Modelling the local spread of invasive plants: importance of including spatial distribution and detectability in management plans

1. The success of locally applied treatments for exotic weed control depends on the effectiveness of the method used and workers' abilities to find plants within infested sites. Detectability of exotic plants, however, depends on aspects of a plant's spatial distribution such as the number and size of patches. We lack an explicit examination of how incomplete detection affects the spread of exotic weeds under a range of realistic field conditions. 2. We developed a model of spatial spread of the exotic plant, Lespedeza cuneata, based on 3 years of data in a Kansas, USA, grassland. We then expanded and generalized the model to study how treatment intensity, spatial distribution of stems and detectability influenced control efforts. 3. When left untreated, occupancy and abundance were higher with randomly distributed infestations than with patchy distributions. Control treatments slowed spread, but only the most intense treatments reduced occupancy and abundance. 4. The greatest spread occurred when low detectability was accompanied with low treatment intensity because many small patches were not treated; this phenomenon was particularly common with random spatial distributions that initially lacked large, easily detectable patches. In contrast, high treatment intensity led to similarly slow spread for both spatial patterns and a range of detectability functions. 5. Synthesis and applications. We developed a model to simultaneously manipulate the spatial distribution of the invading plant, the intensity of control methods used to manage the population and the detectability of occupied areas at a site; different combinations of these three factors led to very different rates of exotic spread. Managers will reasonably try to implement intensive weed control. However, poor timing of treatments, for example, could lead to variation in treatment effectiveness. To maximize success, managers should explicitly consider detectability, especially where small patches are scattered throughout a site. To quantify the probability of detection, managers could perform multiple observer surveys; such information could help to determine the effort needed for effective control. Creating weed maps may increase the detection of existing patches from year to year, but workers would still need to search the entire site for new or previously undetected patches.