Land managers commonly assume that nonindigenous plant species (NIS) are rapidly increasing in population size in all environments in which they occur. In fact, these plant species have differing levels of invasiveness depending on environment. A method was developed that quantifies invasiveness of a plant population based on annual changes in plant density and area occupied, within a series of permanently placed 1 m² (10.76 ft²) monitoring plots. An invasiveness index (I) was calculated from the change in proportion of cells occupied and the proportions of cells that had growth rates > 1 and < 1; the possible value is restricted from −4 to 4. The method was tested on populations of yellow toadflax over 6 yr on a total of six populations within three distinct environments (Ridge, Valley, and Forest). Invasiveness values were different between environments. The Ridge populations had the highest mean level of invasiveness (I  =  0.31), followed by the Valley (I  =  0.26), and then the Forest (I  =  −0.90). Invasiveness also varied by year. The highest annual value of invasiveness was at the Ridge (I  =  1.77) and the lowest was at the Forest (I  =  −1.90), both in 2005. Values of invasiveness were correlated (Pearson's correlation coefficient  =  0.82) with the traditional calculations of population growth rate, but our method provides an enhanced measure of invasiveness because it includes information on both change in population area and density. This research shows that populations of yellow toadflax are not equally invasive in different environments or through time, although consistent patterns can be observed. The method presented and tested was implemented in approximately three person-days per year at less than $500 per year, and can be used to quantify the invasiveness of plant populations and thus allow land managers to prioritize the most invasive populations for management.

Nomenclature: Yellow toadflax, Linaria vulgaris Mill. LINVU.