Accentuating the positive while eliminating the negative of alien tree invasions: a multiple ecosystem services approach to prioritising control efforts.
Invasive plants can have both positive and negative impacts on ecosystem services (ES), with decisions on control often being characterised by conflicts over loss of their perceived positive impacts on individual ES following removal. We present an analytical framework to aid in reducing such conflicts by allocating control effort to both minimise negative impacts and to maximise positive impacts on multiple ES. We used spatial models to map the negative impacts of invasive conifers on biodiversity, perceived landscape quality, and water yield and their positive impacts on erosion protection and carbon storage across a major catchment in the South Island of New Zealand. We tested the effect of distribution type (i.e. hotspot vs Gaussian) on trade-offs among these ES. We also tested whether using a non-linear function optimisation algorithm to fit variable weights to individual ES significantly reduced trade-offs. We show that an optimised multiple-ES approach could considerably reduce conflicts around invasive tree management arising from their contrasting impacts on different ecosystem services (i.e. by reducing trade-offs between ES), but cannot remove such conflict altogether. Our results are consistent with studies showing that ES with a hot-spot type distribution are the most vulnerable to trade-offs in multi-ES prioritisation, and hence will be the most likely to cause conflict in invasive tree control decisions. Our approach also shows that giving higher priority to ES with hot-spot distributions could reduce conflicts (by reducing tradeoffs). We argue that, when ES data are available, including estimates of ES impacts should be among the "due diligence" requirements for developing invasive tree control strategies.