Working smarter, not harder: Objective-dependent management of an invasive thistle, Carduus nutans

Many invasive species managers state that their objective is to control an invader. However, the appropriate choice of a management option requires a more explicit statement of management objectives, in terms of both the relevant time horizon and spatial scale. Using data from a 2-yr mowing experiment, we show that the most effective management strategy for controlling an invasive thistle depends fundamentally on the management goals. We integrate field data from a two-cohort experiment with modeling to assess 14 mowing treatments (differing in intensity, frequency, and timing, and thus also in their required logistical effort) based on their effectiveness in (1) reducing population density of the existing cohort, (2) decreasing projected long-term population growth, and (3) limiting projected population spread of an invasive thistle, musk thistle (Carduus nutans L.). The treatment with high intensity and a single late mow caused the largest reduction in plant survival (and density of existing adult plants); the treatment with high intensity and an early mow in addition to a late mow was most effective at reducing population growth rate and population spread. Against expectation and conventional wisdom, the most frequent mowing treatment did not provide the most effective management outcome for any stated objective. This study highlights the necessity of clearly defined management aims; the term control is too vague to be truly useful. The results also provide important insights for the management of this invasive species.