Quantifying changes in density and spatial extent of multiple Centaurea maculosa Lam (spotted knapweed) populations

Background/Question/Methods: Exotic species vary in their rate of establishment, spread and impact at both the local and landscape scale. Changes in exotic species’ density and spatial extent over time are often used as surrogate measures of impact; hence, it is important to quantify these rates across a range of environments. We studied the spatial and temporal growth of Centaurea maculosa (Lam.) populations across four different natural plant communities. At each site we measured changes in C. maculosa presence/absence and density in 33 0.25 m² plots (132 plots total). Temporal population growth rate was measured as the change in population density (N) over the change in time (t) = δN/δt thus δN = N_t+1 - N_t or λ. Spatial growth of populations was measured in two ways: (1) change in spatial extent over time, and (2) local colonization and extinction rates. The probability of colonization was measured as the number of plots unoccupied in t that became occupied in t + 1 divided by the number of plots observed. The probability of extinction was measured as the number of occupied plots in t that became unoccupied in t + 1, divided by the number of plots occupied in time t.  We also examined the relationships between temporal growth, spatial growth and habitat suitability for C. maculosa (generated from a logistic model of presence/absence data and environmental variables). We hypothesized that not all populations of C. maculosa would increase in density and spatial extent. We also hypothesized a positive relationship between the population growth rate (λ) and habitat suitability.

Results/Conclusions: Of the 132 plots, 7 unoccupied in t became occupied in t +1, and 7 occupied in t became unoccupied in t +1, meaning the probability of extinction equals the probability of colonization for these populations. Twenty-one percent of populations had a λ <1, 61 percent had a λ > 1, and 18 percent were stable. Populations with λ >1 were typically increasing in spatial extent as well. We found a significant positive relationship between λ and habitat suitability. Of the 13 patches studied, 8 decreased in spatial extent and 5 increased in spatial extent. These results highlight the fact that not all exotic species consistently increase in density and spatial extent in all habitats. The positive relationship between population growth rates and habitat suitability predictions suggest these predictions could be used for exotic plant management prioritization.