Parsing propagule pressure: The effects of multiple aspects of propagule pressure on the success of an invasive species
Propagule pressure is known to be a strong driver of successful biological invasions. This commonly used measure typically is thought of as combining the number of non-native individuals released at discrete events (intensity) and the number of release events (frequency). However, additional aspects to propagule pressure are rarely considered: the timing and duration of propagule arrival may also influence invasion success. We examined both intensity and frequency in conjunction with invasion timing, duration, and genetic diversity to determine whether they have an effect on establishment of invasive musk thistle (Carduus nutans). Over a five week period in the fall of 2013, 37 planting treatments, including a control, were replicated 10 times for a total of 370 plots measuring 25 x 25cm. Each treatment had one of three different intensities (propagules of 60, 30, and 15 seeds per plot), one of two different levels of genetic variation (seeds from one or ten mothers), and one of six planting regimes that varied in the timing and duration of the plantings (all seeds at one time, or spread out over 3 or 5 weeks). Censuses were conducted throughout the growing season to assess individual growth, survival, flowering, and fecundity of each plot.
Of the 12600 seeds that were planted, 1209 rosettes established during the fall and 22 flowered the following summer. Treatment effects were analyzed for plants establishment in late November. Maternal genetics did not show an effect, contrary to expectations. Intensity, however, increased establishment. The different timing treatments also influenced success, with significantly lower establishment in plots that received seeds over longer periods. These results suggest that timing of propagule arrival may have a greater effect on establishment than previously realized. Only considering one or two aspects of propagule pressure may provide a misleading view of the factors affecting invasion and lead to the selection of management strategies that are not the most efficient for control. Understanding the relative importance of these components of propagule pressure is key to effectively manipulating propagules to minimize invasion. The information from this study could help to better elucidate the complicated mechanisms underlying invasion and hence allow land managers to improve control of invasive species.