Grubbing and biological control strategies for Nassella trichotoma evaluated using a matrix population model

Abstract

AbstractA stage‐structured matrix population model for the invasive perennial grass Nassella trichotoma (nassella tussock) was used to compare a wide range of hypothetical grubbing and biocontrol management options in a population of 19 visible plants ha−1 on a typical sheep and beef cattle‐grazed farm in North Canterbury, New Zealand. The model has two soil seed banks (ephemeral and persistent), five plant size classes, seed production and ground cover‐dependent population growth reaching a carrying capacity of 35,000 plants ha−1 (80% ground cover) in the absence of management. Elasticity analyses showed that population growth (λ) was most sensitive to plant size class transitions and relatively insensitive to the soil seed bank transitions, seedling recruitment and seed production, implying management targeting the growth and/or survival of plants will be most effective. This conclusion was supported by population trajectory simulations which revealed that a biological control agent attacking seeds or seedlings would need to be 99.8% effective to achieve the population stability achieved under current management where 34.4% of the plants are grubbed annually. By contrast, a biological control agent attacking tillers and reducing plant transitions (growth) across all size classes by 30% annually gave a population trajectory equivalent to that achieved by annual grubbing. In the presence of annual grubbing, a combination of seed‐ and tiller‐feeding agents giving 10% reductions in fecundity and plant growth, respectively, reduced the population density by 60%. Overall, the modelling shows that relaxing the current practice of annual grubbing (by reducing the rate or frequency of grubbing) whether in spring or autumn will result in the growth of N. trichotoma populations in North Canterbury. It also indicates that a biocontrol agent that impedes tiller production in N. trichotoma plants could potentially be an effective replacement for, or adjunct to, annual grubbing.