mathematical model for the dynamics and control of a plant invasion to determine whether prioritizing heavily invaded or recently controlled areas better achieves each of two management objectives: minimizing the total number of detectable invaded sites (eradication) or heavily invaded sites only (beautification). We provide general guidelines for management by considering how invader traits, budgetary and time constraints, and control efficacy influence prioritization of control efforts and discuss their applicability to long-established invasive plants. For a wide range of invaders and control types, we find that prioritizing heavily-invaded sites performs better for meeting the beautification objective, while re-treating previously controlled sites better achieves eradication. However, combinations of invader traits can lead to the alternate strategy being favored for each management objective, while the optimal choice of management strategy can switch depending on the time horizon of control and annual budget. We summarize model findings to provide general guidelines for invasion managers on how to efficiently allocate limited resources based on invader life history, control cost, and mode of action.