Nassella trichotoma in modified tussock grasslands in New Zealand: a case study in landscape‐scale invasive plant population monitoring

An extensive metapopulation of Nassella trichotoma (nassella tussock) distributed over 878 properties, mostly pastoral farms, in the Hurunui District of North Canterbury, New Zealand, has been the subject of a regional management plan since 1941, that aimed at ‘eradication’ and more recently ‘progressive control’. Here, we describe the implementation, analysis and interpretation of a monitoring programme designed to quantify the expected decline in the weed's population. For this purpose, an annual stratified random survey was initiated in 1998. Properties were allocated to strata primarily according to their N. trichotoma plant population sizes, and sample size per stratum was determined using an optimal allocation procedure. Annually, the N. trichotoma plants on random transects within ‘homogeneous areas’ on each of about 50 randomly selected properties were counted, and the metapopulation density estimated. After adjustment for ‘ease of detection’ during monitoring, regression analysis revealed that the metapopulation of N. trichotoma has been stable from 1998 until 2014. The adaptive management response could be either to change the management to achieve population decline or change the objective to one of maintaining the stability in the metapopulation. This case study has many species‐specific features, but nevertheless, serves as an illustration of a general methodology for monitoring.